Immunohistochemical localization of feline immunodeficiency virus using native species antibodies

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

Feline immunodeficiency virus (FIV) infection in cats results in life-long viral persistence and progressive immunopathology. We have previously described a cohort of experimentally infected cats demonstrating a progressive decline of peripheral blood CD4+ T-cell over six years in the face of apparent peripheral viral latency. More recently we reported findings from this same cohort that revealed popliteal lymph node tissue as sites for ongoing viral replication suggesting that tissue reservoirs are important in FIV immunopathogenesis during the late asymptomatic phase of infection. Results reported herein characterize important tissue reservoirs of active viral replication during the late asymptomatic phase by examining biopsied specimens of spleen, mesenteric lymph node (MLN), and intestine from FIV-infected and uninfected control cats. Peripheral blood collected coincident with harvest of tissues demonstrated severe CD4+ T-cell depletion, undetectable plasma viral gag RNA and rarely detectable peripheral blood mononuclear cell (PBMC)-associated viral RNA (vRNA) by real-time PCR. However, vRNA was detectable in all three tissue sites from three of four FIV-infected cats despite the absence of detectable vRNA in plasma. A novel in situ hybridization assay identified B cell lymphoid follicular domains as microanatomical foci of ongoing FIV replication. Additionally, we demonstrated that CD4+ leukocyte depletion in tissues, and CD4+ and CD21+ leukocytes as important cellular reservoirs of ongoing replication. These findings revealed that tissue reservoirs support foci of ongoing viral replication, in spite of highly restricted viral replication in blood. Lentiviral eradication strategies will need address tissue viral reservoirs.

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

Background

Examination 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).

Results

PLN-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.

Conclusion

Collectively 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.

Intracellular nucleotide levels and the control of retroviral infections

Retroviruses consume cellular deoxynucleoside triphosphates (dNTPs) to convert their RNA genomes into proviral DNA through reverse transcription. While all retroviruses replicate in dividing cells, lentiviruses uniquely replicate in nondividing cells such as macrophages. Importantly, dNTP levels in nondividing cells are extremely low, compared to dividing cells. Indeed, a recently discovered anti-HIV/SIV restriction factor, SAMHD1, which is a dNTP triphosphohydrolase, is responsible for the limited dNTP pool of nondividing cells. Lentiviral reverse transcriptases (RT) uniquely stay functional even at the low dNTP concentrations in nondividing cells. Interestingly, Vpx of HIV-2/SIVsm proteosomally degrades SAMHD1, which elevates cellular dNTP pools and accelerates lentiviral replication in nondividing cells. These Vpx-encoding lentiviruses rapidly replicate in nondividing cells by encoding both highly functional RTs and Vpx. Here, we discuss a series of mechanistic and virological studies that have contributed to conceptually linking cellular dNTP levels and the adaptation of lentiviral replication in nondividing cells.

FIV establishes a latent infection in feline peripheral blood CD4+ T lymphocytes in vivo during the asymptomatic phase of infection

Background

Feline immunodeficiency virus (FIV) is a lentivirus of cats that establishes a lifelong persistent infection with immunologic impairment.

Results

In an approximately 2 year-long experimental infection study, cats infected with a biological isolate of FIV clade C demonstrated undetectable plasma viral loads from 10 months post-infection onward. Viral DNA was detected in CD4+CD25+ and CD4+CD25- T cells isolated from infected cats whereas viral RNA was not detected at multiple time points during the early chronic phase of infection. Viral transcription could be reactivated in latently infected CD4+ T cells ex vivo as demonstrated by detectable FIV gag RNA and 2-long terminal repeat (LTR) circle junctions. Viral LTR and gag sequences amplified from peripheral blood mononuclear cells during early and chronic stages of infection demonstrated minimal to no viral sequence variation.

Conclusions

Collectively, these findings are consistent with FIV latency in peripheral blood CD4+ T cells isolated from chronically infected cats. The ability to isolate latently FIV-infected CD4+ T lymphocytes from FIV-infected cats provides a platform for the study of in vivo mechanisms of lentiviral latency.

Utilizing the FIV model to understand dendritic cell dysfunction and the potential role of dendritic cell immunization in HIV infection

Dendritic cells (DC) are potent antigen presenting cells which initiate and coordinate the immune response making them central targets of and attractive candidates for manipulation in chronic lentiviral infections. Emerging evidence suggests that DC immune function is disrupted during both acute and chronic infection with human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). Despite some early promising data, the use of DC for lentiviral immunotherapy has not fulfilled its expected potential and has been complicated by the large number of variables involved in DC harvesting, purifying, and antigen loading. Pre-clinical studies aimed at identifying successful strategies for DC augmentation of current HIV treatment protocols are needed. Over the past two decades, the FIV model for HIV infection has increased the understanding of retroviral pathogenesis, and studies have begun using the FIV model to study DC dysfunction and DC-mediated immunotherapy. Careful consideration of the many variables involved in DC function and therapy should help develop protocols to explore the potential of DC vaccine-based therapies for lentiviral infection.

