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

Sequence variation of the feline immunodeficiency virus genome and its clinical relevance

The ongoing evolution of feline immunodeficiency virus (FIV) has resulted in the existence of a diverse continuum of viruses. FIV isolates differ with regards to their mutation and replication rates, plasma viral loads, cell tropism and the ability to induce apoptosis. Clinical disease in FIV-infected cats is also inconsistent. Genomic sequence variation of FIV is likely to be responsible for some of the variation in viral behaviour. The specific genetic sequences that influence these key viral properties remain to be determined. With knowledge of the specific key determinants of pathogenicity, there is the potential for veterinarians in the future to apply this information for prognostic purposes. Genomic sequence variation of FIV also presents an obstacle to effective vaccine development. Most challenge studies demonstrate acceptable efficacy of a dual-subtype FIV vaccine (Fel-O-Vax FIV) against FIV infection under experimental settings; however, vaccine efficacy in the field still remains to be proven. It is important that we discover the key determinants of immunity induced by this vaccine; such data would compliment vaccine field efficacy studies and provide the basis to make informed recommendations on its use.

Role of Env in resistance of feline immunodeficiency virus (FIV)-infected cats to superinfection by a second FIV strain as determined by using a chimeric virus

A more or less pronounced resistance to superinfection by a second strain of the infecting virus has been observed in many lentivirus-infected hosts. We used a chimeric feline immunodeficiency virus (FIV), designated FIVchi, containing a large part of the env gene of a clade B virus (strain M2) and all the rest of the genome of a clade A virus (a p34TF10 molecular clone of the Petaluma strain modified to grow in lymphoid cells), to gain insights into such resistance. FIVchi was infectious and moderately pathogenic for cats and in vitro exhibited the neutralization specificity of the env donor. The experiments performed were bidirectional, in that cats preinfected with either parental virus were challenged with FIVchi and vice versa. The preinfected animals were partially or completely protected relative to what was observed in naïve control animals, most likely due, at least in part, to the circumstance that in all the preinfecting/challenge virus combinations examined, the first and the second virus shared significant viral components. Based on the proportions of complete protection observed, the role of a strongly matched viral envelope appeared to be modest and possibly dependent on the time interval between the first and the second infection. Furthermore, complete protection and the presence of measurable neutralizing antibodies capable of blocking the second virus in vitro were not associated.

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.

New challenges for the diagnosis of feline immunodeficiency virus infection

Vaccination of cats against feline immunodeficiency virus (FIV) with a whole-virus vaccine results in rapid and persistent production of antibodies that are indistinguishable from those used for diagnosis of FIV infection. There are no diagnostic tests available for veterinary practitioners at the present time to resolve the diagnostic dilemma posed by use of whole-virus vaccines for protection of cats against FIV. There is a great need for development of commercially available rapid diagnostic tests that conform to differentiation of infected from vaccinated animals standards.

FIV infection induces unique changes in phenotype and cellularity in the medial iliac lymph node and intestinal IEL

Studies of human immunodeficiency virus-1 (HIV-1)-infected patients and simian immunodeficiency virus (SIV)-infected macaques have identified profound depletion of CD4(+) T cells and expansion of CD8(+) T cells in the gastrointestinal lamina propria. Less attention has been given to CD8(+) intraepithelial lymphocytes (IEL), and no studies have concurrently examined inductive sites such as draining lymph nodes. Our preliminary data in the feline immunodeficiency virus (FIV) animal model suggested additional changes in IEL, and marked differences in the responses of lymph nodes draining different mucosal sites. To address this, we quantified the absolute leukocyte yield and examined the phenotype of cells from small intestinal IEL, mesenteric lymph node (MLN), and medial iliac lymph node (ILN) from chronically FIV-infected cats. The cellularity of the ILN was increased 530% in FIV-infected animals with an expansion of CD62L(+) cells, suggesting an increased population of naive T cells. The number of CD4(+), as well as CD8(+), T cells was increased in the ILN, resulting in a CD4:CD8 ratio greater than 1:1. In contrast, reduced cellularity, specific loss of CD4(+) T cells, and inversion of the CD4:CD8 ratio was observed in the MLN, which drains the intestine. In IEL, loss of CD8alpha, CD8beta, and CD4-expressing T cells was found in FIV-infected cats. Furthermore, expression intensity of CD8alpha and CD5, markers known to be important in T cell function, was markedly decreased on IEL. These findings expand the array of immune alterations induced by lentiviral infection and indicate that characterization of multiple mucosal sites will be necessary to fully understand the pathogenesis of HIV-1 infection.

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.

Feline immunodeficiency virus neuropathogenesis: from cats to calcium

Invasion of human immunodeficiency virus (HIV) into the central and peripheral nervous system produces a wide range of neurological symptoms, which continue to persist even with adequate therapeutic suppression of the systemic viremia. The development of therapies designed to prevent the neurological complications of HIV require a detailed understanding of the mechanisms of virus penetration into the nervous system, infection, and subsequent neuropathogenesis. These processes, however, are difficult to study in humans. The identification of animal lentiviruses similar to HIV has provided useful models of HIV infection that have greatly facilitated these efforts. This review summarizes contributions made from in vitro and in vivo studies on the infectious and pathological interactions of feline immunodeficiency virus (FIV) with the nervous system. In vivo studies on FIV have provided insights into the natural progression of CNS disease as well as the contribution of various risk factors. In vitro studies have contributed to our understanding of immune cell trafficking, CNS infection and neuropathogenesis. Together, these studies have made unique contributions to our understanding of (1) lentiviral interactions at the blood-cerebrospinal fluid (CSF) barrier within the choroid plexus, (2) early FIV invasion and pathogenesis in the brain, and (3) lentiviral effects on intracellular calcium deregulation and neuronal dysfunction. The ability to combine in vitro and in vivo studies on FIV offers enormous potential to explore neuropathogenic mechanisms and generate information necessary for the development of effective therapeutic interventions.

