Featured Review Cellscience Reviews Vol.2 No.3 ISSN 1742-8130

Feline immunodeficiency virus vaccines: Evolving concepts in vaccine approaches

Janet K. Yamamoto , Eiji Sato, Ruiyu Pu, James Coleman, and Marcus Martin

Introduction

Feline immunodeficiency virus (FIV) causes a natural infection of domestic cats that results in acquired immunodeficiency syndrome (AIDS) resembling human immunodeficiency virus (HIV) infection in humans [1-4]. The worldwide prevalence of FIV infection in domestic cats is reported to be 1-26% in the high-risk populations and 0.7-16% in the healthy (minimal-to-no risk) population [5-11]. Hence, an effective FIV vaccine has an important impact on veterinary medicine in addition to being a small animal AIDS model for humans. Since the discovery of FIV reported in 1987 [1], FIV vaccine research was pursued by testing both molecular and conventional vaccine designs [12,13]. Many problems faced in the development of an effective FIV vaccine are similar to those confronted by HIV vaccine researchers. One of which is the vaccine efficacy against diverse populations of FIV isolates found throughout the world. Based on genetic sequence similarities, these populations are classified into groups called subtypes or clades. Although various vaccine approaches have succeeded in protection against homologous virus (i.e. vaccine virus) [12,13], the vaccine protection against strains from different subtypes was difficult to achieve.

The recent approval of the dual-subtype FIV vaccine by U.S. Department of Agriculture (USDA) and its subsequent commercial release in 2002 were the outcome of extensive collaboration among FIV researchers worldwide and between selected veterinary vaccine manufacturers and academia [13]. The availability of the domestic cats, the natural host for FIV that can be used in vaccine research, was clearly one of the two most important factors in the early development of a commercial FIV vaccine in comparison to HIV vaccines. In light of the recent failure of the HIV-1 envelope (Env) vaccines [14,15], the prospect of an HIV vaccine approved by Federal Drug Administration (FDA) to be released for prophylactic use in humans is not imminent. The findings from FIV vaccine research are reviewed to provide new insights into the development of effective HIV-1 and HIV-2 vaccines.

FIV infection: a natural infection of domestic cats and a model for HIV vaccine

FIV genomic sequence is distantly related to HIV-1, HIV-2, and simian immunodeficiency virus (SIV) sequences (Fig. 1), but its genomic organization and the presence of a few regulatory genes are similar to these AIDS lentiviruses [16,17]. Similar to HIV infection in humans, FIV immunopathogenesis in domestic cats consists of CD4⁺ T-cell loss which leads to immunosuppression and secondary infections [2-4]. FIV infects many types of cells similar to those infected by HIV-1, such as CD4⁺ T cells, macrophages/monocytes, and glial cells [4,18-20]. In addition, FIV infects B cells and CD8⁺ T cells [4,18]. FIV uses CD134 as a primary receptor and CXCR4 as a co-receptor on T cells [21,22]. However, FIV can infect CD134-negative cells using CXCR4 and possibly CCR5 [23,24]. These co-receptors for FIV are similar to those reported for HIV-1 [25]. In fact, FIV can use human CXCR4 and CCR5 to infect human cells [24]. The diverse cell tropisms of FIV have made the development of FIV vaccine as difficult as the development of HIV vaccines.

