Inhibition of anti-class I cytotoxicity by anti-class II monoclonal antibodies (MoAb). I. Blocking of bulk non-DR and non-DQ-directed cytotoxic T cells by MoAb against DR, DQ, and DP

We previously demonstrated that although human cytotoxic T lymphocytes (CTL) generated against a lymphoblastoid cell line (LCL) mutant that had lost DR and DQ (formerly known as MB) expression were directed primarily at class I antigens, the lytic activity of such CTL could be inhibited by monoclonal antibodies (MoAb) against monomorphic determinants on DR molecules. Furthermore, the inhibitory effect was found to occur by binding of the MoAb to the target cell, not the effector cell, because lysis of LCL target cells not expressing DR was not inhibited. In this paper we extend this phenomenon to show (i) that MoAbs directed against DQ and DP class II gene products also inhibit the lysis of LCL target cells by CTLs recognizing class I antigens; and (ii) that not all DR-specific MoAbs blocked target cell lysis by non-DR specific CTLs. In addition, when PBLs were used to stimulate the generation of CTLs in mixed leukocyte culture (MLC), inhibition using anti-class II (DR, DQ, and DP) MoAb occurred only when LCLs but not PHA blasts were used as targets. This might be explained by elevated expression of class II molecules on LCLs compared to PHA blasts.