Review Immune mechanisms in the pathogenesis of …...(MHC) class I transgene [3]. These non-immune mechanisms have been addressed in a recent review paper [4]. New data are constantly

of 12(page number not for citation purposes)

Available online http://arthritis-research.com/content/9/2/208

AbstractIdiopathic inflammatory myopathies (IIMs), comprising polymyositis,dermatomyositis, and inclusion-body myositis, are characterized byinflammatory cell infiltrates in skeletal muscle tissue, muscleweakness, and muscle fatigue. The cellular infiltrates often consistof T lymphocytes and macrophages but also, in some cases,B lymphocytes. Emerging data have led to improved phenotypiccharacterization of the inflammatory cells, including their effectormolecules, in skeletal muscle, peripheral blood, and other organsthat are frequently involved, such as skin and lungs. In this reviewwe summarize the latest findings concerning the role ofT lymphocytes, B lymphocytes, dendritic cells, and other antigen-presenting cells in the pathophysiology of IIMs.

IntroductionIdiopathic inflammatory myopathies (IIMs) are characterizedby mononuclear inflammatory cell infiltrates in skeletal muscletissue, by muscle weakness, and by muscle fatigue. They areoften subclassified into three major groups on the basis ofclinical and histopathological differences: polymyositis,dermatomyositis, and inclusion-body myositis. The cellularinfiltrates in muscle tissue are mainly composed of T lympho-cytes and macrophages but also, in some cases, B lympho-cytes. This observation, together with frequently detectedautoantibodies particularly in polymyositis and dermato-myositis, suggests that the inflammatory myopathies areimmune-mediated; they are believed to be triggered byenvironmental factors in genetically susceptible individuals.The varying clinical features and the different predominatinghistopathological features such as localization andphenotypes of inflammatory infiltrates, or rimmed vacuoles asseen in inclusion-body myositis, suggest that there aredifferent pathophysiological mechanisms leading to myositis.Despite these differences the inflammatory moleculesproduced in muscle tissue are highly similar in chronic

inflammatory myopathies, suggesting that some molecularpathways are shared between the subsets of inflammatorymyopathies.

In the inflammatory myopathies there are also signs ofmicrovascular involvement. The involvement of microvesselswas first reported in patients with dermatomyositis ascapillary loss and recognized by the presence of themembrane attack complex (MAC) [1,2]. Later, activatedcapillaries with increased expression of adhesion molecules(intercellular cell-adhesion molecule-1 and/or vascular cell-adhesion molecule-1) and IL-1α were also seen in patientswithout skin rash, in polymyositis and inclusion-body myositis.Damage or activation of blood vessels could indicate that themicrovessels are targets of the immune reaction in somesubsets of patients with IIM.

It has long been recognized that the inflammatory cellinfiltrates and muscle fiber damage are patchy and aresometimes not detected in muscle biopsies. This is a clinicalproblem in the diagnostic procedure. Moreover, the lack ofcorrelation between the degree of inflammatory infiltrates andmuscle weakness has led to a search for mechanisms otherthan immune-mediated muscle fiber damage that could causemuscle weakness. One such non-immune mechanism,endoplasmic reticulum stress, has been proposed, on thebasis of observations both from human studies and from ananimal model for myositis, the major histocompatibility complex(MHC) class I transgene [3]. These non-immune mechanismshave been addressed in a recent review paper [4].

New data are constantly emerging, leading to improvedcharacterization of the phenotypes of the inflammatory cellsand their effector molecules that are expressed in IIMs, not

ReviewImmune mechanisms in the pathogenesis of idiopathicinflammatory myopathiesCecilia Grundtman, Vivianne Malmström and Ingrid E Lundberg

Rheumatology Unit, Department of Medicine, Karolinska University Hospital Solna, Karolinska Institutet, SE-171 76 Stockholm, Sweden

Corresponding author: Cecilia Grundtman, [email protected]

Published: 26 March 2007 Arthritis Research & Therapy 2007, 9:208 (doi:10.1186/ar2139)This article is online at http://arthritis-research.com/content/9/2/208© 2007 BioMed Central Ltd

APC = antigen-presenting cell; DC = dendritic cell; ICOS = inducible co-stimulator; ICOS-L = ICOS ligand; IFN = interferon; IIM = idiopathicinflammatory myopathy; IL = interleukin; ILD = interstitial lung disease; MAC = membrane attack complex; MHC = major histocompatibility complex.


Arthritis Research & Therapy Vol 9 No 2 Grundtman et al.

only in the major target organ, the skeletal muscle, but also inperipheral blood and in other organs that are frequentlyinvolved, such as skin and lungs. This increasing knowledgehas a great potential to improve our understanding of the roleof these inflammatory cells in disease mechanisms in IIMs. Inthis review we summarize the latest findings concerning therole of T lymphocytes, B lymphocytes, dendritic cells, andother antigen-presenting cells (APCs) in the pathophysiologyof IIMs.

T lymphocytesT lymphocyte functionT lymphocytes recognize antigens on APCs through theT-cell antigen receptor in a MHC-restricted fashion. Peptidesfrom intracellular pathogens proliferating in the cytoplasm arecarried to the cell surface by MHC class I molecules andpresented to cytotoxic (CD8+) T lymphocytes, which oncefully activated have the capacity to lyse infected target cells.In contrast, peptide antigens from pathogens in intracellularvesicles, and those derived from ingested extracellularbacteria and toxins, are carried to the cell surface by MHCclass II molecules and presented to CD4+ T helper cells.These can then differentiate into effector cells, such as TH1,TH2, and TH17 cells [5]. Pathogens that accumulate insidemacrophages and dendritic cells (DCs) tend to stimulate thedifferentiation of TH1 cells and the production of IgGantibodies by B lymphocytes. Conversely, extracellularantigens tend to stimulate the production of TH2, which cansubsequently stimulate the production of IgA and IgE. TH17 isa recently described effector T lineage that has beensuggested to contribute to chronic inflammatory settings.CD8+ T lymphocytes do not have as distinct sublineages andare cytotoxic cells working in a perforin/granzyme-dependentmanner; interestingly, CD4+ lymphocytes can sometimes alsodisplay cytotoxic effector functions.

T lymphocytes in idiopathic inflammatory myopathiesAlthough prominent T lymphocyte infiltrates are not alwaysfound in muscle biopsies, two types of cellular infiltrate havebeen recognized in IIMs, one being endomysial inflammatoryinfiltrates consisting mainly of CD8+ T lymphocytes andmacrophages invading non-necrotic muscle fibers expressingMHC class I antigens [6-8]. These are typically, but notexclusively, found in inclusion-body myositis and polymyositis.The other type of mononuclear cell infiltration isperivascular/perimysial and has become a characteristic ofdermatomyositis; it consists predominantly of CD4+ Tlymphocytes, occasionally together with B lymphocytes andmacrophages [6,9]. The deposition of complementcomponents is also mainly localized to the perivascularregions of muscular or cutaneous lesions. However, the‘classical’ T lymphocyte picture in IIM is, as the authors say,much more complex and an oversimplification of reality [6,9].Independent of T lymphocyte localization, their presencesuggests an involvement of the adaptive immune system inthese disorders (Figures 1 and 2).

