Vaccination of cats with attenuated feline immunodeficiency virus proviral DNA vaccine expressing gamma interferon

Characterization of antiviral T cell responses during primary and secondary challenge of laboratory cats with feline infectious peritonitis virus (FIPV)

Background

Feline infectious peritonitis (FIP) is considered highly fatal in its naturally occurring form, although up to 36% of cats resist disease after experimental infection, suggesting that cats in nature may also resist development of FIP in the face of infection with FIP virus (FIPV). Previous experimental FIPV infection studies suggested a role for cell-mediated immunity in resistance to development of FIP. This experimental FIPV infection study in specific pathogen free (SPF) kittens describes longitudinal antiviral T cell responses and clinical outcomes ranging from rapid progression, slow progression, and resistance to disease.

Results

Differences in disease outcome provided an opportunity to investigate the role of T cell immunity to FIP determined by T cell subset proliferation after stimulation with different viral antigens. Reduced total white blood cell (WBC), lymphocyte and T cell counts in blood were observed during primary acute infection for all experimental groups including cats that survived without clinical FIP. Antiviral T cell responses during early primary infection were also similar between cats that developed FIP and cats remaining healthy. Recovery of antiviral T cell responses during the later phase of acute infection was observed in a subset of cats that survived longer or resisted disease compared to cats showing rapid disease progression. More robust T cell responses at terminal time points were observed in lymph nodes compared to blood in cats that developed FIP. Cats that survived primary infection were challenged a second time to pathogenic FIPV and tested for antiviral T cell responses over a four week period. Nine of ten rechallenged cats did not develop FIP or T cell depletion and all cats demonstrated antiviral T cell responses at multiple time points after rechallenge.

Conclusions

In summary, definitive adaptive T cell responses predictive of disease outcome were not detected during the early phase of primary FIPV infection. However emergence of antiviral T cell responses after a second exposure to FIPV, implicated cellular immunity in the control of FIPV infection and disease progression. Virus host interactions during very early stages of FIPV infection warrant further investigation to elucidate host resistance to FIP.

Replacement of feline foamy virus bet by feline immunodeficiency virus vif yields replicative virus with novel vaccine candidate potential

BACKGROUND

Hosts are able to restrict viral replication to contain virus spread before adaptive immunity is fully initiated. Many viruses have acquired genes directly counteracting intrinsic restriction mechanisms. This phenomenon has led to a co-evolutionary signature for both the virus and host which often provides a barrier against interspecies transmission events. Through different mechanisms of action, but with similar consequences, spumaviral feline foamy virus (FFV) Bet and lentiviral feline immunodeficiency virus (FIV) Vif counteract feline APOBEC3 (feA3) restriction factors that lead to hypermutation and degradation of retroviral DNA genomes. Here we examine the capacity of vif to substitute for bet function in a chimeric FFV to assess the transferability of anti-feA3 factors to allow viral replication.

RESULTS

We show that vif can replace bet to yield replication-competent chimeric foamy viruses. An in vitro selection screen revealed that an engineered Bet-Vif fusion protein yields suboptimal protection against feA3. After multiple passages through feA3-expressing cells, however, variants with optimized replication competence emerged. In these variants, Vif was expressed independently from an N-terminal Bet moiety and was stably maintained. Experimental infection of immunocompetent domestic cats with one of the functional chimeras resulted in seroconversion against the FFV backbone and the heterologous FIV Vif protein, but virus could not be detected unambiguously by PCR. Inoculation with chimeric virus followed by wild-type FFV revealed that repeated administration of FVs allowed superinfections with enhanced antiviral antibody production and detection of low level viral genomes, indicating that chimeric virus did not induce protective immunity against wild-type FFV.

CONCLUSIONS

Unrelated viral antagonists of feA3 cellular restriction factors can be exchanged in FFV, resulting in replication competence in vitro that was attenuated in vivo. Bet therefore may have additional functions other than A3 antagonism that are essential for successful in vivo replication. Immune reactivity was mounted against the heterologous Vif protein. We conclude that Vif-expressing FV vaccine vectors may be an attractive tool to prevent or modulate lentivirus infections with the potential option to induce immunity against additional lentivirus antigens.