Figure 1. Comparative gag phylogenetic tree of FIV, HIV-1, HIV-2, and SIV. An unrooted phylogenetic tree was created using the BLOSUM matrix and neighbor-joining algorithm based on the Kimura two-parameter correction [72,73]. Support at each internal node was assessed using 1000 bootstrap samplings and each tree was visualized using Tree View [74,75]. Branch lengths, drawn to scale, are based on number of synonymous substitutions per site. The gag tree is based on sequences from NCBI data bank (FIV accession numbers NC_001482, AY684181, M36968, D37820, DQ365596, AY13911, AY139112, AY139110, D37819, D37821, D37823, D37824, AF474246, AY679785, D37822, AB027302, AB027304, AB027303; SIV accession numbers AF301156, L06042, AF468659, AF075269, AF131870, M27470, AF328295, 349680, M30931, L40990, M66437, U58991, AF334679, AF077017, M80194, U72748, U79412, M32741, M33262, M19499, M83293, AJ271369, AF103818, U42720; HIV-1 accession numbers L39106, M62320, M38429, M93258, M17449, L31963, AF004394, U43096, U23487, AY679786, AF321523, AB023804, M22639, K03454, U88826, AF005496, AJ006022, L20587, AJ302647, L20571; HIV-2 accession numbers M31113, AF082339, M30502, D00835, X52223, J03654, M30895, Z48731, X05291, J04498, U22047, X62240, U27200, L07625, U75441, AF208027). HIV-1 distribution is shown in major groups (Group O for outlier group, Group M for main group; Group N for non-M group); while HIV-2 distribution is shown in subtypes (subtypes C, D and E missing). SIVcol, SIVSYK, SIVGSN, SIVHOEST, SIVSUN, SIVMND, SIVRCM, SIVSM, and SIVAGM are SIV species isolated from Colobus, Sykes, greater-spot-nosed, L’Hoest, sun-tailed, mandril, red-capped mangabeys, sooty mangabeys, and African green monkeys, respectively. SIVmac belongs to SIV SM group and are isolates from captive macaques [76]. The gag phylogenetic tree shows close relationship between SIVSM/SIVmac and HIV-2 and between SIVcpz and HIV-1, but distant relationship between FIV and primate AIDS lentiviruses.

Diverse populations of FIV are found in domestic cats throughout the world. Much like HIV-1 classification of at least seven subtypes, the worldwide populations of FIV are classified into five subtypes, with subtype B being the predominant one in the USA and in the world [26-28]. Pathogenic FIV strains are now available to provide more rapid assessment of prophylactic and therapeutic approaches using the FIV-cat model [25-27]. Like pathogenic strains of HIV-1 and SIV, high virus load, early CD4⁺ T -cell loss, and clinical disease are the common features of pathogenic FIV strains (Fig. 2) [29-31]. Mutation rates in individual cats appear to be much slower than those in humans [32]. Nevertheless, superinfections and recombinations have been reported for FIV much like those reported for HIV-1 [26,33-36]. Hence, the development of FIV vaccines has similar challenges as those of HIV vaccines.

Figure 2. Pathogenic FIV strains for mucosal challenge studies. FIVBang and FIVFC1 were successfully used as vaginal inocula [30] (panels A-D). FIVFC1 is more pathogenic than FIVBang by causing symptoms (e.g., uveitis, upper respiratory disease) and earlier or severer CD4⁺ T-cell loss. FIVFC1-infected cats develop lymphadenopathy starting 20 weeks post-inoculation (wpi) and are susceptible to infection even at low-dose IV challenge of 15 CID50. The pathogenicity of vaginal FIVFC1 inoculation is shown (panels A-D). Two SPF cats each received vaginal inoculation with 1x10⁶ of either FIVFC1-infected cat PBMCs (#QVI, #QVK) or FIVBang-infected cat T cells (#405, #439). All inoculated cats became infected by 3-6 wpi and developed CD4⁺ T-cell loss by 9-12 wpi when compared to age-matched control cats (#IV4, #427, panel D). An FIV FC1–infected cat with major CD4 loss (panel D) developed uveitis at 10 wpi (panels A-C).

