Mouse cytomegalovirus is infectious for rats and alters lymphocyte subsets and spleen cell proliferation

The Smith strain of mouse cytomegalovirus (MCMV) was infectious for infant and mature DA strain laboratory rats as judged by development of neutralizing antibodies and specific spleen cell proliferation on stimulation with MCMV antigen. An i.p. inoculum of 10(6) PFU of MCMV was fatal for more than two-thirds of infant mice (1-7 days of age), and disseminated viral infection was documented by isolation of virus from body organs. In contrast, weanling and adult rats did not become ill as a result of infection with a larger inoculum of 10(7) PFU. However, these older MCMV infected rats did show transient reversals of T helper/suppressor cell ratios and alterations of immune cell function as detected by in vitro spleen cell proliferation assays. Seven days after MCMV infection, there was a generalized increase in 3H-thymidine incorporation by spleen cells in both resting (unstimulated) cultures and cultures exposed to mitogens (Con A, PHA, LPS) and to MCMV antigen. At 14 days, the spleen cell proliferation in the unstimulated cultures returned to normal but was depressed compared to controls in response to Con A. These observations show that laboratory rats are susceptible to MCMV infection and that asymptomatic infection may occur and cause transient alterations in lymphocyte subsets and in their reactivity to mitogens.