CHAPTER 8

ROLE OF THE MAJOR HISTOCOMPATIBITY

COMPLEX IN THE IMMUNE RESPONSE

Coico, R., Sunshine, G., (2009)

Immunology : a short course, 6th Ed. ,

Wiley-Blackwell 1

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CHAPTER 8 : MHC (Major Histocompatibility Comples)

Introduction

Discovery of the MHC

Structure of MHC Molecules

Antigen Processing & Presentation

Diversity of MHC Molecules Genes of HLA Region

Many pathogens, such as viruses, bacteria, and parasites, invade

host cells and live at least part of their life cycle inside them.

Antibodies do not enter the cell, antibodies are an ineffective

defense. The phase of immune response to pathogens inside host

cells in the domain of T cells and their products.

T cells interact with antigens expressed on the surface of host cells.

TcR interacts with two components, ie linear foreign peptide(antigen)

bound on major histocompatibility complex(MHC)

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inbred strain A mouse inbred strain B mouse P AA x BB

Transplantation ○↘ ↖X X↗ ↙○

F1 AB

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- inbred mouse strains are syngeneic or identical at all genetic loci

- two strains are considered congenic if they are genetically identical except at a single

genetic locus or region

&

&

twins

inbres

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Pomp and Mohlke Journal of Biology 2008 7:36 doi:10.1186/jbiol93 6

1) MHC molecule

⑴ MHC molecule은 target cell에 존재하는 membrane bound glycoprotein으로 항원 peptide와 결합된 상태로 세포 표면에 존재 한다. "antigen presentation" ⑵ Antigen presenting molecule로 T cell이 인식하는 도구이다. [조직이식 거부반응] ⑶ Class I 과 Class II가 존재한다. ⑷ Human의 MHC는 "HLA", Mosue의 MHC는 "H-2“

2) MHC Class I molecules

⑴ 모든 세포에 존재 (even in CTLs) ⑵ CD8+ T cells recognize MHC-I & antigenic peptide complex MHC-I is self & antigenic peptide is non-self

3) MHC Class II molecules

⑴ Professional antigen presenting cells에 존재 ⑵ CD4+ T cells recognize MHC-II & antigenic peptide complex MHC-II is self & antigenic peptide is non-self

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How The MHC Got Its Name

H-antigen : grafted cells [Acts as a transplantation or (histocompatibility) Ag]

* more than 30 loci in mice, there are more than 30 genetically

of H-antigen distinguishable loci for the H antigens

(Mouse; H-2, Human; HLA< human leukocyte antigen>)

H-2 locus

① encodes Ags that elicit intense reactions leading to rapid graft rejection

(in about 10 - 12 days)

② encoded H-2 proteins binds with foreign antigenic peptide and recognized by

T cells

③ among multiple H-2 proteins some has the strongly recognized and showed

the major graft rejection ⇒ Major Histocompatibility Complex

* Other loci except H-2 (ie, H-1 etc) encoded Ags elicit weak graft rejection responses

(weeks or months after grafting) ⇒ minor histocompatibility complex

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Different MHC Molecules Interact with

Different Sets of T Cells

Function of MHC molecules ;

1) Bind with “processed linear antigenic peptide” at inside

of the host cell.

2) Present the peptides to T cells

MHC Restriction ;

1) MHC class I ─ CD8+ T cell (ie CTL)

2) MHC class II ─ CD4+ T cell (ie Th)

MHC expression ;

1) Class I MHC ; all nucleated cells (* not on RBC)

2) Class II MHC ; Professional APCs only

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FIGURE 8.2. Cells expressing MHC class I interact with CD8 cells,which kill

infected host cells; cells expressing MHC class II interact with CD4 T cells, which

synthesize cytokines. 10

1) MHC-I

⑴ α-chain ; α1, α2, α3, extracellular domain & transmembrane domain

⑵ β2m ; β2 microglobulin (다른 chromosome)

