Microglial response in the trigeminal ganglion of mice corneally inoculated with herpes simplex virus (HSV) was investigated. Virus-infected neurons of the trigeminal ganglion did not exhibit apoptotic signal, while those of the trigeminal sensory brainstem nucleus did. Cells expressing ionized calcium binding adapter molecule 1 (Iba1), a specific marker of microglia/macrophages, increased in number in the virally infected region of the trigeminal ganglion, with morphological transformation to an activated phenotype, frequently detected as perineural satellites. Further microglial transformation to macropahges was not evident. Iba1-immunopositive perineural satellites also appeared in the vicinity of virally infected region. Such activated microglia expressed basic fibroblast growth factor (bFGF) molecules. The reverse transcription-polymerase chain reaction (RT-PCR) detected upregulated synthesis of mRNA for bFGF in the trigeminal ganglion. In contrast, in the trigeminal sensory brainstem nucleus, a small number of bFGF-producing cells appeared only in the vicinity of virally infected area. Collectively, Iba1-bearing microglia exist in the mouse trigeminal ganglion and respond to herpes simplex virus infection, most likely conferring neuroprotective functions upon the trigeminal ganglion.
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