Page 574 - Veterinary Immunology, 10th Edition
P. 574

VetBooks.ir  Effector T Cell Memory





               In contrast to the prolonged antibody response, the effector phase
               of T cell responses is relatively brief. Indeed, cytotoxicity is seen

               only in the presence of antigen. This is logical. Sustained cytotoxic
               activities or overproduction of cytokines could cause severe tissue
               damage.
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                  Naïve CD8 T cells are long-lived resting cells that continuously
               recirculate between the bloodstream and lymphoid organs. Once

               they encounter antigen, they multiply rapidly in an effort to keep
               pace with the growth of invading pathogens. They reach a peak 5 to
               7 days after infection when pathogen-specific, cytotoxic T cells can
                                                                 +
               make up 50% to 70% of the total CD8  T cells. As with other
               lymphocytes, asymmetric division results in two daughter cells
               with different fates. In the case of CD8+ cells it results in the
               generation of effector and memory effector T cells. The effector cells
               are derived from the daughter cell closest to the antigen-presenting

               cell. The memory T cells are derived from the daughter distal to the
               APC. The proximal cell has increased glycolytic activity and
               increased expression of effector molecules. The distal cell has

               increased lipid metabolism, increased expression of anti-apoptotic
               molecules, and lives very much longer.
                  Once the infection has cleared, most of the effector cells are
               superfluous. Therefore up to 95% of them undergo apoptosis 1 to 2
               weeks after infection. Elimination of these excess T cells is a tightly

               controlled process involving the CD95 pathway.
                  The number of surviving memory cells is directly related to the
               intensity of the primary response. In general only 5% to 10% of the

               peak number of cytotoxic T cells survive as memory T cells.
               Survival may be a function of duration of exposure to antigen. Cells
               exposed to antigen for prolonged periods may die, whereas cells
               exposed only briefly may live. The observation that chronic viral
               infections can exhaust T cells and impair both cytotoxicity and

               memory is consistent with this idea.
                  Memory T cells can be distinguished from naïve T cells by their
               phenotype, by secreting a different mixture of cytokines, and by
                                                                                       +
               their behavior. For example, memory T cells are CD44  and express




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