Viral Reservoirs in Lymph Nodes of FIV-Infected Progressor and Long-Term Non-Progressor Cats during the Asymptomatic Phase

Abstract

BackgroundExamination of a cohort of cats experimentally infected with feline immunodeficiency virus (FIV) for 5.75 years revealed detectable proviral DNA in peripheral blood mononuclear cells (PBMCs) harvested during the asymptomatic phase, undetectable plasma viral RNA (FIV gag), and rarely detectable cell-associated viral RNA. Despite apparent viral latency in peripheral CD4+ T cells, circulating CD4+ T cell numbers progressively declined in progressor animals. The aim of this study was to explore this dichotomy of peripheral blood viral latency in the face of progressive immunopathology. The viral replication status, cellular immunophenotypes, and histopathologic features were compared between popliteal lymph nodes (PLNs) and peripheral blood. Also, we identified and further characterized one of the FIV-infected cats identified as a long-term non-progressor (LTNP).ResultsPLN-derived leukocytes from FIV-infected cats during the chronic asymptomatic phase demonstrated active viral gag transcription and FIV protein translation as determined by real-time RT-PCR, Western blot and in situ immunohistochemistry, whereas viral RNA in blood leukocytes was either undetectable or intermittently detectable and viral protein was not detected. Active transcription of viral RNA was detectable in PLN-derived CD4+ and CD21+ leukocytes. Replication competent provirus was reactivated ex vivo from PLN-derived leukocytes from three of four FIV-infected cats. Progressor cats showed a persistent and dramatically decreased proportion and absolute count of CD4+ T cells in blood, and a decreased proportion of CD4+ T cells in PLNs. A single long-term non-progressor (LTNP) cat persistently demonstrated an absolute peripheral blood CD4+ T cell count indistinguishable from uninfected animals, a lower proviral load in unfractionated blood and PLN leukocytes, and very low amounts of viral RNA in the PLN.ConclusionCollectively our data indicates that PLNs harbor important reservoirs of ongoing viral replication during the asymptomatic phase of infection, in spite of undetectable viral activity in peripheral blood. A thorough understanding of tissue-based lentiviral reservoirs is fundamental to medical interventions to eliminate virus or prolong the asymptomatic phase of FIV infection.

Peripheral and central immune cell reservoirs in tissues from asymptomatic cats chronically infected with feline immunodeficiency virus

“…This animal has been characterized as a FIV-infected long-term non-progressor (LTNP) cat. [9,11] Additionally, we have demonstrated methylation and deacetylation of histone proteins physically associated with the FIV-promoter in peripheral blood CD4+ T cells isolated during the asymptomatic phase, which is consistent with a condensed chromatin pattern and viral latency. [12] Recently, we demonstrated evidence of active FIV replication in the popliteal lymph nodes in the face of an apparent absence of active viral replication in peripheral blood.…”

Central and peripheral reservoirs of feline immunodeficiency virus in cats: a review

“…Infection of bone marrow cells in naturally infected asymptomatic cats has also been reported, although the identity of infected subsets was not determined [25]. Two recent investigations into tissue reservoirs by our laboratory demonstrated the presence of proviral DNA within the popliteal lymph node, mesenteric lymph node, spleen and mucosa of the small intestines during the late asymptomatic phase of infection (5-6 years post inoculation), in the face of a relatively quiescent or inactive viral harbour replication-competent provirus in spite of relatively quiescent or inactive viral replication in the peripheral blood [79,92,109]. Interestingly, our studies revealed that, firstly, lymph node germinal centres contained an increased frequency of CD3+ T cells compared to uninfected cat tissues.…”

“…Evidence that proviral DNA loads may shift between cellular subsets as disease progresses, especially as CD4+ T cells are depleted, has been presented in a limited number of reports [26,27,79]. The hallmark of FIV infection in the acute and early asymptomatic phases of infection is an inverted peripheral blood CD4:CD8 T cell ratio initially due to a decrease in CD4+ T cells and in some cases a concurrent expansion of CD8+ T cells [60,[80][81][82].…”

“…However, findings from these few studies, including experimental and natural infections, show that CD4+ T cell counts continue to progressively decline to very low levels in blood and lymphoid tissues and may be associated with declining peripheral CD8+ T cell frequencies and clinical signs of immunodeficiencies [7,19,83]. Interestingly, significant declines in CD4+ T cells counts are not always associated with clinical signs of acquired immunodeficiencies and other factors such as environmental stresses may determine disease onset [7,24,79].…”

“…Anatomical reservoirs frequently discussed in the HIV field are the brain, gut-associated lymphoid tissue (GALT), lymph nodes and the reproductive tract [13,14,88,91]. Many of these tissues have been investigated as tissue reservoirs in the FIV-infected cat as well, particularly during the acute and early asymptomatic phases of infection [60,62,79,92]. A thorough understanding of tissue reservoir distribution and biology requires an understanding of the early pathogenesis and dissemination of virus.…”