The secretion of immunoglobulin by plasma cells has been considered a classical example of the “non-regulated” pathway of protein secretion, in which newly synthesized protein is processed by the Golgi, packaged into small vesicles, and immediately secreted without intracellular storage. In the case of lymphokine secretion by T lymphocytes, it is generally not clear whether this non-regulated pathway is also being used, as opposed to the “regulated” pathway which has been proposed to operate in the cytotoxic lymphocyte mechanism. In this case, as in mast cells and endocrine cells, proteins are synthesized and then stored in cytoplasmic granules. The secretion is triggered (regulated) by a membrane receptor-ligand interaction, which for the cytotoxic lymphocytes is part of the target cell binding process. In cytotoxic T lymphocytes, this secretion process can be measured by following the appearance of a granule serine protease in the medium, and it has been shown to be triggered by target cells or by immobilized antibodies which bind the T celt receptor complex. In addition to cytotoxic lymphocytes, cloned T helper cells contain this serine protease in cytoplasmic granules with a low internal pH. Helper lymphocytes secrete this enzyme in response to (1) soluble antigen which has been processed by cells bearing the appropriate MHC antigens; (2) immobilized antibodies against the T cell receptor complex; (3) a combination of phorbol ester and calcium ionophore. Thus in both helper and cytotoxic lymphocytes, the regulated pathway of protein secretion clearly operates after triggering by the T cell antigen receptor.

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