Feline immunodeficiency virus clade C mucosal transmission and disease courses

Combination Antiretroviral Therapy and Immunophenotype of Feline Immunodeficiency Virus

Feline Immunodeficiency Virus (FIV) causes progressive immune dysfunction in cats similar to human immunodeficiency virus (HIV) in humans. Although combination antiretroviral therapy (cART) is effective against HIV, there is no definitive therapy to improve clinical outcomes in cats with FIV. This study therefore evaluated pharmacokinetics and clinical outcomes of cART (2.5 mg/kg Dolutegravir; 20 mg/kg Tenofovir; 40 mg/kg Emtricitabine) in FIV-infected domestic cats. Specific pathogen free cats were experimentally infected with FIV and administered either cART or placebo treatments (n = 6 each) for 18 weeks, while n = 6 naïve uninfected cats served as controls. Blood, saliva, and fine needle aspirates from mandibular lymph nodes were collected to quantify viral and proviral loads via digital droplet PCR and to assess lymphocyte immunophenotypes by flow cytometry. cART improved blood dyscrasias in FIV-infected cats, which normalized by week 16, while placebo cats remained neutropenic, although no significant difference in viremia was observed in the blood or saliva. cART-treated cats exhibited a Th2 immunophenotype with increasing proportions of CD4+CCR4+ cells compared to placebo cats, and cART restored Th17 cells compared to placebo-treated cats. Of the cART drugs, dolutegravir was the most stable and long-lasting. These findings provide a critical insight into novel cART formulations in FIV-infected cats and highlight their role as a potential animal model to evaluate the impact of cART on lentiviral infection and immune dysregulation.

Pathogenesis of oral FIV infection

Feline immunodeficiency virus (FIV) is the feline analogue of human immunodeficiency virus (HIV) and features many hallmarks of HIV infection and pathogenesis, including the development of concurrent oral lesions. While HIV is typically transmitted via parenteral transmucosal contact, recent studies prove that oral transmission can occur, and that saliva from infected individuals contains significant amounts of HIV RNA and DNA. While it is accepted that FIV is primarily transmitted by biting, few studies have evaluated FIV oral infection kinetics and transmission mechanisms over the last 20 years. Modern quantitative analyses applied to natural FIV oral infection could significantly further our understanding of lentiviral oral disease and transmission. We therefore characterized FIV salivary viral kinetics and antibody secretions to more fully document oral viral pathogenesis. Our results demonstrate that: (i) saliva of FIV-infected cats contains infectious virus particles, FIV viral RNA at levels equivalent to circulation, and lower but significant amounts of FIV proviral DNA; (ii) the ratio of FIV RNA to DNA is significantly higher in saliva than in circulation; (iii) FIV viral load in oral lymphoid tissues (tonsil, lymph nodes) is significantly higher than mucosal tissues (buccal mucosa, salivary gland, tongue); (iv) salivary IgG antibodies increase significantly over time in FIV-infected cats, while salivary IgA levels remain static; and, (v) saliva from naïve Specific Pathogen Free cats inhibits FIV growth in vitro. Collectively, these results suggest that oral lymphoid tissues serve as a site for enhanced FIV replication, resulting in accumulation of FIV particles and FIV-infected cells in saliva. Failure to induce a virus-specific oral mucosal antibody response, and/or viral capability to overcome inhibitory components in saliva may perpetuate chronic oral cavity infection. Based upon these findings, we propose a model of oral FIV pathogenesis and suggest alternative diagnostic modalities and translational approaches to study oral HIV infection.

Feline Immunodeficiency Virus Vif N-Terminal Residues Selectively Counteract Feline APOBEC3s

Feline immunodeficiency virus (FIV) Vif protein counteracts feline APOBEC3s (FcaA3s) restriction factors by inducing their proteasomal degradation. The functional domains in FIV Vif for interaction with FcaA3s are poorly understood. Here, we have identified several motifs in FIV Vif that are important for selective degradation of different FcaA3s. Cats (Felis catus) express three types of A3s: single-domain A3Z2, single-domain A3Z3, and double-domain A3Z2Z3. We proposed that FIV Vif would selectively interact with the Z2 and the Z3 A3s. Indeed, we identified two N-terminal Vif motifs (12LF13 and 18GG19) that specifically interacted with the FcaA3Z2 protein but not with A3Z3. In contrast, the exclusive degradation of FcaA3Z3 was regulated by a region of three residues (M24, L25, and I27). Only a FIV Vif carrying a combination of mutations from both interaction sites lost the capacity to degrade and counteract FcaA3Z2Z3. However, alterations in the specific A3s interaction sites did not affect the cellular localization of the FIV Vif protein and binding to feline A3s. Pulldown experiments demonstrated that the A3 binding region localized to FIV Vif residues 50 to 80, outside the specific A3 interaction domain. Finally, we found that the Vif sites specific to individual A3s are conserved in several FIV lineages of domestic cat and nondomestic cats, while being absent in the FIV Vif of pumas. Our data support a complex model of multiple Vif-A3 interactions in which the specific region for selective A3 counteraction is discrete from a general A3 binding domain.

IMPORTANCE

Both human immunodeficiency virus (HIV) and feline immunodeficiency virus (FIV) Vif proteins counteract their host's APOBEC3 restriction factors. However, these two Vif proteins have limited sequence homology. The molecular interaction between FIV Vif and feline APOBEC3s are not well understood. Here, we identified N-terminal FIV Vif sites that regulate the selective interaction of Vif with either feline APOBEC3Z2 or APOBEC3Z3. These specific Vif sites are conserved in several FIV lineages of domestic cat and nondomestic cats, while being absent in FIV Vif from puma. Our findings provide important insights for future experiments describing the FIV Vif interaction with feline APOBEC3s and also indicate that the conserved feline APOBEC3s interaction sites of FIV Vif allow FIV transmissions in Felidae.

