Th2 response in Schistosoma

Prepared by: Shalimar Shadeed

Blood flukes belonging to the genus Schistosoma

are important parasites of humans: Three species affect humans:a. S. mansoni: large intestineb. S. japonicum: small intestinec. S. haematobium: urinary bladder

Flash Back- Schistosoma

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Central Role of Helper T-cells in the Immune System

Innate Immunity

penetration of skin

Specific Immunity adults take on host antigens; Soluble Egg Antigens (SEA) CD4+ Th2 (cytokines according to the subtype activated) Complement system (classical or alternative)

The most important aspect to be discussed throughout this presentation is how this parasite can interfere with the regulatory mechanisms of Th2 . Conseqently, the immune system response.

Slippery Little Suckers Aren’t They…” ~ Julia Roberts

CD4+ is classified into different subsets depending on

which type of cytokines Th2 activates. Understanding the

mechanisms responsible for controlling T cell- dependent

events such eosinophilia and increased IgE levels that had

for some time been recognized as hallmarks of

schistosomiasis, led to the realization that schistosome

infection induces a Th2 response.

Analysis has shown the prominence of Th2 at the time of

egg production (SEA) are able to directly induce Th2

responses when injected into mice.

The deposition of Schistosome eggs is associated with the emergence of a strong Th2 response

In animals infected with S. Mansoni , eggs trapped in tissue become

focal points for inflammatory infiltrates in which forms a

Cirucumova granuloma, these lesions reflect Th2 dominance;

(CD4+ T cells, B cells and fibroblasts, and eosinophils).

This was exp. proven when the granuloma developments was

impacted in mice lacking Th2 was analyzed; the findings shown that

inflammatory infiltrates around eggs were small and compromised.

Granulomas serve an important role protective role in

sequestering the eggs away from the surrounding tissue.

CD4+ Th2 cells play a crucial host-protective roleduring infection

Mice T-cell depleted, infected with S. Mansoni were rendered unable to form

granuloma and developed necrosis in the liver and intestinal mucosa

between 6-7 weeks of infection. Additionally, these mice shown

microvesicular cytoplasmic damage with accompanying nuclear

pyknosis(irreversible mythelations of histones upon cell necrosis); this is

accompanied by elevated serum transaminase concentrations (indication of

liver damage).

Granuloma formation around eggs that have been embolized to the lungs is

dependent on Chemokines and chemokine ligand, which is preferentially

expressed on Th2 cells). For example, CCL2 and CCR4 respectively.

Cont.

1- It’s thought that granuloma formation walls off the eggs and prevent toxic molecules produced reaching the surrounding tissue.2- IL-4 regulates the proinflamatory and cellular response. This was evident in mice lacking IL4; mice died due to the excessive response of the above mentioned mechanisms.

How are CD4 T cell protective (host)?

Data and observations implied that IL-4 suppresses the emergence of an INF-γ secreting CD4+ cell

and/or plays an important role in limiting access of TLR agonists into diseased tissues; the absence

of IL-4 as demonstrated:

Classical Pathway activation in the absence of IL-4:

TLR agonists & IFN-γ activates the Macrophage to produce( NO, ROS,TNF-α).

This pathway is implicated with the disease severity; Hence, we can appreciate the importance of

IL-4 in regulating the secretion of IFN-γ Secreting CD4 cells. Also, it’s hypothesized that that it plays

important role in limiting the access of TLR agonist into diseased tissue.

The likely source of TLR agonists in this setting is the translocation of intestinal microflora into

the tissues and vasculature as a result of diminished barrier function in the intestinal

epithelium

IL-4

Collected date and observations shows the importance of macrophages protective role other than the INF-γ/TLR agonists; IL-4 and IL-13 initiate a distinct “alternative activation” (AA) pathway in macrophages that leads to the expression of a characteristic panel of genes. Arginase 1 (Arg1) is a key marker of AA macs.

L-arginine Arg1 L-ornithine (Urea Cycle)Ornithine is precursor in proline synthesis, and proline and hydroxyproline are major components of collagen, this enzyme was originally suspected to play a role in the development of hepatic fibrosis and, therefore, to play a role in liver disease in schistosomiasis.

IL-4 and IL-13 role in activating the AA pathway in Macrophage

Deletion of Arg1 in macrophages led to significantly greater granulomatous

inflammation, liver fibrosis, and portal hypertension. Arg1 expressing AA macrophages

can successfully compete with Th2 cells for arginine and, in so doing, limit T cell

proliferation. It was evident that in infected mice lacking Arg1 in immune cells also

develops more severe intestinal inflammation following infection with S. mansoni. This

is associated with the increased expression of classical macrophage.

