FD-891, a structural analogue of concanamycin A that does not affect vacuolar acidification or perforin activity, yet potently prevents cytotoxic T lymphocyte-mediated cytotoxicity through the blockage of conjugate formation

FD-891 belongs to a group of 18-membered macrolides, and is a structural an alogue of a specific inhibitor of vacuolar type H+-ATPase, concanamycin A ( CMA). In our previous work, we have shown that CMA specifically inhibits pe rforin-dependent cytotoxic T lymphocyte (CTL)-mediated cytotoxicity through the degradation and inactivation of perforin, although CMA does not affect Fas ligand (FasL)-dependent cytotoxicity. Here, we show that FD-891 potent ly prevents not only perforin-dependent but also FasL-dependent CTL-mediate d killing pathways by blocking CTL-target conjugate formation. In contrast to CMA, FD-891 was unable to inhibit vacuolar acidification and only slight ly decreased the perforin activity in lytic granules. FD-891 blocked granul e exocytosis in response to anti-CD3, mainly owing to the lack of CTL bindi ng to immobilized anti-CD3. The conjugate formation was markedly inhibited only when effector cells were pretreated with FD-891. Consistent with these observations, fluorescence-activated cell sorter (FACS) analysis for cell surface receptors revealed that FD-891 significantly reduced the expression of the T-cell receptor (TCR)/CD3 complex. These data suggest that the bloc kage of conjugate formation and subsequent target cell killing might be at least partly owing to FD-891-induced down-regulation of the TCR/CD3 complex .