Cellular Immunity

Major histocompatibility complex (MHC)

• important in self vs non-self recognition

• T cell recognition of Ag includes recognizing self-MHC

• non-self MHC=foreign, these cells are destroyed

HLA

HLA=human leukocyte antigens=human MHC

3 classes of MHC gene locus

class I MHC, class II MHC, and class III MHC

Class I MHC

-on most nucleated cells

-HLA-A, HLA-B, and HLA-C (in humans)

Class II MHC

-mostly expressed by professional antigen presenting cells (APCS=macrophages, B cell, DC)

HLA-DP, HLA-DQ, HLA-DR (in humans)

Class III MHC

-secreted proteins that are found in serum

-complement proteins, TNF, proteins in Ag processing

MHC Class I Protein

-single chain associated with another protein NOT encoded for by MHC locus (ex: beta2 microglobulin)

-CD8+ T cells (CTLs)

MHC Class II Proteins

-heterodimers encoded for by alpha and beta genes in MHC locus

-CD4+ T cells (helper T)

Class II MHC genes

Class I MHC genes

Class III MHC genes

Co-dominant

both alleles are expressed at once, both give us the haplotype

MHC genes are:

• polygenic

• have multiple similar genes (A,B,C)

• (in humans) 6 from class I MHC, 12 from class II MHC

Allotype

genetic sequence of a gene within a species

HLA locus genes are:

Classical and non-classical

Classical MHC genes

classical=T cell recognition of Ag

ex: HLA-A,B,C and HLA-DP,DQ,DR

Non-classical MHC genes

NK cells and others

Heterodimer of MHC class II proteins

-alpha and beta chains

-protein is divided into alpha1/alpha2 and beta1/beta2

CD4 contacts the class II molecule

MHC class I vs MHC class II antigenic peptides

class I: closed-ended structure accommodates 8-9 AA

class II: open-ended structure accommodates 18-20 AA

Endogenous intracellular peptides

• ER is where class I MHC and Ag peptide associate

• Nonamers (9 AA) are preferred peptide length and bind the strongest

How peptides associate with MHC class I

• proteins originate in cell

• proteasome complexes degrade proteins

• degraded proteins go to rER (TAP proteins) and associate with MHC class I

Exogenous peptides

• peptides degraded by endocytic pathway

• MHC class II expressed on: B cells, macrophages, DCs

How peptides associate with MHC class II

• proteins/cells are endocytosed

• endosome fuses with lysosome

• once CLIP is release, Ag peptide can sit in MHC class II

• MHC class II/Ag then moves to cell surface

Human MHC genes are:

polymorphic=allow population to respond to the largest possible number of antigens

Antigen recognition for T cells

T-cell receptor (TCR)

T cells

• TCR is always cell bound

• TCRs recognize Ag in association to MHC

MHC class I T cell activation and function

MHC class II T cell activation and function

TCR gene rearrangement

• occurs in thymus

• controlled by: RAG1 and RAG2

• 2 possibilities: alpha-beta or gamma-beta

TCR structure

• Heterodimer

• alpha-beta and gamma-beta chains are similar to Fab of Igs

• anchored by disulfide bonds

• variable and constant regions

• 3 CDRs (complimentary determining regions)

TCR gene rearrangement

similar to Ig gene rearrangement

CD3 complex

Signaling function (similar to immunoglobulin Ig-alpha/Ig-beta)

TCRs are members of:

Ig gene superfamily, bc of Ig fold (intrachain disulfide bond)

T cell maturation and differentiation

• progenitor T cells originate in bone marrow and move to thymus

• thymus undergoes ‘education‘ process for alpha-beta T cells

Lack of thymus

• causes T cell defects

• thymectomy

• DiGeorge syndrome (deletion of part of chromosome 22=thymic aplasia)

Positive thymic selection of T cells

• Select T cells that can interact with self-MHC

• cells that bind get a protective signal and survive

Negative thymic selection of T cells

• T cells with high affinity for self-MHC and T cells are reactive to self-peptides=die with apoptosis