This is associated with the increased expression of classical macrophage activation and

the development of Th17-related IL-12/IL- 23p40-dependent neutrophil-associated

inflammation around eggs in the intestinal wall and resultant endotoxemia.

Mechanism-Arg(1)

Interestingly, an additional AA macrophage product, RELM-α (FIZZ1), which is

additionally made by eosinophils and epithelial cells in sites of Th2-mediated

inflammation, appears to function in a somewhat similar manner to Arg1 since in

RELM-α-deficient mice, granulomatous inflammation and fibrosis around

schistosome eggs, as well as Th2 response intensity, are markedly increased.

Together, these findings point toward a strong, protective immunoregulatory role

for AA macrophages during schistosomiasis, in which the cells function to prevent

hyperresponsiveness within the Th2 cell compartment while promoting wound

healing within the intestine by antagonizing classical pro-inflammatory

macrophage activation.

Other AA pathways

T cells has a very role important in helping the development of a B cell

response that protective at later stages during infection.

previous study has shown that cytotoxic effects can be prevented in the

absence of T cells via passive immunization with antibodies from infected

immunologically intact mice or from mice immunized with egg antigens.

The possible explanation for this is that the protective role of the T cell-

dependent cellular infiltrate around eggs is required only transiently until

the antibody response is able to develop to a point where it can neutralize

egg toxins.

Th2 & B-cells

Recent studies have revealed the presence in the liver of F4/80+

macrophage-like cells that can be stained for IgG1 and it is possible that

these cells are “regulatory macrophages” which are able to play an anti

inflammatory role by responding to immune complexes by producing IL-10

and TGF-β1.

As an extension on the body of evidence, IgG1+ F4/80+ cells (putatively

macrophages) in chronically infected mice are transcribing IL-10 in vivo and

secrete this cytokine ex-vivo.

F40+/80 and it’s protective role

It was previously thought that the population of CD4+ T

cells making IL-4 elicited by infection is functionally uniform, such that cells producing IL-4 to alternatively activate macrophages in the liver would also be capable of helping B cells within lymphoid organs.

However, this is done by the B cell help is the specialized role of T follicular helper (Tfh) cells and that the IL-4-producing CD4+ Tfh cells that enter B cell follicles are functionally distinct from the Th2 cells that migrate to peripheral tissues.

The extent to which either Tfh or Th2 cells retain the plasticity to assume each other’s roles during infection remains unclear, but is of considerable interest since, given the protective role of antibody

More in the mechanism

Age prevalence curves showed that patients living in a areas

endemic for schistosomiasis slowly develop resistance to reinfection. This observation can be explained by the fact that a protective response is initiated by the death of adult worms and is boosted as worms naturally age and die.

Successive rounds of chemotherapy accelerates this process, especially when administered repeatedly over time to individuals who are being continuously re-exposed; the strongest correlate of resistance to further infection is the titer of IgE, and specifically of IgE against adult worm antigens such as Sm Tegumteal Allergen Like.

By inference, IL-4-producing Tfh cells play an important role in resistance to reinfection

As discussed before, the CD4 has a very important role in regulating Th2 response

in which protects form hepatic infections. However, it’s clear that there are

detrimental effects associated with strong Th2 responses in the context of

infection; Th2 responses are linked to the development of hepatic fibrosis, which

underpins severe morbidity in chronic Schistosomiasis.

As a small revision on what have been discussed with regard IL-4 and IL-13, is that

both can stimulate fibroblasts to make collagen, it is IL-13 that plays a particularly

important pro-fibrotic role in Schistosomiasis. In the absence of this cytokine, or in

settings where the ability of IL-13 to bind to its cell-associated receptor is blocked,

hepatic fibrosis is very significantly reduced and the prognosis is improved.

Th2 cells cause pathologic changes

The outcome of inflammation and fibrosis is dependent not only on the

extent of collagen production but also on the regulation of degradation

through the expression of matrix metalloproteinases (MMPs) by AA

macrophages.

In this context, MMP12 promotes the development of more severe

disease by regulating the expression of MMP2 and MMP13, both of

which act to degrade the extracellular matrix

Cont.

This cytokine inhibits dendritic cell (DC) activation and, in such action, it limits

their ability to activate T cells. Thus, the production of IL-10 by Th2 cells, regulatory T cells, and innate cells such as macrophages was seen as a compelling mechanism for the inhibition of Th2 responses; this is well documented as an exaggerated immune response in case of mice lacking this cytokine during late stages of the infection.

