Roles of M1 and M2 Macrophages in Herpes Simplex Virus 1 Infectivity

Macrophages are the predominant infiltrate in the corneas of mice that have been ocularly infected with herpes simplex virus 1 (HSV-1). However, very little is known about the relative roles of M1 (classically activated or polarized) and M2 (alternatively activated or polarized) macrophages in ocular HSV-1 infection. To better understand these relationships, we assessed the impact of directed M1 or M2 activation of RAW264.7 macrophages and peritoneal macrophages (PM) on subsequent HSV-1 infection. In both the RAW264.7 macrophage and PM in vitro models, HSV-1 replication in M1 macrophages was markedly lower than in M2 macrophages and unstimulated controls. The M1 macrophages expressed significantly higher levels of 28 of the 32 tested cytokines and chemokines than M2 macrophages, with HSV-1 infection significantly increasing the levels of proinflammatory cytokines and chemokines in the M1 versus the M2 macrophages. To examine the effects of shifting the immune response toward either M1 or M2 macrophages in vivo, wild-type mice were injected with gamma interferon (IFN-gamma) DNA or colony-stimulating factor 1 (CSF-1) DNA prior to ocular infection with HSV-1. Virus replication in the eye, latency in trigeminal ganglia (TG), and markers of T cell exhaustion in the TG were determined. We found that injection of mice with IFN-gamma DNA, which enhances the development of M1 macrophages, increased virus replication in the eye; increased latency; and also increased CD4, CD8, IFN-gamma, and PD-1 transcripts in the TG of latently infected mice. Conversely, injection of mice with CSF-1 DNA, which enhances the development of M2 macrophages, was associated with reduced virus replication in the eye and reduced latency and reduced the levels of CD4, CD8, IFN-gamma, and PD-1 transcripts in the TG. Collectively, these results suggest that M2 macrophages directly reduce the levels of HSV-1 latency and, thus, T-cell exhaustion in the TG of ocularly infected mice. IMPORTANCE Our findings demonstrate a novel approach to further reducing HSV-1 replication in the eye and latency in the TG by modulating immune components, specifically, by altering the phenotype of macrophages. We suggest that inclusion of CSF-1 as part of any vaccination regimen against HSV infection to coax responses of macrophages toward an M2, rather than an M1, response may further improve vaccine efficacy against ocular HSV-1 replication and latency.