Block of Death-Receptor Apoptosis Protects Mouse Cytomegalovirus from Macrophages and Is a Determinant of Virulence in Immunodeficient Hosts

The inhibition of death-receptor apoptosis is a conserved viral function. The murine cytomegalovirus (MCMV) gene M36 is a sequence and functional homologue of the human cytomegalovirus gene UL36, and it encodes an inhibitor of apoptosis that binds to caspase-8, blocks downstream signaling and thus contributes to viral fitness in macrophages and in vivo. Here we show a direct link between the inability of mutants lacking the M36 gene (Delta M36) to inhibit apoptosis, poor viral growth in macrophage cell cultures and viral in vivo fitness and virulence. Delta M36 grew poorly in RAG1 knockout mice and in RAG/IL-2-receptor common gamma chain double knockout mice (RAG gamma C-/-), but the depletion of macrophages in either mouse strain rescued the growth of Delta M36 to almost wild-type levels. This was consistent with the observation that activated macrophages were sufficient to impair Delta M36 growth in vitro. Namely, spiking fibroblast cell cultures with activated macrophages had a suppressive effect on Delta M36 growth, which could be reverted by z-VAD-fmk, a chemical apoptosis inhibitor. TNF alpha from activated macrophages synergized with IFN gamma in target cells to inhibit Delta M36 growth. Hence, our data show that poor Delta M36 growth in macrophages does not reflect a defect in tropism, but rather a defect in the suppression of antiviral mediators secreted by macrophages. To the best of our knowledge, this shows for the first time an immune evasion mechanism that protects MCMV selectively from the antiviral activity of macrophages, and thus critically contributes to viral pathogenicity in the immunocompromised host devoid of the adaptive immune system.