Cytotoxic CD8+ T lymphocytes can release three differentcytotoxic proteins: perforin, granzyme, and granulysin. It isknown from earlier studies that in polymyositis and inclusion-body myositis, CD8+ T lymphocytes and macrophagessurrounding the non-necrotic muscle fibers expressing MHCclass I antigen do express perforin. Perforin may cause a leakin the sarcolemmal surface through which granzymes couldinvade the sarcoplasm to initiate muscle fiber necrosis [10-12]. Recently, granulysin has also been demonstrated in bothpolymyositis and inclusion-body myositis [13]. The presenceof granulysin-expressing CD8+ T lymphocytes tended tocorrelate with steroid resistance in polymyositis [13].Interestingly, perforin/granzyme-expressing CD4+ T lympho-cytes have also been demonstrated [10,14]. A schematicsummary of the potential role of different immune cells in thecontext of chronic muscle inflammation is presented inFigure 3.

The T lymphocyte repertoire in blood seems to differ betweenpolymyositis and dermatomyositis [15]. In peripheral blood ofactive dermatomyositis, a decreased percentage of CD3+

and CD8+ T lymphocytes and decreased IFN-γ expression byCD4+ and CD8+ T lymphocytes but an increase in B lympho-cytes and IL-4-producing CD4+ T lymphocyte frequencieswere found. These features were not seen in the inactive formof the disease. In patients with polymyositis with relapsingdisease, markedly perturbed T cell repertoires were seen. Theexpanded T lymphocytes clonally displayed a memoryphenotype, expressed intracellular perforin, and responded tostimulation by IL-2, which indicates that they have thepotential for reactivation under appropriate conditions [16].This suggests that a continuous autoantigen-driven processmight be prominent in this disease and that in patients withpolymyositis a relapse would more probably be associatedwith the reactivation of clones, which are present at diseaseonset, rather than with the emergence of new ones. Theconcept that clonally expanded muscle-infiltrating CD8+

T lymphocytes could recirculate into the blood is true not onlyfor patients with polymyositis but also for those withinclusion-body myositis [17]. Another possibility for therecirculation of T lymphocytes seen in polymyositis andinclusion-body myositis might be that the same antigen, forexample a virus, could independently induce the sameexpansion in the blood and in the muscle tissue if the antigenis present at both sites.

In patients with IIMs, T lymphocytes are found not only ininflammatory muscle tissues or blood but also in lungs inpatients with interstitial lung disease (ILD), which is afrequent manifestation in patients with polymyositis anddermatomyositis; this was found in up to 60 to 70% of suchpatients when sensitive techniques such as high-resolutioncomputed tomography and pulmonary function tests wereemployed [18]. ILD seems to be less common in patientswith inclusion-body myositis, although no reports are availablein which newly diagnosed patients have been investigated for


lung involvement with these techniques. In the lungs CD8+ Tlymphocytes were distributed both in and around lymphoidfollicles and in the walls of normal-appearing alveoli, whereasCD4+ T lymphocytes and B lymphocytes were seen inassociation with lymphoid follicles. CD4+ T lymphocytes havealso been demonstrated in reconstructed thick alveolar walls[19]. The CD4+/CD8+ ratio in bronchoalveolar lavage fluidwas low [20,21]; most of the CD8+ T lymphocytes were ofHLA-DR+ [21] and CD25+ [22] types, suggesting that theywere activated. In a small study [23], a low CD4+/CD8+ ratiowas seen in bronchoalveolar lavage fluids in all patients withradiographic signs of ILD early in the disease course ofpatients with polymyositis or dermatomyositis. The function ofthese T lymphocytes is not known. It can only be speculatedthat they might interact with antigens (from viruses or othersources) expressed in various pulmonary cells (Figure 3). It isinteresting in this context that there is a reported tissuevariation of histidyl-tRNA synthetases with a higherexpression in normal lungs than in other organs, and thatpatients with autoantibodies against histidyl-tRNA synthetase– anti-Jo-1 antibodies – have ILD in close to 100% of cases.

Furthermore, a restricted accumulation of T lymphocytesexpressing selected T-cell antigen receptor V-gene segmentswas recorded in skeletal muscle and lung but not in peripheralblood, which suggests a common target antigen in theseorgans [23]. Moreover, a role for histidyl-tRNA synthetase inthe disease mechanism is supported by the observation thatthis antigen can serve as a chemokine for DCs and Tlymphocytes when cleaved by certain proteases [24].

Even though the histopathological picture in polymyositis anddermatomyositis is often different, a similar clonal expansionof T lymphocytes is seen in bronchoalveolar lavage fluid [25].In addition, T lymphocytes have been extracted from muscletissue of these patients. The established T cell lines showed avariable proportion of CD4+ and CD8+ T lymphocytes, whichdid not correlate with diagnosis [26]. Examples of CD4+ andCD8+ T lymphocyte localization in polymyositis, dermato-myositis, and inclusion-body myositis are shown in Figures 1and 2. As demonstrated here, a similar pattern with a mixtureof cell populations and both endomysial and perimysiallocalization may be found despite the diagnosis of myositis.


Figure 1

Endomysial localization of CD4+ and CD8+ T lymphocytes. Immunohistochemical staining of samples from patients with polymyositis (PM) (a–c),dermatomyositis (DM) (d–f), and inclusion-body myositis (IBM) (g–i). (a) Hematoxylin and eosin (H&E) staining to localize inflammatory cellinfiltrates in a patient with polymyositis. (b) CD4+ T lymphocytes stained with a monoclonal SK3 mouse IgG1 antibody (Becton Dickinson, SanJose, CA, USA) in the same area as in (a) but further down in the biopsy. (c) CD8+ T lymphocytes stained with a monoclonal SK1 mouse IgG1antibody (Becton Dickinson) in a consecutive section to that in (b). (d) H&E staining to localize inflammatory cell infiltrates in a patient withdermatomyositis. (e) CD4+ T lymphocytes stained with a monoclonal SK3 mouse IgG1 antibody in the same area as in (d) but further down in thebiopsy. (f) CD8+ T lymphocytes stained with a monoclonal SK1 mouse IgG1 antibody in a consecutive section to that in (e). (g) H&E staining tolocalize inflammatory cell infiltrates in a patient with inclusion-body myositis. (h) CD4+ T lymphocytes stained with a monoclonal SK3 mouse IgG1antibody in the same area as in (g) but further down in the biopsy. (i) CD8+ T lymphocytes stained with a monoclonal SK1 mouse IgG1 antibody ina consecutive section to that in (h). Original magnifications: ×250 (a–f) and ×312.5 (g–i).

When evaluating the presence of CD8+ T lymphocytes inbiopsies it is important to consider which reagents have beenused. Although the CD8 molecule is the classic lineagemarker for cytotoxic T lymphocytes, this is only true if the CD8molecule consists of an α and a β subunit (CD8αβ) and not ifit is a CD8αα homodimer. The homodimer can sometimes befound on activated CD4+ T lymphocytes [27], especially inthe gastrointestinal tract. Thus if the reagent used to detectCD8+ T lymphocytes is an antibody against the α subunit itcould detect both classical CD8+ T lymphocytes but alsoactivated CD4+ T lymphocytes. This information was notalways available when immunophenotyping of lymphocytes inmuscle tissue was presented. However, if the analysisincludes the detection of a cytotoxic agent this caveat ispartly avoided. A summary of selected T lymphocyte studiesin IIMs is presented in Table 1.