The Significance of Interferon-γ in HIV-1 Pathogenesis, Therapy, and Prophylaxis

Interferon-γ (IFNγ) plays various roles in the pathogenesis of HIV/AIDS. In an HIV-1 infected individual, the production of IFNγ is detected as early as the acute phase and continually detected throughout the course of infection. Initially produced to clear the primary infection, IFNγ together with other inflammatory cytokines are involved in establishing a chronic immune activation that exacerbates clinical diseases associated with AIDS. Unlike Type 1 IFNs, IFNγ has no direct antiviral activity against HIV-1 in primary cultures, as supported by the in vivo findings of IFNγ therapy in infected subjects. Results from both in vitro and ex vivo studies show that IFNγ can instead enhance HIV-1 replication and its associated diseases, and therapies aimed at decreasing its production are under consideration. On the other hand, IFNγ has been shown to enhance cytotoxic T lymphocytes and NK cell activities against HIV-1 infected cells. These activities are important in controlling HIV-1 replication in an individual and will most likely play a role in the prophylaxis of an effective vaccine against HIV-1. Additionally, IFNγ has been used in combination with HIV-1 vaccine to augment antiviral immunity. Technological advancements have focused on using IFNγ as a biological marker to analyze the type(s) of immunity generated by candidate HIV vaccines and the levels of immunity restored by anti-retroviral drug therapies or novel immunotherapies. Hence, in addition to its valuable ancillary role as a biological marker for the development of effective HIV-1 prophylactic and therapeutic strategies, IFNγ has a vital role in promoting the pathogenesis of HIV.

Polymorphisms and tissue expression of the feline leukocyte antigen class I loci FLAI-E, FLAI-H, and FLAI-K

Cytotoxic CD8+ T-cell immunosurveillance for intracellular pathogens, such as viruses, is controlled by classical major histocompatibility complex (MHC) class Ia molecules, and ideally, these antiviral T-cell populations are defined by the specific peptide and restricting MHC allele. Surprisingly, despite the utility of the cat in modeling human viral immunity, little is known about the feline leukocyte antigen class I complex (FLAI). Only a few coding sequences with uncertain locus origin and expression patterns have been reported. Of 19 class I genes, three loci--FLAI-E, FLAI-H, and FLAI-K--are predicted to encode classical molecules, and our objective was to evaluate their status by analyzing polymorphisms and tissue expression. Using locus-specific, PCR-based genotyping, we amplified 33 FLAI-E, FLAI-H, and FLAI-K alleles from 12 cats of various breeds, identifying, for the first time, alleles across three distinct loci in a feline species. Alleles shared the expected polymorphic and invariant sites in the α1/α2 domains, and full-length cDNA clones possessed all characteristic class Ia exons. Alleles could be assigned to a specific locus with reasonable confidence, although there was evidence of potentially confounding interlocus recombination between FLAI-E and FLAI-K. Only FLAI-E, FLAI-H, and FLAI-K origin alleles were amplified from cDNAs of multiple tissue types. We also defined hypervariable regions across these genes, which permitted the assignment of names to both novel and established alleles. As predicted, FLAI-E, FLAI-H, and FLAI-K fulfill the major criteria of class Ia genes. These data represent a necessary prerequisite for studying epitope-specific antiviral CD8+ T-cell responses in cats.

Novel approaches to vaccination against the feline immunodeficiency virus

Inadequate antigen presentation and/or suboptimal immunogenicity are considered major causes in the failure of human immunodeficiency vaccine to adequately protect against wild-type virus. Several approaches have been attempted to circumvent these hurdles. Here we reviewed some recent vaccinal strategies tested against the feline immunodeficiency virus and focused on: (i) improving antigen presentation by taking advantage of the exquisite ability of dendritic cells to process and present immunogens to the immune system; (ii) boosting immune responses with vaccinal antigens presented in a truly native conformation by the natural target cells of infection. Significance of the studies, possible correlates of protection involved, and implications for developing anti-human immunodeficiency virus vaccines are discussed.

