A Review. MHC Originally discovered in the 1930s by Peter Gorer (gorer, 1936) MHC was genetically...
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MHC A Review
A Review. MHC Originally discovered in the 1930s by Peter Gorer (gorer, 1936) MHC was genetically defined by George Snell (1948) Human Leukocyte Antigen
MHC Originally discovered in the 1930s by Peter Gorer (gorer,
1936) MHC was genetically defined by George Snell (1948) Human
Leukocyte Antigen (HLA) was sequences by Betck et al. in 1999 MHC
is a complex multigene family comprising loci encoding receptors on
the surface of a variety of immune and nonimmune cells These
receptors bind amino acid fragments (or peptides) from foreign
pathogens to start an immunological response
Slide 3
MHC The diversity of MHC genes underlies resistance to the
diverisyt of infectious pathogens (Zuckerkandle and Pauling)
Classical is used to indicate that an MHC gene is polymorphic and
highly expressed Nonclassical genes are MHC genes that cluster with
other MHC genes phylogenetically They are usually not expressed at
high levels and are not polymorphic
Slide 4
General Organization and Inheritance of MHC Every mammalian
species studied possesses a tightly linked cluster of genes, the
major histocompatibility complex (MHC) Associated with
Intercellular recognition Self/nonself discrimination MHC plays a
central role in the development of both humoral and cell-mediated
immune responses
Slide 5
General Organization and Inheritance of MHC Role in antigen
recognition by T-cells Antigen-presenting structures T H and Tc
cells Partly determines the response of an individual to antigens
of infectious organisms and the MHC has been implicated in the
susceptibility to disease and in the development in
autoimmunity
Slide 6
Location and Function of MHC Regions Chromosome 6 in humans =
HLA The most gene-rich region, with over 200 loci identified
Chromosome 17 in mice = MHC/ H-2 3 classes of molecules Class I MHC
genes Encodes glycoproteins expressed on the surface of nearly all
nucleated cells Present peptide antigens of altered self-cells
necessary for the activation of Tc cells
Slide 7
Class I MHC genes Encodes glycoproteins expressed on the
surface of nearly all nucleated cells Present peptide antigens of
altered self-cells necessary for the activation of T C cells
Encoded by the A, B and C regions in humans
Slide 8
Class II MHC genes Encodes glycoprteins expressed primarily on
antigen- presenting cells Macrophages, dendritic cells and B cells
Presents antigenic peptides to T H cells Encoded by DP, DQ and DR
regions in humans
Slide 9
Class III MHC genes Encodes secreted proteins associated with
the immune process including soluble serum proteins, components of
the complement system and tumor necrosis factor C4, C2, Bf
(complement proteins) in humans
Slide 10
MHC Individuals inherit the alleles as two sets, one from each
parent. Each set of alleles if referred to as a haplotype In an
outbred population of humans the offspring are generally
heterozygous at many loci and will express both maternal and
paternal MHC alleles Co-dominantly expressed on the same cell
Slide 11
Generations If two inbred strains of mice having different MHC
haplotypes are bred, the F1 generation inherits haplotypes from
both parental strains and expresses both parental alleles at each
MHC locus Congenic Mice = two strains that are genetically
identical except at a single locus or region The first letter in a
congenic strain refers to the strain providing the genetic
background and the second letter to the strain providing the
genetically different MHC region.