A role of T lymphocytes in polymyositis and dermatomyositisis further supported by the clinical improvement aftertreatment with immunosuppressive drugs that are known toaffect T lymphocytes. In contrast, patients with inclusion-bodymyositis rarely display improved muscle function after

treatment with immunosuppressive drugs, which is why therole of T lymphocytes in disease mechanisms is morequestionable in this form of myositis. Therapeutic agents thatdo not solely target T lymphocytes, but in all mentionedexamples affect T lymphocyte populations, have been shownto be effective in polymyositis and dermatomyositis; these aremethotrexate, cyclosporin A, tacrolimus, and anti-thymocyteglobulin. Methotrexate is one of the most commonly usedsecond-line immunosuppressive drugs given to patients withIIM. It is known to be well tolerated and effective inpolymyositis and dermatomyositis, although no placebo-controlled trials have yet been performed [28]. There are alsocase series showing beneficial effects of tacrolimus and anti-thymocyte globulin [29,30]. In addition, topical cutaneoustacrolimus therapy has also effectively been applied to skinlesions in patients with dermatomyositis [31]. Although theclinical improvement with these drugs could indicate that Tlymphocytes have a role in polymyositis and dermatomyositis,there are no data available to show that these therapies haveeffects on inflammatory cell infiltrates or molecular expressionin muscle tissue that correlate with the clinical effects, whichwould strengthen such a hypothesis.


Page 4 of 12(page number not for citation purposes)

Figure 2

Perivascular localization of CD4+ and CD8+ T lymphocytes. Immunohistochemical staining of samples from patients with polymyositis (PM) (a–c),dermatomyositis (DM) (d–f), and inclusion-body myositis (IBM) (g–i). (a) Hematoxylin and eosin (H&E) staining to localize inflammatory cellinfiltrates in a patient with polymyositis. (b) CD4+ T lymphocytes stained with a monoclonal SK3 mouse IgG1 antibody in the same area as in (a)but further down in the biopsy. (c) CD8+ T lymphocytes stained with a monoclonal SK1 mouse IgG1 antibody in a consecutive section to that in(b). (d) H&E staining to localize inflammatory cell infiltrates in a patient with dermatomyositis. (e) CD4+ T lymphocytes stained with a monoclonalSK3 mouse IgG1 antibody in the same area as in (d) but further down in the biopsy. (f) CD8+ T lymphocytes stained with a monoclonal SK1 mouseIgG1 antibody in a consecutive section to that in (e). (g) H&E staining to localize inflammatory cell infiltrates in a patient with inclusion-bodymyositis. (h) CD4+ T lymphocytes stained with a monoclonal SK3 mouse IgG1 antibody in the same area as in (g) but further down in the biopsy.(i) CD8+ T lymphocytes stained with a monoclonal SK1 mouse IgG1 antibody in a consecutive section to that in (h). Original magnifications:×312.5 (a–c, g–i) and ×250 (d–f).

B lymphocytesB lymphocyte functionB lymphocytes have a major role in the immunological patho-genesis of autoimmune diseases. Not only can theirdifferentiated progeny, plasma cells [32,33], synthesize andsecret large quantities of immunoglobulins, they could alsoregulate other cell types, secrete cytokines, and presentantigens. B lymphocyte activation requires both binding of anantigen by surface immunoglobulin – the B-cell receptor –and an interaction with antigen-specific T lymphocytes(CD4+). CD4+ T lymphocytes then can induce vigorous Blymphocyte proliferation and direct the differentiation of theclonally expanded progeny of naïve B lymphocytes into eitherplasma cells or memory B lymphocytes. Both cytokines

released by CD4+ T lymphocytes and somatic hypermutationof V-region genes can influence antibody isotype switching orthe antigen-binding properties of the antibody, respectively,resulting in the production of antibodies of various isotypesthat can be distributed to various body compartments.

B lymphocytes in idiopathic inflammatory myopathiesIn 1984 and 1990, Arahata and Engel showed that B lympho-cytes were more frequent in perivascular sites than in endo-mysial sites [6,9]. Furthermore, B lymphocytes were morecommon in muscle tissue from patients with dermatomyositisthan from those with polymyositis or inclusion-body myositis.This observation was further supported by another study inwhich perivascular B lymphocytes were only found in patients



Figure 3

Hypothetical involvement of T lymphocytes, B lymphocytes, and dendritic cells (DCs) in idiopathic inflammatory myopathies. (1) An unknown trigger(for example viral infection or ultraviolet radiation) in the respiratory tract or through the skin leads to the cleavage of histidyl-tRNA synthetase bygranzyme B through antiviral CD8+ T lymphocytes in the lungs. (2) Immature DCs carry receptors on its surface that recognize common features ofmany pathogens. When a DC takes up a pathogen in infected tissue it becomes activated and migrates to the lymph node. (3) Upon activation, theDC matures into a highly effective antigen-presenting cell (APC) and undergoes changes that enable it to activate pathogen-specific lymphocytesin the lymph node. T lymphocytes become activated and B lymphocytes, with active help from CD4+ T lymphocytes, proliferate and differentiateinto plasma cells. (4) Activated DCs, T lymphocytes, and B lymphocytes could release cytokines into the bloodstream. (5) The activated Tlymphocyte, on which the DC-MHC–antigen complex is bound, itself binds to specialized endothelial cells called high endothelial venules (HEV).For this purpose it uses the VLA-4 (very late activation antigen-4) and LFA-1 (lymphocyte function associated antigen-1) molecules on its surface tointeract with adhesion molecules (vascular cell-adhesion molecule-1 (VCAM-1) and intercellular cell-adhesion molecule-1 (ICAM-1)) on HEVs,where they can penetrate into peripheral lymphoid tissues. (6,7) Naïve T lymphocytes and B lymphocytes that have not yet encountered theirspecific antigen circulate continuously from the blood into the peripheral lymphoid tissues. (8,9) Various cytokines from the bloodstream orproduced locally could affect the muscle tissue or cell in many different ways. However, it is not clear whether the muscle cell itself could produceand release cytokines. (10-12) DCs, macrophages (Mφ), and B lymphocytes can interact with T lymphocytes in various ways. T lymphocytes couldpossibly also bind to muscle cells through inducible co-stimulators (ICOS), CD40 ligand (CD40-L), CD28, and CTLA-4 (CD152) on T lymphocytesto ICOS ligand (ICOS-L), CD40, and BB-1 antigen on the muscle cell. In that fashion, the muscle cell would function as an APC. (13) Plasma cells(CD138+) can be found in the muscle tissue of certain subgroups of patients with idiopathic inflammatory myopathy, but whether these cells couldproduce autoantibodies locally is not yet known. (14) T lymphocytes have been shown to bind in close contact with muscle cells and to releaseperforin, granzyme A, and granulysin, which may cause necrosis of muscle tissue or cells.



