T Cell Function

T Cell Activation

• T cells do not directly recognize microbes; they recognize antigens (short peptides) presented on host cell surfaces.
• Antigen presentation is crucial because only a few naïve T cells (out of $10^5$ - $10^6$) are specific for any one antigen and most are not at the site of infection.
• Sentinel and tissue-resident cells recognize microbes at infection sites and migrate to secondary lymphoid organs (e.g., lymph nodes) to present antigens to T cells, thus called antigen-presenting cells (APCs).
• Antigens entering the bloodstream are captured by APCs in the spleen.
• T cells migrate to be in the right place at the right time, mediated by adhesion molecules (selectins & integrins) and chemokines.
• Naïve T cells preferentially leave the blood and enter lymph nodes.
• APCs bearing antigens enter the lymph nodes where naïve T cells recognizing their cognate antigen are activated.
• Activated T cells return to circulation and preferentially leave the blood to enter peripheral tissues, moving to sites of infection or inflammation.

Antigen Presentation - 3 Components

• APC presents peptide to T cell via MHC.
• T cell receptor (TCR) on the T cell contacts the MHC-peptide complex.
• The MHC molecule has a polymorphic residue and a "pocket" for the peptide.
• The peptide has an anchor residue that binds to the MHC.

Professional Antigen Presenting Cells

• Specialized cells that capture, process, and display protein antigens to CD4+ T cells.
• They express MHC Class II (loaded with peptides) and co-stimulatory molecules to activate CD4+ T cells.
• After activation, CD4+ T-cells "help" the APC to enhance their function by:
  • Expression of CD40L (co-stimulator)
  • Production of IFN$γ$ (cytokine)
• Examples: Dendritic cells, macrophages, B cells

Dendritic Cells

• Capture, endocytose, process, and present antigens to CD4+ T-cells (both naïve and memory).
• The ONLY APC that can activate naïve T-cells.
• Resident in tissue sites commonly invaded by microbes (e.g., skin & epithelium of the GI tract).
• Express receptors (e.g., TLR) which help them identify & capture microbes.
• Migrate to regional (draining) lymph nodes to present antigens to T cells.

Macrophages

• Phagocytose microbes, process and present antigens to CD4+ T-cells.
• CD4+ T-cells signal to the macrophage through CD40L and IFN$γ$.
• Microbicidal activity of the macrophage is enhanced, leading to a cell-mediated immune response.
• E.g., Alveolar macrophages in the lung phagocytose microbes (e.g., M. tuberculosis) and attract T cells to the site using cytokines.
• Circulating monocytes migrate to site of infection and differentiate into macrophages.

B Cells

• Recognize unprocessed and soluble (free) antigens via their B cell receptor (BCR, membrane-bound antibody).
• Internalize (endocytose) and process proteins into peptides.
• Present peptides on MHC-II to CD4+ T-cells.
• CD4+ T-cells signal to the B cells through CD40L and IL-4.
• Signaling then activates B-cells to differentiate into antibody-producing cells, resulting in a T-cell-dependent humoral immune response.

MHC and Antigen Presentation

• All nucleated cells can present antigen to CD8+ T cells via MHC-I.
• MHC class I presents peptides to CD8+ T-cells.
• MHC class II presents peptides to CD4+ T-cells.
• MHC class I present on all nucleated cells and MHC Class II are present on Professional APC.

MHC Genes

• MHC class I genes: HLA-A, HLA-B, HLA-C

• MHC class II genes: HLA-DP, HLA-DQ, HLA-DR

• A set of MHC alleles in an individual determines their MHC haplotype.

• MHC locus is located on the short arm of chromosome 6.

• MHC class I and class II genes are the most polymorphic genes of the human genome.

• Each individual expresses the MHC alleles that they inherited from their two parents.

• Each individual contains only a few different MHC molecules (6 class I and about 8 or more class II molecules in a heterozygous individual).

• A single MHC molecule can bind multiple peptides to maximize the types of different antigens that can be shown to T cells.

