EFFECTS OF PHARMACOLOGICAL AND LENTIVIRUS-INDUCED IMMUNE SUPPRESSION ON ORBIVIRUS PATHOGENESIS - ASSESSMENT OF VIRUS BURDEN IN BLOOD MONOCYTES AND TISSUES BY REVERSE TRANSCRIPTION IN-SITU PCR

We investigated the effects of pharmacological and lentivirus-induced immunosuppression on bluetongue virus (BTV) pathogenesis as a mechanis m for virus persistence and induction of clinical disease. Immunologic ally normal and immunosuppressed sheep were infected subcutaneously wi th BTV serotype 3 (BTV-3), a foreign isolate with unknown pathogenicit y in North American livestock, and with North American serotype 11 (BT V-11), Erythrocyte-associated BTV RNA was detected earlier and at grea ter concentrations in sheep treated with immunosuppressive drugs. Simi larly, viral RNA and infectious virus were detected in blood monocytes earlier and at higher frequency in immunosuppressed animals: as many as 1 in 970 monocytes revealed BTV RNA at peak viremia, compared to <1 in 10(5) monocytes from immunocompetent sheep. Animals infected with BTV-3 had a higher virus burden in monocytes and lesions of greater se verity than those infected with BTV-11, BTV RNA was detected by in sit u hybridization in vascular endothelial cells and cells of monocyte li neage, but only in tissues from immunocompromised animals, and was mos t abundant in animals infected with BTV-3. In contrast, reverse transc ription-in situ PCR showed BTV RNA from both viral serotypes in high n umbers of tissue leukocytes and vascular endothelial cells from both i mmunosuppressed and, to a lesser extent, immunocompetent animals. Coll ectively, these findings show that BTV infection is widely distributed during acute infection but replication is highly restricted in animal s with normal immunity. These findings also suggest that in addition t o virulence factors that define viral serotypes, immunosuppression cou ld play a role in the natural history of orbivirus infection, allowing for higher virus burden, increased virus persistence, and greater pot ential for acquisition of virus by the arthropod vector.