Slide 12
MHC Molecules and Genes Class I and Class II MHC molecules are
membrane- bound glycoproteins Antigen presenting molecules Class II
MHC molecules are soluble proteins and exhibit much less
polymorphism than class I and II
Slide 13
Structure of Class I Molecule Contains a large chain associated
noncovalently with the much smaller 2-microglobulin molecule. The
chain is a polymorphic transmembrane glycoproteins 45 kilodaltons
(kDa) Encoded by genes within the A, B and C region of the HLA in
humans Anchored in the membrane by hydrophobic transmembrane
segment and hydrophilic cytoplasmic tail 2-microglobulin is an
invariant protein of about 12kDa encoded by a gene located on a
different chromosome
Slide 14
Structure of Class I Molecule chain molecules are organized
into three external domains (1, 2, 3) each containing approximately
90 amino acids A transdomain of about 40 amino acids and a
cytoplasmic anchor segment of 30 amino acids 2-microglobulin is
similar to 3 external domain Papain (enzyme) cleaves the chain 13
residues proximal to its transmembrane domain
Slide 15
Structure of Class I Molecule 1 and 2 domains interact for form
a platform of eight antiparallel strands spanned by two long
-helical regions This structure forms a deep groove or cleft
approximately 25 X 10 X 11 The long helices as sides and the
strands of the sheet as the bottom The Peptide Binding Cleft is
located on the top surface of the class I MHC molecule Can bind
8-10 amino acids
Slide 16
Peptide Binding by MHC Molecules Several hundred different
allelic variants of class I MHC molecules in humans Each individual
expresses only up to 6 different class I molecules and up to 12
different class II molecules A given MHC molecule can bind numerous
different peptides Some peptides can bind to several different MHC
molecules Class I molecules bind peptides containing 8 10 amino
acid resides
Slide 17
Class I MHC-Peptide Interaction Class I MHC molecules bind
peptides Present these to CD8 + T cells A nucleated cell expresses
about 10 5 copies of each class I molecule Many different peptides
will be expressed simultaneously on the surface of a nucleated cell
by class I MHC molecules Class I MHC can bind over 2,000 distinct
peptides Presented with a frequency of 100-4,000 copies per
cell
Slide 18
Polymorphism of Class I Molecules Polymorphism Multiple alleles
at a given genetic locus within a species MHC expressed by an
individual does not change over time One of the most polymorphic
genetic complexes Possesses an extraordinarily large number of
different alleles at each locus The alleles differ in their DNA
sequences from one individual to another The gene products
expressed by different individuals have unique structural
differences
Slide 19
Polymorphism of Class I Molecules Up to 20 amino acid residues
contributing to the uniqueness of each allele Analysis of HLA class
I molecules has so far revealed 59 A alleles, 111 B alleles, and 37
C alleles. The current estimate of actual polymorphism in humans is
on the order of 100 alleles for each locus The binding site for
antigens is the most polymorphic location on MHC Class I
Slide 20
MHC Genes Chromosome 6 in humans and 17 in mice Nearly 100
genes spanning some 4000kb 2000kb for Class I Contains
approximately 20 genes HLA-A, HLA-B, HLA-C HLA-A has 250 and HLA-B
has 500 known alleles Non-classical MHC class I include: HLA-E,
HLA-F, HLA-G, HLA-H, HLA-J, HLA-X and MIC Many of these are
pseudogenes and do not encode a protein product The non-classical
gene products are categorized as class Ib molecules
Slide 21
Class Ib Molecules Have an chain that associates with 2
-microglobulin Less polymorphic Expressed at lower levels Tissue
distribution is more limited than classical class I molecules The
function of non-classical class I MHC molecules is largely
unknown
Slide 22
Classical Class I MHC Molecules Expressed on most somatic cells
Level of expression varies Highest level of expression are by
lymphocytes 1% of the total plasma-membrane proteins, 5X10 5
molecules per cell
Slide 23
Regulation of MHC Expression Interferons (alpha, beta and
gamma) and tumor necrosis factor increase expression of class I MHC
molecules on cells MHC expression is increased or more commonly,
decreased by a number of viruses, ie. Cytomegalovirus, hepatitis B
virus and adenovirus 12.