2) MHC-II

⑴ α-chain ; α1, α2 extracellular domain & transmembrane domain

⑵ β-chain ; β1, β2, extracellular domain & transmembrane domain

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[3-1

] M

HC

Cla

ss I

mo

lec

ule

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[3-2

] M

HC

Cla

ss I

I m

ole

cu

le

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MHC class I MHC class II

peptide

binding

region

α1 & α2(S-S bond) domains α1 & β1 domains

● Four anti-parallel β-strands

and one α-helix from one

domain (total 8 anti-parallel β-

strands & 2 α-helices from α

chain )

● Size; 25Å x 10Å x 11Å (8-11 amino acid peptides can

bind, 9 is optimal)

● Oligosaccharide modified

● Four anti-parallel β-strands and

one α-helix from each domain

(total 8 anti-parallel β-strands

& 2 α-helices from α-chain and

βchain)

● Size; similar to Class I but both

ends are open form (13-25

residue peptides)

● Oligosaccharide modified at α1 & β1 domain.

Ig-like

region

α3 domain, (β2m) α2 domain & β2 domain

● Interact with β2m

● Recognize CD8

● Highly conserved

● CD4 interacts with β2 domain

● Highly conserved 14

● Recognized by TCR and CD4

molecules on Th cells

● Recognized by TCR and

CD8 molecules on CTLs. Remarks

● 13-25 amino acids in length

(processed linear peptide)

● 9-11 amino acids in length

(processed linear peptide)

ligand

peptide

● Variable in length

● Short and hydrophillic tail

● No phosphorylation site.

● ~30 amino acids

● Variable but consensus

phosphorylation sites

● Intracellular trafficking

cytoplasmi

c region

Both of α and β chains,

● ~25 amino acids (hydrophobic

and basic residues)

● α-helix

● ~25 amino acids

(hydrophobic and basic

residues)

● α-helix

trans-

membrane

region

MHC class Ⅱ MHC class I

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Figure 3-13

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DRB1 "B1" ; locus

DRB1*04 ; * 를 한 후의 2단위 숫자 ; allele group DRB1*0401 ; allel group의 subtype

[3-3] Anchor Residues / Peptide Motifs

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Figure 3-22

Pattern of MHC Molecule Expression in Different cells

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- Exogenous antigen endocytosis as endocytic vesicle(or phagosome)

↓ Lysosomal fusion and digestion [phagolysosome]

* Containing phagocytosed antigenic protein/lysosomal proteases /

HLA-DM (or H-2M in the mouse)

* Lysosome ; pH 4.5-5, contains ca. 40 digestive enzymes (ex, cathepsin

or leupeptin) endosome cycling to surface ; 10-15 min/cycle

↓ MHC class II and CD74[invariant chain(Ii)] assembly in ER

↓ Vesicle of MHC class II─ digested Ii (ie. CLIP chaperone function 수행)

migrates from Golgi to the cytosol.

↓ Fusion of phagolysosome and MHC-II vesicle

(CLIP and antigen peptide exchange catalyzed by HLA-DM)

↓ transported to cell surface membrane

↳ Recognized by CD4+ Helper T cells

[4-1] Responses to Exogenous Antigens:

Generation of MHC Class II-peptide Complexes

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Class II- associated Invariant Polypeptide

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Immunodominant T cell epitopes

각각의 Peptide들은 서로 다른 HLA class II 의 epitope일 수 있으며, 따라서 사람마다 다른 T cell epitope를 인식할 수 있다.

hydrophobic peptides

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1) Intracellular (endogenous) antigen 의 운명

- Endogenous antigens are digested by proteasme (ubiquitin complex)

↓ Transported into ER by TAP

↓ Antigen peptide + MHC ClassⅠ(α-chain:β2m)

↓ transported to cell surface membrane

↳ Cytotoxic T cells recognize & lysis of the cell

⑴ Intracellular antigen ; viral infection, aged or mutated cytosolic proteins

⑵ Proteasome ; 28 protein subunits(~35kD)로 구성 [700kD, 1500kD form]

(Class II MHC region gene product)

Ubiquitin tagged proteins degradation

※ Proteasomes typically generate peptides between 5 and 11 amino

acids long.

