2. MHC CLASS I and CLASS II MOLECULE
VIRUS INFECTED CELL ANTIGEN PRESENTING CELL
3. ABBREVIATIONS
MHC - Major Histocompatibility Complex
HLA - Human Leucocyte Antigen
APC - Antigen Presenting Cell
NK - Natural Killer Cell
CD - Cluster Differentiation
TNF - Tumor Necrosis Factor
IFN - Interferon
KIR - Killer cell Immunoglobulin like Receptor
TAP - Transporter associated with Antigen Processing
LMP - Latent Membrane Protein
ERAP - Endoplasmic Reticulum Amino Peptidase
MIIC - MHC class II containing Compartment
CLIP - CLass II associated Invariant chain Peptide
RAGE - Receptor for Advanced Glycation End Products
NKG2 - Natural Killer Group 2
4. HISTORY
Genetically determined structures on the cell surface that
regulate immunological response.
• 1980 – Nobel Laureates For Medicine Or Physiology
BARUJ BENACERRAFJEAN DAUSSETGEORGE D. SNELL
5. HISTORY
ROLF M.ZINKERNAGEL PETER C.DOHERTY
1996 – NOBEL LAUREATES FOR MEDICINE OR PHYSIOLOGY
Specificity of the cell mediated immune defense
6. DON WILEY
Structural studies done by DON
WILEY and OTHERS showed that
different MHC proteins bind and
present different antigen fragments
HISTORY
IN 1980 K. ZIEGLER AND E.R.UNANUE showed that intra
cellular processing of APC is required to activate T cells
8. CLASSIFICATION
Class I & II MHC Molecules –
membrane bound glycoproteins
closely related in both structure and function
Class III MHC Molecule-
Groups of unrelated proteins
Do not share any similarity to class I and II
molecules.
Participates in other aspects of immune response
9. Class I MHC Molecules
All nucleated cells, platelets
Present intracellular antigens to CD8+ T cells
Consists of two polypeptide chains
α chain – 45 kDa
β chain – 12 kDa
10. Class I MHC MOLECULE
α CHAIN
3 external domains - each approximately 90 amino
acids long.
A transmembrane domain - 25 hydrophobic amino
acids and a short chain of charged hydrophilic
aminoacids.
A cytoplasmic anchor segment- 30 amino acid long.
11. CLASS I MHC MOLECULE
β2 Microglobulin –
Similar in size and organization to α3 domain
No transmembrane region
Bound non covalently to α domain
12. CLASS II MHC MOLECULE
Antigen presenting cells
Present extracellular antigens to CD4+ T cells
Consists of two different polypeptide chains
α chain – 33 kDa
β chain – 28 kDa
13. CLASS II MOLECULE
α CHAIN
▪ contains two extra cellular domains – α1
and α2 domain
▪ a transmembrane segment
▪ a cytoplasmic anchor segment
β CHAIN
▪ similar to α chain
14. PEPTIDE BINDING GROOVE
MHC CLASS I MOLECULE –
▪ formed by α1 and α2 domains
▪ socket like opening
▪ bind a peptide of 8 to 10 amino acids
MHC CLASS II MOLECULE –
▪ formed by α1 and β1domains
▪ open-ended groove
▪ bind a peptide of 13 to 18 amino acids
15.
16. MHC CLASS I MOLECULE MHC CLASS I PEPTIDE BINDING
GROOVE
17. MHC CLASS II MOLECULE MHC CLASS II PEPTIDE BINDING
GROOVE
19. MAJOR HISTOCOMPATIBILITY
COMPLEX
MHC is a collection of genes arrayed within a
long continuous stretch of DNA on
▪ chromosome 6 – humans
▪ chromosome 17 – mice
MHC is referred to as
▪ HLA complex – humans
▪ H-2 complex – mice
21. MHC CLASS I MOLECULE
▪ classical class I loci (A, B, C) - α chain
▪ non classical class I loci - HLA-E, F, G molecule
▪ β2 microglobulin - chromosome 15
MHC CLASS II MOLECULE
▪ classical class II loci (DP, DQ, DR) – both α and β chain
▪ non classical class II loci (DM, DO) – HLA-DM, HLA-DO
24. POLYMORPHISM - many alternative forms of each
gene or allele exists with in the population
• Many alternative forms of each allele exits in
the population.
26. LINKAGE DISEQUILIBRIUM
Non random association of alleles of two or more
different genes, at two different loci with a
frequency greater than would be expected by
chance.
Usually these alleles are located close to one
another in the same chromosome
Depends on,
▪ natural selection
▪ population size
▪ mutation rate
31. The MHC molecules expressed by an individual are fixed.
However the promiscuity of antigen binding ensures the
enormous flexibility of MHC molecules to bind to various
peptides.
It is rare for any two unrelated individuals to have identical
sets of HLA genes.
