Immune Modulation of Adult Neurogenesis during Experimental Herpes Simplex Encephalitis

Abstract

Herpes Simplex Virus-1 (HSV-1) is the primary cause of sporadic viral encephalitis in the United States. While prompt treatment results in high survival rates, greater than fifty percent of patients develop severe, life-altering neurological deficits subsequent to Herpes Simplex encephalitis (HSE). Inflammatory cues generated during non-HSE viral encephalitis have been shown to alter reparative neurogenesis in humans and mice. However, interactions between induced immune responses and the normal processes of adult neurogenesis remain unexplored during HSE. The present thesis hypothesized that inflammation induced by experimental HSV-1 infection of laboratory mice alters neural stem/progenitor cell (NSC) proliferation. The response of NSCs was dynamic throughout HSV-1 infection of adult BALB/c mice, with a significant increase in the NSC population during acute HSE at 6 d p.i. and a significant decrease by 15 d p.i.. The 6 d p.i. and 15 d p.i. time points coincided with macrophage infiltration and CD8(+) T cell infiltration respectively. At 6 d p.i., infiltrating macrophages were observed to be alternatively activated (M2) whereas macrophages at 15 d p.i. were classically activated (M1 macrophages). Treatment with M2 macrophages resulted in significant increases in the NSC population size both in vivo and in vitro. In vitro, the increase in NSC proliferation was found to be mediated via Wnt5a. In vivo, transplanted M2 macrophages increased the number of NSCs and Wnt5a(+)CD11b(+) macrophages were observed in the brains of 5 d p.i. mice. On the other hand, M1 macrophages activated with interferon-γ did not support NSC proliferation, which suggested that macrophage effects may be modulated by other immune factors present in the infected brain. CD8(+) T cells are the largest population of infiltrating immune cells during chronic HSE and produce robust amounts of IFN-�. The interactions of CD8(+) T cells and NSCs were addressed in vitro and in vivo experiments. Co-culture of activated CD8(+) T cells and NSCs abrogated NSC proliferation in vitro while in vivo depletion of CD8(+) T cells restored the NSC population in infected mice. Further evaluation of the effect of CD8(+) T cells on NSC proliferation was dependent on IFN-� both in vivo and in vitro, as IFN-� receptor 1 (IFN-�R1) knockout mice were unresponsive to CD8(+) T cells in culture and infected IFN-�R1 knockout mice had no reduction in the size of the NSC population. To continue evaluation of interactions of NSC proliferation and IFN-�, alternative models of HSE were assessed in C57BL/6 mice. The kinetics of immune cell and NSC proliferation were similar between BALB/c and C57BL/6 mice, but macrophage and T cell phenotypes were different between the strains. Macrophages infiltrating the brain, at any time point examined, were persistently classically activated with an M1 phenotype, and T cell infiltration was dominated by CD4(+) T cells in C57BL/6 mice. On the other hand, NSC proliferation was significantly decreased when IFN-� was expressed suggesting that the interactions of IFN-� and NSCs were similar in both strains of mice tested. The studies presented in this dissertation are the first to suggest a connection between IFN-� produced by infiltrating T cells and impaired NSC proliferation. The mechanism by which T cells mediate this effect may be direct or through interaction with other cell types in the brain. However, there are implications that NSC proliferation and associated reparative neurogenesis may be impacted in any model of brain damage that exhibits prominent IFN-� expression.