Cell-Mediated Immunity An adaptive immune response mediated by specific cells of the immune system –Primarily T lymphocytes ( T cells ), but also macrophages and NK cells. –Formally defined as immunity that can be transferred from one organism to another by lymphoid cells, but not by serum antibody.

T cells Main coordinators and effectors of cellular immunity Defined by their development in the t hymus and the presence of a T-cell receptor (TCR) complex

T cells (continued) Two main types: 1. CD4 + : Stimulate other immune cells. 2. CD8 + Cytotoxic T cells: Kill intracellularly-infected cells. Two major types of CD4+ T cells: 1. T H 1: Inflammatory T cells -- Stimulate macrophages and promote inflammatory responses. 2. T H 2: Helper T cells -- Stimulate B-cells to produce antibodies. (A third type, T H 3, has recently been shown to promote IgA production.)

T cells develop in the thymus and undergo positive and negative selection Positive selection: T cells which can react to self MHC (major histocompatability complex) carrying peptides are allowed to live. Those that cannot undergo apoptosis (suicide). Negative selection: T cells that react strongly to self-antigens on MHC are eliminated. Only those T cells that can react to MHC, but do not bind strongly to self-antigens emerge as mature T cells from the thymus. Only about 2% of immature T cells make it through positive and negative selection.

T cell development T cell development in the thymus: Cortex Medulla Immature double-negative T cells (CD8 -, CD4 - ) Immature double-positive T cells (CD8 +, CD4 + ) Positive selection/ negative selection CD8 + T cellsCD4 + T cells Mature T cells

The T cell Receptor Similar in structure to Immunoglobulins (similar to a single F ab fragment. Composed of two glycoprotein chains (  /  or  /  ). Most mature T cells have TCRs composed of an  chain and a  chain (they are called  /  T cells). Each chain has a constant region and a variable region, similar to an antibody light chain. A TCR recognizes a small (8-13 aa) peptide epitope displayed on MHC  chain  chain Epitope-binding site Variable region Constant region Transmembrane region

TCR compared to Immunoglobulins Similarities Both have specific Antigen-binding region created by the variable regions of two polypeptide chains. Both display great potential for diversity via genetic recombination at the genome level Differences A TCR is monovalent (has one binding site). An Ig is bivalent (has two binding sites). The TCR has no secreted form. It is always membrane-bound. The TCR does not recognize free antigen. Antigen must be presented to a T cell on an MHC molecule (next week). There is no class switching for the TCR. Once made, the TCR does not change.  chain  chain Epitope-binding site Variable region Constant region Transmembrane region T cell Receptor Immunoglobulin

The TCR only recognizes specific peptide/MHC complexes expressed on the surfaces of cells A TCR complex is composed of one heterodimeric TCR (ususally  /  ), plus a 5-polypeptide CD3 complex which is involved in cell signalling for T cell activation. Each TCR is produced through genetic recombination and recognizes one small peptide epitope (about 8-13 amino acids). One T cell expresses only one specific type of TCR. The T cell Receptor, cont. CD3 is the activation complex for the TCR Binding of antigen/MHC to the TCR stimulates CD3. CD3 then sends an activation signal to the inside of the T cell.       TCR: Antigen recognition CD3 Cell signaling 

Responses to infection -- T cell component Infection Innate immunity (0-4 hours) Early induced response (4-96 hours) Late adaptive response >96 hours) Protective immunity Immunological memory Recognition by pre-formed, non- specific effectors Recruitment of effector cells Transport of antigen to lymphoid organs Recognition by pre-formed, Ab and T cells Recognition by memory B cells and T cells Removal of infectious agent Removal of infectious agent Removal of infectious agent Removal of infectious agent Removal of infectious agent Recognition and activation of effector cells Recognition by naïve B and T cells Clonal expansion and differentiation to effector cells Rapid expansion and differentiation to effector cells This chart is not intended to be memorized The adaptive immune response involving antigen-specific T cells and B cells is only one part of the immune response and is required to protect against pathogens. A pathogen is by definition an organism that can cause disease. In other words, a pathogen is an organism that can bypass innate immunity and requires an adaptive immune response for clearance.

Generation of an adaptive immune response During an adaptive immune response,T cells which recognize specific antigen(s) are selected for differentiation into armed effector cells which undergo clonal expansion to produce a battery of antigen- specific cells. Clonal expansion refers to the process by which antigen- specific T cells or B cells are stimulated to reproduce clones of themselves to increase the system’s repertoire of antigen-specific effectors.

Generation of an adaptive immune response Activation of antigen-specific T cells (the initiation of the adaptive response) occurs in the secondary lymph tissues (lymph nodes and spleen). This activation depends upon antigen presentation by a professional antigen presenting cell (APC) along with simultaneous co-stimulation. (eg., B7 on the APC, CD28 on the T cell).

Initiation of the adaptive immune response The first step is the draining of antigen into the lymph node(s). In the lymph node(s) (or spleen), antigens are trapped by professional APCs which display them to T cells.

The antigen presenting cells, continued Dendritic Cell MacrophageB cell Note: this B cell is not a plasma cell -- a plasma cell is shown above. Plasma cells do not present antigen. They simply pump out antibody for a few days then die.