Env-expressing autologous T lymphocytes induce neutralizing antibody and afford marked protection against feline immunodeficiency virus

The envelope (Env) glycoproteins of HIV and other lentiviruses possess neutralization and other protective epitopes, yet all attempts to induce protective immunity using Env as the only immunogen have either failed or afforded minimal levels of protection. In a novel prime-boost approach, specific-pathogen-free cats were primed with a plasmid expressing Env of feline immunodeficiency virus (FIV) and feline granulocyte-macrophage colony-stimulating factor and then boosted with their own T lymphocytes transduced ex vivo to produce the same Env and interleukin 15 (3 x 10(6) to 10 x 10(6) viable cells/cat). After the boost, the vaccinees developed elevated immune responses, including virus-neutralizing antibodies (NA). Challenge with an ex vivo preparation of FIV readily infected all eight control cats (four mock vaccinated and four naïve) and produced a marked decline in the proportion of peripheral CD4 T cells. In contrast, five of seven vaccinees showed little or no traces of infection, and the remaining two had reduced viral loads and underwent no changes in proportions of CD4 T cells. Interestingly, the viral loads of the vaccinees were inversely correlated to the titers of NA. The findings support the concept that Env is a valuable immunogen but needs to be administered in a way that permits the expression of its full protective potential.

DNA vaccines in veterinary use

DNA vaccines represent a new frontier in vaccine technology. One important application of this technology is in the veterinary arena. DNA vaccines have already gained a foothold in certain fields of veterinary medicine. However, several important questions must be addressed when developing DNA vaccines for animals, including whether or not the vaccine is efficacious and cost effective compared with currently available options. Another important question to consider is how to apply this developing technology in a wide range of different situations, from the domestic pet to individual fish in fisheries with several thousand animals, to wildlife programs for disease control. In some cases, DNA vaccines represent an interesting option for vaccination, while in others, currently available options are sufficient. This review will examine a number of diseases of veterinary importance and the progress being made in DNA vaccine technology relevant to these diseases, and we compare these with the conventional treatment options available.

Vaccination with vif-deleted feline immunodeficiency virus provirus, GM-CSF, and TNF-alpha plasmids preserves global CD4 T lymphocyte function after challenge with FIV

Feline immunodeficiency virus (FIV) DNA vaccine approaches that included a vif-deleted FIV provirus (FIV-pPPRDeltavif) and feline cytokine expression plasmids were tested for immunogenicity and efficacy by immunization of specific pathogen free cats. Vaccine protocols included FIV-pPPRDeltavif plasmid alone; a combination of FIV-pPPRDeltavif DNA and feline granulocyte macrophage-colony stimulating factor (GM-CSF) and tumor necrosis factor (TNF)-alpha expression plasmids; or a combination of FIV-pPPRDeltavif and feline interleukin (IL)-15 plasmids. Cats immunized with FIV-pPPRDeltavif, GM-CSF and TNF-alpha plasmids demonstrated an increased frequency of FIV-specific T cell proliferation responses compared to other vaccine groups. Immunization with FIV-pPPRDeltavif and IL-15 plasmids was distinguished from other vaccine protocols by the induction of antiviral antibodies. Suppression of virus loads was not observed for any of the FIV-pPPRDeltavif DNA vaccine protocols after challenge with the FIV-PPR isolate. However, prior immunization with FIV-pPPRDeltavif, GM-CSF, and TNF-alpha plasmids resulted in preservation of CD4 T cell functions, including mitogen-induced cytokine expression and antigen-specific proliferation upon infection with FIV. These findings justify further examination of cytokine combinations as adjuvants for lentiviral DNA vaccines.

Co-immunization with IL-15 enhances cellular immune responses induced by a vif-deleted simian immunodeficiency virus proviral DNA vaccine and confers partial protection against vaginal challenge with SIVmac251

Simian immunodeficiency virus (SIV) infection of rhesus macaques is a valuable animal model for human immunodeficiency virus (HIV)-1 vaccine development. Our laboratory recently described the immunogenicity and limited efficacy of a vif-deleted SIVmac239 proviral DNA (SIV/CMVDelta vif) vaccine. The current report characterizes immunogenicity and efficacy for the SIV/CMVDelta vif proviral DNA vaccine when co-inoculated with an optimized rhesus interleukin (rIL)-15 expression plasmid. Macaques co-inoculated with rIL-15 and SIV/CMVDelta vif proviral plasmids showed significantly improved SIV-specific CD8 T cell immunity characterized by increased IFN-gamma ELISPOT and polyfunctional CD8 T cell responses. Furthermore, these animals demonstrated a sustained suppression of plasma virus loads after multiple low dose vaginal challenges with pathogenic SIVmac251. Importantly, SIV-specific cellular responses were greater in immunized animals compared to unvaccinated controls during the initial 12 weeks after challenge. Taken together, these findings support the use of IL-15 as an adjuvant in prophylactic anti-HIV vaccine strategies.