Tab

le 1

Sel

ecte

d a

rtic

les

on

imm

un

oh

isto

chem

ical

loca

lizat

ion

of

infl

amm

ato

ry c

ells

in id

iop

ath

ic in

flam

mat

ory

myo

pat

hie

s

Ref

eren

ceP

olym

yosi

tisD

erm

atom

yosi

tisIn

clus

ion-

body

myo

sitis

T ly

mph

ocyt

es[9

]53

pat

ient

s. T

lym

phoc

ytes

wer

e le

ast a

bund

ant a

t 31

pat

ient

s. T

he p

ropo

rtio

n of

T ly

mph

ocyt

es w

as

48 p

atie

nts.

T ly

mph

ocyt

es w

ere

leas

t abu

ndan

t at

periv

ascu

lar a

nd m

ost a

bund

ant a

t end

omys

ial s

ites.

The

lo

wes

t at p

eriv

ascu

lar s

ites

and

high

est a

t pe

rivas

cula

r site

s an

d m

ost a

bund

ant a

t end

omys

ial

CD

4/C

D8

ratio

was

hig

hest

at p

eriv

ascu

lar s

ites

and

low

est

endo

mys

ial s

ites.

The

CD

4/C

D8

ratio

was

hig

hest

si

tes.

CD

4/C

D8

ratio

was

hig

hest

at p

eriv

ascu

lar

at e

ndom

ysia

l site

s. T

he p

ropo

rtio

n of

T ly

mph

ocyt

es

at p

eriv

ascu

lar s

ites

and

low

est a

t end

omys

ial s

ites.

si

tes

and

low

est a

t end

omys

ial s

ites.

The

pro

port

ion

estim

ated

to b

e Ia

+, o

r act

ivat

ed, w

as n

early

twic

e as

hig

h at

M

any

infla

mm

ator

y ce

lls d

ispl

ay th

e Ia

mar

ker,

but

of T

lym

phoc

ytes

est

imat

ed to

be

Ia+, o

r act

ivat

ed,

endo

mys

ial s

ites

as a

t per

ivas

cula

r site

son

ly o

ne-s

ixth

of p

eriv

ascu

lar l

ymph

ocyt

es a

nd

was

nea

rly tw

ice

as h

igh

at e

ndom

ysia

l site

s as

at

one-

fifth

of e

ndom

ysia

l T ly

mph

ocyt

es w

as Ia

+, o

r pe

rivas

cula

r site

sac

tivat

ed

[8]

7 pa

tient

s. E

ndom

ysia

l inf

iltra

tes

of T

lym

phoc

ytes

wer

e 21

pat

ient

s. S

ever

e m

uscl

e fib

er n

ecro

sis,

–

pred

omin

ant.

Par

tly in

vade

d no

n-ne

crot

ic c

ells

wer

e al

so

pred

omin

ant p

eriv

ascu

lar T

lym

phoc

yte

infil

trat

es,

seen

; som

e of

thes

e fib

ers

also

exp

ress

ed M

HC

cla

ss I

fibro

sis

and

perif

asci

cula

r atr

ophy

. MH

C c

lass

I w

ere

loca

lized

to p

erifa

scic

ular

fibe

rs

[11]

5 pa

tient

s. P

erfo

rin w

as c

o-lo

caliz

ed w

ith g

ranz

yme

A a

nd

5 pa

tient

s. T

here

wer

e ve

ry fe

w p

erfo

rin+

and

2 pa

tient

s. P

erfo

rin w

as c

o-lo

caliz

ed w

ith g

ranz

yme

occa

sion

ally

inva

ded

non-

necr

otic

mus

cle

fiber

s. T

he

gran

zym

e A

-pos

itive

cel

ls in

DM

A a

nd o

ccas

iona

lly in

vade

d no

n-ne

crot

ic m

uscl

e pe

rcen

tage

of p

erfo

rin+

cells

am

ong

the

endo

mys

ial C

D8+

fiber

s. T

he p

erce

ntag

e of

per

forin

+ce

lls a

mon

g th

e ce

ll po

pula

tion

was

9.9

%en

dom

ysia

l CD

8+ce

ll po

pula

tion

was

12.

5%

[10]

5 pa

tient

s. C

o-lo

caliz

atio

n of

per

forin

with

CD

8, C

D4,

4

patie

nts.

Co-

loca

lizat

ion

of p

erfo

rin w

ith C

D8,

–

CD

3, a

nd C

D2

was

ana

lyze

d. T

he g

ener

al d

istr

ibut

ion

of

CD

4, C

D3,

and

CD

2 w

as a

naly

zed.

The

gen

eral

in

flam

mat

ory

cells

was

end

omys

ial.

The

over

all C

D4/

CD

8 di

strib

utio

n of

infla

mm

ator

y ce

lls w

as p

eriv

ascu

lar

ratio

was

1.1

and

the

tota

l num

ber o

f CD

8+in

flam

mat

ory

and

perim

ysia

l. Th

e ov

eral

l CD

4/C

D8

ratio

was

1.2

ce

lls in

9 ra

ndom

mic

rosc

opic

fiel

ds w

as 2

36. M

ore

than

90%

an

d th

e to

tal n

umbe

r of C

D8+

infla

mm

ator

y ce

lls in

of

per

forin

+ce

lls w

ere

CD

2+C

D3+

, and

per

forin

was

exp

ress

ed

9 ra

ndom

mic

rosc

opic

fiel

ds w

as 1

67. M

ore

than

bo

th in

non

-inva

sive

inte

rstit

ial T

lym

phoc

ytes

and

in a

utoi

nvas

ive

90%

of p

erfo

rin+

cells

wer

e C

D2+

CD

3+, a

nd 5

0% o

f C

D8+

T ly

mph

ocyt

es. O

f all

perfo

rin+

cells

, 60%

wer

e C

D8+

and

all p

erfo

rin+

cells

wer

e C

D4+

and

50%

wer

e C

D8+

. 40

% C

D4+

. Con

vers

ely,

75%

of t

he C

D8+

cells

and

50%

of t

he

Con

vers

ely,

80%

of t

he C

D4+

cells

and

90%

of t

he

CD

4+ce

lls w

ere

perfo

rin+. 4

3% o

f the

CD

8+T

lym

phoc

ytes

C

D8+

cells

wer

e pe

rforin

+. P

erfo

rin w

as d

istr

ibut

ed

that

con

tact

ed a

mus

cle

fiber

exp

ress

ed p

erfo

rin v

ecto

rially

ra

ndom

ly in

the

cyto

plas

m o

f the

infla

mm

ator

y to

war

d th

e ta

rget

mus

cle

fiber

T ly

mph

ocyt

es

[13]

17 p

atie

nts.

The

num

bers

of C

D4,

CD

8, a

nd C

D56

cel

ls p

er

–7

patie

nts.