Successes and failures of single-strain FIV vaccines

Many vaccine approaches similar to those being tested for HIV vaccines were tested in FIV-cat system as a vaccine for veterinary medicine and as a model for HIV vaccines. Single-subtype or single-strain FIV vaccines in the form of deletion-mutant DNA vaccines, inactivated whole-virus (IWV) vaccines, and inactivated whole-infected-cell (IWC) vaccines conferred sterilizing protection against homologous virus challenge [12,13,37]. The vaccine strain, dose, route, immunization protocol, adjuvant, and protection rate of major FIV vaccine trials are extensively described in our previous reviews [12,13,37]. The results from deletion-mutant FIV DNA vaccines are similar to the findings of a nef-deletion SIV vaccine in the macaque system, which is considered to be the most effective vaccine in the SIV-rhesus macaque system against homologous subtype strains [38,39]. However, in comparison to the replication-competent attenuated viruses, DNA vaccines do not express viral proteins in the vaccinated host at high levels and over prolonged period of time. Hence, protection with these FIV DNA vaccines may be less effective than the one conferred by attenuated vaccines. Additional studies with more strenuous challenge systems are still required for FIV DNA vaccines. In contrast, attenuated vaccines have safety concerns of potential reversion as well as recombination with live virus [13,40,41]. Consequently, the use of attenuated vaccines for FIV and HIV has practical concerns in comparison to inactivated vaccines and subunit vector vaccines. Subunit vector vaccines consist of targeted sections of FIV gene carried by vector to be expressed in the immunized host. Such vaccines are so far unsuccessful at eliciting strong immunity against homologous challenge [12,13,37].

Inactivated FIV vaccines consisting of IWC and IWV conferred sterilizing protection against homologous and closely related FIV strains from the same subtype [12,37]. Initial studies showed protection with IWV SIV vaccines, but this protection was due to the immunity against human cellular antigens that were present in vaccine and challenge virus since both were grown in human cells [42,43]. IWC and IWV FIV vaccine protection was not due to cellular antigens in the vaccine preparations because cats immunized with uninfected cells used for vaccine production were not protected against the challenge inoculum grown in vaccine cell line or in primary feline peripheral blood mononuclear cells (PBMCs) from outbred cats (Table 1) [44-47].

Table 1. Single-subtype FIVPet vaccine protection against homologous and closely related strain but not against heterologous subtype strain^a Notes: ^aSpecific-pathogen-free (SPF) cats were immunized with either inactivated whole-infected-cell vaccine based on FIV-Petaluma (IWC-Pet), inactivated whole-virus vaccine based on FIV-Petaluma (IWV-Pet), uninfected vaccine cells (Uninfected FeT-1), or placebo (phosphate buffered saline, PBS). These vaccine immunogens were mixed in Syntext Adjuvant Formulation supplemented with threonyl-muramyl dipeptide (tMDP-SAF) or adenyl-muramyl dipeptide (aMDP-SAF). Unless specified otherwise, all immunizations were performed at two-three-week intervals and challenged two-three weeks after the last boost. In one study, cats were immunized three-times at two-three-week intervals followed by one boost at 41 weeks after the third immunization (3X+1X) [46]. FIV-Petaluma (Pet), FIV-Dixon (Dix), and FIV-Shizuoka (Shi) are nonpathogenic strains based on their inability to induce CD4⁺ T-cell depletion or immunosuppressive disease in the first two-three years post inoculation with high dose (>100 median cat infectious dose, CID50). FIV challenge inocula were grown in vitro in either IL-2-dependent feline T-cell line 1 (FeT-1) or peripheral blood mononuclear cells (PBMCs). Challenge dose is based on CID50, which was determined by titration of each diluted inoculum in two-three cats per group using either intraperitoneal (IP) or intranasal (nasal) inoculation. ^bReference(s) for each of the studies are shown next to the first group in the last column (Ref #).

IWV vaccine approach based on a single strain was not successful at conferring protection in SIV-macaque systems [42,43]. Single-strain IWV vaccines of SIVmac and SIVAGM conferred no protection in rhesus macaques and African green monkeys (AGMs) against homologous SIV challenge grown in PBMCs from rhesus macaques or AGMs, respectively [43]. However, similar experiments with HIV-2-cynomolgus system were successful [48]. IWV vaccines based on pathogenic FIV strains were not successful even against homologous challenges [45,49]. The discrepancies in vaccine efficacy between these studies and successful trials were initially attributed to the differences in vaccine antigen dose and adjuvant [12,13]. However, recent FIV vaccine studies suggest that the virus strain used may have an effect [13,37]. The IWV vaccine based on pathogenic FIV-Glasgow 8 (subtype A FIVGL8) was less effective against FIVGL8 challenge than IWV vaccine based on non-pathogenic FIV-Petaluma (subtype A FIVPet) [13,50]. A recent study suggests that FIVGL8 wildtype is unable to retain the surface (SU) Env glycoprotein on the virus [51], whereas higher amounts of SU Env remain on FIVPet [13]. If important vaccine epitopes reside in SU Env or requires SU Env interaction with other viral components (e.g., transmembrane Env) to be exposed [52,53], then the difference in the strains may have contributed to the different vaccine efficacy between IWV-FIVPet and IWV-FIVGL8 vaccines [51]. In general, IWV vaccines derived from pathogenic strains have been less successful at vaccine protection than those derived from non-pathogenic strains [13,37]. Both IWC and IWV vaccines derived from pathogenic FIV-Bangston (FIVBang) had little-to-no prophylactic efficacy against homologous challenge (Table 1) [13,37]. However, the potential exists that pathogenic strains are more difficult to be protected by vaccine immunity.

