Ineffectual immunity in a resurrected mouse model of persistent viremia.

Viruses that establish persistent (i.e., chronic) infections have evolved sophisticated strategies to avoid clearance by the host immune system. This is particularly true for viruses that infect immunocompetent mammals and sustain high infectious burdens in body sites under intense immune surveillance (i.e., the blood, a.k.a., "viremia"). Historically, lymphocytic choriomeningitis virus (LCMV) infection of laboratory mice has served as a powerful model to understand mechanisms of failed immunity, but other viruses may have unique and underappreciated persistence strategies. Here, we resurrect a bygone model of viral persistence-lactate dehydrogenase-elevating virus (LDV)-and use modern transgenic mouse technologies to investigate various aspects of anti-viral immunity. We find that interferons have a modest impact on LDV replication, with interferon-alpha blunting LDV viremia in the acute phase of the infection and interferon-gamma reducing LDV viral loads in the chronic phase of infection, but only when paired with an intact interferon-alpha response. Adaptive immunity, assessed in Rag-knockout mice, had only a modest impact on LDV viremia, and only during the sub-acute phase of infection. Mice lacking the critical immune checkpoint molecule PD-1 showed no signs of disease and supported LDV viral loads at levels equivalent to their wild-type counterparts. Altogether, these results point to a novel and highly effective mechanism of persistence that is minimally impacted by conventional aspects of anti-viral immunity or immune exhaustion-a rarity among persistent viruses. Given the relative paucity of chronic infection models in the laboratory mouse, LDV infection may be useful for exploring unique modes of immune system failure. IMPORTANCE: Viruses that infect a host over long periods of time have evolved unique strategies to evade the host immune system. Of particular interest are viruses that cause persistent infection in the laboratory mouse-the most well-developed tool for studying the mammalian immune system. Here, we resurrected a model of persistent RNA virus infection (lactate dehydrogenase-elevating virus, LDV) and applied modern tools of mouse immunology to further characterize its persistence. We found that host factors that typically have a dramatic effect on viral infections-e.g., the interferon system and lymphocytes-had very little impact on LDV infection. Removing "checks" on immune activation also had little effect on the virus or host health. Altogether, these findings imply that LDV uses a unique and highly effective mechanism to avoid immune clearance. Understanding this mechanism has implications for understanding ways in which the immune system fails.