Determinants of FIV and HIV Vif sensitivity of feline APOBEC3 restriction factors

Background

Feline immunodeficiency virus (FIV) is a global pathogen of Felidae species and a model system for Human immunodeficiency virus (HIV)-induced AIDS. In felids such as the domestic cat (Felis catus), APOBEC3 (A3) genes encode for single-domain A3Z2s, A3Z3 and double-domain A3Z2Z3 anti-viral cytidine deaminases. The feline A3Z2Z3 is expressed following read-through transcription and alternative splicing, introducing a previously untranslated exon in frame, encoding a domain insertion called linker. Only A3Z3 and A3Z2Z3 inhibit Vif-deficient FIV. Feline A3s also are restriction factors for HIV and Simian immunodeficiency viruses (SIV). Surprisingly, HIV-2/SIV Vifs can counteract feline A3Z2Z3.

Results

To identify residues in feline A3s that Vifs need for interaction and degradation, chimeric human–feline A3s were tested. Here we describe the molecular direct interaction of feline A3s with Vif proteins from cat FIV and present the first structural A3 model locating these interaction regions. In the Z3 domain we have identified residues involved in binding of FIV Vif, and their mutation blocked Vif-induced A3Z3 degradation. We further identified additional essential residues for FIV Vif interaction in the A3Z2 domain, allowing the generation of FIV Vif resistant A3Z2Z3. Mutated feline A3s also showed resistance to the Vif of a lion-specific FIV, indicating an evolutionary conserved Vif–A3 binding. Comparative modelling of feline A3Z2Z3 suggests that the residues interacting with FIV Vif have, unlike Vif-interacting residues in human A3s, a unique location at the domain interface of Z2 and Z3 and that the linker forms a homeobox-like domain protruding of the Z2Z3 core. HIV-2/SIV Vifs efficiently degrade feline A3Z2Z3 by possible targeting the linker stretch connecting both Z-domains.

Conclusions

Here we identified in feline A3s residues important for binding of FIV Vif and a unique protein domain insertion (linker). To understand Vif evolution, a structural model of the feline A3 was developed. Our results show that HIV Vif binds human A3s differently than FIV Vif feline A3s. The linker insertion is suggested to form a homeo-box domain, which is unique to A3s of cats and related species, and not found in human and mouse A3s. Together, these findings indicate a specific and different A3 evolution in cats and human.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0274-9) contains supplementary material, which is available to authorized users.

Identification of a novel subtype of feline immunodeficiency virus in a population of naturally infected felines in the Brazilian Federal District

The feline immunodeficiency virus (FIV) is a retrovirus that is found worldwide, and it can be assigned to six subtypes (A, B, C, D, E, and a putative subtype F) based on sequencing analysis of the env and gag genes. Subtypes A and B are the most common worldwide. In Brazil, several authors have isolated only subtype B, and its prevalence differs markedly among investigated populations. Blood samples from 200 domestic felines from the Federal District in Brazil were analyzed by PCR. Samples that tested positive for FIV were then cloned, sequenced, and analyzed phylogenetically and statistically. The results represent the first description of FIV infection in the Central Region of Brazil and suggest that only 2 % of felines in this region are positive for the virus. In addition, the analysis showed that one out of the four positive samples that we detected could not be assigned to any of the six classical subtypes. This sample was taken as a putative novel subtype of the FIV virus. The remaining three positive samples were assigned to subtype B, with differences existing among these samples.

Strain-specific viral distribution and neuropathology of feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a naturally occurring lentivirus of domestic cats, and is the causative agent of feline AIDS. Similar to human immunodeficiency virus (HIV), the pathogenesis of FIV involves infection of lymphocytes and macrophages, and results in chronic progressive immune system collapse and death. Neuropathologic correlates of FIV infection have not yet been elucidated, and may be relevant to understanding HIV-associated neurologic disease (neuroAIDS). As in HIV, FIV strains have been shown to express differential tendencies towards development of clinical neuroAIDS. To interrogate viral genetic determinants that might contribute to neuropathogenicity, cats were exposed to two well-characterized FIV strains with divergent clinical phenotypes and a chimeric strain as follows: FIV(PPR) (PPR, relatively apathogenic but associated with neurologic manifestations), FIV(C36) (C36, immunopathogenic but without associated neurologic disease), and Pcenv (a chimeric virus consisting of a PPR backbone with substituted C36 env region). A sham inoculum control group was also included. Peripheral nerve conduction velocity, CNS imaging studies, viral loads and hematologic analysis were performed over a 12 month period. At termination of the study (350 days post-inoculation), brain sections were obtained from four anatomic locations known to be involved in human and primate lentiviral neuroAIDS. Histological and immunohistochemical evaluation with seven markers of inflammation revealed that Pcenv infection resulted in mild inflammation of the CNS, microglial activation, neuronal degeneration and apoptosis, while C36 and PPR strains induced minimal neuropathologic changes. Conduction velocity aberrations were noted peripherally in all three groups at 63 weeks post-infection. Pcenv viral load in this study was intermediate to the parental strains (C36 demonstrating the highest viral load and PPR the lowest). These results collectively suggest that (i) 3' C36 genomic elements contribute to viral replication characteristics, and (ii) 5' PPR genomic elements contribute to CNS manifestations. This study illustrates the potential for FIV to provide valuable information about neuroAIDS pathogenesis related to genotype and viral kinetics, as well as to identify strains useful to evaluation of therapeutic intervention.