In general, IWV and IWC FIV vaccines were effective against homologous and closely related FIV strains from the same subtype (Table 3) [12,13,37,46]. Moreover, IWC vaccine based on subtype B FIV-MB conferred protection against contact challenge with cats naturally infected with other subtype B viruses [54]. However, single-strain IWV FIV vaccine based on subtype A FIVPet had minimal-to-no efficacy against homologous challenge with FIVPet inoculum derived directly from infected cats (i.e., in vivo-derived inoculum) and against subtype D FIV-Shizuoka (FIVshi) and subtype A/B recombinant FIVBang (Tables 1-3) [49]. Hence, a revised vaccine approach was required to broaden vaccine immunity against pathogenic homologous and heterologous subtype viruses.

Table 2. Triple-subtype and dual-subtype FIV vaccine protection against heterologous pathogenic strain challenges [45] ^a Notes: ^a SPF cats were immunized with either triple-subtype vaccines, dual-subtype vaccines, single-strain vaccines, uninfected vaccine cells (FeT-J), or placebo (PBS). All immunizations were performed at two-three-week intervals and challenged three weeks after the last boost. FIV-Bangston (Bang) and FIV-Glasgow 8 (GL8) are pathogenic strains, which cause CD4⁺ T-cell loss within two years post inoculation at moderate (50 CID50) to high (>100 CID50) doses. FIV-Bang is a recombinant virus with env (V4-V9) of subtype B with remaining regions consisting of subtype A. FIV challenge inocula were either grown in primary peripheral blood mononuclear cells (In vitro PBMC) or derived directly from plasma of experimentally infected cats (In vivo Plasma). Challenge dose is based on CID50, which was determined by titration of individual inoculum in cats using intravenous inoculation. ^b Abbreviations: dual-subtype vaccine consisting of IWC based on FIV-Petaluma and IWC based on FIV-Shizuoka (IWC-Pet+IWC-Shi); triple-subtype vaccine consisting of IWC-Pet, IWC based on FIV-Bangston, and IWC-Shi (IWC-Pet+IWC-Bang+IWC-Shi); triple-subtype vaccine consisting of IWV based on FIV-Petaluma, IWV based on FIV-Bangston, and IWV based on FIV-Shizuoka (IWV-Pet+IWV-Bang+IWV-Shi); dual-subtype vaccine consisting of IWV-Pet and IWV-Shi; IL-2-independent feline T-cell line (FeT-J) derived from FeT-1 and used as vaccine cell line for IWC-Shi and IWC-Bang; Syntex MF59 adjuvant (MF59).