Slide 24
Chemosensation and Genetic Individuality Olfactory cues play a
fundamental role is social interactions in a wide variety of
mammals from rodents to primates (Brown, 1979) Each mammal has an
individual odour or signature for individual recognition (Porter,
1985) Olfactory signatures have a function in mammalian
reproduction (MacDonald and Brown, 1985) Chemical signals can be
deposited on external substrates to mark territory Olfactory cues
can still be effective in darkness (Porter, 1999)
Slide 25
Immunobiology of Classical MHC class I antigens Outbreeding
mammals have a degree of polymorphism that ensures that in each
species there are more than 10 9 unique class I antigenic
phenotypes in a population The heavy chain consists of three
domains (1, 2 and 3) The light chain consists of a single domain,
2- microglobulin, attached non-covalently to the 3 domain
Slide 26
Immunobiology of Classical MHC class I antigens The 1 and 2
domains consist of an -helix and a - pleated sheet. The two sheets
join together to make a continuous platform which supports the two
-helices defining an antigen binding cleft The cleft is occupied by
a self-peptide of 8-10 amino acids
Slide 27
MHC-associated Odours and Individual Recognition Inbred strains
consist of animals that are genetically uniform and homozygous for
all their genes Each chromosome pair is identical Resulting from
mating brother and sister pairs for at least 20 consecutive
generations The genotype is nearly identical Congenic strains are
two strains that are identical except for allelic differences at a
particular locus Any measurable characteristic that differs between
the two congenic strains must be ascribed to the genes at that
locus Formed by repeated backcrosses of a donor strain to an inbred
strain with selection for a locus of interest
Slide 28
The Y-maze Paradigm Mice are trained to enter alternative
chambers scented by an airflow through odour boxes occupied by MHC-
congenic mice Mice are trained by water deprivation and reward The
mouse first learns to differentiated between two strong odours like
juniper and cinnamon and is rewarded with a drink It is correct if
it chooses the correct arm at least 80% of the time The mice learn
to distinguish inbred strains and then distinguish congenic strains
that differ only in the alleles at the MHC
Slide 29
The Y-maze Paradigm Findings: No sensory perception other than
olfaction is required to distinguish between MHC types (Yamazaki,
1979) Urine is a good source of MHC-specific odours (Yamaguchi,
1981) Males and females had equal ability to distinguish between
MHC types (Yamaguchi, 1981) Olfactory acuity of mice could enable
the discrimination of MHC Class I molecules that differ only in
three amino acids (Yamazaki, 1983a) No amount of training could
enable a mouse to distinguish between individuals of the same
inbred strain (Yamaguchi, 1981)
Slide 30
The Y-maze Paradigm Mice could discriminate between allelic
differences in the class II and Q/T/M regions, indicating that all
three genetic loci of the MHC can contribute to individual odours
(Yamazaki, 1982, 1984) The haemopoietic system is a source of MHC-
associated odours (Yamazaki, 1985) X and Y chromosomes are also
discriminated between, but are much less potent in scent marking
than the MHC (Yamazaki, 1986a) Ease of training indicates that MHC
is a potent source of individual odour (Boyse, 1987)
Slide 31
Habituation-Dishabituation Paradigm Most mammal are usually
inquisitive when presented with a novel stimulus, but the novelty
can wear off upon extended exposure to the stimulus The test
involves placing an experimental animal in a test arena with a
removable wire lid The animal is given opportunity to explore Then
it is presented with the first urine odour on a piece of filter
paper attached to the wire lid A novel stimulus
Slide 32
Habituation-Dishabituation Paradigm The animal rears on its
hind legs and sniffs The urine sample is presented three times By
the 3 rd exposure it no longer rears to investigate It has
habituated to the odour A novel urine sample is presented to the
animal If it perceives the odour of the new sample to be different
from the first there is a rapid dishabituation The rat lifts its
head, sniffs the air and moves toward the new odour and rears to
investigate it
Slide 33
MHC-associated odours and reproduction The male will choose a