Not all alleles encode proteins.
Human class II genes are highly polymorphic and in some
cases an individual can inherit different number of genes.
The theoretical number of combinations possible are 1.7
billion class I haplotypes, 1015 class II haplotypes. Thus
bringing a total of more than 1.7 x1027 combinations.
High level of MHC polymorphism provide better survival
advantage.
32. FUNCTIONAL RELEVANCE FOR MHC
POLYMORPHISM
• Polymorphism in the MHC is clustered largely
with in the membrane distal α1 and α2
domains of class I, α1 and β1 domains of class
II molecules.
• Allelic differences contribute to the observed
differences in the ability of MHC molecules to
interact with a given peptide ligand
33. Genetic diversity of MHC loci in human
population
(2013)
(2605)
(1551)
(1200)
(7)
34.
35. MHC CLASS I EXPRESSION
Nucleated cells and platelets
Level of expression differs among different cell
types
Highest – lymphocytes (5x105 molecules /cell)
Low – fibroblasts, myocytes, hepatocytes,
neural cells.
Different individual with in a species present
different viral peptides.
36. MHC CLASS II MOLECULE
Antigen presenting cells
Level of expression significantly increases after
activation.
pAPCs differ by
▪ mechanism of antigen uptake
▪ constitutive expression of class II MHC
molecule.
▪ inherent co-stimulatory activity.
37. PROFESSIONAL APC
DENDRITIC CELLS –
▪ most effective
▪ constitutively express high levels of MHC class II molecule
▪ inherent co-stimulatory activity
▪ activate naïve TH cells.
MACROPHAGES –
to be activated before they express sufficient level of MHC
class II molecule and co-stimulatory activity
B CELLS –
▪ constitutively express MHC class II molecule
▪ to be activated for co-stimulatory activity.
NON PROFESSIONAL APC
1.Fibroblasts
2.Glial cells
3.Pancreatic β cells
4.Thymic epithelial cells
5.Thyroid epithelial cells
6.vascular endothelial cells
38. VARIOUS REGULATORS
GENETIC REGULATORY COMPONENTS –
CIITA, RFX – class II MHC transactivators. If defective causes
bare lymphocyte syndrome.
VIRAL INTERFERENCE –
Viral infection interfere with MHC class I expression by,
▪ decreased level of components needed for peptide
transport,
▪ or MHC class I assembly
▪ decreased transcription
CYTOKINE MEDIATED SIGNALLING –
▪ early stages of infection – IFN α and TNF α
▪ later stages of infection – IFN γ
39. The alleles in MHC haplotype
determine which fragment of
peptide to be presented
This variability in immune response
to an antigen among different
individuals is explained by two
models viz.,
▪ determinant-selection model
▪ holes-in-the-repertoire model
40. Determinant-selection model –
Different class II MHC molecules differ in their
ability to bind particular processed antigens.
And some peptides may be more crucial to
eliminate the pathogen than others.
Holes-in-the-repertoire model –
T cells bearing receptors that recognize certain
foreign antigens which happen to closely
resemble self antigens may be eliminated during
T cell development, leaving the organism without
the cells/receptors for future responses to foreign
molecules.
41. PEPTIDE BINDING
Each individual has 6 different class I molecule
and 12 or more different class II molecule.
Peptide binding by class I and II molecules
does not exhibit fine specificity.
A given MHC molecule can bind to several
different peptides and some peptides can bind
to several different MHC molecules.
So the binding between peptide and MHC is
often referred to as promiscuous.
42. In both types of MHC molecules peptide ligands are held in
largely extended conformation that runs along the length of
the groove.
43. MHC CLASS I and CLASS II MOLECULE
with BOUND PEPTIDES
46. ANCHOR RESIDUES – The amino acid residues of the peptide
that anchor the peptide to MHC class I binding groove.
▪ There is a carboxy terminal anchor usually formed by a
hydrophobic amino acid and amino terminal anchor either in 2nd
or 2nd and 3rd position also formed by hydrophobic amino acid.
The amino acid residues lining the binding site vary among
different class I allelic molecule which determines the
classical identity of anchor residues.
50. ENDOGENOUS PATHWAY
STANDARD PROTEASOME – 14 subunits.
IMMUNOPROTEASOME –
▪ present in pAPCs and infected cells
▪ in response to the cytokines IFN γ and TNFα the
genes LMP2 and LMP7 encode replacement catalytic
protein subunits that convert standard proteasomes to
immunoproteasomes.
▪ increased level of protein degradation and turn
over more rapidly.
The proteasome complex cleaves peptide bonds in an
ATP-dependent manner.
52. TAP PROTEIN
Membrane spanning heterodimer.
Consists of two proteins : TAP1 and TAP2
Has multiple transmembrane segments, a domain
projecting into the lumen of RER and an ATP-binding
domain that projects into the cytosol.