Designing of dual-subtype FIV vaccines

The revised approach was undertaken starting 1991 by combining single-strain vaccines from different subtypes into dual-subtype and triple-subtype FIV vaccines (Table 2) [45]. The multi-subtype vaccine approach was based on a concept that vaccine immunogens from different subtypes can overlap at conserved protective epitopes as well as contain subtype-specific new protective epitopes, which may broaden immunity to vaccine epitopes. Such an approach was successfully applied to multi-serotype avian vaccine against Marek’s disease [55]. Dual-subtype FIV vaccine, instead of triple-subtype FIV vaccine, conferred the strongest immunity against both non-pathogenic and pathogenic isolates from homologous-subtype strains and subtype A/B recombinant (Tables 2,3) [13,45,49]. Furthermore, such protection was achieved against intravenous (IV) challenges with in vivo-derived inocula. Simple addition of different subtype strains to dual-subtype FIV vaccine did not increase the vaccine efficacy (Table 2). The best efficacy was achieved with dual-subtype FIV vaccine consisting of FIV strains (subtype A FIVPet and subtype D FIVShi) isolated from long-term survivor (LTS) cats of FIV infection (Tables 2,3). Addition of subtype A/B FIVBang IWV to the dual-subtype (A+D) IWV vaccine did not enhance protection against FIVBang but instead decreased vaccine efficacy (Table 2). Similar results were observed with addition of FIVFC1 p24 (core protein of Gag) to dual-subtype (A+D) vaccine (Table 3). The vaccines containing pathogenic strains (FIVBang and FIVFC1) were less effective than those containing only non-pathogenic strains (FIVPet and FIVShi) (Tables 2,3). These preliminary results are similar to the finding with the single-strain vaccine trials showing more efficacy with vaccines derived from non-pathogenic strains than from pathogenic strains [13,37]. Such observation is supported by the findings of broad antiviral immunity present in long-term survivors of less pathogenic HIV-1 infection [56]. If selection of the vaccine viruses is essential to multi-subtype inactivated vaccine approach, then such observation may be important for the HIV vaccine development. Additional FIV studies with pathogenic and nonpathogenic strain-based IWV vaccines are in progress to confirm this important finding.

Table 3. Commercial and prototype dual-subtype FIV vaccine protection against intravenous challenge with in vivo-derived pathogenic strains ^a Notes: ^a SPF cats were immunized with either commercial dual-subtype vaccine (Fel-O-Vax FIV), prototype dual-subtype vaccine (IWV-Pet + IWV-Shi), single-strain vaccines (IWV-Pet or IWV-Shi), commercial vaccine supplemented with FIV-FC1 p24 (Fel-O-Vax FIV + FC1-p24), uninfected vaccine cells (FeT-J), or placebo (PBS). Fel-O-Vax FIV vaccine consisted of 1.25 x10⁷ -2.5x10⁷ IWC plus <50µg of IWV per dose and are shown as (2x10⁷ + <50). The vaccine immunogens were formulated in Fort Dodge-1 (FD-1) adjuvant, which is identical to the adjuvant used in commercial Fel-O-Vax FIV vaccine [31]. Due to unavailability of the FD-1 adjuvant, uninfected vaccine cells (FeT-J) were formulated with Ribi adjuvant [31]. All immunizations were performed at three-week intervals and challenged three weeks after the last boost. FIV-Bangston (Bang) and FIV-FC1 (FC1) are pathogenic strains and FIV-Petaluma (Pet) and FIV-Shizuoka (Shi) are nonpathogenic strains. In vitro-derived PBMC inoculum was obtained by culturing primary PBMC with virus for not more than 18 passages (In vitro PBMC). In vivo-derived FIV challenge inocula were either pooled plasma (In vivo Plasma) or pooled PBMC (In vivo PBMC) obtained directly from experimentally infected cats. Challenge dose was based on CID50, which was determined by titration of each inoculum in cats using intravenous administration. ^b Unless specified otherwise, the reference for each of the studies is shown next to the first group in the right column (Ref #). Although Fel-O-Vax FIV + FC1-p24 vaccinated group was performed with remaining three groups and described in reference 59, only Fel-O-Vax FIV-vaccinated group, FeT-J-immunized group, and PBS-immunized group were described in reference 31 (*).

