Background:  Xenotropic Murine Leukemia Virus-related Retrovirus (XMRV) is a human retrovirus recently discovered in familial prostate cancer tissue using DNA array based Virochip technology. Understanding viral replication kinetics, tissue tropism, and the host immune response is fundamental to establish the etiology of XMRV infection in human disease. Development of serologic assays to detect XMRV-specific antibodies would facilitate epidemiologic studies.

Methods:  Five rhesus macaques were inoculated intravenously with XMRV. Blood was collected throughout the course of infection, and tissue from multiple organs was harvested at necropsy. Two macaques were necropsied at day 6 or 7 and one at day 144 post infection. The remaining 2 animals were re-inoculated with XMRV on day 158 and necropsied on day 291. XMRV-specific immunoreactivity was monitored by Western blot using viral lysate. Recombinant env gp70, p15E and gag p30 were utilized to develop serologic assays on the high-throughput automated ARCHITECT instrument system (Abbott Diagnostics).

Results:  XMRV inoculation resulted in low transient plasma viremia, although proviral DNA persisted in circulating peripheral blood mononuclear cells for several weeks. Of interest, the earliest leukocyte targets were CD4⁺ T cells and NK cells followed by CD8⁺ enriched T and CD20⁺ enriched B cells (50% positive); CD14⁺ monocytes were negative. Animals sacrificed at the acute stage showed evidence of viral replication in spleen, lung, lymph nodes and liver. In contrast, sacrifice of 2 animals at 19 weeks post XMRV re-inoculation showed greater dissemination of XMRV DNA and RNA in various organs including the GI and urinary tract as well as in vaginal tissue of the one female. By Western blot analysis, all 3 chronically infected macaques developed antibody responses to env and gag proteins. The serologic assays demonstrated 100% sensitivity by detecting all Western blot positive serial bleeds from the XMRV-infected macaques. Preliminary results showed evidence of detectable reactivity to all 3 antigens in a low proportion (~0.1%) of US blood donors.

Conclusions:  These data suggest that lymphocytes are a primary target for replication persistence (low grade replication) of XMRV in the absence of detectable plasma viremia. This study identified specific serological markers useful for detection of antibodies induced by XMRV infection. The prototype antibody assays will facilitate large-scale epidemiological studies.