Prior mucosal exposure to heterologous cells alters the pathogenesis of cell-associated mucosal feline immunodeficiency virus challenge

BACKGROUND

Several lines of research suggest that exposure to cellular material can alter the susceptibility to infection by HIV-1. Because sexual contact often includes exposure to cellular material, we hypothesized that repeated mucosal exposure to heterologous cells would induce an immune response that would alter the susceptibility to mucosal infection. Using the feline immunodeficiency virus (FIV) model of HIV-1 mucosal transmission, the cervicovaginal mucosa was exposed once weekly for 12 weeks to 5,000 heterologous cells or media (control) and then cats were vaginally challenged with cell-associated or cell-free FIV.

RESULTS

Exposure to heterologous cells decreased the percentage of lymphocytes in the mucosal and systemic lymph nodes (LN) expressing L-selectin as well as the percentage of CD4+ CD25+ T cells. These shifts were associated with enhanced ex-vivo proliferative responses to heterologous cells. Following mucosal challenge with cell-associated, but not cell-free, FIV, proviral burden was reduced by 64% in cats previously exposed to heterologous cells as compared to media exposed controls.

CONCLUSIONS

The pathogenesis and/or the threshold for mucosal infection by infected cells (but not cell-free virus) can be modulated by mucosal exposure to uninfected heterologous cells.

Acute mucosal pathogenesis of feline immunodeficiency virus is independent of viral dose in vaginally infected cats

BACKGROUND

The mucosal pathogenesis of HIV has been shown to be an important feature of infection and disease progression. HIV-1 infection causes depletion of intestinal lamina propria CD4+ T cells (LPL), therefore, intestinal CD4+ T cell preservation may be a useful correlate of protection in evaluating vaccine candidates. Vaccine studies employing the cat/FIV and macaque/SIV models frequently use high doses of parenterally administered challenge virus to ensure high plasma viremia in control animals. However, it is unclear if loss of mucosal T cells would occur regardless of initial viral inoculum dose. The objective of this study was to determine the acute effect of viral dose on mucosal leukocytes and associated innate and adaptive immune responses.

RESULTS

Cats were vaginally inoculated with a high, middle or low dose of cell-associated and cell-free FIV. PBMC, serum and plasma were assessed every two weeks with tissues assessed eight weeks following infection. We found that irrespective of mucosally administered viral dose, FIV infection was induced in all cats. However, viremia was present in only half of the cats, and viral dose was unrelated to the development of viremia. Importantly, regardless of viral dose, all cats experienced significant losses of intestinal CD4+ LPL and CD8+ intraepithelial lymphocytes (IEL). Innate immune responses by CD56+CD3- NK cells correlated with aviremia and apparent occult infection but did not protect mucosal T cells. CD4+ and CD8+ T cells in viremic cats were more likely to produce cytokines in response to Gag stimulation, whereas aviremic cats T cells tended to produce cytokines in response to Env stimulation. However, while cell-mediated immune responses in aviremic cats may have helped reduce viral replication, they could not be correlated to the levels of viremia. Robust production of anti-FIV antibodies was positively correlated with the magnitude of viremia.

CONCLUSIONS

Our results indicate that mucosal immune pathogenesis could be used as a rapid indicator of vaccine success or failure when combined with a physiologically relevant low dose mucosal challenge. We also show that innate immune responses may play an important role in controlling viral replication following acute mucosal infection, which has not been previously identified.

Improved health and survival of FIV-infected cats is associated with the presence of autoantibodies to the primary receptor, CD134

We analyzed antibody responses in sera from feline immunodeficiency virus (FIV)-infected and uninfected cats. A strong antiviral response to the viral surface glycoprotein (SU) was noted in both natural and experimental infections. In addition, 143 of 226 FIV-infected animals (63%) also expressed antibodies to the primary binding receptor, CD134, whereas cats infected with other feline RNA viruses, including calicivirus, coronavirus, herpesvirus, and feline leukemia virus, did not. Both affinity-purified anti-CD134 and anti-SU antibodies blocked FIV infection ex vivo. FACS analyses revealed that the anti-CD134 antibodies bound to a cryptic epitope on the receptor that was only exposed when SU bound to CD134. Anti-CD134 binding caused displacement of SU from the surface of the cell and inhibition of infection. The presence of antibodies to CD134 correlated with lower virus loads and a better overall health status in FIV(+) cats, whereas anti-SU antibodies were present independent of health status. The findings are consistent with a role for antireceptor antibodies in protection from virus spread and disease progression.

Replication properties of clade A/C chimeric feline immunodeficiency viruses and evaluation of infection kinetics in the domestic cat

Feline immunodeficiency virus (FIV) causes progressive immunodeficiency in domestic cats, with clinical course dependent on virus strain. For example, clade A FIV-PPR is predominantly neurotropic and causes a mild disease in the periphery, whereas clade C FIV-C36 causes fulminant disease with CD4(+) T-cell depletion and neutropenia but no significant pathology in the central nervous system. In order to map pathogenic determinants, chimeric viruses were prepared between FIV-C36 and FIV-PPR, with reciprocal exchanges involving (i) the 3' halves of the viruses, including the Vif, OrfA, and Env genes; (ii) the 5' end extending from the 5' long terminal repeat (LTR) to the beginning of the capsid (CA)-coding region; and (iii) the 3' LTR and Rev2-coding regions. Ex vivo replication rates and in vivo replication and pathologies were then assessed and compared to those of the parental viruses. The results show that FIV-C36 replicates ex vivo and in vivo to levels approximately 20-fold greater than those of FIV-PPR. None of the chimeric FIVs recapitulated the replication rate of FIV-C36, although most replicated to levels similar to those of FIV-PPR. The rates of chloramphenicol acetyltransferase gene transcription driven by the FIV-C36 and FIV-PPR LTRs were identical. Furthermore, the ratios of surface glycoprotein (SU) to capsid protein (CA) in the released particles were essentially the same in the wild-type and chimeric FIVs. Tests were performed in vivo on the wild-type FIVs and chimeras carrying the 3' half of FIV-C36 or the 3' LTR and Rev2 regions of FIV-C36 on the PPR background. Both chimeras were infectious in vivo, although replication levels were lower than for the parental viruses. The chimera carrying the 3' half of FIV-C36 demonstrated an intermediate disease course with a delayed peak viral load but ultimately resulted in significant reductions in neutrophil and CD4(+) T cells, suggesting potential adaptation in vivo. Taken together, the findings suggest that the rapid-growth phenotype and pathogenicity of FIV-C36 are the result of evolutionary fine tuning throughout the viral genome, rather than being properties of any one constituent.