Mucosal immunization against ovine lentivirus using PEI-DNA complexes and modified vaccinia Ankara encoding the gag and/or env genes

Sheep were immunized against Visna/Maedi virus (VMV) gag and/or env genes via the nasopharynx-associated lymphoid tissue (NALT) and lung using polyethylenimine (PEI)-DNA complexes and modified vaccinia Ankara, and challenged with live virus via the lung. env immunization enhanced humoral responses prior to but not after VMV challenge. Systemic T cell proliferative and cytotoxic responses were generally low, with the responses following single gag gene immunization being significantly depressed after challenge. A transient reduction in provirus load in the blood early after challenge was observed following env immunization, whilst the gag gene either alone or in combination with env resulted in significantly elevated provirus loads in lung. However, despite this, a significant reduction in lesion score was observed in animals immunized with the single gag gene at post-mortem. Inclusion of IFN-gamma in the immunization mixture in general had no significant effects. The results thus showed that protective effects against VMV-induced lesions can be induced following respiratory immunization with the single gag gene, though this was accompanied by an increased pulmonary provirus load.

Prevention of immunodeficiency virus induced CD4+ T-cell depletion by prior infection with a non-pathogenic virus

Immune dysregulation initiated by a profound loss of CD4+ T-cells is fundamental to HIV-induced pathogenesis. Infection of domestic cats with a non-pathogenic lentivirus prevalent in the puma (puma lentivirus, PLV or FIV(pco)) prevented peripheral blood CD4+ T-cell depletion caused by subsequent virulent FIV infection. Maintenance of this critical population was not associated with a significant decrease in FIV viremia, lending support to the hypothesis that direct viral cytopathic effect is not the primary cause of immunodeficiency. Although this approach was analogous to immunization with a modified live vaccine, correlates of immunity such as a serum-neutralizing antibody or virus-specific T-cell proliferative response were not found in protected animals. Differences in cytokine transcription profile, most notably in interferon gamma, were observed between the protected and unprotected groups. These data provide support for the importance of non-adaptive enhancement of the immune response in the prevention of CD4+ T-cell loss.

Vaccination of rhesus macaques with a vif-deleted simian immunodeficiency virus proviral DNA vaccine

Studies in non-human primates, with simian immunodeficiency virus (SIV) and simian/human immunodeficiency virus (SHIV) have demonstrated that live-attenuated viral vaccines are highly effective; however these vaccine viruses maintain a low level of pathogenicity. Lentivirus attenuation associated with deletion of the viral vif gene carries a significantly reduced risk for pathogenicity, while retaining the potential for virus replication of low magnitude in the host. This report describes a vif-deleted simian immunodeficiency virus (SIV)mac239 provirus that was tested as an attenuated proviral DNA vaccine by inoculation of female rhesus macaques. SIV-specific interferon-gamma enzyme-linked immunospot responses of low magnitude were observed after immunization with plasmid containing the vif-deleted SIV provirus. However, vaccinated animals displayed strong sustained virus-specific T cell proliferative responses and increasing antiviral antibody titers. These immune responses suggested either persistent vaccine plasmid expression or low level replication of vif-deleted SIV in the host. Immunized and unvaccinated macaques received a single high dose vaginal challenge with pathogenic SIVmac251. A transient suppression of challenge virus load and a greater median survival time was observed for vaccinated animals. However, virus loads for vaccinated and unvaccinated macaques were comparable by twenty weeks after challenge and overall survival curves for the two groups were not significantly different. Thus, a vif-deleted SIVmac239 proviral DNA vaccine is immunogenic and capable of inducing a transient suppression of pathogenic challenge virus, despite severe attenuation of the vaccine virus.

Advances in FIV vaccine technology

Advances in vaccine technology are occurring in the molecular techniques used to develop vaccines and in the assessment of vaccine efficacy, allowing more complete characterization of vaccine-induced immunity correlating to protection. FIV vaccine development has closely mirrored and occasionally surpassed the development of HIV-1 vaccine, leading to first licensed technology. This review will discuss technological advances in vaccine designs, challenge infection assessment, and characterization of vaccine immunity in the context of the protection detected with prototype and commercial dual-subtype FIV vaccines and in relation to HIV-1.

