Prolonged microglial cell activation and lymphocyte infiltration following experimental herpes encephalitis

Experimental murine herpes simplex virus (HSV)-1 brain infection stimulates microglial cell-driven proinflammatory chemokine production which precedes the presence of brain-infiltrating systemic immune cells. In the present study, we investigated the phenotypes and infiltration kinetics of leukocyte trafficking into HSV-infected murine brains. Using real-time bioluminescence imaging, the infiltration of luciferase-positive splenocytes, transferred via tail vein injection into the brains of HSV-infected animals, was followed over an 18-day time course. Flow cytometric analysis of brain-infiltrating leukocytes at 5, 8, 14, and 30 days postinfection (d.p.i.), was performed to assess their phenotype. A predominantly macrophage (CD45^highCD11b⁺Ly6C ^high) and neutrophil (CD45^highCD11b⁺Ly6G ⁺) infiltration was seen early during infection, with elevated levels of TNF-α mRNA expression. By 14 d.p.i., the phenotypic profile shifted to a predominantly lymphocytic (CD45^highCD3⁺) infiltrate. This lymphocyte infiltrate was detected until 30 d.p.i., when infectious virus could not be recovered, with CD8⁺ and CD4⁺ T cells present at a 3:1 ratio, respectively. This T lymphocyte infiltration paralleled increased IFN-γ mRNA expression in the brain. Activation of resident microglia (CD45^intCD11b⁺) was also detected until 30 d.p.i., as assessed by MHC class II expression. Activated microglial cells were further identified as the predominant source of IL-1β. In addition, infected mice given primed immunocytes at 4 d.p.i. showed a significant increase in mortality. Taken together, these results demonstrate that intranasal infection results in early macrophage and neutrophil infiltration into the brain followed by prolonged microglial activation and T lymphocyte retention. Similar prolonged neuroimmune activation may contribute to the neuropathological sequelae observed in herpes encephalitis patients.