Activation and Function Of T and B cells

Activation and Function Of T and B cells • Activation of T and B cells expressing Agspecific receptors results in: – proliferation. – Differentiation into effector cells and memory cells. • The activation of T cells leads to the synthesis and secretion of cytokines that affect many different cell types. • The activation of B cells results in Ab production.

Activation and Function of T and B cells

APCs - dendritic cells - Macrophages - B cells APCs take up Ag, process and present it to T cells and provide costimulator signals that activate naive T cells. Dendritic cells express high levels of MHC class II and class I. They are motile moving from sites where they are exposed to Ag to lymph nodes, where they can interact with T cells.

Fig. shows that the interaction of Ag with dendritic cells in tissues results in the maturation of the dendritic cell, ultimately leading to the Ag-bearing cell moving out of the tissue and to the lymph node draining this tissue site. In the fig. the Ag "G-bacteria" is taken up by immature dendritric cell, Ag interact with toll-like receptors "TLRs" expressed on dendritric cells.

Activation of TLRs by microbial components leads to activation of both branches of immunity "innate and adaptive" through the production of cytokines *Toll-like receptors (TLR) are considered as part of “pattern recognition molecules”. They fit with a distinct pattern of corresponding piece of a pathogen. They serve as flags alerting responding cells to the presence of invading microorganism.

They facilitate phagocytosis. TLR activate phagocytes and tissue dendritic cells in response to pathogens. They are large family of receptors each of which recognizes specific microbial molecular patterns. Activation of these cells facilitates initiation of acquired immune response due to the production of proinflammatory cytokines “ this illustrates the important relationship between the innate and acquired immune system”

Coating microbes with complement makes phagocytosis easy”opsonization” Foreign Ags induce high levels of costimulatory molecules while self Ags do not induce high levels of costimulatory molecules, thus naive T cells are not activated.

Interactions at the surface of APC and CD 4+ T cells

1) First signal for T cell activation § Interaction between MHC class II molecules on APC + peptide and "V + V " of TCR on T cell. § This signal is necessary but is not enough for T cell activation. 2) – Interaction of MHC class II with the coreceptor CD 4 which makes T cell 100 fold more responsive to Ag. § CD 4 is moved closer to TCR after binding of peptide. MHC to TCR, a process enhancing signal transduction.

3. Costimulator pairs "B 7 -CD 28, CD 152/CD 40, CD 154". Costimulator molecules are required for activation of naive T cells. CD 40 -CD 154 interaction increases the expression of B 7 on APC and enhances B 7 -CD 28 interaction between the APC and the T cell. 4. Adhesion molecules. CD 54 + ICAM-1 "intercellular adhesion molecule "on APC and CD 11 a/CD 18 = LFA-1 "leukocyte function-associated antigen" on T cell. Adhesive interactions slow down the movement apart of the APC and T cell.

5. The Immunological Synapse. § The area of contact between APC and T cell. § The synapse is required for sustained intracellular signaling, lasting until the two cells split apart after 8 hours of contact. 6. T cells membrane lipids "cholesterol, glycosphingolipids" are mobilized to the synapse and draw with them intracellular signaling.

Intracellular Events in CD 4+ T cell Activation

1. Binding of MHC II peptide to TCR transmits a signal via the tightly associated CD 3 and Zeta molecule into the interior of the T cell. 2. The nature of the signal across the membrane may involve aggregation of multiple TCR into the cell membrane or a conformational change in the transmembrane region of the TCR chain. 3. Activation of TKs which then activate proteins by adding P group to tyrosine residues.

3. TK associated with CD 3 -zeta = Fyn - Src family TK associated with CD 4 -zeta = Lck - Sark family 4. Fyn, Lck activated they cluster with region of CD 3 and zeta chains that contain *ITAMS and phosphorylate them. They act as docking sites for another TK "ZAP 70". 5. Lck activated ZAP 70 which phosphorylates adaptor molecules proteins "LAT, SLP-76" on the cell membrane forming larger complex of signal transduction molecules at the immuno-logical synapse.

Activation of Intracellular Signaling • Activated adaptor molecules activate intracellular signaling pathways. • They bind PLC- "phosphorylated by ZAP-70" PIP 2 "phosphitidyl inosital biphosphate" DAG "Diacyl glyceral" which activates transcription factor NF -KB.

* immunoreceptor tyrosine-based activation motif. • Activates membrane-associated protein kinase, activate transcription factor NF-KB and IP 3 "inositol trip" which increases intracellular Ca++ which activates Calcineurin which activates transcription factor NF-AT. This pathway is clinically significant because cyclosporine-A "a drug preventing graft rejection" binds to calcineurin and inhibits T cell activation.

• Ras & Rac "guanosine-nucleotide binding proteins" activate cytoplasmic cascade of mitogen. • Activated protein "MAP" kinase leading to activation of Transcription factor Ap-1. • Activation of genes enlargement of T cells IL-2 which is growth factor for T cells & binds to high affinity IL-2 receptor on the same or on different T cell

Other ways of T cell Activation 1. Superantigens. 2. Mitogens § Concanavalin A "Con A" ……… lectin § Phytohemagglutinin "PHA" ……… lectin § Pokeweed mitogen ……… lectin 3. Anti CD 3 Abs. CD 4+ T cell subsets

TH 1 and TH 2 are generated from the Ag driven differentiation of THO cells. TH 1 cells synthesize IL-2, IFN and TNFβ(lymphotoxin). • Cytokines synthesized by TH 1 cells activate cells involved in CMI: CD 8+ T cells, NK cells and M. • TH 2 cells synthesize IL-4, IL-5, and IL-13. • IL-4 and IL-13 influence b-cell class switch to Ig. E Ig. G 4 in humans and IL-5 activates eosinophils.

