Page 557 - Veterinary Immunology, 10th Edition

production and clonal expansion. Presumably cytotoxic T cells need
VetBooks.ir to be highly sensitive to viral peptides so that they can kill infected

cells as soon as possible. These differences in signal thresholds are
probably due to the structure of the immunological synapses

formed when a cytotoxic T cell encounters a target.
When cytotoxic T cells encounter a target cell, a synapse forms at
the point of contact (Fig. 18.7). This synapse has two “centers.” One
part of the central zone contains clustered TCR-CD8 complexes. The

other attracts secretory lysosomes that release their contents into
the synaptic space and so destroy the target cell. Both are
surrounded by a pSMAC rich in adhesion molecules that forms a
“gasket,” preventing the accidental escape of cytotoxic molecules.

Once the synapse forms, cytotoxic T cell killing is efficient. Within
seconds after contacting a T cell, the organelles and the nucleus of
the target show apoptotic changes, and the target is dead in less
than 10 minutes. Cytotoxic T cells can disengage and move on to

kill other targets within 5 to 6 minutes. In addition, several
cytotoxic cells may join in killing a single target.

FIG. 18.7 Structure of the immunological synapse that forms
between a cytotoxic T cell and its target. The outer ring of adhesive
proteins forms an effective “gasket” that prevents leakage of
cytotoxic molecules into tissue fluid. There are, however, two central
SMACs. One is dedicated to signaling and contains the TCR

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