female that is different from him at the MHC (Yamazaki, 1976)
Females can also choose mates and prefer to mate with males of
different MHC (Potts, 1991) Bruce Effect the pregnant female is
introduced to a strange male during the preimplantation stage of
development and spontaneous abortion occurs (Yamazaki, 1983b) The
effect is likely due to a neuroendocrine disturbance
Slide 34
MHC-associated odours: the carrier hypothesis MHC-based
developmental variations give rise to distinctive odour profiles
(Boyse, 1987) It is possible to discriminate between urine samples
taken from germ-free MHC congenic mice, which lack commensal flora
(Yamazaki, 1990) Fetal MHC-associated odours are evident in the
urine of mothers as early as day 9 of gestation (Beauchamp, 1994)
Before the fetus has a functioning immune system (Owen, 1969)
Slide 35
MHC-associated odours: the carrier hypothesis A hypothetical
route of class I MHC molecules from the membrane to the circulation
and into the urine Cleavage of the molecule at the juxta-membranous
protease site removes the transmembrane region and releases the
molecule into the circulation in soluble form The molecule does not
aggregate and circulates as a soluble heterodimer consisting of and
chains Further enzymatic cleavage at the junction of the 2 and 3
domains removes most of the 3 domain with the attached 2 -
microglobulin This cleavage allows relaxation of the binding
platform, opening of the cleft and loss of bound peptide
Slide 36
MHC-associated odours: the carrier hypothesis Further enzymatic
cleavage at the junction of the 2 and 3 domains removes most of the
3 domain with the attached 2 -microglobulin This cleavage allows
relaxation of the binding platform, opening of the cleft and loss
of bound peptide The resulting molecule with a naked binding cleft
is capable of binding a cocktail of small aromatic molecules such
as volatile-odorants and is small enough to pass into the
urine
Slide 37
MHC-associated odours: the carrier hypothesis In the serum the
molecule is a heterodimer with a heavy chain of 39kDa associated
non-covalently with 2-microglobulin Present in the serum as a
concentration between 350 390 ng/ml A half-life of 2.7 h Excreted
via the kidneys In the urine degradation occurs giving rise to a
major molecular mass species of 27kDa Cleavage at the papain
cleavage site
Slide 38
MHC-associated odours: the carrier hypothesis Class I molecules
are large and consequently lack a vapour pressure Class I molecules
might associate with smaller molecules, in an allelic-specific way,
and transport them from the blood into the urine Serum has an odour
but cannot be used to discriminate between MHC types (Brown, 1987,
Yamazaki, 1999) Treat with proteases to liberate odour Carboxylic
acids may be a source of the odour
Slide 39
In Search of the Chemical Basis for MHC Odourtypes (Kwak, 2010)
Lewis Thomas (1975) Two different classes of compounds have been
proposed: Small volatile molecules and non-voltaile MHC peptide
ligans (Boehm, 2005, Restrepo, 2006) Volatile molecules are
non-ionic and have a finite vapour pressure between ambient and
250C Their molecular weights are generally less than 300Da
Slide 40
Odourtype Volatile molecules are non-ionic and have a finite
vapour pressure between ambient and 250C Their molecular weights
are generally less than 300Da MHC peptide ligands are Ionic and
Composed of inne amino acids Their molecular weights exceed
1000Da
Slide 41
Chemical Investigations of MHC Odourtype in Mice Mice can
recognize the MHC-driven odour cues from urine and blood Studies
have failed to find specific behavioral effects of MHC variation in
wild mice (Cheetham, 2007, Sherborne 2007, Thom 2008, Potts
1991)
Slide 42
Chemical Investigations of MHC Odourtype in Mice Using anion
exchange chromatography confirmed that acidic compounds were
responsible for the MHC odour cues Some voltaile organic acids are
regulated by MHC genetic variation (Singer, 1997) GC/MS study of
about 80 compounds associated with MHC (Willse, 2005) MHC
associated odour cue can be recognized in spite of variation in
background strains (Beauchamp, 1990, Yamazaki, 1994, Eggert, 1996,
Willse, 2006)
Slide 43
Chemical Investigations of MHC Odourtype in Mice MHC-regulated
metabolic odour signal must have a background-independent component
GC/MS experiments on 3 MHC-congenic mice derived from different
