engleski

Cytomegalovirus evasion of DNAM-1 dependent immune control by inflammatory monocytes and NK cells

The poliovirus receptor (PVR, CD155) is a highly conserved and ubiquitously expressed glycoprotein involved in cellular adhesion and immune recognition. PVR is constitutively expressed on the majority of somatic cells under physiological conditions and its expression is modulated as a consequence of viral infections and oncogenesis. Interestingly, while tumors frequently exhibit abnormally high PVR cell surface levels, some viruses (for example HCMV or HIV) downregulate PVR surface expression presumably to avoid immune cell recognition. PVR serves as a ligand for three receptors: DNAM-1 (CD226), an activating receptor expressed on the majority of immune cells ; TIGIT, receptor that inhibits NK and T cell cytotoxicity and CD96 (Tactile), receptor with both activating and inhibitory functions on NK cells. Therefore, the precise mechanism that balances activating and inhibitory signals gathered through these receptors, as well as consequences of the PVR modulation in vivo are important unresolved issues of the PVR biology that might explain differences in tumor and viral modulation of PVR. To assess the effect of balancing mechanism mediated by PVR receptors in vivo, we took advantage of murine model of cytomegalovirus infection. Our results showed that similar to human cytomegalovirus (HCMV), mouse cytomegalovirus (MCMV), downregulates the surface PVR. We have also characterized the molecular mechanism of this viral regulation that includes PVR retention in endoplasmic reticulum and proteasomal degradation. In addition, using a panel of MCMV deletion mutants it was possible to attribute this function to a novel MCMV protein, within the predicted m20 gene region that we named m20.1. Viral mutant lacking this regulator was severely attenuated in vivo, and this attenuation was reduced or abolished in DNAM KO mice, indicating the dominance of the activating receptor DNAM-1 in deciding the outcome of the modulation of PVR levels. The early attenuation of mutant viruses lacking the PVR inhibitor was only partially dependent on NK cells, which can be explained by the fact that these cells induce both activating and inhibitory PVR receptors upon infection. However, depletion of mononuclear phagocytes abolished the virus control which correlates with dramatic upregulation of DNAM-1 and the absence of inhibitory PVR receptors on these cells, even upon infection. In particular, we identified CCL2 dependent inflammatory monocytes as the major subpopulation controlling virus lacking PVR inhibitor via induction of iNOS. Overall, our data provide the strongest evidence so far for CMV control by mononuclear phagocytes and NK cells in which DNAM-1–PVR pathway plays an essential role and demonstrates novel mechanism of viral regulation of paired receptors. Hence, these results may be instrumental to identifying novel intervention targets and in designing novel vaccines and vaccine vectors.

MCMV; evasion; immune control; DNAM-1; PVR; NK cell; monocyte

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