⑶ TAP (transporter associated with antigen processing);

TAP-1:TAP-2 complex

[4-2] Endogenous Antigens: Generation of MHC Class I-peptide Complexes

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2) Chaperone proteins and MHC class I molecule in ER

chaperone: 단백질의 정확한 고차구조형성(folding) 및 복합체 형성을 돕지만 최종적 구조체에 는 끼어들지 않는 단백질. Heat shock protein family가 포함됨 [chaperonin ; calnexin, Hsp60, GroEL등]

- calnexin:α-chain

↓ calnexin……α-chain:β2m…calreticullin

↳ α-chain:β2m:calreticullin……tapasin…TAP-1 (waiting for Ag peptide)

⑴ Calnexin ; calcium dependent carbohydrate binding protein (lectin)

ER내에서 TcR, Ig등의 peptides가 생성될 때 partly folded상태 유지

⑵ calretricullin , tapasin은 MCH class I molecule이 peptide와 결합하면 set free

⑶ Most of the peptide transported by TAP failed in binding to MHC class I and are cleared out of the ER

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peptide bound MHC class I molecule in the ER moves via the Golgi apparatus

to the cell surface, where it is presented to a CD8+ T Lymphocytes 29

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[4-3] More About MHC & Binding Peptides

1) Decreased MHC Class I Expression in Virus-Infected and Tumors

Some viruses (herpes simplex virus, adenovirus, cytomegalovirus etc.)

produce peptides that decreases expression of MHC class I by

- inhibit the synthesis of MHC class I

- Interrupt the transport of peptide-MHC class I complex to the cell surface

Tumor cells frequently show decreased expression of MHC class I.

However, decreased MHC-I expression triggers NK-cell activation

- MHC-I::KIR(killer inhibitory receptor on NK cell) prevent killing by NK.

Diseases related with MHC class I antigen presentation

⑴ non functional TAP ; no peptides entering ER

→ reduced MHC-I on cell surface (less than 1% of normal levels)

→ poor CD8+T cell responses → example, chronic resporatory infections

⑵ Herpes simplex virus ; TAP binding & inhibit TAP function

⑶ Adenovirus ; MHC-I binding protein생성으로 binding에 의해 ER leaving을 못하게 함.

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Fig. 8.8. A dendritic cell takes up (or pinocyotsis) an exogenous antigen, for example derived from a

virus-infected cell, but processed peptides associate with MHC class I molecules inside the cell and

are presented to CD8+ T cells.

2) Cross-Presentation : Exogenous Antigens Presented in the MHC Class I Pathway

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3) Which Antigens Trigger Which T-Cell Responses?

What is the discrimination of “endogenous” and “exogenous” ?

단백질 항원의 분해(processing)와 그 산물의compartment(구획)간 이동경로에 의해 구분 - Exogenous ;

taken up by professional APCs →MHC class II pathway → CD4+ T cells

(bacteria, virus, allergens, harmless antigens..)

- Endogenous ;

cytosolic antigens→ MHC class I pathway → CD8+ T cells

(create epitopes from infectious pathogens via the endogenous or cross-presentation

pathways )

2) Cross-Presentation : Exogenous Antigens Presented in the MHC Class I Pathway

바이러스항원처럼 원래 endogenous 경로의 항원단백(MHC class I::Tc)이지만 그림과 같이 2차적으로 pAPC에 탐식되어지면 exogenous 항원단백처럼 MHC class II를 통한 CD4 Th 세포가 인식하여 cytokines 분비를 유발할 수 있다.

본 경로는 pAPC에 감염되지 않았지만 “virus에 대해 조직의 감염세포를 인식하는 CD4 Th 세포의 형성”에 대한 기전으로 이해된다.

동시에 MHC class I에 인식되는 peptide가 존재하면 이를 통해 CD8 Tc cell이 인식하는 경로도 가능하다.

