Mechanisms of inhibition of Cryptococcus neoformans by human lymphocytes

Recently, our laboratory and others have demonstrated that human peripheral blood T and NK lymphocytes directly inhibit the growth of Cryptococcus neoformans. In this study, we further define the conditions under which lymphocyte-mediated fungistasis against C. neoformans occurs and examine whether mechanisms implicated in lymphocyte-mediated activities against other target cells are also involved in anticryptococcal activity. The addition of whole or broken heat-killed C. neoformans modestly inhibited lymphocyte-mediated fungistasis, whereas other particulates had no effect. The hydroxyl radical scavenger catechin, but not diethyl urea or propyl gallate, profoundly inhibited fungistasis. Salicylic acid inhibited fungistasis in a dose-dependent fashion. However, two other cyclooxygenase inhibitors, piroxicam and indomethacin, had no effect, suggesting that the mechanism of inhibition by salicylic acid was cyclooxygenase independent. Reagent prostaglandin E2, at concentrations shown by others to inhibit NK cell-mediated bactericidal and tumorlytic activities, had no effect on lymphocyte-mediated fungistasis. The addition of selected monoclonal antibodies or ligands reactive with receptors on human lymphocytes had no significant effect on lymphocyte-mediated fungistasis. Acapsular, small-capsuled, and large-capsuled C. neoformans organisms were inhibited by lymphocytes to an approximately equal extent. These data demonstrate that lymphocyte-mediated activity against C. neoformans proceeds regardless of the presence of capsule and by mechanisms at least in part dissimilar from those seen with other target cells.