Encoded by TAP1 and TAP2 genes within class II MHC region
and different allelic forms of these regions exists.
Both TAP1 and TAP2 belong to the family of ATP-binding
cassette proteins.
These proteins mediate ATP dependent transport of
aminoacids, sugars, ions, and peptides.
TAP has affinity for peptides containing 8 to16 aminoacids.
56. INVARIANT CHAIN
CD 74
Non MHC encoded protein
Interacts with class II molecule preventing any
endogenously derived peptide from binding
Also involved in the folding of class II α andβ
chains, their exit from RER, and subsequent
routing.
60. CROSS PRESENTATION
First reported by Michael Bevan and later
described in detail by Peter Cresswell and
colleagues.
Internalized antigens that would normally be
handled by the exogenous pathway leading to
class II MHC presentation is redirected to class I
peptide loading pathway.
When this form of antigen presentation leads to
the activation of CTL responses, it is referred to as
cross-priming.
Dendritic cells are most efficient cross-presenters.
61. MECHANISM
Two hypothesis
1. Cross presenting cells possess special-processing
machinery that allows loading of exogenously
derived peptides onto class I MHC molecules.
2. Specialized endocytosis machinery that can send
internalized antigen directly to an organelle
where the peptides are loaded to class I MHC
molecule
Though it has not been resolved cross presented
antigens has been found to enter the cytoplasm.
62. LICENSED DENDRITIC CELLS
Dendritic cells first present the antigen by
classical exogenous pathway to CD4+ T cells.
These activated helper cells might then return the
favour by inducing costimulatory molecules in DC
and by cytokine secretion, supplying a “second
opinion” that it licenses the DC to cross present
the antigen to CD8+ T cells.
This helps to avoid accidental induction of CTLs to
self-proteins.
65. CD1
The nonpeptide antigens are presented by
members of CD1 family of nonclassical class I
molecules.
Structurally similar to classical MHC class I but
overlap functionally with MHC class II.
Five human CD1 genes are known in
chromosome 1.
Very little polymorphism
Expressed by thymocytes, B cells, DCs,
hepatocytes and epithelial cels.
66. The peptide binding groove as both deeper and narrower than
classical MHC molecules
Short chain self lipids with relatively low affinity are
loadedonto CD1 molecules in ER and allow CD1 folding.
When they encounter long chain, high affinity lipid antigens
self lipids are exchanged.
These newly loaded CD1 molecules return to the surface for
recognition by CD1-restricted T cells.
67. NON CLASSICAL MHC CLASS I
MOLECULES
HLA-E –
They are transported to cell surface only when they
are bound to the peptide derived from HLA-A, HLA-B,
or HLA-C.
The amount of HLA-E on the surface is an indicator of
the overall level of class I MHC biosynthesis in the
cell.
They are recognized by CD94-NKG2A receptor of
natural killer cell
68. HLA-F
Present in cells of thymus. Spleen and tonsil.
Comes to the surface only when the lymphocytes are activated.
HLA-G
Present in fetal cells at maternal – fetal interface.
They are credited with inhibiting the rejection by
maternal CD8+ T cells by protecting the fetus from
identification as foreign, which may occur when
paternally derived antigens begin to appear on
developing fetus.
69. HLA DM
Non classical MHC class II molecule
Catalyze the exchange of CLIP with antigenic
peptides.
It is a heterodimer and relatively non
polymorphic
Found predominantly in the endosomal
compartment
70. HLA DO
Observed in B cells, dendritic cells
Relatively non polymorphic class II molecule
Expression is not induced by IFNγ.
Negative regulator of antigen binding by
modulating the function of HLA-DM.
Following cell activation DO expression is
down regulated.
71. CLASS III MHC MOLECULE
Extremely gene dense
Contain about 62 genes
The genes C4A and C4B are highly polymorphic.
Some important products include
I. C4, C2 and factor B
II. TNF α and β
III. NOTCH-4
IV. PBX2 gene product – regulating the expansion of
hematopoietic precursor
V. RAGE – upregulated in vasculopathies, atherosclerosis,
and vasculitis.
73. INHIBING RECEPTORS -
KIR receptors recognize HLA-B or HLA-C
CD94-NKG2A receptors recognize HLA-E
ACTIVATING RECEPTORS –
NKG2D – ligands are nonpolymorphic MHC class I like
molecules that do not associate with β2 microglobulin.
They are found in cells undergoing stress either due to
DNA damage or VIRAL infection.
Editor's Notes
CIITA, RFX – class II MHC trans activators. Defects in these ta causes BARE LYMPHOCYTE SYNDROME
Eg – 1. in CMV viral proteins bind to β2M preventing assembly and transport of MHC class I molecule.
2. ADENO virus – reduced transcription of TAP gene.