• Viruses and bacteria favor the production of TH 1 cells, whereas allergens and parasites favor TH 2 cell induction. • Cytokines produced by TH 1 can inhibit the function of TH 2 and vice versa. • The recently characterized TH 17 subset synthesizes IL-17 family cytokines, which induce pro-inflammatory responses in many different cell types. TH 17 cells also respond to fungi and some extracellular bacteria.

• Treg cells inhibit the function of the other subsets of CD 4+ T cells by cell contact and by the synthesis of inhibitory cytokines TGF- β and IL-10. • Cytokines Influence Differentiation into a particular Subset of CD 4+ T Cells(Fig. below).

Cytokines synthesized by cells of the innate immune system influence the differentiation into specific subsets of CD 4+ T cells, particularly from dendritic cells. Thus, cells of the innate immune system have a critical role in shaping the pattern of the adaptive immune response. The critical role played by these cytokines is often referred to as a (third signal) that works in conjunction of the first (peptide-MHC-TCR) and second(costimulatory) signals in driving the differentiation of activated CD 4+ T cells.

T – B Cooperation

• All proteins are thymus dependent "TD" Ags, because they require CD 4+ T cells to "help" or cooperate with B cells to synthesize Abs. • The set of CD 4+ cells that participate in Ab responses to TD Ags is referred to as helper T cells (TH). • T & B cells must be specific for a particular Ag. • Epitopes must be part of the same protein sequence.

• TH and B cells may respond to different epitopes in the antigen; the TH to an internal epitope generated during the processing of the antigen , the B cell to an external epitope. T – B cooperation in the response to a TD Ag is also known as linked recognition • B cells can act as APC for the CD 4+ T cell "Fig. 10. 6".

• Fig 10. 7: – Interactions between B and T cells are critical for the mutual activation of T and B cells, as a result T cells synthesize cytokines and the B cells synthesize Abs. – CD 40 -CD 40 L interactions are required for T and B cell activation and for the B cell to switch from Ig. M synthesis to other isotypes.

Activation of CD 8+ T cells

• CD 8+ T cells can kill cells infected with bacteria or viruses, as well as transplanted foreign cells. • CD 8+ T cells recognize a combination of peptide in association with MHC class I molecules. • CD 8+ T cells also synthesize cytokines "IFN- , TNF - , • IL-4. • Activation of CD 8+ T cells involve first signal "peptide-MHC-TCR", the second or costimulatory signals, in addition of cytokine IL-12"( third signal), this cytokine is synthesized by cells of the innate immune response, it is critical for activation of CD 8.

• The paired adhesion molecules expressed on the Tcell and target cell surface help to maintain contact between the cells for several hours. • Activated CD 8+ T cells contains granules that contain cytotoxic proteins and expresses CD 178 "Fas ligand". • Killing by CD 8+ T cells occurs by two pathways 1. Exocytosis onto target cells "perforin and granzymes". § Perforin form ringlike transmembrane channels or pores in the target cell membrane.

§ Granzyme "serine esterase" passes into the cells and induces apoptosis. § Granulysin is a small protein which can kill intracellular pathogens such as Listeria and Mycobacteria that live inside macrophages and dendritic cells. 2. Target cell killing via the interaction of the molecule CD 178 "Fas ligand" on CD 8+ T cells with CD 95 "Fas", a surface molecule expressed on may cells.

3. T cell can kill itself via Fas. L interactions. 4. Once T CD 8+ cells finishes the killing process, it detaches from the target cell to attack and kill additional Target cells. Harmless or noninfectious Ags "killed virus" do not trigger CD 8+ T cells responses.

Conjugate Vaccine Conjugate vaccines utilize the principle of linked recognition to generate TD response. Fig. 10. 11 shows that a purified polysaccharide from the bacterium is conjugated i. e physically linked to a carrier protein. Tetanus toxoid is used in the H. influenzae b conjugate vaccine and many others. The carrier protein generates T-cell epitopes that activate T helper cells which interact with B cells specific for the polysaccharide. The resulting TD response involves switching to isotypes such as Ig. G and the development of long term memory.

Notes: Two antigen-specific cells , a CD 4+ T helper and a B cell, need to interact to generate Ab in response to TD Ags. The epitopes the T helper and the B cell recognize must be part of the structure of the same Ag(linked recognition). T-B cell cooperation involves interactions between pairs of molecules on the surface of the CD 4+ T cell and the B cell: an Ag specific first signal ( peptide. MHC class II expressed on the B cell with the TCR) and critical second or costimulator signals, which

include CD 40 -CD 40 ligand ICOS-ICOS ligand. The T cell and the B cell are mutually activated, the T cell to sythesize cytokines and the B cell to synthesize Ab. The cytokine synthesized by the T cell determines the isotype of the Ab sythesized by the B cell. Some Ags _such as polysaccharides that have many repeating epitopes_ are thymus- independent-TI- and activate B cell Ab synthesis without significant T help

Responses to TI Ags are predominantly Ig. M and immunological memory does not develop. People lacking a functional thymus or T cells make responses to TI Ags. The intracellular pathways involved in B cell activation are similar to activation pathways of CD 4+ and CD 8+ T cells. The signal through the Ag-specific BCR can be modulated positively (via the B cell coreceptor) or negatively( for example via the low affinity Fc receptor for Ig. G).