Influence of Xenogeneic 2-Microglobulin on Functional Recognition of H-2Kb by the NK Cell Inhibitory Receptor

PR-MHC-I molecules are short lived and, for H-2Kb, comprise 10% of total MHC-I. In the present study, we confirm that signaling through the mouse NK inhibitory receptor Ly49C requires the presence of PR-Kb and that this signaling is prevented when PR-Kb is ablated by pulsing with a peptide that can bind to it with high affinity. Although crystallographic data indicate that Ly49C can engage H-2Kb loaded with high-affinity peptide, our data suggest that this interaction does not generate an inhibitory signal. We also show that no signaling occurs when the PR-Kb complex has mouse 2-microglobulin (2m) replaced with human 2m, although replacement with bovine 2m has no effect. Furthermore, we show that 2m exchange occurs preferentially in the PR-Kb component of total H-2Kb. These conclusions were reached in studies modulating the sensitivity to lysis of both NK- resistant syngeneic lymphoblasts and NK-sensitive RMA-S tumor cells. We also show, using an in vivo model of lymphocyte recirculation, that engrafted lymphocytes are unable to survive NK attack when otherwise syngeneic lymphocytes express human 2m. These findings suggest a qualitative extension of the "missing self" hypothesis to include NK inhibitory receptors that are restricted to the recognition of unstable forms of MHC-I, thus enabling NK cells to respond more quickly to events that decrease MHC-I synthesis. The Journal of Immunology, 2005, 175: 3542-3553.