Cell-Mediated Immunity and T Lymphocytes

T Lymphocytes and Cell-Mediated Immunity

• T lymphocytes perform multiple functions in defending against infections by various kinds of microbes.
• They play a major role in cell-mediated immunity, which provides defense against infections by intracellular microbes.
• Cell-mediated immunity is necessary to eliminate infections that occur inside cells, such as those caused by viruses and some bacteria.

Phases of T Cell Responses

• Naive T lymphocytes recognize antigens in peripheral lymphoid organs, which initiates proliferation and differentiation into effector and memory cells.
• Effector cells perform their functions when they are activated by the same antigens in peripheral tissues or lymphoid organs.
• The responses of naive T lymphocytes to cell-associated microbial antigens consist of a series of sequential steps, including:
  • Antigen recognition
  • Cytokine secretion and receptor expression
  • Clonal expansion
  • Differentiation into effector and memory cells

Antigen Recognition and Costimulation

• The initiation of T cell responses requires multiple receptors on the T cells recognizing ligands on antigen-presenting cells (APCs).
• The T cell receptor (TCR) recognizes MHC-associated peptide antigens.
• CD4 or CD8 coreceptors on the T cells recognize MHC molecules on the APC and help the TCR complex to deliver activating signals.
• Adhesion molecules strengthen the binding of T cells to APCs.
• Costimulators, such as B7-1 and B7-2, on APCs promote the responses of T cells to antigens.
• Cytokines amplify the T cell response and direct it along various differentiation pathways.

Role of Adhesion Molecules and Costimulation

• Adhesion molecules, such as integrins, on T cells recognize ligands on APCs and stabilize the binding of T cells to APCs.
• The binding of T cells to APCs must be stabilized for a sufficiently long period to achieve the necessary signaling threshold.
• Costimulation ensures that naive T lymphocytes are activated fully by microbial antigens and not by harmless foreign substances or self-antigens.

Inhibitory Receptors of T Cells

• Inhibitory receptors, such as CTLA-4 and PD-1, are critical for limiting and terminating immune responses.
• These receptors function to terminate responses of activated T cells and prevent immune responses against self-antigens.
• Genetic deletion or blockade of these molecules can result in systemic autoimmune disease.

Stimuli for Activation of CD8+ T Cells

• The activation of CD8+ T cells is stimulated by recognition of class I MHC-associated peptides and requires costimulation and helper T cells.
• The initiation of CD8+ T cell activation often requires cytosolic antigen from one cell (e.g., virus-infected or tumor cells) to be cross-presented by dendritic cells.
• The differentiation of naive CD8+ T cells into fully active cytotoxic T lymphocytes (CTLs) and memory cells may require the concomitant activation of CD4+ helper T cells.### Helper T Cells and CD8+ T Cells
• Human immunodeficiency virus (HIV) kills CD4+ but not CD8+ T cells, which is likely the explanation for the defective CTL responses to many viruses in HIV patients.
• CD8+ T cells can respond to some viruses without the help of CD4+ T cells.

Biochemical Pathways of T Cell Activation

• T cell activation leads to protein synthesis, differentiation, and effector functions.
• Antigen recognition triggers several biochemical mechanisms, including:
  • Kinase activation
  • Adaptor protein recruitment
  • Production of active transcription factors
• The TCR complex, CD4/CD8 coreceptors, and CD28 coalesce at the center of the immune synapse, while integrins move to the periphery.
• The immune synapse is required for optimal induction of activating signals in the T cell.

Signaling Pathways

• CD4 and CD8 coreceptors facilitate signaling through the protein tyrosine kinase Lck.
• Lck phosphorylates ITAMs (immunoreceptor tyrosine-based activation motifs) in the CD3 and ζ chains.
• The phosphorylated ITAMs in the ζ chain become docking sites for the tyrosine kinase ZAP-70.
• ZAP-70 phosphorylates various adaptor proteins and enzymes, leading to additional signaling events.
• The major signaling pathways linked to ζ-chain phosphorylation and ZAP-70 are:
  • Calcium-NFAT pathway
  • Ras- and Rac-MAP kinase pathways
  • PKCθ-NF-κB pathway
  • PI-3 kinase pathway

Calcium-NFAT Pathway

• NFAT (nuclear factor of activated T cells) is a transcription factor that is present in an inactive form in the cytoplasm of resting T cells.
• NFAT activation and nuclear translocation depend on the concentration of Ca2+ ions in the cytosol.
• The signaling pathway is initiated by ZAP-70-mediated phosphorylation and activation of phospholipase Cγ (PLCγ).
• PLCγ catalyzes the hydrolysis of PIP2, generating IP3, which binds to IP3 receptors on the ER membrane, stimulating the release of Ca2+ from the ER.

