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Spontaneous activation of latent cytomegalovirus from murine spleen explants. Role of lymphocytes and macrophages in release and replication of virus.

Cytomegalovirus (CMV) is a major pathogen in the compromised host where many infections result from activation of latent virus. Because latent CMV infection has been difficult to study in humans, murine models have been developed and investigated. Here, we describe the events involved in activation of latent murine CMV (MCMV) from spleen explants in vitro. Infectious virus was no longer detectable in murine organs 4 mo after inoculatioN of 10(5) plaque-forming units of MCMV. 8-10 d after establishment of spleen explants, phagocytic macrophages covered 70-80% of the surface of tissue culture dishes, and lymphocytes were continuously released, reaching titers of 10(6) cells/ml. MCMV was produced spontaneously after 12-18 d from spleen explant cultures of 33 of 34 mice. Virus replicated to titers above 10(4) plaque-forming units/ml, remained at that level for 4-5 wk, and gradually disappeared as macrophages were lysed. Although MCMV was shown to be replicating in macrophages, these cells were never found to be the source of latent virus. Cell separation studies indicated that latent virus was initially released from 70% of lymphocyte cultures and was associated with the B cell enriched fraction. We conclude that MCMV establishes nonreplicating dormant infection in B lymphocytes, activates from these cells in spleen explant cultures, and is augmented in titer by replication in permissive macrophages.