Rat cytomegalovirus major immediate-early enhancer switching results in altered growth characteristics

It has been hypothesized that the major immediate-early (MIE) enhancer of c ytomegalovirus (CMV) is important in determining virus tropism and latency because of its essential role in initiating the cascade of early gene expre ssion necessary for virus replication. Although rat CMV (RCMV) and murine C MV (MCMV) exhibit extreme species specificity in vivo, they differ in their ability to replicate in tissue culture. MCMV can replicate in a rat embryo fibroblast (REF) cell line while RCMV does not grow in murine fibroblasts. The tropism is not due to a block in virus entry into the cell. We have co nstructed a recombinant RCMV in which the RCMV MIE enhancer has been replac ed with that of MCMV. Growth of the recombinant virus in tissue culture rem ains restricted to rat cells, suggesting that other viral and/or host facto rs are more important in determining in vitro tropism. Unlike findings usin g recombinant MCMV in which the human CMV (HCMV) MIE enhancer substitutes f or the native one (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal , J. Virol. 72:8502-8509, 1998), infection with our recombinant virus at a low multiplicity of infection resulted in a substantial decrease in virus r eplication. This occurred despite comparable or increased MIE transcription from the recombinant virus. In vivo experiments showed that the recombinan t virus replicates normally in the spleen during acute infection. Notably, the recombinant virus appears to be deficient in spreading to the salivary gland, suggesting a role for the MIE enhancer in tropism for certain tissue s involved in virus dissemination. Four months after infection, recombinant virus with the foreign MIE enhancer was reactivated from spleen explants.