Preclinical evaluation of a dual-acting microbicidal prodrug WHI-07 in combination with vanadocene dithiocarbamate in the female reproductive tract of rabbit, pig, and cat

The mucosal safety of the combination antiretroviral spermicide,WHI-07 [5-bromo-6-methoxy-5,6-dihydro-3'-azidothymidine-5'-(p-bromophenyl)-methoxy alaninyl phosphate] and vanadocene dithiocarbamate (VDDTC), was evaluated in 3 different animal models. Twenty-seven NZW rabbits in four subgroups were exposed intravaginally to a gel-microemulsion (GM) with and without three dose levels of WHI-07 plus VDDTC (0.5+0.06%, 1.0+0.12% and 2.0+0.25%) or 4% nonoxynol-9 (N-9; Conceptrol) for 14 consecutive days. Ten nonestrus gilts (Duroc) in three subgroups received either a single or daily intravaginal application of GM with and without 2.0% WHI-07 plus 0.25% VDDTC or 2.0% benzalkonium chloride (BZK)-containing gel for 6 and 4 consecutive days, respectively. Five cats received a single intravaginal application of GM incorporating 2.0% WHI-07 plus 0.25% VDDTC. Genital tract histopathology was performed in the pig and rabbit at the end of dosing period but after 18 weeks post-dosing in the cat. Porcine cervicovaginal lavage (CVL) fluid was obtained for up to 72 hours after a single exposure and changes in the levels of inflammatory cytokines (IL-1beta, IL-8, IFN-gamma, and TNF-alpha) were quantitated by a multiplexed chemiluminescence-based immunoassay. Rabbit vaginal tissues were evaluated for localized cellular inflammation and in situ apoptosis by immunohistochemical staining for CD45, nuclear factor (NF)-kappa B, and terminal deoxynucleotidyl transferase-mediated FITC-deoxyuridine triphosphate nick-end labeling (TUNEL) using confocal laser scanning microscopy (CLSM), respectively. Vanadium content in selected organs and body fluids from rabbits and pigs was determined by atomic absorption spectroscopy. When compared with 4% N-9 (total irritation score 13-14 out of a possible 16), none of the rabbits given WHI-07 plus VDDTC intravaginally, developed histological alterations such as epithelial erosion, edema, leukocyte influx or vascular congestion characteristic of inflammation (total irritation score 4-6). CD45 and NF-kappa B immunoreactivity was limited to cells within the vascular lumen of both control and WHI-07 plus VDDTC-treated vaginal tissues. TUNEL assay revealed lack of increased apoptotic cells in vaginal mucosa exposed to increasing concentrations of WHI-07 plus VDDTC. Basal levels of proinflammatory cytokines (IL-1beta, IL-8, IFN-gamma and TNF-alpha) in porcine CVL were unaffected by intravaginal exposure to WHI-07 plus VDDTC when compared with BZK used as a positive control. Endpoint histology of the reproductive tract from cats and pigs after a single or repeated intravaginal exposure to WHI-07 plus VDDTC, respectively, revealed lack of irritation/inflammation in the epithelium, subepithelium/lamina propria, vessels/perivascular tissues, and underlying/surrounding muscles. Vanadium was not preferentially incorporated into rabbit or porcine tissues and body fluids at levels above 1 microg/g. Based on comparative histologic data and surrogate markers for inflammation, repeated intravaginal administration of WHI-07 plus VDDTC via a gel-microemulsion did not result in vaginal irritation, mucosal toxicity, or systemic absorption of vanadium. Therefore, the combined use of WHI-07 and VDDTC via gel-microemulsion appears safe for topical use as a prophylactic anti-HIV microbicide.

Molecular mechanisms of FIV infection

Feline immunodeficiency virus (FIV) is an important viral pathogen worldwide in the domestic cat, which is the smallest animal model for the study of natural lentivirus infection. Thus, understanding the molecular mechanisms by which FIV carries out its life cycle and causes an acquired immune deficiency syndrome (AIDS) in the cat is of high priority. FIV has an overall genome size similar to HIV, the causative agent of AIDS in man, and shares with the human virus genomic features that may serve as common targets for development of broad-based intervention strategies. Specific targets include enzymes encoded by the two lentiviruses, such as protease (PR), reverse transcriptase (RT), RNAse H, and integrase (IN). In addition, both FIV and HIV encode Vif and Rev elements essential for virus replication and also share the use of the chemokine receptor CXCR4 for entry into the host cell. The following review is a brief overview of the current state of characterization of the feline/FIV model and development of its use for generation and testing of anti-viral agents.