The dual-subtype IWV FIV vaccine consisting of inactivated FIVPet and FIVShi is the prototype of the USDA-approved dual-subtype FIV vaccine (Fel-O-Vax FIV vaccine) released for veterinary use [13]. The commercial vaccine consists of IWC and IWV immunogens and differs from prototype IWV vaccine by inducing higher levels of virus-neutralizing (VN) antibodies to homologous strains and closely related strains than the prototype vaccine [57]. Based on immunoblot analysis, cats immunized with commercial vaccine also developed consistently higher levels of antibodies to SU Env than cats immunized with prototype vaccine [13,57]. This was expected since previous studies with IWC and IWV FIVPet vaccines showed higher levels of VN antibodies in sera from IWC-vaccinated cats than sera from IWV-vaccinated cats [13]. Hence, humoral immunity consisting of VN antibodies may be involved in IWC and IWV vaccine protections against homologous challenge and closely related strains. However, both commercial and prototype dual-subtype FIV vaccines induced little-to-no VN antibodies to distinctly heterologous strains from different subtypes [13,31,49,57]. Protection conferred by prototype dual-subtype FIV vaccine against pathogenic subtype A/B recombinant (FIVBang) occurred in the presence of little-to-no VN antibody titers to FIVBang [49]. The commercial vaccine protected cats against IV challenge with pathogenic subtype B FIV-FC1 (FIVFC1) and against contact challenge with cats infected with subtype B FIV-Aomori2 (FIVAo2). Majority of the protected cats had no VN antibody titers to the challenge viruses [31,58,59]. Hence, protection against heterologous subtype challenges occurred in the absence or in the minimal presence of VN antibodies, suggesting that such vaccine protection was most likely mediated by antiviral cellular immunity. Prototype dual-subtype FIV vaccine was reported to induce strong virus-specific cellular immunity [60,61]. Recent preliminary studies suggest that commercial FIV vaccine also induces strong cellular immunity as determined by interferon-γ (IFNγ) ELISpot analysis [59-61].

Protective vaccine immunity and vaccine antigens

Both commercial and prototype FIV vaccines induced high levels of VN antibodies to homologous strains and closely related strains [57]. Pooled sera from single-strain IWV FIVPet-vaccinated cats protected unvaccinated naïve cats against homologous FIVPet challenge [62]. A preliminary study recently demonstrated passive protection with sera from commercial dual-subtype vaccinated cats [63]. In addition, both single-subtype (FIVPet) and dual-subtype (FIVPet + FIVShi) vaccines induced strong virus-specific cellular immunity, consisting of T-cell immunity [59-61]. Adoptive transfer of whole blood cells from IWV FIVPet -vaccinated cats conferred protection against homologous FIVPet challenge in major histocompatibility complex (MHC)-matched recipients [64]. In contrast, adoptive transfer of cells from unvaccinated cats did not protect either MHC-matched or MHC-unmatched recipients [64]. Recent studies demonstrate that the protection against homologous virus challenge was conferred by adoptive transfer of B-cell depleted, T-cell enriched population from MHC-matched, vaccinated donors [60]. Vaccine-induced antiviral activity of the T cells appears to be responsible for the adoptive-transfer protection for the following reasons: 1) Protection was observed between MHC-matched donor-recipient pairs, suggesting the involvement of MHC-restricted T-cell function(s) [60,63]. 2) The cell population that provided adoptive-transfer protection was not B cells and was enriched for T cells. 3) Studies have shown that PBMCs from dual-subtype IWV-vaccinated cats had IFNγ, IL-2, and perforin production responses upon in vitro stimulation with FIV antigens, suggesting the induction of T cells, specifically T-helper 1 (TH1) cells and potentially cytotoxic T lymphocytes (CTL) [61]. Hence, dual-subtype vaccine protection against homologous FIVPet challenge most likely involves both VN antibody immunity and antiviral T-cell immunity. However, vaccine protection against heterologous subtype viruses (FIVFC1, FIVAo2) is most likely the result of antiviral T-cell immunity without VN antibody immunity.

The vaccine approaches that conferred the best vaccine protection for SIV-macaque model and FIV-cat model were nef-deletion attenuated SIV vaccine and IWV/IWC FIV vaccines, respectively [13,38]. The vaccine composition of these vaccines is a whole virus. Consequently, the virus component(s) responsible for these vaccine protections may reside in any of the viral proteins. A recent study showed partial protection against low-dose FIVBang challenge with a FIVPet and FIVShi p24 protein vaccine [63]. Furthermore, a recent finding demonstrates HIV-1 p24 vaccine protection in cats against low-dose FIVBang challenge. Taken together, these results suggest the importance of including appropriate p24 as part of vaccine composition. Future studies will determine which virus proteins and epitopes are involved in the dual-subtype vaccine protection against heterologous subtype challenges.