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).

IgG from acutely infected cats blocks mucosal feline immunodeficiency virus infection

We have previously shown an absence of detectable systemic or local infection in cats exposed to an infectious (100 TCID(50)) feline immunodeficiency virus (FIV) plasma inoculum via either the rectal or vaginal mucosa. In contrast, this same plasma inoculum was infectious via parenteral inoculation. Moreover an equivalent dose of cell-free tissue culture-origin virus inoculum infected 100% of cats by either the rectal or vaginal exposure route. To evaluate this phenomena, we used a tissue culture system to identify a heat-stable factor in the plasma of cats acutely (3 weeks) infected with FIV that blocked infection of naive peripheral blood mononuclear cells (PBMC) by either cell-free or cell-associated FIV in vitro. A single application of as little as a 1:200 dilution of either heparinized or Alsevier's anticoagulated plasma effectively inhibited production of FIV p26 in culture over a 21-day co-culture period. Depletion of antibody using a protein A column abrogated the inhibitory effect of FIV plasma against in vitro FIV infection. Co-inoculation of heat-inactivated plasma with 400 TCID(50) FIV-B-2542 cell-free supernatant virus onto the vaginal mucosa of two cats resulted in complete inhibition of infection in one cat and increased time to infection in the second. Thus, antibody found in the plasma of cats acutely infected with FIV blocks cell-associated and cell-free infection, inhibits virus production in previously infected cells, and reduces mucosal transmission efficiency in vivo. Extrapolation may help explain the relatively inefficient mucosal transmission of human immunodeficiency virus-1 (HIV) and other lentiviruses.

Assessment of CD4+ and CD8+ IFN-gamma producing cells by ELISPOT in naïve and FIV-infected cats

IFN-gamma is critical for the development of antiviral cell-mediated immunity in HIV infected humans and FIV infected cats. The ELISPOT has proven to be a technically straightforward assay to quantify the number of IFN-gamma producing cells and offers a reasonable alternative for the quantitative measurement of T-cell function in cats. We used a feline-specific ELISPOT to identify constitutive as well as Con A stimulated IFN-gamma production in T-cell subsets and determine if there were differences between purified (positively sorted) and negatively depleted populations from naïve and FIV infected cats. We found no difference in the total number of PBMC constitutively producing IFN-gamma in naïve and FIV+ cats. Con A exposure was associated with increased numbers of IFN-gamma producing PBMC in naïve, but not FIV+, cats. Equivalent numbers of CD4+ and CD8+ T cells constitutively expressed IFN-gamma in naïve cats. However, in FIV+ cats, the number of IFN-gamma producing CD8+ T-cells was approximately two-fold over that seen for CD4+ T-cells. We found minimal differences between purified (e.g. CD4+ or CD8+) and corresponding depleted (e.g. CD8- or CD4-) populations in samples from FIV+ cats. In contrast, depleted populations from naïve cats showed greater response to Con A than did purified populations. Thus, while determination of the number of IFN-gamma producing cells by feline-specific ELISPOT is a useful tool for the evaluation of the feline immune response, determination of the initial sample population and T-cell subset is critical for optimal interpretation of the IFN-gamma ELISPOT.

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: protection from an intraclade challenge administered systemically or mucosally by an attenuated vaccine

Feline immunodeficiency virus (FIV) infection of domestic cats represents a valuable system through which to investigate criteria for antilentiviral vaccines in a natural host species. Here, we examined whether vaccination with a strain of FIV attenuated as a result of prolonged growth in vitro could protect against a fully virulent, highly heterologous intraclade challenge. The results indicated that the vaccine virus produced a low-grade infection with no detectable pathological effects and afforded a long-lasting sterilizing immunity if the challenge was delivered intraperitoneally as cell-free virus but not against a cell-associated intravaginal challenge. In the latter case, however, the replication and pathological consequences of the challenge virus were markedly suppressed. Together with similar results obtained in rhesus monkey models, these findings should give impulse to the development of attenuated FIV vaccines to be tested in controlled studies in field cats. Field studies may provide answers to some of the existing safety concerns surrounding attenuated AIDS vaccines in humans.

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.

FIV vaccine development and its importance to veterinary and human medicine: a review FIV vaccine 2002 update and review

Feline immunodeficiency virus (FIV) is a natural infection of domestic cats that results in acquired immunodeficiency syndrome resembling human immunodeficiency virus (HIV) infection in humans. The worldwide prevalence of FIV infection in domestic cats has been reported to range from 1 to 28%. Hence, an effective FIV vaccine will have an important impact on veterinary medicine in addition to being used as a small animal AIDS model for humans. Since the discovery of FIV reported in 1987, FIV vaccine research has pursued both molecular and conventional vaccine approaches toward the development of a commercial product. Published FIV vaccine trial results from 1998 to the present have been compiled to update the veterinary clinical and research communities on the immunologic and experimental efficacy status of these vaccines. A brief report is included on the outcome of the 10 years of collaborative work between industry and academia which led to recent USDA approval of the first animal lentivirus vaccine, the dual-subtype FIV vaccine. The immunogenicity and efficacy of the experimental prototype, dual-subtype FIV vaccine and the efficacy of the currently approved commercial, dual-subtype FIV vaccine (Fel-O-Vax FIV) are discussed. Potential cross-reactivity complications between commercial FIV diagnostic tests, Idexx Snap Combo Test and Western blot assays, and sera from previously vaccinated cats are also discussed. Finally, recommendations are made for unbiased critical testing of new FIV vaccines, the currently USDA approved vaccine, and future vaccines in development.