Mucosal challenge with cell-associated or cell-free feline immunodeficiency virus induces rapid and distinctly different patterns of phenotypic change in the mucosal and systemic immune systems

The majority of human immunodeficiency virus type 1 (HIV-1) infections occur via mucosal transmission through contact with genital secretions containing cell-associated and cell-free virus. However, few studies have assessed whether exposure to cells, HIV-1 infected or uninfected, plays a role in the sexual transmission of HIV-1. This study examined phenotypic changes in mucosal and systemic lymphoid tissue 24 hr after vaginal exposure to in vitro equilibrated infectious doses of cell-associated or cell-free feline immunodeficiency virus, uninfected heterologous cells, or medium alone. We found that even at this early time-point, mucosal exposure to virus induced substantial alterations in the phenotype and distribution of leucocytes, particularly in the tissues of the mucosal immune system. Second, we found that the type of virus inoculum directly influenced the phenotypic changes seen. Vaginal exposure to cell-free virus tended to induce more generalized phenotypic changes, typically in the peripheral immune system (blood and systemic lymph nodes). In contrast, exposure to cell-associated virus was primarily associated with phenotypic shifts in the mucosal immune system (gut and mucosal/draining lymph nodes). In addition, we found that exposure to uninfected heterologous cells also induced alterations in the mucosal immune system. These data suggest that significant immune changes occur within the first 24 hr of virus exposure, well before substantial replication would be anticipated. As the mucosal immune system, and particularly the gut, is an early and persistent target for lentiviral replication, these findings have substantial implications for HIV-1 pathogenesis and vaccine development.

Limitations of the Human-PBL-SCID Mouse Model for Vaginal Transmission of HIV-1

PROBLEM

SCID mice reconstituted with human peripheral blood lymphocytes (PBL) are amenable to vaginal transmission of HIV-1. We investigated the effectiveness of this model to establish systemic HIV-1 infection.

METHOD OF STUDY

Eighty progesterone-primed C.B-17 SCID mice were reconstituted with human-PBLs and intravaginally inoculated with CCR5 HIV-1 (BaL or 92BR09) infected human-PBLs in the presence of human semen. After two weeks, viral RNA load in spleen, peritoneal lavage (PL), and serum was quantitated by the nucleic acid sequence-based amplification method.

RESULTS

In five independent experiments, spleen from 8/60 (13.3%), PL from 7/60 (11.6%), and serum from 16/56 (28.5%) mice were positive for BaL HIV-1 infection. Similarly, spleen from 4/20 (20%), PL from 1/20 (5%) and serum from 5/20 (25%) mice vaginally inoculated with 92BR09-infected human-PBLs were positive for HIV-1. A one-sided power analysis using normal approximation revealed that at 5% significance level, the overall response rate need to increase form 0.29 to 0.9 and 80% of the control groups needs to achieve a response rate between 6/10 and 9/10 to make the assay feasible.

CONCLUSION

The incidence of vaginal transmission of CCR5 HIV-1 in the human-PBL-SCID mouse was low and variable, which constitutes a major disadvantage for preclinical evaluation of vaginal microbicides.

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.

Transient or occult HIV infections may occur more frequently than progressive infections: changing the paradigm about HIV persistence

Evidence of transient HIV infections was found in 8 subjects at high-risk for HIV infection among 47 longitudinally studied over 2-5 (average approximately 3.5) years, whereas only two subjects developed progressive infection. All of these subjects developed serum antibodies (Ab) to conformational epitopes of HIV gp41 (termed "early HIV Ab"), but the 8 transiently infected subjects lost this Ab within 4-18 months, and did not seroconvert to positivity in denatured antigen EIA or Western Blot (WB). However, the two progressively infected subjects eventually seroconverted in the EIA and WB tests within one to two months after the appearance of "early HIV Ab". HIV env and nef sequences were directly PCR amplified from the peripheral blood mononuclear cells (PBMCs) of two of the eight transiently infected subjects during the time of "early HIV Ab"-postivity, and these showed significant sequence divergence from the HIV strains in the laboratory, indicating that they were not laboratory contaminants. Genome identity typing ("paternity-typing") of PBMC samples obtained at the time of "early HIV Ab"-positivity, and later when Ab was absent from each of the 8 subjects, showed that blood samples were not mixed-up. This provides further evidence that transient or occult infection with HIV does occur, and perhaps at a greater frequency than do progressive infections.

Induction of serum and mucosal FIV-specific immune responses by intranasal immunization with p24Gag

We examined the ability of FIV p24Gag to induce systemic and mucosal FIV-specific immune responses when delivered as a nasal immunogen alone, or with a mucosal adjuvant, Escherichia coli heat labile toxin LT(R192G). Nasal immunization with p24Gag alone induced FIV-specific immune responses but overall responses were weak, transient, and/or present only in a few animals. Co-administration of LT(R192G) resulted in strong FIV-specific serum IgG and enhanced salivary IgA responses. Moreover, FIV-specific IgA was detected in vaginal wash fluid from 6/6 cats co-immunized with LT(R192G) and p24Gag versus 1/6 immunized with p24Gag alone. This is the first report detailing induction of systemic or mucosal FIV-specific immune responses by nasal immunization alone. As such, this study demonstrates that nasal immunization of cats can be a relevant and effective route for the delivery of candidate vaccines. However, while nasal immunization of cats with p24Gag induces antigen-specific systemic immune responses, development of strong systemic and mucosal immune responses requires co-administration of a mucosal adjuvant, such as LT(R192G).

