Determinant Heterogeneity of L-1, L-2 and L-3 Antigen Molecules on Human T Cells as Defined by Monoclonal Antibodies and Their Roles in T Cell-Mediated Immune Functions

A variety of monoclonal antibodies (MoAbs) were generated, directed to three distinct antigen species, designated L-1, L-2 and L-3, respectively, present on human T cells or on their subsets. Whereas L-1 was expressed on most human thymus and T cells, and was identified to be a human homologue of mouse Lyt-1 antigen with a molecular weight (MW) of 70 killodaltons (KD), L-2 was expressed on a T cell subset which contained 30-40% of peripheral T cells, and was thought to correspond to a human homologue of mouse Lyt-2, 3 antigens consisting of 32 KD MW components. L-3 was expressed on a T cell subset negative for L-2, and was regarded as a helper/inducer T cell antigen, previously reported for Leu-3 or OKT4. Precise serological analysis using binding-inhibition assays with fluorescein- or radio-labeled MoAbs demonstrated that these MoAbs developed in this study defined a number of different antigenic determinants present on L-1, L-2 and L-3 antigen molecules, respectively. Thus, two MoAbs (5B3 and 7C6) detected two distinct determinants existing on the same L-1 molecule, one of which seemed to be identical to that defined by Leu-1 MoAb. At least four distinct determinants were found on L-2 antigen molecules with 7 MoAbs directed to this molecular species, and one out of 4 determinants appeared identical to that detected by OKT8 MoAb. Among these 7 MoAbs, two (3D6 and 4A2) failed to react with L-2 antigen present on a number of cultured leukemic T cell lines, though these two MoAb clearly reacted with the L-2 positive thymus and T cells in a lesser number than that recognized by the other 5 L-2 MoAbs. We also produced 3 MoAbs against the L-3 antigen molecule, all of which recognized a single determinant identical or closely linked to that determined by OKT4A MoAb. The possible functional role of T cell antigens or their antigen determinants defined by MoAbs as described above was then studied by measuring the effects of these MoAbs on a variety of human T cell responses including proliferative reaction and cytotoxic T cell (CTL) induction in autologous mixed lymphocyte cultures (AMLC) stimulated with autologous Epstein-Barr virus-transformed B (EBV-B) cells as well as on T cell-mediated cytotoxicity. Whereas the L-1 MoAb tested had little effect on human T cell functions as described above, L-2 and L-3 MoAbs exerted their inhibitory effects on T cells either proliferating in AMLC or cytotoxic to autologous EBV-B cells. Thus, L-2 MoAb, despite their different reactivities with antigen determinants on the L-2 molecule, L-2 MoAb had significant inhibitory effects on cytotoxic reactions mediated by L-2 positive CTL and probably on CTL induction. L-3 MoAb, on the other hand, significantly suppressed the proliferation of T cells ?in AMLC as well as cytotoxicity mediated by L-3 positive CTL. These data suggest that L-2 and L-3 antigen molecules have important roles in antigen recognition and manifestation of the effector function mediated by human T cells