Should accessory proteins be structural components of lentiviral vaccines? Lessons learned from the accessory ORF-A protein of FIV

The FIV regulatory protein Rev and accessory proteins Vif and ORF-A are essential for efficient viral replication and full-blown pathogenesis. Expressed at very low level during viral replication, they are nevertheless processed for recognition by cytotoxic T-lymphocytes (CTLs) and trigger cellular immune responses in FIV-infected cats. The observation that the accessory ORF-A protein of FIV is continuously expressed during viral replication and targeted by cellular immune responses in natural FIV infection, prompted us to investigate the protective potential of this protein. To this aim cats were immunized with three different strategies (protein alone in alum adjuvant, DNA alone, or DNA prime-protein boost) and generated clearly detectable immune responses. Upon challenge with ex vivo homologous FIV, ORF-A immunized cats showed distinct enhancement of acute-phase infection possibly due to an increased expression of the FIV receptor CD134. However, at subsequent sampling points plasma viremia was reduced and CD4+ T-lymphocytes in the circulation declined more slowly in ORF-A immunized than in control animals. These findings support the contention that a multicomponent vaccine, with the inclusion of both accessory and structural proteins, can not only improve the host's ability to control lentivirus replication and slow down disease progression but also draw attention to the fact that even simple immunogens that eventually contribute to protective activity can transiently exacerbate subsequent lentiviral infections.

A feline immunodeficiency virus vif-deletion mutant remains attenuated upon infection of newborn kittens

This report characterizes lentivirus attenuation associated with a vif mutation by inoculation of newborn kittens with a vif-deleted feline immunodeficiency virus provirus plasmid (FIV-pPPRDeltavif). Virus in peripheral blood, antiviral antibody or CD4 T-cell count alterations were not detected in kittens inoculated with FIV-pPPRDeltavif plasmid, with the exception of one kitten that demonstrated FIV Gag antibody production at 42 weeks after inoculation. In contrast, wild-type FIV-pPPR-infected kittens were viraemic, seropositive and exhibited a decrease in the CD4 T-cell subset in peripheral blood. Interestingly, FIV-specific T-cell proliferative responses detected at 32 and 36 weeks after infection were comparable for both FIV-pPPRDeltavif- and wild-type FIV-pPPR-inoculated kittens and suggested the possibility of a discreet tissue reservoir supporting sustained FIV-pPPRDeltavif expression or replication. Overall, these findings confirmed that the severe virus attenuation for both replication and pathogenicity exhibited by a vif-deleted FIV mutant is similar for both neonatal and adult hosts.

Virally delivered cytokines alter the immune response to future lung infections

Respiratory syncytial virus (RSV) is an important cause of infant morbidity and mortality worldwide and is increasingly recognized to have a role in the development and exacerbation of chronic lung diseases. There is no effective vaccine, and we reasoned that it might be possible to skew the immune system towards beneficial nonpathogenic responses by selectively priming protective T-cell subsets. We therefore tested recombinant RSV (rRSV) candidates expressing prototypic murine Th1 (gamma interferon [IFN-gamma]) or Th2 (interleukin-4 [IL-4]) cytokines, with detailed monitoring of responses to subsequent infections with RSV or (as a control) influenza A virus. Although priming with either recombinant vector reduced viral load during RSV challenge, enhanced weight loss and enhanced pulmonary influx of RSV-specific CD8+ T cells were observed after challenge in mice primed with rRSV/IFN-gamma. By contrast, rRSV/IL-4-primed mice were protected against weight loss during secondary challenge but showed airway eosinophilia. When rRSV/IL-4-primed mice were challenged with influenza virus, weight loss was attenuated but was again accompanied by marked airway eosinophilia. Thus, immunization directed toward enhancement of Th1 responses reduces viral load but is not necessarily protective against disease. Counter to expectation, Th2-biased responses were more beneficial but also influenced the pathological effects of heterologous viral challenge.

Vaccination against the feline immunodeficiency virus: the road not taken

Natural infection of domestic cats by the feline immunodeficiency virus (FIV) causes acquired immunodeficiency syndrome (AIDS). FIV is genetically related to human immunodeficiency virus (HIV), and the clinical and biological features of infections caused by feline and human viruses in their respective hosts are highly analogous. Although the obstacles to vaccinating against FIV and HIV would seem to be of comparable difficulty, a licensed vaccine against feline AIDS is already in widespread use in several countries. While this seemingly major advance in prevention of AIDS would appear to be highly instructive for HIV vaccine development, its message has not been heeded by investigators in the HIV field. This review endeavours to relate what has been learned about vaccination against feline AIDS, and to suggest what this may mean for HIV vaccine development.