SPECIFICITY OF HLA CLASS-I ANTIGEN RECOGNITION BY HUMAN NK CLONES - EVIDENCE FOR CLONAL HETEROGENEITY, PROTECTION BY SELF AND NON-SELF ALLELES, AND INFLUENCE OF THE TARGET-CELL TYPE

Prior studies using polyclonal populations of natural killer (NK) cell s have revealed that expression of certain major histocompatibility co mplex (MHC) class I molecules on the membrane of normal and transforme d hematopoietic target cells can prevent NK cell-mediated cytotoxicity . However, the extent of clonal heterogeneity within the NK cell popul ation and the effect of self versus non-self MHC alleles has not been clearly established. In the present study, we have generated more than 200 independently derived human NK cell clones from four individuals of known human histocompatibility leukocyte antigens (HLA) type. NK cl ones were analyzed for cytolytic activity against MHC class I-deficien t Epstein Barr virus (EBV) transformed B lymphoblastoid cell lines (B- LCL) stably transfected with several HLA-A, -B, or -C genes representi ng either self or non-self alleles. All NK clones killed the prototypi c HLA-negative erythroleukemia K562 and most lysed the MHC class I-def icient C1R and 721.221 B-LCL. Analysis of the panel of HLA-A, -B, and -C transfectants supported the following general conclusions. (a) Wher eas recent studies have suggested that HLA-C antigens may be preferent ially recognized by NK cells, our findings indicate that 70% or more o f all NK clones are able to recognize certain HLA-B alleles and many a lso recognize HLA-A alleles. Moreover, a single NK clone has the poten tial to recognize multiple alleles of HLA-A, HLA-B, and HLA-C antigens . Thus, HLA-C is not unique in conferring protection against NK lysis. (b) No simple patterns of HLA specificity emerged. Examination of a l arge number of NK clones from a single donor revealed overlapping, yet distinct, patterns of reactivity when a sufficiently broad panel of H LA transfectants was examined. (c) Both autologous and allogeneic HLA antigens were recognized by NK clones. There was neither evidence for deletion of NK clones reactive with self alleles nor any indication fo r an increased frequency of NK clones recognizing self alleles. (d) Wi th only a few exceptions, protection conferred by transfection of HLA alleles into B-LCL was usually not absolute. Rather a continuum from e ssentially no protection for certain alleles (HLA-A0201) to very stri king protection for other alleles (HLA-B5801), with a wide range of i ntermediate effects, was observed. (e) Whereas most NK clones retained a relatively stable HLA specificity, some NK clones demonstrated vari able and heterogeneous activity over time. (f) NK cell recognition and specificity cannot be explained entirely by the presence or absence o f HLA class I antigens on the target cell. For example, HLA-B5801, an allele that strongly protected two different B-LCL targets, had no ef fect on NK cell susceptibility when expressed in two NK sensitive tumo r targets, K562 and jurkat. These results suggest the possibility that NK cells may possess cytotoxic mechanisms that are both MHC dependent and MHC independent, depending on the nature of the target cell.