On the other hand, immunomodulation was seen to occur normally in chronically infected IL-10−/− mice.

It is apparent that the mechanism is ultimately cell autonomous in that the cells remain hypoproliferative even when transferred into acutely infected mice (where resident Th2 cells are proliferating strongly) [86]. Increased expression over time of grail (gene related to anergy) [86], and of inhibitory CD200 receptors [13], possibly as a result of repetitive stimulation with antigen (a common theme in chronic infection), is implicated in the loss of Th2 proliferative capacity late in infection

Regulation of Th2 responses by IL-10

Treg compartment expands during infection following schistosome eggs.

Expansion is dependent on TLR2; in the absence of this pattern recognition

receptor, immunopathologic changes are more severe, and there is a

prolonged Th1 response during acute infection.

Loss of Th2 responsiveness at week 8 of infection in schistosomiasis is

interesting because the decline occurs despite the fact that parasitic worms

live for years, continuing to produce eggs such that there is effectively an

ongoing increase in the amount of antigen to which the host is being

exposed.

The regulation of Th2 responses by Treg cells and the overall pic

In this regard, immunoregulation in schistosomiasis is reminiscent of T cell exhaustion in

chronic viral infections where the interaction of PD-1 on T cells with its receptors (PD-L1/PD-

L2) on other cells plays an important role in inhibiting T cell function. Nevertheless, in other

worm infections, PD-L2 expression by AA macrophages has been implicated in the loss of

responsiveness within the responding T cell population, and consistent with this, in

schistosomiasis, PD-L2 expression is linked to the development of less severe disease. It is

also possible that PD-L1 is playing a role in T cell non-responsiveness during schistosomiasis .

The production of IL-10 by Treg cells has been implicated in limiting the development of

immune mechanisms, measured as the increased production of Th1, Th17, and Th2

cytokines, increased IgE, IgG2b, and IgG1, and increased eosinophilia, that are correlated

with the expression of resistance to reinfection in infected mice following chemotherapy.

Cont.

Studying how Schistosoma has the ability to interfere with the Treg in such

the development of Treg cells and the effects of schistosome antigens on

dendritic cells and macrophage.

the outcome of allergic and autoimmune disease. Infection with S.mansoni,

or the injection of schistosome eggs, or of SEA, in settings where animals have

been manipulated to develop autoimmune diseases or are genetically

predisposed to do so have, in some cases, had notably beneficial effects.

Exploiting schistosomiasis-associated immune responses to treat other diseases

Many studies have shown that schistosoma eggs induces Th2

response. The response becomes most noticeable after egg production begins during infection, and eggs isolated from infected mice are able to strongly induce a Th2 response when injected, without adjuvant, into naïve mice.

A SEA is similarly immunogenic. DCs are also crucially important in the induction of Th2 responses during infection since following their deletion the immune response becomes dominated by IFN-γ production. However, SEA fails to activate DCs in the classical way, but rather is able to inhibit TLR-initiated activation and generally appears to maintain DCs in an immature state.

The mechanisms underpinning Th2 response induction during schistosomiasis

Intriguingly, Omega-1 (produced by the parasite) is a molecule that has

hepatotoxic properties and is the major target of antisera that protect infected T cell-deficient mice against liver damage.

Recent findings indicate that Omega-1 is taken up by DCs via its glycans through the CTL (C-Type Lectin) mannose receptor (MR) and thereafter inhibits protein synthesis by degrading RNA and that these processes result in DCs that preferentially induce a Th2 response.

MR has also been implicated in the response of DCs to glycans from the infectious stage of the parasite that leads to the suppression of IFN-γ production by T cells. Other possible mechanisms for the inhibition of DC activation by schistosome products have been reviewed recently [26] and include the degradation of TLRs within endosomes by endocytosed schistosome protease or engaging CTL that modulate canonical NF-kB translocation following TLR stimulation

Cont. how does the Parasite inactivates IR?

During a primary infection of S. mansoni, the immune system responds vigorously,

especially once significant tissue damage associated with egg production begins,

however this response doesn’t eliminate the parasite. Nevertheless, the response

protects the liver against toxic molecules released by eggs and promotes healing in

the intestinal epithelium that is being continuously perforated by eggs as they pass

from the vasculature into the intestinal lumen. These beneficial mechanisms are a

key for the survival of this parasite for a long periods of time (chronic).

Understanding how Th2 responses are induced and regulated in schistosomiasis (and

relating this to the regulation of other Th2-mediated diseases) remains a priority.

Summary

Questions?