Sustained generation of tissue dendritic cells from cats using organ stromal cell cultures

Currently most dendritic cells (DC) for in vitro study are generated from bone marrow or peripheral blood by culture in high concentrations of GM-CSF and other cytokines. However, in mice it is also possible to generate DC from spleen cells using long-term stromal cell cultures. To determine whether tissue DC could be also be generated from cats, we established stromal cell cultures from a number of different tissues of newborn cats. We found that stromal cell cultures from spleen, lung, liver, kidney, brain, and lymph node tissues were all capable of spontaneously generating DC over long periods of time (months), without requiring the addition of exogenous cytokines. The tissue DC generated from these stromal cell cultures could be readily isolated at high purity by simple mechanical detachment. The feline tissue DC expressed high levels of CD11c, CD11b, and MHC Class II and variable levels of CD80 and CD14 and exhibited high levels of spontaneous macropinocytosis. Moreover, DC from spleen stromal cell cultures, but not DC from lung or liver stromal cell cultures, stimulated mixed-lymphocyte reactions. The DC generated from the stromal cell cultures were relatively independent of GM-CSF for survival and proliferation, indicative of a dependence on other growth factors produced by the stromal cells. These results suggest that tissues of young cats contain a population of resident DC progenitor cells that under appropriate conditions are capable of spontaneous proliferation and generation of immature DC.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Low-dose interferon-alpha treatment for feline immunodeficiency virus infection

Feline immunodeficiency virus sustains an AIDS-like syndrome in cats, which is considered a relevant model for human AIDS. Under precise enrolment requirements, 30 naturally infected cats showing overt disease were included in a trial of low-dose, oral human interferon-alpha treatment. Twenty-four of them received 10 IU/Kg of human interferon-alpha and 6 placebo only on a daily basis under veterinary supervision. The low-dose human interferon-alpha treatment significantly prolonged the survival of virus-infected cats (p<0.01) and brought to a rapid improvement of disease conditions in the infected hosts. Amelioration of clinical conditions was neither correlated with plasma viremia, nor with proviral load in leukocytes. A good survival of CD4+ T cells and a slow increase of CD8+ T cells were also observed in human interferon-alpha-treated cats. Interestingly, the improvement of the total leukocyte counts showed a much stronger correlation with the recovery from serious opportunistic infections. As shown in other models of low-dose interferon-alpha treatment, there was a rapid regression of overt immunopathological conditions in virus-infected cats. This hints at a major role of interferon-alpha in the control circuits of inflammatory cytokines, which was probably the very foundation of the improved clinical score and survival despite the unabated persistence of virus and virus-infected cells.

Gene-expression changes induced by Feline immunodeficiency virus infection differ in epithelial cells and lymphocytes

Infection of cats with Feline immunodeficiency virus (FIV) is an important model for understanding comparative lentivirus biology. In vivo, FIV infects lymphocytes and monocyte/macrophages, but in vitro infection is commonly investigated in epithelial Crandell-Reese Feline Kidney (CRFK) cells. In this study, the transcriptional responses of CRFK cells and primary lymphocytes to infection with FIV 34TF, a cloned subtype A virus, and FIV USgaB01, a biological subtype B isolate, were determined. Reverse-transcribed mRNA from both cell types was hybridized to microarrays containing 1700 human expressed sequence tags in duplicate and data were analysed with Significance Analysis of Microarrays (sam) software. Results from six experiments assessing homeostatic cross-species hybridization excluded 3.48 % inconsistently detected transcripts. Analysis of data from five time points over 48 h after infection identified 132 and 24 differentially expressed genes in epithelial cells and lymphocytes, respectively. Genes involved in protein synthesis, the cell cycle, structure and metabolism were affected. The magnitude of gene-expression changes ranged from 0.62 to 1.62 and early gene induction was followed by downregulation after 4 h. Transcriptional changes in CRFK cells were distinct from those in lymphocytes, except for heat-shock cognate protein 71, which was induced at multiple time points in both cell types. These findings indicate that FIV infection induces transcriptional changes of a modest magnitude in a wide range of genes, which is probably reflective of the relatively non-cytopathic nature of virus infection.

Progression of chronic hepatitis and preneoplasia in Helicobacter hepaticus-infected A/JCr mice

Helicobacter hepaticus infection induces sustained inflammation and carcinoma of the liver in A/JCr mice, and serves as a model of human cancers associated with viral hepatitis and H. pylorichronic gastritis. Here we describe the pathogenesis of premalignant disease in A/JCr mice infected with H. hepaticus. We inoculated dams intragestationally and/or pups postnatally, and evaluated offspring at 3, 6, or 12 months. Mice infected at or before 3 weeks of age, but not at 12 weeks, developed disease. Male mice were most affected, but expressed a bimodal pattern of susceptibility. Males exhibited lobular necrogranulomatous and interface (chronic active) hepatitis, while females usually developed intraportal (chronic persistent) hepatitis. Portal inflammation was slowly progressive, with tertiary lymphoid nodule development by 12 months. Hepatic bacterial load and preneoplastic lesions, including clear and tigroid cell foci of cellular alteration, were correlated with lobular hepatitis severity. No extrahepatic surrogate disease marker reliably predicted individual hepatitis grade. In conclusion, gender and bacterial exposure timing are key determinants of H. hepaticus disease outcomes. Intrahepatic inflammation is driven by local signals characterized by a vigorous but nonsterilizing immune response. Continued study of chronic hepatitis progression may reveal therapeutic targets to reduce the risk of hepatocellular carcinoma.

Oligo(dA-dT)-dependent signal amplification for the detection of proteins in cells

An ultrasensitive protein detection system in situ named the ImmunoAT-tailing method was developed. It consists of three elementary processes: (i) detection of a protein by a primary antibody and a biotinylated secondary antibody; (ii) linking of biotinylated 15-base oligo(dA-dT) to the biotinylated immunocomplex via streptavidin; and (iii) self-priming elongation of oligo(dA-dT) by the Klenow fragment, 3' to 5' exo-. After the elongation reaction in the presence of dATP, dTTP, and dye-labeled dUTP, the protein was labeled with a large number of the dye molecules. The poly(dA-dT) elongated without the labeled nucleotides was detected by 4',6-diamidino-2-phenylindole (DAPI) staining. By combining the different labelings, double staining was possible. This ImmunoAT-tailing method has a specificity and sensitivity higher than that of tyramide signal amplification.