T helper cell cytoplasmic granules. Exocytosis in response to activation via the T cell receptor.

A series of class II MHC-restricted keyhole limpet hemocyanin-specific Th cell clones were examined for cytoplasmic granules by histochemical techniques and fractionation of their homogenates. All showed granules containing lysosomal enzymes and high levels of trypsin-like activity revealed by a N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester-esterase assay. Using the latter as a marker for granule contents, granule secretion was observed in response to MHC-restricted, Ag-dependent signals presented in vitro, and correlated well with T cell activation as measured by proliferation. However, human rIL-2, a strong inducer of T cell proliferation, did not stimulate N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester-esterase secretion by itself, nor did it influence the secretion mediated by Ag. Other factors found to induce secretion included immobilized antibodies directed against determinants of the TCR complex, as well as Con A. These results suggest that granule enzyme secretion is another indicator of TCR-mediated activation and provides a possible mechanism for Th cell function via a rapid, local delivery of granule contents to adjacent cells.