The amplitude of gammaherpesvirus lytic replication dictates adaptive immune activation: Potential implications for KSHV LANA in immune evasion

Adaptive immune responses to primary Kaposi sarcoma-associated herpesvirus (KSHV) infection are poorly defined. To develop better small-animal models for understanding KSHV pathogenesis and immunity, we previously generated a chimeric virus in which the KSHV latency-associated nuclear antigen (kLANA), a conserved multifunctional protein critical for viral latency, was exchanged for the LANA homolog in murine gammaherpesvirus 68 (MHV68). Despite comparable levels of latent infection between WT and KLKI MHV68, kLANA directly repressed MHV68 lytic replication and reactivation. We therefore hypothesized that suppression of lytic replication by kLANA dampens adaptive immune responses. To test this, mice were infected with equivalent doses of either WT or KLKI MHV68 and adaptive immune responses were evaluated over time. B and T cell activation was starkly reduced following KLKI MHV68 infection, despite a potent virus-specific effector CD8+ T cell response against both viruses. These phenotypes were independent of inoculating dose, as high dose infection with KLKI MHV68 still showed reduced adaptive immune cell activation. In contrast, infection of Ifnar1-/-mice, which support enhanced KLKI MHV68 lytic replication, led to potent adaptive cellular and humoral immune activation by both WT and KLKI viruses, suggesting that the level of viral replication, and not simply amount of virus present, is a major driver of adaptive immunity during GHV infection. Collectively, these data support the hypothesis that kLANA-mediated suppression of lytic replication facilitates immune evasion by holding viral replication below a threshold for potent induction of adaptive immunity. IMPORTANCEKSHV is a gammaherpesvirus that establishes lifelong, chronic infections in humans and increases the risk of virus-associated cancers. Currently, there is little information on how primary KSHV infection influences adaptive immune development in healthy individuals. Rodent models, such as murine gammaherpesvirus 68 (MHV68), provide a valuable laboratory system for studying gammaherpesvirus pathogenesis in vivo. In this study, we report that infection with a previously characterized chimeric KSHV-MHV68 virus expressing KSHV LANA represses lytic viral replication and elicits weak antiviral adaptive immune responses following primary infection, despite efficient latency establishment. Using this chimeric MHV68 virus, we demonstrate that lytic viral amplification must breach a threshold to trigger a potent virus-specific adaptive immune response. We propose that KSHV, through LANA, evades detection by repressing lytic viral replication to remain "below the radar" of adaptive immune defenses during host colonization.