Pre-existing immunity to pathogenic Listeria monocytogenes does not prevent induction of immune responses to feline immunodeficiency virus by a novel recombinant Listeria monocytogenes vaccine

Listeria monocytogenes is an attractive biologic vaccine vector against HIV because it induces a strong cell mediated immune response, can be delivered by mucosal routes, can be readily manipulated to express viral antigens, and is easy and inexpensive to produce. Proof of concept studies have been performed using HIV Gag expressing recombinant L. monocytogenes in the mouse. Here we report the development and validation of recombinant L. monocytogenes to be evaluated in the FIV/cat model of HIV. Using a simplified approach to introduce individual and polyprotein FIV gag genes, we show that recombinant L. monocytogenes containing the entire gag expresses the full-length Gag polyprotein in a soluble secreted form. A DNA vaccine plasmid (pND14-Lc-env) that replicates in Gram positive bacteria and contains the FIV SU (gp100) and the ectodomain of TM (gp40) in a eukaryotic expression cassette was transfected into LM-gag to create LM-gag/pND14-Lc-env. After infection of target cells with LM-gag/pND14-Lc-env in vitro, both FIV Gag and Env proteins were detected in soluble cell lysates. Whether previous exposure to L. monocytogenes affects the immunogenicity of LM-gag/pND14-Lc-env was determined in cats infected with wild-type L. monocytogenes orally and/or subcutaneously. After a single oral dose of LM-gag/pND14-Lc-env, cats with existing anti-L. monocytogenes immune responses developed anti-FIV Gag IgA titers in vaginal secretions, saliva, and feces. Similarly, FIV Gag and Env specific IFN-gamma ELISPOT responses were measurable in spleen and lymph node but at a statistically higher frequency in cats exposed to a single subcutaneous dose of wild-type L. monocytogenes versus cats exposed both subcutaneously and orally. The FIV/cat model will provide a useful challenge system to determine whether recombinant L. monocytogenes can protect against a lentivirus in its natural host after challenge by the routes common to HIV transmission.

Feline lentiviruses demonstrate differences in receptor repertoire and envelope structural elements

Feline immunodeficiency virus (FIV) causes fatal disease in domestic cats via T cell depletion-mediated immunodeficiency. Pumas and lions are hosts for apparently apathogenic lentiviruses (PLV, LLV) distinct from FIV. We compared receptor use among these viruses by: (1) evaluating target cell susceptibility; (2) measuring viral replication following exposure to specific and non-specific receptor antagonists; and (3) comparing Env sequence and structural motifs. Most isolates of LLV and PLV productively infected domestic feline T cells, but differed from domestic cat FIV by infecting cells independent of CXCR4, demonstrating equivalent or enhanced replication following heparin exposure, and demonstrating substantial divergence in amino acid sequence and secondary structure in Env receptor binding domains. PLV infection was, however, inhibited by CD134/OX40 antibody. Thus, although PLV and LLV infection interfere with FIV superinfection, we conclude that LLV and PLV utilize novel, more promiscuous mechanisms for cell entry than FIV, underlying divergent tropism and biological properties of these viruses.

Antiretroviral spermicide WHI-07 prevents vaginal and rectal transmission of feline immunodeficiency virus in domestic cats

WHI-07 [5-bromo-6-methoxy-5,6-dihydro-3'-azidothymidine-5'-(p-bromophenyl)-methoxy alaninyl phosphate] is a novel dual-function aryl phosphate derivative of zidovudine with potent anti-human immunodeficiency virus (HIV) and spermicidal activities. WHI-07 was active against the feline immunodeficiency virus (FIV). This study evaluated whether topical application of WHI-07 as a single agent and in combination with an organometallic vanadium complex, vanadocene dithiocarbamate (VDDTC), via a nontoxic gel microemulsion can block vaginal as well as rectal transmission of feline AIDS (FAIDS) by chronically FIV-infected feline T cells in the natural host model. Genital transmission of FIV was monitored in recipient cats by the appearance of viral antibodies to FIV Gag proteins and by virus isolation of blood leukocytes as measured by FIV reverse transcriptase activity and FIV-specific PCR. Microbicidal activity was considered effective when the treated cats did not show evidence of FIV infection for up to 18 weeks postchallenge. An aggregate analysis of 46 specific-pathogen-free cats revealed that a single dose of the infected cell inoculum efficiently transmitted FIV infection when delivered into the vagina (100%) or rectum (66%). Pretreatment of the vagina or rectum with 2% WHI-07 alone or in combination with 0.25% VDDTC significantly (P = 0.004) protected cats from genital transmission by the highly infectious inoculum (7 million FIV(Bangston)-infected feline T cells). Collectively, using the vaginal and rectal transmucosal model for FAIDS, our studies demonstrated that WHI-07 either alone or in combination with a vanadocene has clinical potential for the development of a dual-function anti-HIV microbicide for sexually active women.

Covert vertical transmission of feline immunodeficiency virus

Covert vertical transmission of feline immunodeficiency virus (FIV), the feline counterpart of human immunodeficiency virus type 1 (HIV-1), was identified in kittens born to FIV-infected cats. DNA PCR detected FIV gag and env sequences in tissues from kittens nonviable at birth, and in viable kittens monitored postnatally and necropsied at either 11 weeks or 1 year of age. Although FIV DNA was detected in initial blood samples from all 16 viable kittens, viral DNA became increasingly difficult to detect over time and infectious virus could rarely be demonstrated. Only maternal FIV antibody was detected in kitten plasma during the entire postnatal observation period, and kittens remained healthy, with normal CD4:CD8 T cell ratios at >14 months of age. Thus, mother-to-offspring FIV exposure, occurring in utero and postnatally, can result in covert infection in kittens with virus sequestered and contained in tissue sites. These findings appear directly relevant to suspected transient HIV infections and reports of HIV-specific cellular immune responses in highly exposed seronegative adults and uninfected infants born to HIV-positive women.

