1718 Recognition of H - 2 Domains by Cytotoxic T Lymphocytes

H-2 antigens encoded by the K and the D end of the major histocompatibility complex (MHC) t are the primary target structures for alloreactive cytolytic T lymphocytes (CTL) (1-3). However, little is known about the repertoire of H-2specific CTL and about which parts and epitopes of the H-2 antigens are recognized by CTL. Recently (4) we were able to identify with a panel of monoclonal antibodies (mAb) two spatially separated domains on the H-2K k molecule that harbor the alloantigenic sites defined by mAb (see Fig. 1). Using target inhibition of H-2Kk-specific CTL generated in a bulk culture system, we found that antibodies to one domain (cluster B) exerted a much stronger inhibitory effect than antibodies to antigenic determinants in cluster B, indicating a preponderance of CTL with specificity for cluster B (5). The presence of each individual mAb during the effector phase resulted in only partial blockade of the heterogeneous CTL population, whereas a mixture of all mAb could almost completely inhibit target cell lysis. These results suggest that the CTL are multiclonal and heterogeneous and that different epitopes on the H-2K k antigen are recognized by different CTL clonotypes. To verify this conclusion in the present report the fine specificity and repertoire of CTL clones were investigated. CTL clones were generated in a limiting dilution system that allows rapid analysis of hundreds of clones. Our results demonstrate the existence of at least three different CTL subpopulations against the H-2K k molecule differing not only in their precursor frequencies but also in their repertoire. Thus the majority of clones in a high frequent subpopulation recognize almost exclusively determinants of cluster A whereas the less frequent population is predominantly specific for cluster B. The generation and expression of these CTL subpopulations appears to be controlled by as yet unknown regulatory mechanisms.