The

num

bers

of C

D4,

CD

8, a

nd C

D56

1,

000

mus

cle

fiber

s at

end

omys

ial s

ites

wer

e 27

5 ±

140

, ce

lls p

er 1

,000

mus

cle

fiber

s at

end

omys

ial s

ites

355

± 1

50, a

nd 2

5 ±

13

(mea

ns ±

SD

) in

all c

ases

. Gra

nuly

sin

wer

e 25

0 ±

120

, 330

± 2

00, a

nd 3

1 ±

16

was

exp

ress

ed in

the

cyto

plas

m o

f inf

iltra

ting

cells

, mos

tly

(mea

ns ±

SD

) in

all c

ases

. Gra

nuly

sin

was

C

D4+

and

CD

8+ce

lls, o

nly

a fe

w C

D56

+ce

lls a

t a h

ighe

r ex

pres

sed

in th

e cy

topl

asm

of i

nfilt

ratin

g ce

lls, m

ostly

le

vel i

n en

dom

ysia

l site

s th

an in

per

ivas

cula

r site

s. N

otab

ly,

CD

4 an

d C

D8+

cells

, onl

y a

few

CD

56+

cells

at a

st

eroi

d-re

sist

ant p

atie

nts

with

PM

had

a h

ighe

r fre

quen

cy o

f hi

gher

leve

l at e

ndom

ysia

l site

s th

an a

t per

ivas

cula

r do

uble

-pos

itive

CD

8 an

d gr

anul

ysin

at e

ndom

ysia

l site

s th

an

site

sst

eroi

d-re

sist

ant p

atie

nts

with

IBM

[61]

2 pa

tient

s. C

D8+

T ly

mph

ocyt

es w

ere

foun

d to

inva

de m

uscl

e 2

patie

nts.

Mos

t IC

OS

and

ICO

S-L

pos

itive

cel

ls

20 p

atie

nts.

CD

8+T

lym

phoc

ytes

wer

e fo

und

to

fiber

s co

-exp

ress

ing

ICO

S-L

and

MH

C c

lass

I. T

he e

ndom

ysia

l w

ere

loca

lized

to th

e pe

rimys

ial r

egio

ns a

nd

inva

de IC

OS

-L a

nd M

HC

cla

ss I

co-e

xpre

ssin

g IC

OS

-pos

itive

CD

8+an

d C

D4+

T ly

mph

ocyt

es w

ere

seen

at a

co

nnec

tive

tissu

e. IC

OS

-exp

ress

ing

CD

4+an

d m

uscl

e fib

ers.

ICO

S-L

exp

ress

ion

was

foun

d on

the

sim

ilar f

requ

ency

and

ratio

to th

ose

in p

atie

nts

with

IBM

CD

8+T

lym

phoc

ytes

wer

e in

frequ

ent a

nd p

rese

nt

surfa

ce o

f alm

ost a

ll fib

ers,

incl

udin

g he

alth

y on

ly in

the

perim

ysiu

m a

nd a

roun

d bl

ood

vess

els,

m

yofib

ers

away

from

infla

mm

atio

n. 1

/20

or 2

/20

but n

ot a

mon

g au

toin

vasi

ve T

lym

phoc

ytes

auto

inva

sive

CD

8+T

lym

phoc

ytes

wer

e do

uble

-po

sitiv

e fo

r IC

OS

and

CD

8. A

sim

ilar p

erce

ntag

e of

C

D4+

T ly

mph

ocyt

es, p

artly

foun

d in

clo

se c

onta

ct

with

CD

8+T

lym

phoc

ytes

, als

o sh

owed

ICO

S

doub

le im

mun

orea

ctiv

ity

Con

tinue

d



Tab

le 1

(co

nti

nu

ed)

Ref

eren

ceP

olym

yosi

tisD

erm

atom

yosi

tisIn

clus

ion-

body

myo

sitis

B ly

mph

ocyt

es

[9]

53 p

atie

nts.

B ly

mph

ocyt

es w

ere

mos

t abu

ndan

t at

31 p

atie

nts.

B ly

mph

ocyt

es w

ere

mos

t abu

ndan

t 48

pat

ient

s. B

lym

phoc

ytes

wer

e m

ost a

bund

ant a

t pe

rivas

cula

r site

s an

d le

ast a

bund

ant a

t end

omys

ial

at p

eriv

ascu

lar s

ites

and

leas

t abu

ndan

t at

periv

ascu

lar s

ites

and

leas

t abu

ndan

t at e

ndom

ysia

l si

tes;

how

ever

, onl

y fe

w B

lym

phoc

ytes

wer

e ex

pres

sed

endo

mys

ial s

ites

site

s; h

owev

er, o

nly

few

B ly

mph

ocyt

es w

ere

expr

esse

d in

com

paris

on w

ith p

atie

nts

with

der

mat

omyo

sitis

com

pare

d w

ith d

erm

atom

yosi

tis

Pol

ymyo

sitis

, 3

patie

nts.

Occ

asio

nal s

catte

red

B ly

mph

ocyt

es (C

D19

and

N

o da

ta w

ere

give

n fo

r 5 p

atie

nts

with

16

pat

ient

s. O

ccas

iona

l sca

ttere

d B

lym

phoc

ytes

de

rmat

o-C

D20

) with

a m

ean

dens

ity o

f 2.8

cel

ls/m

m2

wer

e se

en.

derm

atom

yosi

tis w

ho w

ere

inve

stig

ated

for

(CD

19 a

nd C

D20

) with

a m

ean

dens

ity o

f m

yosi

tis, a

nd

Pla

sma

cells

(CD

138)

sho

wed

a m

ean

dens

ity o

f B

lym

phoc

ytes

(CD

19 a

nd C

D20

) and

pla

sma

4.2

cells

/mm

2w

ere

seen

. Pla

sma

cells

(CD

138)

in

clus

ion-

body

11

.0 c

ells

/mm

2 . C

D13

8+ce

lls w

ere

loca

ted

pred

omin

antly

ce

lls (C

D13

8). H

owev

er, a

com

paris

on w

as d

one

show

ed a

mea

n de

nsity

of 1

7.2

cells

/mm

2 . C

D13

8+

myo

sitis

, [34

]; in

the

endo

mys

ium

. The

re w

ere

3.9-

fold

mor

e pl

asm

a ce

lls

with

ano

ther

stu

dy in

whi

ch it

was

sho

wn

that

IBM

ce

lls w

ere

loca

ted

pred

omin

antly

in th

e en

dom

ysiu

m.

derm

ato-

than

B ly

mph

ocyt

esm

uscl

e tis

sue

had

0.84

-fold

B ly

mph

ocyt

es a

nd

Ther

e w

ere

4.1-

fold

mor

e pl

asm

a ce

lls th

an

myo

sitis

, [49

]4.

3-fo

ld p

lasm

a ce

llsB

lym

phoc

ytes

Den

driti

c ce

lls

Pol

ymyo

sitis

10

pat

ient

s. C

olle

ctio

ns o

f mye

loid

DC

s (B

DC

A-1

+) w

ere

14 p

atie

nts.