• cells NOT reacting=selected to survive and leave thymus

Diversity of TCR (5)

• combinatorial joining (3×10^6 possibilities)

• alternative joining of D segments (beta chain)

• junctional flexibility (imprecise cutting/joining of genes)

• P nucleotides (palindromic bases added)

• N nucleotides: Tdt enzyme addition

• estimates 10^13 possibilities

Major CD4+ T helper cell subsets (5)

• Th0=naive T cells

• Th1= CMI, intracellular microbes

• Th2= humoral, extracellular microbes, worms

• Treg(Th3)= regulate autoimmunity

• Th17= extracellular bacteria, fungi, associated with autoimmunity

Th cells are divided into two major subsets:

• Th1=CMI (IL-2, IFN-gamma, TNF-beta)

• Th2= (IL-4,5,6,10,13)

Th1/Th2 subsets

• Th0 to Th1/Th2 subset is dependent on APC stimulation

• IL-12 from DC stimulates Th1 production

• IL-4 from Th2 stimulates Th2 production

Cytokine activity

• cytokines must be bound to specific cytokine receptor

• once bound, signaling cascade exerts biological activity

• act locally, because short in vivo half life

• cytokine receptors have 2-3 chains

Cytokine receptor redundancy

• in a cytokine receptor family, often share same signaling chain

• get higher affinity with binding of 2-3 chains

• after cell activation=increased cytokine receptor expression

SCID (X-inked severed combined immunodeficiency)

defect in gamma chain gene

Th1

stimulate CMI/inflammation, IL-2, IFN-gamma

Th2

stimulate humoral responses, IL-4,5,6,10

Th3 (Tregs)

regulatory cells, make IL-10, TGF-beta

Th17

extracellular bacteria and fungi

IL-2 receptor family

shares common gamma chain

CD4 and CD8

CD4=class II=beta2, CD8=class I=alpha3

T cell activation/co-stimulation

• T cells need 2 signals to be stimulated

• Signal 1: TCR-Ag (pMHC)

• Signal 2: CD28/B7

• inhibitory signal:CTLA-4 binds to B7 over CD28 and sends suppression signal

Lack of 2 signals causes T cells to:

T cells become anergic (suppressed) or undergo apoptosis

How to killa microbe

• innate defenses: phagocytosis

• adaptive responses: antibodies, cytotoxic T lymphocytes (CTLs), NK cells, ADCC

Effector cell

• effector cell is a cell with a ‘job‘

• effector cells are not naive (increased expression of adhesion molecules)

Cytotoxic T cells

• important in eliminating altered self cells (like virally infected cells or tumor cells)

B and T cell ‘jobs‘

B cell= is the plasma cell

T cell= is the CTL, Th1 and Th2 cell

CTL-mediated killing of target cells

• is the release of cytotoxic mediators at interface between Tc (target cell)

perforin

pore forming

granzymes

enter target cell through pores

FasL on CTL and Fas on target cell

• cascade events= apoptosis

• TNF-alpha binds to Fas =apoptosis

Perforin activity

NK cell-mediated killing of altered-self cells

• similar to CTL but no AG specific recognition

• NK cells active before CTLs

• produce cytokines to increase macrophage activity

• produce perforin and granzymes

• NK cells detect alterations in expression on surface of normal proteins

Antibody-dependent cellular cytotoxicity (ADCC)

• Ab serves the specificity to deliver Ag to non-specific cells to target'

• FcR binding

• NK cells, neutrophils, eosinophils, macrophages

• release of granules=cell death

impact of HIV on T helper cells

NK cells kill altered cells via:

reduction in MHC class I and presence of stress proteins

ADCC

when antibody binds to target cell, Fc region can interact with FcRs on surface of cells (neutrophils, NK, macrophages, eosinophils), these cells release toxic chemicals that kill the target cell