Paired altered-self MHC I recognition receptors enhance NK cell immunity to acute viral infection

Natural killer (NK) cells are essential mediators of host defense against murine cytomegalovirus (MCMV) infection. We have shown that licensed Ly49R+ Ly49G2+ NK cells mediate MHC I Dk-dependent control of MCMV in C57BL/6 (B6) mice bearing a C57L-derived NK gene complex (NKC). However, precise roles for the Ly49R activation receptor and the Ly49G2 inhibitory receptor in Dk-dependent MCMV control are not defined. Furthermore, a basis for Dk-dependent NK cell sensing of MCMV-infected targets and control of MCMV infection remains unclear.

To delineate a role for Ly49G2 in MHC I Dk resistance, we generated Ly49g2C57L mutant mice via CRISPR/Cas9 gene-editing. A single cytosine insertion resulted in Ly49G2 truncation and undetectable NK cell surface expression. Ly49G2-deficient mice exhibited increased mortality due to abrogated MCMV control by NK cells, despite an otherwise unmodified Ly49 repertoire and normal NK cell development. Our findings thus demonstrate for the first time a vital role for an inhibitory receptor in NK cell-mediated antiviral immunity.

The MCMV immunoevasin m04/gp34 escorts MHC class I (MHC I) molecules to the surface of infected cells where these complexes bind Ly49 inhibitory receptors and prevent NK cell attack. Nonetheless, we have found that Ly49G2 promotes robust Ly49R+ NK cell expansion and antiviral immunity during MCMV infection. We thus examined a role for gp34 in Ly4R+ Ly49G2+ NK cell sensing of MCMV infection. We discovered that Ly49R was selectively triggered during MCMV infection on antiviral NK cells licensed by the Ly49G2 IR. We further found that Ly49R recognition of MCMV-infected targets was dependent on MHC I Dk and MCMV gp34 expression. Remarkably, although Ly49R was critical for Ly49G2-dependent antiviral immunity, blockade of the activation receptor in Ly49G2-deficient mice had no impact on virus control, suggesting that paired Ly49G2 MCMV-sensing might enable Ly49R+ NK cells to better engage viral targets. Indeed, MCMV gp34 facilitated Ly49G2 binding to infected cells, and the IR was required to counter gp34-mediated immune evasion. A specific requirement for Ly49G2 in antiviral immunity was further explained by its capacity to license cytokine receptor signaling pathways and enhance Ly49R+ NK cell proliferation during infection. These findings advance our understanding of the molecular basis for functionally disparate self-receptor enhancement of antiviral NK cell immunity.