FIV-cat mucosal model for HIV vaccine

The large majority of the single-subtype and dual-subtype FIV vaccine trials was performed using IV, intraperitoneal, and intramuscular challenges [12,13]. These challenge routes are appropriate for developing FIV vaccines for veterinary use in domestic pet cats, since natural FIV transmission occurs mainly by contaminated blood through biting [3,13]. These challenge routes may simulate the transmission of HIV by IV drug users, but they do not simulate the mucosal route of sexual transmission, the predominant transmission mode of HIV. FIV infections by vaginal and rectal routes were demonstrated in pathogenesis, vaccine, and contraceptive-drug therapy studies [30,47,65,66] (Fig. 2). FIV vaccine studies against mucosal challenges are limited [12,13,47,65]. Both commercial and prototype dual-subtype FIV vaccines have not been tested against mucosal challenge. However, single-subtype IWV FIVPet conferred some protection against mucosal nasal challenge with homologous FIVPet. Furthermore, mucosal immunization has yet to be tested with dual-subtype FIV vaccines. Studies are in progress testing mucosal immunizations of dual-subtype vaccines against mucosal vaginal challenge with pathogenic FIV strains.

The use of inactivated vaccine approach: its significance to FIV and HIV vaccine development

The two most important factors in the early development of a commercial FIV vaccine in comparison to HIV vaccines are the availability of the domestic cats for vaccine-challenge trials and the willingness of the veterinary medicine to use inactivated vaccine approach for retroviral vaccines [13]. The precedent of using inactivated vaccine approach for retrovirus was established in veterinary medicine by the first commercial release of feline leukemia virus (FeLV) vaccine in 1985 [13]. Many of the subsequent FeLV vaccines were also based on inactivated vaccine approach. FeLV belongs to the same Retrovirinae family as FIV and HIV but is classified in a different genus of gammaretrovirus, which causes cancer in infected animals [13]. However, like FIV and HIV, FeLV will integrate into host genome and cause T-cell depletion and immunosuppression [67]. An important observation made in the last 20 years is that no known cases of accidental infection due to improper inactivation of the vaccine virus has been reported with FeLV vaccines [13].

Due to safety concerns, the inactivated HIV-1 vaccine approach has not been tested as a prophylactic vaccine in HIV-1 negative individuals. However, single-strain IWV HIV-1 vaccine depleted of Env glycoprotein (Remune^TM) was tested as part of immunotherapeutic regimen in HIV-positive subjects [68-71]. Significant increase in TH-cell responses to the viral antigens was observed without any affect on the virus load [68-70]. In contrast, a recent unblinded study in Spain showed beneficial virologic effect in asymptomatic HIV-infected subjects treated with Remune^TM in combination with antiretroviral therapy (ART) when compared to subjects with ART alone [71]. Although immune system of HIV-positive subjects is generally affected by the infection, testing of inactivated vaccine approaches in HIV-positive subjects may be the appropriate compromise needed to identify the most effective vaccine virus(es) and vaccine epitopes for both prophylactic and post-HIV exposure immunotherapeutic uses. The dual-subtype FIV vaccines are currently being tested in FIV-positive cats to evaluate whether IWV/IWC vaccines provide therapeutic benefits. Such studies may eventually aid in identifying the most effective viral antigens for prophylactic and immunotherapeutic HIV-1 vaccine.

Acknowledgements

This work was funded by NIH R01 AI30904 and JKY Miscellaneous Donors Fund. J. Yamamoto is the inventor of record on a University of Florida held patent and may be entitled to royalties from companies that are developing commercial products that are related to the research described in this paper. Janet K. Yamamoto is the corresponding author and to whom reprint requests should be made. Phone number (352) 392-4700 ext. 3945; FAX (352) 392-7128

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