Date Published: May 29, 2014
Publisher: Public Library of Science
Author(s): Catherine A. Forbes, Anthony A. Scalzo, Mariapia A. Degli-Esposti, Jerome D. Coudert, Lewis L. Lanier.
Natural Killer (NK) cells are crucial in early resistance to murine cytomegalovirus (MCMV) infection. In B6 mice, the activating Ly49H receptor recognizes the viral m157 glycoprotein on infected cells. We previously identified a mutant strain (MCMVG1F) whose variant m157 also binds the inhibitory Ly49C receptor. Here we show that simultaneous binding of m157 to the two receptors hampers Ly49H-dependent NK cell activation as Ly49C-mediated inhibition destabilizes NK cell conjugation with their targets and prevents the cytoskeleton reorganization that precedes killing. In B6 mice, as most Ly49H+ NK cells do not co-express Ly49C, the overall NK cell response remains able to control MCMVm157G1F infection. However, in B6 Ly49C transgenic mice where all NK cells express the inhibitory receptor, MCMV infection results in altered NK cell activation associated with increased viral replication. Ly49C-mediated inhibition also regulates Ly49H-independent NK cell activation. Most interestingly, MHC class I regulates Ly49C function through cis-interactions that mask the receptor and restricts m157 binding. B6 Ly49C Tg, β2m ko mice, whose Ly49C receptors are unmasked due to MHC class I deficient expression, are highly susceptible to MCMVm157G1F and are unable to control a low-dose infection. Our study provides novel insights into the mechanisms that regulate NK cell activation during viral infection.
In humans, cytomegalovirus (CMV) is a pathogen responsible for causing significant mortality in immunocompromised patients  and in individuals lacking Natural Killer (NK) cells . Mouse cytomegalovirus (MCMV) is a natural pathogen of mice. The similarities in structure and biology between human and mouse CMV make the latter a widely utilized model for human infection . The study of MCMV has provided valuable insights into how the immune system responds to infection, and has helped to define the immune evasion mechanisms used by CMV to ensure that viral replication proceeds. NK cells play a crucial role in the early control of MCMV infection in resistant mouse strains; they limit viral replication and mortality during acute infection. The ability of NK cells to control viral infection is tightly regulated by their activating and inhibitory receptors . Activating NK cell receptors include activating forms of killer cell immunoglobulin-like receptors (KIRs) in humans, and Ly49 receptors in mice. Both humans and mice express CD94/NKG2C which recognizes MHC class I molecules, and NKG2D which can be triggered by stress-induced ligands. NK cells also possess inhibitory receptors specific for MHC class I that permit discrimination of normal healthy cells from diseased ones, such as virus-infected cells, that display reduced MHC class I expression. These receptors include KIR in humans and members of the Ly49 family in mice, and LIR-1 and CD94/NKG2A in both species (reviewed in ). Inbred strains of mice express distinct NK cell receptor repertoires; NK cell receptors are encoded within a polygenic cluster in which each receptor gene is subject to polymorphism between the mouse strains; this variability results in resistance or susceptibility to specific viral infections.
We previously showed, and confirmed in this study, that m157 from the K181 and the G1F MCMV strains bind Ly49H with similar affinities . Here, we analyzed the kinetics of binding of m157G1F to Ly49H or Ly49C. We found that the kinetics of association of m157G1F to Ly49C was slightly quicker than to Ly49H. In addition, the binding profiles suggested similar affinities for the two receptors, consistent with the results obtained by surface plasmon resonance analysis . Kinetics of dissociation of m157 indicated that interactions with Ly49H were more sustained. This could suggest that after some degree of initial inhibition, Ly49H activation would proceed unchallenged when NK cells coexpress the two receptors. However, since Ly49C is not internalized upon engagement, unlike Ly49H, it is likely that the Ly49C receptors that have dissociated from m157 will establish new interactions and induce inhibitory signals as long as the NK cell-target pairs remain conjugated. Nevertheless, these different dissociation rates could lead to competition in a mixed population where some NK cells express exclusively Ly49H or Ly49C that would ultimately favor Ly49H+ engagement and thus lead to an efficient anti-viral NK cell response. This could explain why infection with either MCMV K181 or MCMV m157G1F resulted in similar viral replication in B6  and in Cmv1r mice as shown here.