Peptide Immunodominance & Binding

• Multiple possible epitopes exist within an antigen.
• Antigen processing generates multiple peptides, one of which can bind to a class II allele.
• T cells respond to the immunodominant peptide epitope.
• Each MHC has a single peptide-binding cleft.
• Each MHC can bind many different peptides with similar structure (size).
• Peptide size is typically 8-11 residues for MHC-I and 10-30 residues for MHC-II.

Antigen Processing (MHC-I→CD8)

• TAP = transporter associated with antigen processing
• ER = endoplasmic reticulum
• ERAP = ER-associated peptidase

Antigen Processing (MHC-II→CD4)

• Uptake of extracellular proteins into vesicular compartments of APC.
• Processing of internalized proteins in endosomal/lysosomal vesicles.
• Biosynthesis and transport of class II MHC molecules to endosomes.
• Association of processed peptides with class II MHC molecules in vesicles.
• Expression of peptide-MHC complexes on the cell surface.
• li = invariant chain; CLIP = class II-associated invariant chain peptide
• HLA-DM facilitates peptide loading.

Antigen Processing Summarized

T Cell Activation

1. Antigen presentation (MHC → TCR)
   • Class I → CD8 + T cells
   • Class II → CD4 + T cells
2. Co-stimulatory molecules
   • B7 (CD80/86) → CD28
   • CD40 → CD40 ligand (CD40L)
3. Cytokines
   • Proliferation (IL-2)
   • Differentiation

T Cell Co-stimulators (signal 2)

1. B7:CD28 (B7.1=CD80 or B7.2=CD86)
   • “Resting” APC have no or low B7 expression
     • T cell anergy - “unresponsiveness”
     • Maintain tolerance to self antigens
   • Expression of B7 increases after activation of APC
     • Microbes binding to innate receptors (e.g. TLR)
     • Activated by cytokines (e.g. IFN$γ$)
2. CD40:CD40L
3. ICOS:ICOSL (follicular helper T-cell-dependent antibody response)

B7 and CD28 Interaction

• Promotes T-cell survival
  • ↑ anti-apoptotic proteins (e.g. Bcl-2)
• Enhances proliferation
  • ↑ IL-2 and IL-2R, ↑ cyclins, ↓ cell cycle inhibitors
• Promotes differentiation into effector and memory cells
  • multiple mechanisms

CD40 and CD40L Interaction

• T cells expressing CD40L activate APC to be better stimulators by increased expression of B7 molecules and induction of cytokine production (e.g. IL-12) to promote T cell differentiation.

B7 and CD28 Inhibition

• Cytotoxic T-Lymphocyte Antigen-4 (CTLA-4)
  • Inhibitory receptor of the CD28 family
  • It has a higher affinity for B7 molecules than CD28
  • It competitively inhibits binding of CD28 to B7 molecules
• Programmed death receptor-1 (PD-1)
  • Inhibitory receptor of the CD28 family
  • Binds PD-L1 or PD-L2 on APC and inhibits T-cell response
• Timing is important
  • CD28 receptors are constitutively expressed on T cells
  • CTLA-4/PD-1 are only expressed after T-cell activation (1-2 days)
• Failure to regulate T-cell responses may result in:
  • Inflammation → No/low expression of inhibitory receptors
  • Impaired immune response → High expression of inhibitory receptors

Therapeutic Immunomodulators

• Rheumatoid arthritis (inflammation):
  • Too much immune activation, not enough inhibitory signal
  • Treat with CTLA-4-Ig (mimics CTLA-4 receptor), blocking B7 interactions with CD28 and inhibiting T-cell activation
• Cancer (impaired anti-tumor response):
  • Too much immune inhibition
  • Treat with anti-CTLA4 (blocking antibody), blocking B7 interactions with CTLA-4 and promoting T-cell activation

Cytokines and T Cell Proliferation/Differentiation

• IL-2 is important for T cell proliferation.
• Antigen recognition leads to lymphocyte clonal activation and expansion.
• Cytokines (e.g., IL-2) are important for T-cell proliferation.
• Naïve CD4+ and CD8+ T cells can differentiate into effector and memory cells.
• Effector CD4+ T cells activate macrophages, B cells, and other cells, causing inflammation.
• Effector CD8+ T cells (CTL) kill infected target cells and activate macrophages.
• Memory T cells:
  1. Long-lived cells
  2. Enhance ability (faster) response to antigens
  3. Generate new effector cells when re-encounter with the same antigen (e.g. vaccination)

