Central and Peripheral Reservoirs of Feline Immunodeficiency Virus During the Asymptomatic Phase

Infection with the lentivirus feline immunodeficiency virus (FIV) results in lifelong viral persistence and progressive immunopathology in the cat. During the initial infection, FIV quickly disseminates to multiple cell types and anatomical compartments, collectively comprising the in vivo viral reservoir. The relative importance of each infected cellular subtype, anatomic compartment, along with the size of the reservoir are crucial areas of investigation for developing effective viral suppression and eradication therapies. The first chapter of this dissertation reviews what is currently known about FIV reservoirs in the infected cat, and emphasizes the utility of the FIV-infected cat as a model for the HIV-infected human.;FIV-infection in the cat typically progresses through three clinical phases- the acute, asymptomatic and terminal immunodeficiency phases. During the acute phase of infection, there is prolific viral replication with broad viral distribution throughout the cells and tissues of the infected animal. Clinically, the acute phase is characterized by high plasma viremia, a transient drop in peripheral blood CD4+ T cell numbers, transient fever and transient lymphadenopathy. The acute phase is followed by the asymptomatic phase, which may last months to years. During the asymptomatic phase, the FIV-infected cat generally demonstrates no outward sign of clinical disease (clinical latency). However, on closer examination FIV-infected cats in the asymptomatic phase demonstrate progressively declining peripheral blood CD4+ T cell numbers, an inverted CD4/CD8 ratio, cytokine aberrations and leukocyte dysfunction. In the terminal immunodeficiency phase (FAIDS), viral replication rebounds in association with collapse of the adaptive immune system, resulting in re-emergent plasma viremia, intercurrent infections and/or neoplasia.;Our laboratory has established a cohort of experimentally FIV-infected cats where viral activity and immunopathology in the peripheral blood have been closely monitored over time. Approximately ten months post inoculation (PI) the cats entered the asymptomatic phase characterized by undetectable plasma viremia suggesting that viral replication in the peripheral blood during the asymptomatic phase is either absent or present at a level below the limits of detection. In the second chapter of this thesis, we investigated the viral replication status and viral genomic stability over time in peripheral blood and popliteal lymph node (PLN)-derived cells.;In spite of undetectable plasma and rare to undetectable viral gag RNA in peripheral blood mononuclear cells (PBMCs), this cohort demonstrated instability of the 5' viral genome, suggesting that blood may not accurately represent viral activity in asymptomatic FIV-infected cats. In spite of undetectable plasma viremia and rare to undetectable viral gag RNA in PBMCs, circulating CD4+ T cell numbers progressively declined in most of the FIV-infected animals (progressors). The aim of chapter three was to explore this dichotomy by examining lymph node as a possible viral tissue reservoir. The viral replication status, cellular immunophenotypes, and histopathologic features were compared between popliteal lymph nodes (PLNs) surgically obtained from FIV infected cats and peripheral blood. PLN-derived leukocytes demonstrated active viral transcription, FIV protein production, and contained replication competent provirus. Similar to the peripheral blood, PLNs had a decreased proportion of CD4+ T cells relative to non-infected control animals. Collectively, this data indicates that PLNs harbor important reservoirs of ongoing viral replication during the asymptomatic phase of infection, in spite of undetectable viral replication in the peripheral blood.;The evidence that PLN tissue harbors foci of ongoing viral replication instigated the aim of the fourth chapter in which we sought to further characterize cellular and tissue reservoirs of active viral replication by examining additional tissues including spleen, mesenteric lymph node (MLN), and intestine. In spite of undetectable viral transcription in the blood, FIV gag RNA was detected in all three tissue sites from 3 of 4 FIV-infected cats. A novel in situ hybridization assay identified lymphoid follicular domains as microanatomical foci of ongoing FIV replication in all 4 FIV-infected cats. We found CD4+ leukocyte depletion in tissues and that infected CD4+ and CD21+ leukocytes produce greater amounts of viral RNA than CD8+ leukocytes. These findings support the hypothesis that tissue reservoirs harbor foci of ongoing viral replication in the late asymptomatic phase and these microanatomical foci should be considered as persistent viral reservoirs. Future eradication strategies must take into account their importance in the pathogenesis of lentivirus-associated immunodeficiency disease.