background strains 36 out of 55 peaks from urine were found to be
present in all three strains Background genes, MHC genes and their
interaction regulate the urinary volatile profiles SPME GC/MS on
mice (Kwak 2009)
Slide 44
Chemical Investigations of MHC Odourtype in Mice A compound was
found to be more prominent in one MHC type in one strain and less
prominent in that same MHC type in the other strain MHC regulation
of volatile metabolic phenotypes was modulated by background genes
All published chemical investigations have failed to find
qualitative differences in volatile compounds that are associated
with MHC types, except Eggert, 1996 The patterns of volatile
metabolites vary according to MHC types, inconsistently and in a
complex way
Slide 45
The Major Histocompatibility Complex and the Chemosensory
Signaling of Individuality in Humans Four characteristics of MHC is
responsible for its enormous diversity A number of functional loci
per class Allelopolymorphism (estimated for HLA class I raging up
to 60 different alleles per locus) Codominant expression A high
level of heterozygocity (in natural populations of mice, it is
almost 100%)
Slide 46
The Major Histocompatibility Complex and the Chemosensory
Signaling of Individuality in Humans MHC effects on social behavior
in humans came from three types of studies A series of population
studies on the occurrence of shared HLA types in couples which in
part confirms an influence of the MHC but are not conclusive
Experimental studies on trained rats responses to human urinary
odor which demonstrated preliminary results of an MHC-associated
odor expression in humans A field study in which women were asked
to index others whose body odors elicited a strong response A
higher communality between indexing females and indexed females
than with indexed males Males for whom negatively rated body odors
were reported show a higher genetic similarity than those for whom
positively rated body odors were reported (Eggert, 1994)
Slide 47
Investigate trained rats responses to human odors Rats are able
to distinguish between the urine odors of immunogenetically defined
HLA-homogenous groups of persons HLA-associated urine donors occur
in members of both sexes Stimulus generalization task can be
performed using urine donors with different HLA-types
HLA-associated odor constitution appears not to be limited to a
definite HLA-specificity
Slide 48
Fractionation and Bioassay of Human Odor Types Which fraction
in human urine are chemosensorically active? The role of fractions
containing either volatile or nonvolatile substances Bioassay using
rats sniffing volatiles, smaller than 250Da and nonvolatiles Rats
were able to recognize the odor profile of the training samples in
the fractionated residue The distillate from the urine samples
containing the volatile urine fraction was not recognized
Slide 49
GC of Human Urine Using a GC on human urine 120 peaks were
found 16 out of 120 of specific components associated with the
examined HLA-types were found HLA differences influence the pattern
of ubiquitous metabolites Is the body odor of humans associated
with HLA- types? The constitution of body odors in humans may
provide evidence of an ultimate function of MHC- associated odors
in humans
Slide 50
Fractionation and Bioassay of Human Odor Types All tests showed
that the bioactive information of the odor profile is not included
in the distillate containing only the volatiles. Olfactory
information was not changed by adding a foreign distillate to a
familiar residue Olfactory recognition only occurred when the
residue of the fractionated urine samples were presented Odor
conveyed by the volatiles disappears very quickly. The mechanism of
slowly released volatiles would prevent a rapid disappearance of
the individual scent
Slide 51
MHC-regulated odortypes: Peptide-free urinary volatile signals
Small molecular weight compounds are involved in the signaling of
MHC odortype It appears that MHC genetic variation influences both
volatile and non-volatile (peptide-based) chemical signals It could
be that volatile signals are used to recognize a member of the
species and non-voltaile signals may play a more prominent role
close up Humans may discriminate sweat odors of the subject with
different HLA type and may have HLA- dependent odor
preferences.This also occurs in mice
Slide 52
Olfactory Cues Associated with the MHC Chemosenory signals are