Immunohistochemical localization of feline immunodeficiency virus using native species antibodies

Feline immunodeficiency virus (FIV) is the feline analog of human immunodeficiency virus and a small animal model of human acquired immune deficiency syndrome (AIDS). We sought to identify early in vivo target cells in cats infected with clade B or C FIV. In tissues, however, neither mouse monoclonal nor rabbit polyclonal antibodies suitably detected FIV because of either insensitivity or lack of specificity. We therefore developed an immunohistochemical protocol using high-antibody-titer serum from cats chronically infected with FIV(Petaluma). Native species anti-FIV antibodies were labeled with biotinylated protein A before placement on tissues, and downstream signal was tyramide-amplified. This method revealed many productively infected cells in bone marrow, lymph node, thymus, mucosal-associated lymphoid tissue, and spleen, but few such cells in liver and none in kidney or brain. Concurrent labeling for virus and cell phenotype revealed that antigen-bearing populations were primarily T lymphocytes but included macrophages and dendritic cells. Our results demonstrate that FIV: 1) expands rapidly in T cells, 2) targets long-lived reservoir populations, and 3) is replicatively quiescent in brain at 3 weeks after infection. Use of native species antibodies for immunohistochemical detection of infectious antigens has application to other settings in which xenotypic (eg, mouse and rabbit) antibody sources are inadequate or unavailable.

Synergism between HIV and other viruses in the mouth

The HIV family replicate in and are shed from the mouth. Oral sexual practices potentially contribute to the overall extent of HIV transmission, particularly if high-risk practices are not restricted. Herpesviruses and papillomaviruses that appear in the oral cavity can determine oral HIV replication. The mechanisms probably include heterologous transactivation, enhanced expression of HIV receptors and co-receptors in target cells, release of cytokines and chemokines, and production of superantigens. Oral diseases peculiar to, or more common in, the HIV-infected patient further predispose to heightened oral HIV replication and trafficking. Defining the mechanisms by which oral viruses interact with HIV in the co-infected host should permit intervention measures against oral HIV transmission to be more precisely targeted.

Early pathogenesis of transmucosal feline immunodeficiency virus infection

To identify the early target cells and tissues in transmucosal feline immunodeficiency virus (FIV) infection, cats were exposed to a clade C FIV isolate via the oral-nasal or vaginal mucosa and multiple tissues were examined by virus isolation coculture (VI), DNA PCR, catalyzed tyramide signal-amplified in situ hybridization (TSA-ISH), and immunohistochemistry between days 1 and 12 postinoculation (p.i.). FIV RNA was detected in tonsil and oral or vaginal mucosa as early as 1 day p.i. by TSA-ISH and in retropharyngeal, tracheobronchial, or external iliac lymph nodes and sometimes in spleen or blood mononuclear cells by day 2, indicating that regional and distant spread of virus-infected cells occurred rapidly after mucosal exposure. By day 8, viral RNA, DNA, and culturable virus were uniformly detected in regional and distant tissues, connoting systemic infection. TSA-ISH proved more sensitive than DNA PCR in detecting early FIV-infected cells. In mucosal tissues, the earliest demonstrable FIV-bearing cells were either within or subjacent to the mucosal epithelium or were in germinal centers of regional lymph nodes. The FIV(+) cells were of either of two morphological types, large stellate or small round. Those FIV RNA(+) cells which could be colabeled for a phenotype marker, were labeled for either dendritic-cell-associated protein p55 or T-lymphocyte receptor antigen CD3. These studies indicate that FIV crosses mucous membranes within hours after exposure and rapidly traffics via dendritic and T cells to systemic lymphoid tissues, a pathway similar to that thought to occur in the initial phase of infection by the human and simian immunodeficiency viruses.

Kinetics of early FIV infection in cats exposed via the vaginal versus intravenous route

To determine the influence of route of virus exposure on early pathogenesis of feline immunodeficiency virus (FIV) infection, cats were exposed to either of two FIV isolates (FIV-B-2542 or FIV-A-PPR) by vaginal or intravenous (IV) inoculation. Exposure to either virus clade by either route of inoculation resulted in vaginal and systemic infection. Peak plasma viremia and tissue proviral burden were 1-3 log(10) greater in cats infected with FIV-B-2542 vs. FIV-A-PPR, irrespective of inoculation route. Plasma RNA levels paralleled provirus titers in FIV-B-2542-infected cats and were highest in those exposed IV. In contrast, plasma RNA titers were higher in cats infected vaginally with FIV-A-PPR than in those infected IV. Despite early differences, PBMC provirus titers were similar in all groups by 9 weeks postinfection. In cats infected IV, but not vaginally, CD4(+) lymphocyte counts declined significantly independent of the magnitude of viremia. Mitogen-induced lymphoproliferation was decreased in all infected cats regardless of CD4(+) cell counts; this decline correlated with the magnitude of peak plasma viremia in FIV-B-2542, but not FIV-A-PPR, infected cats. These results establish that the kinetics of early FIV infection differ with route of exposure as well as virus isolate and that properties extrapolated from one virus isolate may not be universal.

Feline immunodeficiency virus Gag- and Env-specific immune responses after vaginal versus intravenous infection

To better understand the correlation of mucosal and systemic immune responses with lentiviral containment, we contrasted the early mucosal and systemic immune responses induced by vaginal versus intravenous exposure of cats to feline immunodeficiency virus (FIV) isolates of differing pathogenicity and clade (i.e., FIV-B-2542 and FIV-A-PPR). We found that despite divergence in viral genotype, the mucosal and systemic immune responses induced differed more with route of exposure than virus isolate. In intravenously exposed cats, Gag-specific antibody (both IgG and IgA isotype) predominated in the serum, saliva, and vaginal wash fluid irrespective of infecting virus isolate. While Env-specific responses were more variable, they were more often detected in vaginally infected cats. Both IgG and IgA directed against Gag and Env were consistently present in vaginal wash fluids independent of route of infection or virus isolate. FIV Gag- and Env-specific cytotoxic lymphocytes (CTLs) were detected in blood and tissue lymphocytes of cats infected with either virus strain but were greatest in intravenously infected animals. Likewise, FIV-specific CTLs were detected in CD8(+) vaginal lymphocytes of animals infected by either route but were also more frequent in intravenously inoculated animals. In summary, we found qualitative differences in the immune responses following vaginal infection but no evidence (1) that mucosal immune responses were enhanced in vaginally exposed cats, (2) that local mucosal infection led to measurably greater immune responses in either compartment; or (3) that more prominent immune responses correlated with lower viral burden.

