Murine cytomegalovirus induces apoptosis in non-infected cells of the developing mouse brain and blocks apoptosis in primary neuronal culture

Cytomegalovirus (CMV) is the most common cause of congenital infection, resulting in birth defects such as microcephaly. In this study, we found that apoptosis is induced in the developing mouse brain infected with murine cytomegalovirus (MCMV) in an association with neuronal cell loss. With the combination of the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) technique and immunohistochemical staining, 3.8% of the TUNEL-positive cells were double-stained with the antibody to neuron-specific enolase, while none of the TUNEL-positive cells were stained with antibodies to the immediate early and early viral antigens of MCMV. Furthermore, distribution pattern of the TUNEL-positive cells was different from that of viral DNA-positive cells detected by the in situ DNA-DNA hybridization. More than 30% of the TUNEL-positive cells were double-stained with the F4/80 antibody specific for microglia/macrophages, which were sometimes swollen, presumably the consequence of engulfment of the neuronal apoptotic cells. In the primary neuronal cultures, MCMV infection inhibited the induction of apoptosis either by serum deprivation or by glutamate treatment. It was also confirmed by the double-staining method that apoptosis was not induced in the viral-infected neuronal cultures. These results suggest that MCMV infection induces apoptosis in non-infected neuronal cells, presumably by indirect mechanisms, and that apoptotic cells are engulfed by microglia/macrophages. The induction and blocking of neuronal apoptosis by viral infection may be important for morphological and functional brain disorders in the congenital CMV infection.