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4) Binding of Peptides Derived from Self-Molecules by MHC

노후 또는 변이된 self proteins의 proteasome 혹은 lysozyme 에 의한 분해물 및 catabolized ribosomal and mitochondrial proteins

실제로, 세포에서 분리한 MHC-I 착물에는 이와같은 self-peptide가 흔히 존재한다.

MHC-II 에도 self-peptide가 결합한다. Autophage : An intracellular pathway in which proteins in the cytoplasm are transported

into lysosomes for degradation.

Self peptide-MHC complex는 일반적으로 T cell 을 활성화시키지 못하는 이유: - T cell education에서 self recognizing TCR will be deleted (negative selection) - self peptides do not express co-stimulator (a second signal) functions to activate T cells.

5) Inability to Respond to an Antigen (항원에 대한 무반응성)

항원에 MHC와 결합할 수 있는 peptide(t-cell epitope)가 없는 경우 The MHC molecule expressed in some people may not bind the particular peptide which

can bind to the other people (vaccine non-responders).

HTLV-1, HBV, leprosy, malaria, 결핵, AIDS, ….

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[4-4] Other Types of Antigen That Activate T-Cell Responses

Other Pathways of Antigen-Processing & Presentation

(A) Super-antigen (bacterial exotoxins)

Staphylococcal enterotoxin binds to

MHC-II and activates CD4+ T cells

(B) Glycolipids (bacterial cell wall )

MΦ and dendrites express CD1 (MHC-I과 구조가 유사하며, β2m 과 결합) binds to glycolipids and activates T cells as MHC-I

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Human MHC Gene

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[5-1] The diversity of MHC molecules in the human population is due to

both polygeny and genetic polymorphism

1) Polygeny(多元遺傳子性) ⑴ expression of several different genes encoding proteins of similar functions ⑵ MHC class I molecules (for human 6 isotypes ; HLA-A, -B, -C, -E, -F, -G),

(for mice, K, D, L)

MHC class II

(for human 5 isotypes, HLA-DP, -DQ, -DR, -DM, -DO)

(for mice, I-A, I-E)

2) Genetic polymorphism (多形現象, 多形成) ⑴ different variants of each gene. ⑵ two haplotypes from each parents

3) allels and allotypes

(mouse의 경우 Kb and Kd but not Db and Kb)

4) 명명법 : HLA-B8*2702 의 경우, MHC class I인 HLA-B association, *뒤 4단위수; allele 의 의미임

(HLA-Cw ; complements와 구분하여 표기하기도 함)

HLA haplotyping method ; 항체를 이용한 혈청학방법 및 PCR법 41

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[5-2] Regulation of Expression of MHC Genes

(1) MHC molecules are co-dominantly expressed :

– from both the maternal and paternal chromosome

-- every nucleated cell expresses up to 6 different HLA class I and 6 (8)

different class II molecules.

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(2) Co-ordinate Regulation of MHC Expression :

– MHC-I and MHC-II genes are regulated separately -- MHC-I molecules can be expressed in the absence of MHC class II molecules.

(3) Inheritance of MHC Genes:

– Inheritance is basically a co-dominant Mendelian fashion -- However, individuals within a family do not usually express identical MHC molecules.

부모와 자식간의 유전자 형도 다르므로 가족간 이식거부반응 발생

In one person, the MHC-I

molecules expressed on every

cells are the same.

Person A has; Person B has

HLA-A2 & -A5 HLA-A4 & -A11

HLA-B5 & -B6 HLA-B8 & -B9

HLA-C1 & -C10 HLA-C1 & -C7

Also, the MHC-II molecules

expressed on every cells are

the same, but different between

individuals.

At least 6 kinds of HLA class II

molecules on each cell.

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형제가 많을 경우 HLA type이 같을 확률이 25% 이지만 실제로 가족 내 일치하는 경우는 없다 - 부모와 자식간의 유전자 형도 다르므로 가족간 이식거부반응 발생 45

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[5-3] Other Genes Within HLA

1. MHC class III gene

2. HLA-G at placental trophoblast cell, preventing rejection of the fetus from maternal host

3. HLA-DM involved in peptide exchange in the MHC class II pathway

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END of Ch 8

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