Targeted lymph node immunization can protect cats from a mucosal challenge with feline immunodeficiency virus

With the rapid spread of human immunodeficiency virus (HIV) infection worldwide it is clear that effective strategies for mucosal vaccination against lentiviruses are urgently required. The aim of the present study is to determine whether protective immune responses against a mucosal challenge by feline immunodeficiency virus (FIV) can be elicited by targeting the immunization to the medial iliac lymph nodes--the principal site of migration of cells from the genital and rectal mucosa. Cats were challenged with homologous FIV via the rectal route. Targeted lymph node immunization was found to be an effective route of immunization eliciting both humoral and proliferative responses to peptide-based and fixed cell vaccines. Vaccination with fixed virus infected cells elicited protection against a cell-free mucosal FIV challenge. In addition, some cats vaccinated with fixed uninfected cells also remained uninfected following a cell-associated FIV challenge.

Virulence differences between two field isolates of feline immunodeficiency virus (FIV-APetaluma and FIV-CPGammar) in young adult specific pathogen free cats

The goal of this study was to identify a strain of feline immunodeficiency virus (FIV) that would be more virulent for adult cats than the prototype FIV-APetaluma and, thereby, enhance the FIV infection model for HIV-1 related research. Diehl et al. reported that one clade C strain of FIV, FIV-CPGammar, was more virulent than other known FIV isolates. Mortalities from 58 to 100% were reported for kittens 12 weeks of age and less following intravenous inoculation. A more variable and somewhat less virulent disease course was observed in neonatal to 8-10-week-old kittens infected orally, intravaginally or intrarectally with this same isolate (Obert and Hoover, 2000). However, no studies have been done with FIV-CPGammar in adult cats. Therefore, the virulence of FIV-CPGammar for young adult cats was compared to that of FIV-APetalulma, the original FIV isolate. One group of five cats were inoculated intraperitoneally with 470 TCID(50) of FIV-CPGammar in the form of pooled plasma from acutely infected cats, while a second group was infected with plasma containing the 750 TCID50 of FIV-APetaluma. The cats were observed for 20 weeks for gross signs of disease, hematologic abnormalities, time of antibody appearance, and plasma and peripheral blood mononuclear cell (PBMC) associated virus levels. Viral RNA and proviral DNA were measured by a real-time PCR, sensitive to 50 copies per milliliter. The only outward sign of disease was lymphadenopathy, which occurred at a similar time and intensity in both groups of cats. Cats infected with FIV-CPGammar were more likely to be neutropenic and lymphopenic during the first 10-12 weeks of infection than cats infected with FIV-APetaluma. Both groups of cats showed similar overall declines in absolute mean CD4 cell counts and identical concomitant increases in CD8 cells. CD4/CD8 cell ratios were also similar. Antibody, as measured by an ELISA against recombinant FIV-TM antigen, appeared in all cats by 4 weeks post-infection. The most significant differences were in plasma viral RNA and PBMC proviral DNA levels. Cats infected with FIV-CPGammar had up to 100 times higher mean levels of viral RNA during the first few weeks of infection than cats infected with FIV-APetaluma. This difference was also mirrored in levels of proviral DNA in PBMC, which were significantly higher in the FIV-CPGammar infected cats. Plasma viral RNA and PBMC proviral DNA levels were virtually identical in both groups of cats at 20 weeks post-infection. However, proviral DNA in tissues such as thymus and popliteal lymph nodes was 10-fold or so higher in FIV-CPGammar infected cats at 20 weeks and histopathologic lesions were more severe. Based on these various parameters, we concluded that FIV-CPGammar was more virulent than FIV-APetaluma in young adult cats during the 20-week study period. However, we were not able to recreate the severe and rapidly progressive disease previously reported for kittens, suggesting an age-related resistance similar to that observed previously for FIV-APetaluma (George et al., 1993).

Relationship of lymphoid lesions to disease course in mucosal feline immunodeficiency virus type C infection

Feline immunodeficiency virus (FIV) infection typically has a prolonged and variable disease course in cats, which can limit its usefulness as a model for human immunodeficiency virus infection. A clade C FIV isolate (FIV-C) has been associated with high viral burdens and rapidly progressive disease in cats. FIV-C was transmissible via oral-nasal, vaginal, or rectal mucosal exposure, and infection resulted in one of three disease courses: rapid, conventional/slow, or regressive. The severity of the pathologic changes paralleled the disease course. Thymic depletion was an early lesion and was correlated with detection of FIV RNA in thymocytes by in situ hybridization. The major changes in thymic cell populations were depletion of p55+/S100+ dendritic cells, CD3- cells, CD4+/CD8- cells, and CD4+/CD8+ cells and increases in apoptosis, CD45R+ B cells, and lymphoid follicles. In contrast to thymic depletion, peripheral lymphoid tissues often were hyperplastic. Mucosally transmitted FIV-C is thymotropic and induces a spectrum of lymphoid lesions and disease mirroring that seen with the human and simian immunodeficiency virus infections.