Date Published: March 13, 2014
Publisher: Public Library of Science
Author(s): Angela Zarama, Natàlia Pérez-Carmona, Domènec Farré, Adriana Tomic, Eva Maria Borst, Martin Messerle, Stipan Jonjic, Pablo Engel, Ana Angulo, Chris A. Benedict.
Receptors of the signalling lymphocyte-activation molecules (SLAM) family are involved in the functional regulation of a variety of immune cells upon engagement through homotypic or heterotypic interactions amongst them. Here we show that murine cytomegalovirus (MCMV) dampens the surface expression of several SLAM receptors during the course of the infection of macrophages. By screening a panel of MCMV deletion mutants, we identified m154 as an immunoevasin that effectively reduces the cell-surface expression of the SLAM family member CD48, a high-affinity ligand for natural killer (NK) and cytotoxic T cell receptor CD244. m154 is a mucin-like protein, expressed with early kinetics, which can be found at the cell surface of the infected cell. During infection, m154 leads to proteolytic degradation of CD48. This viral protein interferes with the NK cell cytotoxicity triggered by MCMV-infected macrophages. In addition, we demonstrate that an MCMV mutant virus lacking m154 expression results in an attenuated phenotype in vivo, which can be substantially restored after NK cell depletion in mice. This is the first description of a viral gene capable of downregulating CD48. Our novel findings define m154 as an important player in MCMV innate immune regulation.
Pathogens have recourse to innumerable tactics for evading host immune surveillance. Viruses, and in particular large DNA viruses such as herpesviruses, are endowed with the capacity to encode multiple products committed to altering, during all stages of their life cycle, several functions of the innate and adaptive immune system. The homeostatic equilibrium achieved between host immune responses and viral immune escape mechanisms empowers these viruses to successfully establish their characteristic lifelong infections. Human cytomegalovirus (CMV), the prototype β-herpesvirus, usually leads to asymptomatic infections in healthy individuals where it remains in a latent state for life, going through sporadic reactivation and leading to severe diseases in immunocompromised patients , . The generation of an efficient host-elicited immune response against CMV includes the induction of natural killer (NK) cells, antibody and T-cell mediated responses . As a consequence, CMV has evolved diverse countermeasures to avoid recognition by T cells, allowing it to interfere with the surface expression of major histocompatibility complex class I (MHC class I) and class II and costimulatory molecules, compromising antigen presentation –. Likewise, the virus counteracts NK cell triggering, primarily by suppressing the expression of ligands for activating receptors while preserving engaged inhibitory receptors –. In addition, CMV alters the function of cytokines and their receptors, and interacts with complement factors. While great strides have been made in recent years in identifying CMV inhibitors of immune response mechanisms, current consensus is that among the vast amount of genetic CMV material still requiring a functional assignment, the virus harbours as yet uncovered immunoevasins directed against already known or new immunological targets. Due to the species-specific nature of human CMV (HCMV) replication, infection of mice with murine CMV (MCMV) has proven to be an invaluable model for studying aspects of the biology underlying CMV infection. In this regard, the MCMV system has been widely used to unveil new immunomodulatory molecules and to explore their roles in infection and viral pathogenesis .
For an effective immune response against many viral infections, antigen-presenting cells such as dendritic cells and macrophages must expose a concerted repertoire of receptors that alert T and NK cells for their efficient activation. In this context, distortion of the surface receptor content is a maneuver widely adopted by numerous viruses to elude the immune system and secure an optimal milieu for their replication and dissemination. In this study we show that several cell-surface molecules of the SLAM family, which operate as co-signalling molecules triggering distinct signal-transduction networks in T, NK and antigen-presenting cells, are targeted by MCMV. Notably, CD48, CD84, CD229 and Ly108 get differentially restricted from the cell surface within the window of time it takes for the virus to complete its life cycle and produce productive progeny. Hence, the fact that CMV might have an active interest in interrupting SLAM interactions through the downregulation of the specific receptors/ligands in the infected cell indicates that, at least for the four SLAM members analyzed in our study, engagement of the corresponding receptors/counter receptors should exert prevailing activating signals in key immune cells during infection.