Tcell activation

Marrack P et al. Genomic-scale analysis of gene expression in resting and activated Tcells. Curr Opin Immunol 2000 12 206-209. 

An important trigger for apoptosis is known as the Fas system. This is used by cytotoxic Tcells, for example, which eliminate infected cells in this way (top left). Most of the body s cells have Fas receptors (CD 95) on their plasma membrane. If a T cell is activated by contact with an MHC presenting a viral peptide (see p. 296), binding of its Fas ligands occurs on the target cell s Fas receptors. Via the mediator protein FADD ( Fas-associated death domain ), this activates cas-pase-8 inside the cell, setting in motion the apoptotic process. 

The various organs of the immune system such as spleen, lymph nodes, thymus and bone marrow containing the cells involved in the various immune responses offer the possibility to harvest these cells and perform in vitro assays for evaluation of effects on the immune system. When part of an in vivo animal study this may indicate a direct toxic effect of pharmaceuticals, that is, immunosuppression (Table 18.2). So, it is feasible to obtain cell suspensions for further evaluation such as determination of cellular subsets of T and B leukocytes by fluorescent activated cell sorter analysis (FACS analysis), and determination of natural killer (NK) cell activity of the spleen cell population. An advantage of this approach is that it may lead to identification of a biomarker to be used in clinical studies. In addition, in vitro stimulation of spleen cells with mitogens activating specific subsets may indicate potential effects on the functionality of splenic cell populations. Concanavalin A (Con A) and phytohemagglutinin (PHA) activate Tcells, while lipopolysaccharide (LPS) activates primarily B cell populations. Blood is collected for total white blood cell (WBC) determination and blood cell differential count. In addition, serum can be obtained for determination of serum immunoglobulins. 

Protection of rats from devl. of Pseudomonas aeruginosa infections Activation of multiple effector pathwaye (Tcells, Bcells, macrophages) Protection of mice from devl. of C. albicans infection (macrophages) Polysaccharides as antineoplastic immunostimulators (NK cells) Proliferative responses of blood lymphocytes Protection of ginseng saponins of immunosuppression in mice Inhibition on bacterial endotoxin-induced embryolethality in rats Induction of neutrophil accumulation in mice... 

In vitro, MDC is chemotactic for a number of cell types including thymocytes (Cambell et al., 1999 Chantry et al., 1999), dendritic cells, monocytes, IL-2-activated NK cells (Godiska et al., 1997), TH2 T cells (Bonecchi et al., 1998 Sallusto et al., 1998 Imai et al, 1998) and CLA + Tcells (Cambell et al, 1999). MDC has also been reported to inhibit HIV replication in vitro (Pal et al, 1997 Struyf et al, 1999), although the physiological significance of this observation is still unclear. 

Fig. 11.11 Subunit structure of the Tcell receptors. The figure shows the different subunits ofT cell receptors in a highly simplified representation. The stoichiometry of the subunits in the complete receptor is not clear. The aft chains are also known as the Tia/ complex the ye and 3e chains together form the CD3 complex. ARAM antigen recognition activation motif.

Fig. 11.12 Overview of signaling pathways associated with activation of lymphocytes. The triggering signal for activation of T lymphocytes is generally antigen binding to the Tcell receptor. The activated receptor passes the signal on to associated tyrosine kinases like Fyn, Lck and ZAP70. These phospho-rylate the transmembrane protein LAT on cytoplasmic tyrosine residues. The LAT phospho-tyro-sine residues are docking sites for adaptors ( She,...

Further effector functions of Tcells are targeted by l,25(OH)2D3 CD178 (FasL) is downregulated on l,25(OFI)2D3 treated T cells thereby conferring protection from activation-induced cell death [105],... 

In vitro, l,25(OH)2D3 potently inhibits upregulation of activation markers after stimulation with mitogens or proinflammatory stimuli (see above). Most patient-related data were therefore also generated from in vitro stimulated cells of patients that were previously treated with l,25(OH)2D3. However, even under those conditions, results were variable or contradicted evidence from experiments where cells were directly exposed to l,25(OH)2D3 in vitro. Thus, the proliferative capacity of Tcells from l,25(OH)2D3-treated hemodialysis patients to mitogens or conventional antigens was found unchanged [150,162]. IL-1 and IL-6 were transiently increased in one study after stimulation with PMA or LPS [163], but decreased according to others [154, 164]. 

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