The NK2.1 receptor is encoded by Ly-49C and its expression is regulated by MHC class I alleles.

A dual receptor system composed of activation and inhibitory receptors apparently controls NK cell-mediated lysis. In the C57BL/6 mouse, the NK1.1 molecule acts as an activation receptor whereas Ly-49A, C and G2 can inhibit NK cell lysis of target cells expressing specific MHC class I molecules. We previously reported that NK2.1 is an activation receptor sharing structural properties with members of the NKR-P1 and Ly-49 receptor families. In this study, we have shown that NK2.1 is encoded by the previously described Ly-49C gene. We also found that the expression level of NK2.1/Ly-49C is modulated by H-2-dependent factors and that this regulation differs from that previously described for Ly-49A. Flow cytometry analyses of NK-enriched spleen cells from MHC congenic strains on C57BL/10 and BALB/c backgrounds indeed revealed that the level of NK2.1/Ly-49C expression, but not the number of positive cells, is low in strains expressing H-2b and H-2k haplotypes as compared to H-2d mice. Similar analyses of splenic NK cells from two series of congenic and congenic recombinant strains on the C57BL/10 background indicate that the main regulatory element(s) are most likely the H-2Kb and H-2Dk alleles. Together with our and others previous observations, these results identify the NK2.1/Ly-49C antigen as a receptor for MHC class I molecules whose expression is regulated by host MHC genes.