Ras- and Rac-MAP Kinase Pathways

• These pathways are initiated by ZAP-70-dependent phosphorylation and accumulation of adaptor proteins at the plasma membrane.
• The pathways lead to the activation of distinct MAP kinases (ERK and JNK), which induce the expression of the transcription factor AP-1.

PKCθ-NF-κB Pathway

• PKCθ is activated by diacylglycerol, which is generated by PLC-mediated hydrolysis of membrane inositol lipids.
• PKCθ acts through adaptor proteins recruited to the TCR complex to activate NF-κB.
• NF-κB is released from its inhibitor IκB and moves to the nucleus, promoting the transcription of several genes.

PI-3 Kinase Pathway

• PI-3 kinase phosphorylates the membrane phospholipid PIP2 to generate PIP3.
• PIP3 is required for the activation of Akt, which has many roles, including stimulating the expression of antiapoptotic proteins and promoting survival of antigen-stimulated T cells.

T Cell Functional Responses

• T cell activation leads to the secretion of cytokines, such as IL-2, which act on the T cells themselves and on other cells involved in immune defenses.
• Cytokines are produced by effector T cells and serve diverse roles in host defense.
• IL-2 is produced by activated CD4+ T cells and stimulates the survival and proliferation of T cells.
• The IL-2 receptor is a three-chain molecule, and its expression is increased on activated T cells.

Clonal Expansion

• T lymphocytes activated by antigen and costimulation begin to proliferate, resulting in the expansion of antigen-specific clones.
• The magnitude of clonal expansion is remarkable, especially for CD8+ T cells.
• The expansion of CD4+ T cells is 100- to 1000-fold less than that of CD8+ cells.

Differentiation of Naive T Cells into Effector Cells

• Some of the progeny of antigen-stimulated, proliferating T cells differentiate into effector cells whose function is to eradicate infections.
• Effector cells of the CD4+ lineage acquire the capacity to produce different sets of cytokines.
• Effector cells of the CD8+ lineage acquire the ability to kill infected cells.

Development of Memory T Lymphocytes

• A fraction of antigen-activated T lymphocytes differentiates into long-lived memory cells.
• Memory cells survive even after the infection is eradicated and antigen is no longer present.
• Memory cells can be found in lymphoid organs, peripheral tissues, and the circulation.

Migration of T Lymphocytes in Cell-Mediated Immune Reactions

• T cells at different stages of their lives have to migrate in different ways.
• Naive T cells migrate between blood and secondary lymphoid organs through HEVs (high endothelial venules).
• HEVs are lined by specialized endothelial cells that express carbohydrate ligands that bind to L-selectin.### Migration of Naive T Cells
• Naive T cells migrate through blood vessels in a multistep sequence similar to other leukocytes
• Engagement of L-selectin with HEV (high endothelial venules) allows chemokines to bind to CCR7 on T cells
• CCR7 transduces signals that activate LFA-1 (leukocyte function-associated antigen 1) on naive T cells, increasing binding affinity for ICAM-1 on HEV
• Firm adhesion and arrest of rolling T cells occurs, followed by exit into the T cell zone of the lymph node

Egress of T Cells from Lymph Nodes

• Sphingosine 1-phosphate (S1P) plays a key role in T cell egress from lymph nodes
• S1P levels are higher in blood and lymph than inside lymph nodes
• S1P binds to its receptor, reducing expression on circulating naive T cells
• When a naive T cell enters a node, S1P receptor expression increases, and the cell exits the node through efferent lymphatic vessels following the S1P gradient

Activation and Differentiation of T Cells

• Activated T cells do not express CCR7 or L-selectin, preventing re-entry into lymph nodes
• Effector T cells migrate to sites of infection, expressing adhesion molecules and chemokine receptors that bind to ligands on vascular endothelium
• Naive T cells do not express ligands for E- or P-selectin or receptors for chemokines produced at inflammatory sites, preventing migration into sites of infection

Site-Specific Immune Response

• Homing of effector T cells to a site of infection is independent of antigen recognition, but lymphocytes that recognize antigens are preferentially retained and activated at the site
• Chemokines produced by macrophages and endothelial cells stimulate motility of transmigrating T cells
• Effector T cells that leave the circulation and recognize microbial antigens presented by local tissue APCs become reactivated and contribute to killing microbes

Decline of the Immune Response

• After an infection is cleared, the immune response subsides, and the system returns to homeostasis
• Survival and proliferation of T cells are maintained by antigen, costimulatory signals from CD28, and cytokines like IL-2
• Once the infection is cleared, stimulated T cells die by apoptosis, and the response subsides within 1-2 weeks