used by many species as markers of individuality helping to
identify members of the same species A marker of individuality has
to stable over longer times and must be very different than a
genetic marker makes sense MHC also allows for degrees of genetic
relatedness (kin recognition) MHC has to be phenotypically
expressed in a way that allows other members of the species to
evaluate the specific MHC-type present
Slide 53
MHC MHC codes for transmembrane glycoproteins Self/non-self
discrimination of the immune system MHC class I is also found in
soluble form in the body fluids Serum, lymph and urine of rats In
human A9;A23, A24 are expressed in large amounts in the soluble
form Expression of odor signals is associated with the MHC and has
been proposed that these mechanisms of natural selection have
formed the development of a mechanism of sexual selection which is
involved in the maintenance of the genetic diversity in the
MHC
Slide 54
MHC Associated Odor Signals Lewis Thomas was the first to
propose that chemosensory signals associated with the MHC serve the
function of discriminative stimuli How are MHC specific odors
generated? Selective binding of odor molecules to soluble
derivatives of MHC class I receptors or MHC specific selective
bacterial colonizing or Coexpression of odor producing genes lying
in the MHC region
Slide 55
MHC Associated Odor Signals In humans There appears to be an
association between the concentration of soluble MHC proteins and
the salience of body odor cues Subjects with the HLA types A9 (A23,
A24) and B15 (B62, B63) occur in a much higher proportion than
expected in samples of subjects whose body odor was described as
more salient than usual These subject also show an elevated
concentration of soluble MHC proteins in their body fluids
Slide 56
MHC Associated Odor Signals The concentration of soluble MHC
proteins appears to be correlated with the reproductive cycle in
women with the highest concentrations within the first half of the
cycle The hedonics of the body odors of other subjects appears to
be associated with the similarity with regard to the MHC
Slide 57
MHC Associated Odor Signals In the same sex, body odors of
similar subjects are rated more positive than body odors of
dissimilar subjects In different sexes the reverse results were
obtained The similarity of the HLA class I genotype of two person
influences social perception measured by ratings of familiarity,
attractiveness or pleasantness
Slide 58
MHC-correlated mate choice in humans MHC in humans is also
called Human Leukocyte Antigen (HLA)
Slide 59
Disassortative Preferences: A Theory Women not using the
contraceptive pill rated as more pleasant the odors of men with MHC
antigens that were dissimilar to their own The smell of MHC
dissimilar men also reminded them of their current or previous
mates smell more frequently than did the smell of MHC similar men
Women using the contraceptive pill preferred the smell of MHC
similar men. Hormonal contraception physiologically mimics
pregnancy and there is a shift in preference during pregnancy
towards kin who would be most likely to assist in offspring
care
Slide 60
Disassortative Preferences: A Theory Women rated the odor of
six T-shirts worn by four males and two females Both men and
normally cycling women showed a tendency to rate the odors of MHC
dissimilar subjects as more attractive through a negative
correlation between the number of shared alleles and pleasantness
ratings Pill users rated MHC similar odors as more pleasant
Slide 61
Disassortative Preferences: A Theory T-shirt odors samples were
collected from 6 men Female raters chose a sample which they would
most and least like to smell on an everyday basis Most preferred
odor donors who shared significantly more alleles (2.3 on average)
A higher preference for the smell of donors carrying the same
alleles as the smellers inherited from their fathers, but not their
mothers (Hutterites)
Slide 62
Processing of MHC correlated odors Brain activity was recorded
in humans who were presented with two HLA similar and one HLA
dissimilar stimuli MHC similar samples were processed faster and
evoked larger potentials than MHC dissimilar samples Avoiding MHC
similar partners (Inbreeding) may have higher biological
significance that attraction to MHC dissimilar partners, whose odor
elicited relatively weaker responses