The m153 gene product stabilizes expression of the inhibitory NKR-P1B ligand, Clr-b, during mouse cytomegalovirus infection

Natural killer (NK) cells are a subset of innate lymphoid cells (ILC) capable of recognizing stressed and infected cells through multiple germline-encoded receptor-ligand interactions. Missing-self recognition involves NK cell sensing of the loss of host-encoded inhibitory ligands on target cells, including MHC class I (MHC-I) molecules and MHC-independent ligands. Mouse cytomegalovirus (MCMV) infection has been shown to promote a rapid loss of the inhibitory NKR-P1B ligand, Clr-b, on infected cells. Here, we provide evidence that an MCMV m145 family member, m153, functions to stabilize Clr-b at the cell surface during MCMV infection. Ectopic expression of m153 in fibroblasts significantly augments Clr-b cell surface levels. Moreover, infections using m153-deficient MCMV mutants (Δm144-m158; Δm153) show an accelerated and exacerbated Clr-b downregulation. Importantly, enhanced loss of Clr-b upon infection with MCMV Δm153-mutants can be reverted to wild-type levels by exogenous m153 complementation in fibroblasts. While the effects of m153 on Clr-b levels are independent of Clec2d transcription, imaging experiments reveal that the m153 and Clr-b proteins only minimally co-localize within the same subcellular compartments, and tagged versions of the proteins were refractory to co-immunoprecipitation using gentle detergents. Indeed, a prominent intracellular vesicular localization of m153 suggests that its effects on Clr-b stabilization may be indirect. In vivo, the Δm153-mutant possesses enhanced virulence, independent of Clr-b and NKR-P1B, suggesting that m153 may modulate other Clr or activating NKR:ligand interactions.