Pla

smac

ytoi

d D

Cs

(BD

CA

-2+) w

ere

20 p

atie

nts

with

IBM

wer

e in

vest

igat

ed. C

olle

ctio

ns

and

incl

usio

n-pr

esen

t in

90%

, wid

ely

dist

ribut

ed a

cros

s th

e se

ctio

n w

ith

seen

in 1

0/14

pat

ient

s an

d w

ere

mai

nly

loca

lized

of

mye

loid

DC

s (B

DC

A-1

+) w

ere

pres

ent i

n 17

/20

body

myo

sitis

, ad

ditio

nal h

igh-

dens

ity a

ccum

ulat

ions

. Hig

h-de

nsity

to

end

omys

ial a

nd p

reiv

ascu

lar l

ocat

ions

patie

nts

wid

ely

dist

ribut

ed a

cros

s th

e se

ctio

n w

ith

[48]

; der

mat

o-ac

cum

ulat

ions

wer

e ty

pica

lly e

ndom

ysia

l and

eith

er

addi

tiona

l hig

h-de

nsity

acc

umul

atio

ns. T

hese

hig

h-m

yosi

tis, [

49]

surr

ound

ed m

yofib

ers

and

som

etim

es in

vade

d ap

pare

ntly

de

nsity

acc

umul

atio

ns w

ere

typi

cally

end

omys

ial a

nd

non-

necr

otic

myo

fiber

s. T

hey

wer

e al

so e

xpre

ssed

in d

ense

ei

ther

sur

roun

ded

myo

fiber

s an

d so

met

imes

inva

ded

colle

ctio

ns o

f cel

ls (a

lway

s so

me

CD

3+ce

lls in

thes

e de

nse

appa

rent

ly n

on-n

ecro

tic m

yofib

ers.

The

y co

uld

also

co

llect

ions

) bet

wee

n m

yofib

ers.

Har

dly

any

plas

mac

ytoi

d be

exp

ress

ed in

den

se c

olle

ctio

ns o

f cel

ls (a

lway

s D

Cs

(BD

CA

-2+) w

ere

seen

som

e C

D3+

cells

in th

ese

dens

e co

llect

ions

) be

twee

n m

yofib

ers.

Har

dly

any

plas

mac

ytoi

d D

Cs

(BD

CA

-2+) w

ere

seen

Der

mat

o-–

14 p

atie

nts.

MxA

was

exp

ress

ed in

per

ifasc

icul

ar

20 p

atie

nts.

A q

uant

itativ

e st

udy

has

been

don

e m

yosi

tis, [

49];

area

s. E

ight

pat

ient

s sh

owed

MxA

exp

ress

ion

betw

een

patie

nts

with

IBM

and

DM

who

had

not

in

clus

ion-

body

by

myo

fiber

s in

a p

refe

rent

ially

per

ifasc

ular

re

ceiv

ed im

mun

osup

pres

sive

trea

tmen

t. Th

e m

yosi

tis, [

48,4

9]di

strib

utio

n in

bot

h at

roph

ic a

nd n

orm

al-s

ized

nu

mbe

rs o

f mye

loid

and

pla

smac

ytoi

d D

Cs

wer

e fib

ers

but a

lso

in c

apill

arie

s an

d ot

her b

lood

si

mila

r in

periv

ascu

lar a

nd p

erim

ysia

l are

as in

IBM

. ve

ssel

s in

13/

14 p

atie

nts.

In re

gion

s of

H

owev

er, t

he e

ndom

ysia

l loc

atio

ns o

f mye

loid

DC

pe

rifas

cicu

lar a

trop

hy a

ll m

yofib

ers

wer

e po

sitiv

enu

mbe

rs w

ere

a m

ean

of 3

-fold

hig

her t

han

thos

e of

pl

asm

acyt

oid

DC

s. In

con

tras

t, in

DM

ther

e w

ere

16-fo

ld a

nd 3

-fold

mor

e pl

asm

acyt

oid

DC

s th

an

mye

loid

DC

s in

end

omys

ial a

nd p

eriv

ascu

lar s

ites,

re

spec

tivel

y

[47]

6 pa

tient

s. Im

mat

ure

DC

s (C

D1a

) wer

e m

ainl

y fo

und

in

6 pa

tient

s. Im

mat

ure

DC

s (C

D1a

) wer

e m

ainl

y –

lym

phoc

ytic

infil

trat

es in

all

patie

nts.

Mat

ure

DC

s fo

und

in ly

mph

ocyt

ic in

filtr

ates

in a

ll pa

tient

s.

(DC

-LA

MP

/CD

83) w

ere

loca

lized

to p

eriv

ascu

lar i

nfilt

rate

s M

atur

e D

Cs

(DC

-LA

MP

/CD

83) w

ere

loca

lized

to

surr

ound

ing

mus

cle

fiber

spe

rivas

cula

r inf

iltra

tes

surr

ound

ing

mus

cle

fiber

s

The

refe

renc

es w

ere

sele

cted

acc

ordi

ng to

imm

unoh

isto

chem

ical

dat

a of

loca

lizat

ion

of T

lym

phoc

ytes

, B ly

mph

ocyt

es, a

nd d

endr

itic

cells

(DC

s) in

ske

leta

l mus

cle

tissu

e of

pat

ient

s w

ith p

olym

yosi

tis (P

M),

derm

atom

yosi

tis (D

M),

and

incl

usio

n-bo

dy m

yosi

tis (I

BM

). M

HC

, maj

or h

isto

com

patib

ility

com

plex

; IC

OS

, ind

ucib

le c

o-st

imul

ator

; IC

OS

-L, I

CO

S li

gand

.

with dermatomyositis but not in patients with polymyositis orinclusion-body myositis [8]. In contrast, a recent study byGreenberg and colleagues [34] demonstrated the presenceof differentiated B lymphocytes in the form of CD138+ plasmacells predominantly in the endomysium of muscle tissue ofpatients with polymyositis and inclusion-body myositis (nopatients with dermatomyositis were included in this study). Alocal antigen-driven response has been shown to be present ininflammatory myopathies. Clonal expansion, class-switchedsignificant somatic mutation, and variation of local Blymphocyte and plasma cell maturation could occur within theskeletal muscle [35]. These characteristics are hallmarks of anantigen-driven response, which would mean that T lympho-cytes are not the only cell that could drive an intramuscularantigen-specific autoimmunity in inflammatory myopathies.

In peripheral blood, activated B lymphocytes (RP105-negative B cells) were distinctly increased in patients withdermatomyositis in comparison with those with polymyositis[36]. In another study, peripheral blood mononuclear cellsfrom patients with dermatomyositis, but not from those withwith polymyositis, produced significant amounts ofimmunoglobulins in vitro [37].

Thus, earlier studies based on cellular expression in muscletissue and, to some extent, the investigations of peripheralblood suggest that B lymphocytes are important indermatomyositis but may have a less important role inpolymyositis. However, more recent data using stainingmarkers to detect plasma cells indicated a role of Blymphocytes in polymyositis and inclusion-body myositis aswell [34]. Autoantibodies are present in 60 to 70% ofpatients with polymyositis or dermatomyositis, supporting arole for B lymphocytes in these disease entities, but lessoften in patients with inclusion-body myositis. Someautoantibodies found in patients with myositis, such as anti-Ro/SSA, anti-La/SSB, anti-Scl-70, and anti-U1 ribonucleo-protein, are also found in other autoimmune diseases,whereas others, especially anti-synthetase antibodies (forexample anti-histidyl-tRNA synthetase or anti-Jo-1), anti-Mi2,and anti-signal-recognition particle are more specific formyositis. The myositis-specific autoantibodies are oftenassociated with distinct clinical manifestations, such as theanti-synthetase syndrome characterized by myositis, ILD,arthritis, Raynaud’s phenomenon, and skin changes called‘mechanic’s hands’. The most frequent myositis-specificautoantibody is anti-Jo-1, which is more common in patientswith polymyositis but may also be present in patients withdermatomyositis [38,39]. The newly discovered autoantibodyanti-p155 seems to be associated more often withdermatomyositis and para-neoplastic dermatomyositis, and itsfrequency is similarly high in children (29%) and adults (21%)(with para-neoplasy the frequency is 75%) [40].