T cells continuously circulate via the blood and lymph through different lymph nodes until they either find presented antigen or eventually die When a T cell encounters an APC displaying antigen to which it can bind, it stops migrating and binds strongly to the APC. Within about 2 days (48 hours), most antigen-specific T cells have been trapped by antigen and within about 4 to5 days armed effector T cells are migrating out of the lymph node.

Review -- Cytokines produced early in response to infection influence the future functions of activated CD4 + cells TH0TH0 TH2TH2TH1TH1 IL-2 IL-4 IFN-  IL-2 IFN-   F  IL-4 IL-6 IL-10 Cytokines produced by T H 1 cells inhibit T H 2 cells Cytokines produced by T H 2 cells inhibit T H 1 cells An immune response is often dominated by a cell- mediated response or an antibody response. Some pathogens have evolved strategies to shift the immune response toward the less effective type for that pathogen.

Functions of the different T cell types CD8 + cells: Kill virally infected cells CD4 + cells: –T H 1: Activate macrophages to aggressively ingest antigen and to kill ingested microbes. –T H 2: Stimulate B cells to differentiate into antibody- producing plasma cells. B cells will only undergo isotype switching after receiving T cell help. The Ig class that a B cell switches to is specified by the types and balance of cytokines secreted by the helper T cell.

Immunological memory When B cells are activated to reproduce, some differentiate into plasma cells and some become long-term memory cells. An adaptive immune response also produces T cell memory, but the nature of memory T cells is unknown. Two possibilities exist. Memory T cells probably originate from either: –1. A long-lived subset of effector T cells that differentiates into memory T cells -- like memory B cells. –2. The continuous low-level activation of naïve T cells by specific antigen that is retained in the lymph nodes after an infection. This mechanism would suggest that APCs in the lymph node hold on to antigen on a long-term basis after an infection and continuously stimulate T cells at a low level so there is always a small effector population ready to go.

MHC classes I and II Functions: class I MHC: –Displays peptides derived from antigen originating inside the cell (endogenous antigen). –Important in cytotoxic responses (eg, CD8 + -killing of virus-infected cells). Class II MHC: –Displays antigen derived from ingested antigens (exogenous antigen). –Important in humoral (antibody) responses as well in fighting as some intracellular parasites (eg. Mycobacterium tuberculosis and M. leprae) Locations: –Class I MHC found on all nucleated cells (all cells need to be prepared to be killed in case of a viral take-over or tumorigenic transformation). –Class II MHC found only on antigen presenting cells (cells that present antigen to CD4+ T cells --> Macrophages, activated B-cells, dendritic cells.

Antigen Presentation to T cells: MHC Antigens are presented to T cells as short peptide fragments bound to Major Histocompatibility (MHC) molecules. Two types of MHC in humans and mice: –MHC I: presents an 8-10 amino acid peptide to CD8 + T cells. –MHC II: presents a longer peptide (13 aa or more) to CD4 + T cells.

MHC structure Peptide binding cleft 11 22 22 11 Class II MHC Peptide binding cleft 11 22 33  2 -microglobin Class I MHC MHC classes I and II have an almost identical 3-D structure. Both classes of MHC are polygenic (each cell has many MHC genes) and polymorphic (there are many alleles for each locus), but the MHC genes do not undergo recombination. Note: Human MHC are called HLA (human leukocyte antigen).

MHC / T cell interactions The MCH/peptide-TCR interaction is facilitated by the CD4 or CD8 co-receptor. Class II MHC Class I MHC TCR complex CD8 CD8 + T cell target cell CD4 Antigen presenting cell TCR complex CD4 + T cell

Antigen processing: Endogenous pathway All nucleated cells can process endogenous proteins and present fragments on their class I MHC. Endoplasmic reticulum Nucleus Cytoplasmic proteins degradation Vesicle carrying MHC I-peptide Processing in E.R. and complexing with MHC I Display of MHC I + peptide on cell surface

Antigen processing: Exogenous pathway Professional antigen presenting cells ingest microbes and free particles, degrade them in lysozomes, and present fragments to CD4 + T cells on MHC II. Endoplasmic reticulum Nucleus Vesicle carrying MHC II MHC II is assembled in ER Display of MHC II + peptide on cell surface Ingestion of microbe Degradtion in lysozome Vesicle fusion, assembly of peptide/MHC II

CD4 + T cell activation T cells require co-stimulation for activation -- binding of the TCR to MHC/peptide is not enough to activate a T cell by itself. B7 on an APC binds to CD28 on the T cell to deliver a co- stimulatory signal. (see figure 13-8). Activation by peptide/MHC-TCR binding plus a co- stimulatory signal leads to Interleukin-2 (IL-2) release and up-regulation of the IL-2 receptor on the T cell. IL-2 stimulates growth and proliferation of T cells.

CD8+ T cell activation A naïve circulating CD8 + T cell also requires co- stimulation to become an “armed” effector cell. A CD8 + T cell can be activated by an APC displaying MHC I/peptide along with B7 (CD8 + cells also have CD28). Activation of the CD8 + cell causes upregulation of the IL-2 receptor and production of IL-2, leading to growth and proliferation. An activated CD8 + T cell can sustain itself on its own IL-2 production, once activated.