T Cell Activation Overview

1. Antigen presentation (MHC → TCR)
   • Class I → CD8 + T cells
   • Class II → CD4 + T cells
2. Co-stimulatory molecules
   • B7 (CD80/86) → CD28
   • CD40 → CD40 ligand (CD40L)
3. Cytokines
   • Proliferation (IL-2)
   • Differentiation

T Cell Phenotypes (Functions)

1. Memory T cells
2. CD8 + cytotoxic T cells
3. CD4 + helper T cells (Th1, Th2, Th17, Treg)

Development of Memory T Cells

• After stimulation of naïve T-cells by antigens, they differentiate into effector T cells (short-lived) or memory T cells (long-lived).
• Memory T cells display:
  • Increased levels of anti-apoptotic proteins (e.g. Bcl-2)
  • Slow proliferation
  • The ability to self-renew with maintenance dependent on cytokines only (e.g. IL-7)

Characteristics of Memory T Cells

• Different receptor expression patterns than naïve T cells
• Can mount larger and more rapid responses to antigens than naïve T cells
• Are heterogeneous and can be further subdivided:
  • Central memory T cells - CCR7+L-selectin+ (lymph node)
  • Effector memory T cells – CCR7-L-selectin- (mucosal tissues)
  • Th1 (IFN$γ$), Th2 (IL-4, IL-5, IL-13) or Th17 (IL-17)

Development of CD8 T Cells

• Differentiation of naïve CD8+ T cells to effector or memory CD8+ T cells may require CD4+ T cell help
  • DIRECT
  • INDIRECT

Functions of CD8 T Cells

• Perforin/granzyme-mediated cell killing
• Fas/FasL-mediated cell killing
• Granzymes enter target cell cytosol and activate caspases, leading to apoptosis of target cell

Differentiation of CD4 T Cells

• Naïve CD4+ T cells may differentiate into distinct subsets to combat different types of pathogens.
• Differentiation is dependent on the cytokines produced by:
  1. The APC that encounters the microbe (microbe specific)
  2. The T cells themselves and other nearby cells (e.g. NK cells produce IFN$γ$)

Polarisation of CD4 T Cells

• STAGE 1 – Induction by the 3 signals → TCR:MHC, co-stimulators & cytokines
  • Cytokines induce transcription factors
• STAGE 2 – Commitment
  • Epigenetic changes cause genes that encode the subsets’ cytokine to be more accessible and vice versa
• STAGE 3 – Amplification (polarisation)
  • Cytokines produced promote the development of that Th subset and inhibits the development of other Th subsets (e.g. Th1 inhibits Th2)

Th1 CD4 T Cells

• Involved in macrophage activation and production of some antibody isotypes
• Promoted by microbes, IL-12, and IFN-gamma.
• Transcription factors: STAT1, STAT4, T-bet
• Classical macrophage activation (enhanced microbial killing)
• Complement binding and opsonizing IgG antibodies

Th2 CD4 T Cells

• Involved in IgE production and eosinophil activation
• Promoted by helminths, IL-4
• Transcription Factors: STAT6, GATA-3
• Alternative macrophage activation (enhanced fibrosis/tissue repair)
• Mast cell degranulation
• Intestinal mucus secretion and peristalsis

Th17 CD4 T Cells

• Involved in inflammation and barrier function
• Promoted by bacteria, fungi, IL-6, IL-1, IL-23, TGF-$β$
• Transcription Factor: STAT3, ROR$γ$t
• Increased barrier function
• Antimicrobial peptides
• Neutrophil response

Regulatory T Cells

• Involved recognition of self antigen in Peripheral tissues and thymus.
• Natural Tregs: Recognition of self antigen in thymus
• Inducible or adaptive Tregs: Recognition of self antigen in peripheral tissues
• CTLA-4 is important for Inhibition of T cell activation
• IL-10 and TGF-$β$ are important for Inhibition of T cell effector functions