There are also reports on autoantibodies in patients withinclusion-body myositis. In one of these, an increased

frequency of serum monoclonal antibodies reactive to amuscle constituent was demonstrated [41].

The functional role of plasma cells in muscle in polymyositis andinclusion-body myositis is not yet fully elucidated. B lymphocytesand plasma cells could, beside antibody secretion or their roleas APCs, function as stimulatory cells for other immune cells. Indermatomyositis, CD4+ T lymphocytes could release IL-17 andIFN-γ after both polyclonal and oligoclonal activation to acquirea plasma cell-like morphology that is associated with a highsecretory activity, similar to that of plasma cells secretingimmunoglobulins [42]. The different role of immune cells in thecontext of muscle inflammation is presented in Figure 3.

More recent support for a role of B lymphocytes is the goodclinical effect seen with B cell depletion. This was firstreported in a pilot study that included patients withcorticosteroid-refractory dermatomyositis who were treatedwith rituximab (Rituxan®), an anti-CD20 monoclonal antibodythat is approved for treatment of some rheumatic diseases[43]. Later short-term beneficial effects with rituximab werealso demonstrated in a case report of two patients withrefractory polymyositis and one with dermatomyositis [44].

So far, only few studies have investigated and furtheraddressed the roles of B lymphocytes and plasma cells inmyositis. It is, however, certain that further research focusingon the functional role of B lymphocytes and specificautoantibodies in IIMs is warranted; this could provide pivotalinsights in the disease mechanisms for a possible futurespecific targeted treatment strategy. A comparison ofexpression in B lymphocytes in IIMs is presented in Table 1.

Dendritic cells and other antigen-presenting cellsDendritic cell functionTissue DCs that have internalized particulate and solubleantigens at the site of inflammation are induced to mature,and an innate immune response is triggered. These cells areactivated through receptors that signal the presence ofpathogen components or cytokines. DCs are also respon-sible for T lymphocyte activation by migrating to the lymphnode and providing both antigen presentation and co-stimulation. After maturation, DCs lose their ability to capturenew antigens and thus there is a continuous flow of DCs fromtissue to draining secondary lymphoid organs after challengewith antigen. In their mature activated form, DCs are the mostpotent APCs for naïve T lymphocytes. Both macrophagesand B lymphocytes also have the capacity to function asAPCs, but the ability of DCs to take up, process, and presenta variety of pathogens and antigens makes them the mostimportant activators of naïve T lymphocytes.

Dendritic cells and other antigen-presenting cells inidiopathic inflammatory myopathiesThe presence of T lymphocytes in all subsets of IIMsindicates a permanent immune response that requires the



presence of APCs. DCs are central to the development ofinnate and adaptive immune responses. Two main classes ofDC have been classified, myeloid and plasmacytoid DCs.Myeloid DCs are potent APCs and have a function in theadaptive immune system. They are capable of capturing,processing, and presenting antigens and thereby stimulatinglymphocytes to a specific immune response. In contrast,plasmacytoid DCs are important in the innate immune systemand can produce large amounts of IFN-α and IFN-β, both ofwhich have several functions including the stimulation of cellsto produce specialized protein as a defense againstpathogens. IFN-α can transform healthy monocytes into cellswith properties of DCs; this was shown in sera from patientswith active systemic lupus erythematosus [45]. IFN-α canalso contribute to plasma cell differentiation and couldtherefore be important in the generation and sustenance ofantibody responses [46].

Until recently, only few data were available on DCs in IIMs, butthe results of some very recent studies have partly revealedimportant insights on this issue. Physiologically, DCs do notappear in normal muscle tissue, whereas the use of newmarkers for immature and mature DCs (CD1a and DC-LAMP/CD83, respectively) enabled the immature DCs to bedetected in lymphocytic infiltrates in both polymyositis anddermatomyositis muscle tissue samples [47]. Local DCs haverecently been demonstrated in all subsets of IIMs [48]: inmuscle specimens from patients with inclusion-body myositisand patients with polymyositis, myeloid DCs were present insubstantial numbers, frequently surrounding and sometimesinvading intact myofibers. They were part of a dense collectionof cells that also included T lymphocytes. In dermatomyositismuscles, an increased number of plasmacytoid DCs wasfound in comparison with the amount of myeloid DCs [48].The importance of the plasmacytoid DCs in activating the typeI interferon system in dermatomyositis was suggested by itsassociation with the transcriptional response of the IFN-α andIFN-β inducible genes and by the fact that the interferon-induced MxA protein was expressed in muscle tissue of thesepatients [49]. MxA expression was demonstrated in capillariesand in perifascicular myofibers, which are characteristic sitesof dermatomyositis pathology [49].

We have also found MxA expression in muscle tissue indermatomyositis, but also in polymyositis and inclusion-bodymyositis, supporting a role of the type 1 interferon in allsubsets of myositis. We therefore could not confirm aspecific MxA staining pattern able to be used as a diagnostictool that then could distinguish dermatomyositis from theother subsets of myositides, as suggested by Greenberg andcolleagues [34]; this therefore needs to be pursued further. Acomparison between expression patterns of DCs in IIMs ispresented in Table 1.

However, the detailed function of both myeloid andplasmacytoid DCs in IIMs has still not been fully clarified.

Greenberg and colleagues [34] proposed two hypotheses:first, the presence of myeloid DCs invading the non-necrotic-seeming myofiber regions suggests the occurrence of activephagocytosis, endocytosis, pinocytosis, or receptor-mediateduptake of antigen, activities for which these cells arespecialized; and second, that some of these cells are activelypresenting antigen and activating T lymphocytes locally withinmuscle tissue rather than in the lymph node [48].

In addition, there is some evidence that chemokine receptorson DCs can promote autoimmune reactions. This issupported by the observation mentioned above that someautoantigens, such as aminoacyl tRNA synthetases, mayexhibit chemotactic properties for activated monocytes,T lymphocytes, and immature DCs (not mature DCs). Theycould therefore have a capacity to attract inflammatory cells,including immature DCs, to infiltrate affected muscle cells.Taken together, these results suggest that antigens deliveredto receptors on immature DCs are potent immunogenscapable of breaking self-tolerance and able to induceautoimmune diseases [24,50].

As mentioned above, macrophages and B lymphocytes canalso function as APCs. In addition to those, other cell typescan acquire this function, although usually without reachingthe same efficacy as a ‘professional’ APC [51]. Emergingevidence from experiments in vitro and in vivo suggests thatmuscle fibers could function as antigen-specific cells [52-54].A schematic summary of the different roles of immune cells inthe context of muscle inflammation is presented in Figure 3.

