An M2 Rather than a TH2 Response Contributes to Better Protection against Latency Reactivation following Ocular Infection of Naive Mice with a Recombinant Herpes Simplex Virus 1 Expressing Murine Interleukin-4

We found previously that altering macrophage polarization toward M2 responses by injection of colony-stimulating factor 1 (CSF-1) was more effective in reducing both primary and latent infections in mice ocularly infected with herpes simplex virus 1 (HSV-1) than M1 polarization by gamma interferon (IFN-gamma) injection. Cytokines can coordinately regulate macrophage and T helper (T-H) responses, with interleukin-4 (IL-4) inducing type 2 T-H (T(H)2) as well as M2 responses and IFN-gamma inducing T(H)1 as well as M1 responses. We have now differentiated the contributions of these immune compartments to protection against latency reactivation and corneal scarring by comparing the effects of infection with recombinant HSV-1 in which the latency-associated transcript (LAT) gene was replaced with either the IL-4 (HSV-IL-4) or IFN-gamma (HSV-IFN-gamma) gene using infection with the parental (LAT-negative) virus as a control. Analysis of peritoneal macrophages in vitro established that the replacement of LAT with the IL-4 or IFN-gamma gene did not affect virus infectivity and promoted polarization appropriately. Protection against corneal scarring was significantly higher in mice ocularly infected with HSV-IL-4 than in those infected with HSV-IFN-gamma or parental virus. Levels of primary virus replication in the eyes and trigeminal ganglia (TG) were similar in the three groups of mice, but the numbers of gC(+) cells were lower on day 5 postinfection in the eyes of HSV-IL-4-infected mice than in those infected with HSV-IFN-gamma or parental virus. Latency and explant reactivation were lower in both HSV-IL-4- and HSV-IFN-gamma-infected mice than in those infected with parental virus, with the lowest level of latency being associated with HSV-IL-4 infection. Higher latency correlated with higher levels of CD8, PD-1, and IFN-gamma mRNA, while reduced latency and T-cell exhaustion correlated with lower gC(+) expression in the TG. Depletion of macrophages increased the levels of latency in all ocularly infected mice compared with their undepleted counterparts, with macrophage depletion increasing latency in the HSV-IL-4 group greater than 3,000-fold. Our results suggest that shifting the innate macrophage immune responses toward M2, rather than M1, responses in HSV-1 infection would improve protection against establishment of latency, reactivation, and eye disease. IMPORTANCE Ocular HSV-1 infections are among the most frequent serious viral eye infections in the United States and a major cause of virus-induced blindness. As establishment of a latent infection in the trigeminal ganglia results in recurrent infection and is associated with corneal scarring, prevention of latency reactivation is a major therapeutic goal. It is well established that absence of latency-associated transcripts (LATs) reduces latency reactivation. Here we demonstrate that recombinant HSV-1 expressing IL-4 (an inducer of T(H)2/M2 responses) or IFN-gamma (an inducer of T(H)1/M1 responses) in place of LAT further reduced latency, with HSV-IL-4 showing the highest overall protective efficacy. In naive mice, this higher protective efficacy was mediated by innate rather than adaptive immune responses. Although both M1 and M2 macrophage responses were protective, shifting macrophages toward an M2 response through expression of IL-4 was more effective in curtailing ocular HSV-1 latency reactivation.