A mutation in the latency-related gene of bovine herpesvirus 1 leads to impaired ocular shedding in acutely infected calves

Bovine herpesvirus 1 (BHV-1) is an important pathogen of cattle, and infect ion is usually initiated in the ocular or nasal cavity. Like other alphaher pesviruses, BHV-1 establishes latency in sensory neurons but has the potent ial of reactivating from latency and spreading. The only abundant viral tra nscript expressed during latency is the latency-related (LR) RNA, which is alternatively spliced in trigeminal ganglia during acute infection (L. R. D evireddy and C. Jones, J. Virol. 72:7294-7301, 1998). LR gene products inhi bit cell cycle progression (Y. Jiang, A. Hossain, M. T. Winkler, T. Holt, A . Doster, and C. Jones, J. Virol. 72:8133-8142,1998) and chemically induced apoptosis (J. Ciacci-Zannela, M. Stone, G. Henderson, and C. Jones. J. Vir ol. 73:9734-9740, 1999). Although these studies suggest that LR gene produc ts play an important role in the latency/pathogenesis of BHV-1, constructio n of a mutant is necessary to test this hypothesis. Because the bICP0 gene overlaps and is antisense to the LR gene, it was necessary to mutate the LR gene without altering bICP0 expression. This was accomplished by inserting three stop codons near the beginning of the LR RNA, thus interfering with expression of proteins expressed by the LR RNA. The LR mutant virus grew wi th wild-type (WT) efficiency in bovine kidney (MDBK) cells and expressed bI CP0 at least as efficiently as WT BHV-1 or the LR rescued virus. When calve s were infected with the LR mutant, we observed a dramatic decrease (3 to 4 log units) in ocular shedding during acute infection relative to WT or the LR rescued virus. In contrast, shedding of the LR mutant from the nasal ca vity was not significantly different from that of the WT or the LR rescued virus. Calves infected with the LR mutant exhibited mild clinical symptoms, but they seroconverted. Neutralizing antibody titers were lower in calves infected with the LR mutant, confirming reduced growth. In summary, this st udy suggests that an LR protein promotes ocular shedding during acute infec tion of calves.