HUMAN NATURAL-KILLER-CELL RECEPTORS FOR HLA-CLASS-I MOLECULES - EVIDENCE THAT THE KP43 (CD94) MOLECULE FUNCTIONS AS RECEPTOR FOR HLA-B ALLELES

GL183 or EB6 (p58) molecules have been shown to function as receptors for different HLA-C alleles and to deliver an inhibitory signal to nat ural killer (NK) cells, thus preventing lysis of target cells. In this study, we analyzed a subset of NK cells characterized by a p58-negati ve surface phenotype. We show that p58-negative clones, although speci fic for class I molecules do not recognize HLA-C alleles. In addition, by the use of appropriate target cells transfected with different HLA -class I alleles we identified HLA-B7 as the protective element recogn ized by a fraction of p58-negative clones. In an attempt to identify t he receptor molecules expressed by HLA-B7-specific clones, monoclonal antibodies (mAbs) were selected after mice immunization with such clon es. Two of these mAbs, termed XA-88 and XA-185, and their F(ab')(2) fr agments, were found to reconstitute lysis of B7(+) target cells by B7- specific NK clones. Both mAbs were shown to be directed against the re cently clustered Kp43 molecule (CD94). Thus, mAb-mediated masking of K p43 molecules interferes with recognition of HLA-B7 and results in tar get cell lysis. Moreover, in a redirected killing assay, the cross-lin king of Kp43 molecules mediated by the XA185 mAb strongly inhibited th e cytolytic activity of HLA-B7-specific NK clones, thus mimicking the functional effect of B7 molecules. Taken together, these data strongly suggest that Kp43 molecules function as receptors for HLA-B7 and that this receptor/ligand interaction results in inhibition of the NK-medi ated cytolytic activity. Indirect immunofluorescence and FACS(R) analy sis of a large number of random NK clones showed that Kp43 molecules ( a) were brightly expressed on a subset of p58-negative clones, corresp onding to those specific for HLA-B7; (b) displayed a medium/low fluore scence in the p58-negative clones that are not B7-specific as well as in most p58(+) NK clones; and (c) were brightly expressed as in the p5 8(+) clone ET34 (GL183(-)/EB6(+), Cw4-specific). Functional analysis r evealed that Kp43 functioned as an inhibitory receptor only in NK clon es displaying bright fluorescence. These studies also indicate that so me NK clones (e.g., the ET34) can coexpress two distinct receptors (p5 8 and Kp43) for different class I alleles (Cw4 and B7). Finally, we sh ow that Kp43 molecules function as receptors only for some HLA-B allel es and that still undefined receptor(s) must exist for other HLA-B all eles including B27.