Whether muscle cells actually do function as APCs stillremains uncertain. Muscle fibers are incapable of expressingthe ‘classical’ co-stimulatory molecules B7-1 and B7-2, butsome studies indicate that skeletal muscle cells and humanmyoblasts still have the possibility of acting as APCs with acell-to-cell contact between BB-1 antigen on the one hand,and CD28 and CTLA-4 on CD4+ or CD8+ T lymphocytes onthe other [53-55]. However, whether muscle cells that expressBB-1 simultaneously could express MHC class I and/or classII antigens is still not clarified. Furthermore, cDNA for the BB-1antigen has not been isolated. BB-1 is selectively induced bytreatment of myoblasts with pro-inflammatory cytokines suchas IFN-γ or tumor necrosis factor [55]. As both thesecytokines have been detected in muscle tissue from patientswith IIM [56-58], the muscle environment in the inflamedmuscle could possibly provide the necessary signals onmuscle fibers for an antigen presentation to T lymphocytes.

Once a naïve T lymphocyte has been activated, it expressesseveral proteins to sustain or modify the co-stimulatory signalfor the clonal expansion and differentiation of T lymphocytes.Several co-stimulatory signal systems have been identified inmuscle tissue from patients with IIM, such as CD40–CD40ligand (CD40-L), inducible co-stimulator (ICOS)–ICOS ligand(ICOS-L), B7RP-1, B7h, and B7-H2. When a co-stimulatory



receptor is bound by its ligand, an activating signal istransmitted to the T lymphocyte. This also activates the APCto express B7 molecules, which further stimulate Tlymphocyte proliferation. Interestingly, muscle cells andmyofibers have been shown to express some of these co-stimulatory signals, namely CD40-L and ICOS-L (ICOS-Lboth expressed and functional [59]) during pathologicalconditions [59,60]. Schmidt and colleagues [61] demon-strated that muscle fibers expressing MHC class I, taken frompatients with inclusion-body myositis, caused an upregulationof ICOS–ICOS-L molecules in association with enhancedperforin expression by autoinvasive CD8+ T lymphocytes,which could exert a cytotoxic effect. This further supports thehypothesis that muscle fibers could work as APCs.

ConclusionThe molecular basis of IIMs in humans, as in many otherautoimmune rheumatic diseases, is heterogeneous, involvingseveral complex cellular components that probably contributeto differences in disease susceptibility, clinical and histopatho-logical phenotype, and severity. Although this heterogeneitymakes the study of the pathogenesis of IIMs extraordinarilycomplex, it might also provide distinct avenues for noveltherapeutic interventions. Controlling the immune response isas complex as its launching. An essential feature ofphysiological immune response is its self-limitation, by which itis attenuated by several mechanisms. We have only juststarted to understand the orchestrated life of T lymphocytes, Blymphocytes, and DCs in IIMs, but there are still manyunanswered questions about how this usually effective systemcan go awry and result in false immune-mediated reactions.

On the basis of detailed immunohistochemical studies onmuscle biopsies, two major types of inflammatory infiltratewere observed: endomysial and perivascular/perimysial. Inendomysial infiltrates there was a striking dominance of CD8+

T lymphocytes, which could even be the predominatinginfiltrating cell type, followed by macrophages and CD4+

T lymphocytes. These infiltrates often surrounded non-necrotic fibers and sometimes seemed to invade the fibers(Figure 1). This observation suggests an immune reactionthat targets muscle fibers. The perivascular infiltrates, incontrast, were dominated by CD4+ T lymphocytes andmacrophages, and sometimes the presence of B lympho-cytes suggested an immune reaction that targets micro-vessels (Figure 2). A role for B lymphocytes as well as one forCD4+ T lymphocytes in the pathogenesis of IIMs is supportedby frequently detected autoantibodies in polymyositis anddermatomyositis but less often in inclusion-body myositis.These autoantibodies are both non-specific (frequently alsobeing found in other autoimmune disease) and myositis-specific [38,39]. A role for CD4+ T lymphocytes in thedisease mechanism is further supported by the geneticassociation with HLA-DRB1*0301, DQA1*0501, andDQB1*0201, which was particularly seen for subgroups ofpatients with autoantibodies.

The endomysial infiltrates were reported to be characteristicfeatures of polymyositis and inclusion-body myositis, whereasthe perivascular infiltrates were associated with patients withdermatomyositis. However, there are cases with a less distinctlocalization of infiltrates or with combined endomysial andperivascular cellular infiltrates [3,6]. Moreover, in some casesthe inflammatory cell infiltrates are diffusely spread in thetissue, whereas in other cases the infiltrates are very small orare not found at all. In addition, the perivascular changes maybe seen in patients without a skin rash, whereas endomysialinfiltrates are occasionally seen in cases with a skin rash.

Taken together, these results indicate that there may be twomajor pathways: one leading to cellular infiltrates withpredominating endomysial localization, and another with apredominantly perivascular localization often with microvesselinvolvement and capillary loss. The latter is more often seen inpatients with a skin rash and dermatomyositis, but thereseems to be an overlap between clinical phenotypes, histo-pathology, and immunotypes. These observations suggestthat there might be more than just one factor that determinesthe histopathological and clinical phenotypes, for examplegenes and environment.

During the past few years, the results of several advancedhistopathological, molecular, functional, and medical studieshave provided new data that could be of importance inunderstanding the immune mechanisms in IIMs. Takentogether, they demonstrate the complexity of involvement ofthe immune system in these diseases and suggest that boththe innate and adaptive immune systems are involved indermatomyositis, polymyositis, and inclusion-body myositis.This complexity of T lymphocyte populations in muscle tissuein clinical subsets of myositis was demonstrated in the originalobservations of different cellular subsets in muscle tissue byArahata and Engel [6,7,9] and is exemplified in Figures 1 and2. These observations make it necessary to revise the ‘oldhistorical’ hypothesis on the pathogenesis of IIMs. Recently adispute over the most appropriate and accurate diagnosticcriteria has erupted, including the importance of thehistopathological picture and the localization of immune cells[62]. Other phenotypes such as autoantibody may also beimportant for the classification of disease subsets that will helpto enhance our understanding of disease mechanisms andthereby improve the treatment and prognosis for thesepatients. A revision of classification criteria has recently beenstarted in an international interdisciplinary collaboration thatwe believe will facilitate these efforts.

Competing interestsThe authors declare that they have no competing interests.

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AbstractIntroductionT lymphocytesT lymphocyte functionT lymphocytes in idiopathic inflammatory myopathies

Dendritic cells and other antigen-presenting cellsDendritic cell functionDendritic cells and other antigen-presenting cells in idiopathic inflammatory myopathies

B lymphocytesB lymphocyte functionB lymphocytes in idiopathic inflammatory myopathies

ConclusionCompeting interestsReferences

Documents

Review Immune mechanisms in the pathogenesis of …...(MHC) class I transgene [3]. These non-immune mechanisms have been addressed in a recent review paper [4]. New data are constantly