Modulation of Cytolytic T Cell Responses by Heparan Sulfate

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Accumulating evidence suggests that the functional properties of alloactivated T cells may depend upon the microenvironment in which the T cells reside. For instance, we showed previously that heparan sulfate, a biologically active polysaccharide present on cell surfaces and extracellular matrices, modulates the proliferative responses of splenocytes through enhancement of cytokine and prostaglandin production by macrophages. Here we report that under conditions of suboptimal stimulation, heparan sulfate causes discrete alterations in the functional responses of murine cytolytic T cells. When present in a 5-day mixed leukocyte culture (MLC), heparan sulfate mediates an increase, from 3- to 10-fold, in T cell-mediated cytotoxicity. This increase is dose dependent and most pronounced when heparan sulfate is present in the highest concentration during the first 24 hr of the culture period. On the other hand, when added during the last 48-72 hr of an MLC, heparan sulfate decreases cytotoxicity by 3- to 30-fold. Neutralizing antibodies against IL-1α, but not antibodies against IL1β, IL-6, or TNFα/β, abrogate the heparan sulfate-mediated increase in cytotoxicity, suggesting that the increase depended in part upon the production of IL-1α. However, studies in which exogenous IL-1 was added to MLC showed that increased cytotoxicity was not due only to increased cytokine production. Augmentation of cytotoxicity was in part independent of T cell help, as depletion of CD4+ cells from the responder population before MLC, or addition of neutralizing anti-murine IL-2 antibodies plus human IL-2 to the MLC, did not abrogate the stimulatory effect of heparan sulfate. Heparan sulfate-treated CD8+ lymphoblasts isolated after 7 days in MLC demonstrated an increased cytotoxicity, elevated intracellular serine esterase, and perforin levels compared with lymphoblasts from control MLC. The decrease in cytotoxicity observed when heparan sulfate was present during the last several days of an MLC was likely mediated by PGE2, as elevated levels of PGE2 were detected in MLC supernatants of heparan sulfate-treated cultures, and because the decrease was not observed in the presence of indomethacin. Our results are consistent with the idea that the metabolism of heparan sulfate, an endogenous component of parenchymal tissues, may regulate the tempo and magnitude of alloreactive cytotoxic T cell responses.

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