LATENT MURINE CYTOMEGALOVIRUS-INFECTION IN MACROPHAGES

In this study we show that macrophages (M phi) are latently infected w ith murine cytomegalovirus (MCMV). After clearance of acute MCMV infec tion, the predominant form of chronic infection in Balb mice is latenc y rather than persistence. Peritoneal exudate cells (PECs) from latent ly infected Balb mice (3-9 months postinfection) contained MCMV genome and reactivatable virus. Adherent cells from both resident and thiogl ycollate-elicited PECs carried more MCMV DNA (measured by PCR) than no nadherent cells, and were selectively enriched for M phi. FAGS sorted F4/80(+) M phi contained MCMV DNA, while other FAGS sorted cell popula tions from PECs were never positive for MCMV DNA. MCMV reactivated fro m FAGS sorted F4/80(+) M phi in 32% of cocultures with murine embryoni c fibroblasts (MEFs). Since M phi carry MCMV genome and reactivatable virus, but not lytic virus,they are latently infected with MCMV. We de termined the frequency of M phi carrying MCMV genome in PECs (about 1/ 50,000) using a limiting dilution PCR assay. Using this frequency and estimates of the total amount of MCMV genome in populations, we estima te that latently infected M phi carry 1-10 copies of MCMV genome. To e valuate the origin of latently infected M phi, we compared the frequen cy of cells carrying MCMV genome in the resident and elicited PECs. Th e frequency of M phi carrying MCMV DNA was the same in resident and th ioglycollate-elicited PECs, despite the fact that there was a ninefold increase in the number of M phi recovered after thioglycollate elicit ation. This argued for recruitment of bone marrow-derived M phi (BMM p hi) carrying MCMV genome into the peritoneum during inflammatory respo nses. Consistent with this hypothesis, MCMV genome, but not persistent virus, was detected in bone marrow cells from latently infected mice. (C) 1997 Academic Press