Role of natural killer cells in resistance against friend retrovirus-induced leukemia

Immunization with a murine cytomegalovirus based vector encoding retrovirus envelope confers strong protection from Friend retrovirus challenge infection

Immunization vectors based on cytomegalovirus (CMV) have attracted a lot of interest in recent years because of their high efficacy in the simian immunodeficiency virus (SIV) macaque model, which has been attributed to their ability to induce strong, unusually broad, and unconventionally restricted CD8⁺ T cell responses. To evaluate the ability of CMV-based vectors to mediate protection by other immune mechanisms, we evaluated a mouse CMV (MCMV)-based vector encoding Friend virus (FV) envelope (Env), which lacks any known CD8⁺ T cell epitopes, for its protective efficacy in the FV mouse model. When we immunized highly FV-susceptible mice with the Env-encoding MCMV vector (MCMV.env), we could detect high frequencies of Env-specific CD4⁺ T cells after a single immunization. While the control of an early FV challenge infection was highly variable, an FV infection applied later after immunization was tightly controlled by almost all immunized mice. Protection of mice correlated with their ability to mount a robust anamnestic neutralizing antibody response upon FV infection, but Env-specific CD4⁺ T cells also produced appreciable levels of interferon γ. Depletion and transfer experiments underlined the important role of antibodies for control of FV infection but also showed that while no Env-specific CD8⁺ T cells were induced by the MCMV.env vaccine, the presence of CD8⁺ T cells at the time of FV challenge was required. The immunity induced by MCMV.env immunization was long-lasting, but was restricted to MCMV naïve animals. Taken together, our results demonstrate a novel mode of action of a CMV-based vaccine for anti-retrovirus immunization that confers strong protection from retrovirus challenge, which is conferred by CD4⁺ T cells and antibodies.

CMV-based vectors have attracted a lot of attention in the vaccine development field, since they were shown to induce unconventionally restricted CD8⁺ T cell responses and strong protection in the SIV rhesus macaque model. In a mouse retrovirus model, we show now that immunization with a mouse CMV-based vector encoding retrovirus envelope conferred very strong protection, even though it was not designed to induce any CD8⁺ T cell responses. In this MCMV.env immunization, protection relied on the induction of CD4⁺ T cells and the ability to mount a strong anamnestic neutralizing antibody response upon retrovirus infection, but it was restricted to MCMV pre-naïve mice. In our model system, the MCMV based vector shows very high efficacy that is comparable to an attenuated retrovirus-based vaccine, and encourages the pursuit of this vaccination strategy.

Induction of complex immune responses and strong protection against retrovirus challenge by adenovirus-based immunization depends on the order of vaccine delivery

Background

In the Friend retrovirus mouse model we developed potent adenovirus-based vaccines that were designed to induce either strong Friend virus GagL_85–93-specific CD8⁺ T cell or antibody responses, respectively. To optimize the immunization outcome we evaluated vaccination strategies using combinations of these vaccines.

Results

While the vaccines on their own confer strong protection from a subsequent Friend virus challenge, the simple combination of the vaccines for the establishment of an optimized immunization protocol did not result in a further improvement of vaccine effectivity. We demonstrate that the co-immunization with GagL_85–93/leader-gag encoding vectors together with envelope-encoding vectors abrogates the induction of GagL_85–93-specific CD8⁺ T cells, and in successive immunization protocols the immunization with the GagL_85–93/leader-gag encoding vector had to precede the immunization with an envelope encoding vector for the efficient induction of GagL_85–93-specific CD8⁺ T cells. Importantly, the antibody response to envelope was in fact enhanced when the mice were adenovirus-experienced from a prior immunization, highlighting the expedience of this approach.

Conclusions

To circumvent the immunosuppressive effect of envelope on immune responses to simultaneously or subsequently administered immunogens, we developed a two immunizations-based vaccination protocol that induces strong immune responses and confers robust protection of highly Friend virus-susceptible mice from a lethal Friend virus challenge.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0336-7) contains supplementary material, which is available to authorized users.

Tetherin promotes the innate and adaptive cell-mediated immune response against retrovirus infection in vivo

Tetherin/BST-2 is a host restriction factor that could directly inhibit retroviral particle release by tethering nascent virions to the plasma membrane. However, the immunological impact of Tetherin during retrovirus infection remains unknown. We now show that Tetherin influences antiretroviral cell-mediated immune responses. In contrast to the direct antiviral effects of Tetherin, which are dependent on cell surface expression, the immunomodulatory effects are linked to the endocytosis of the molecule. Mice encoding endocytosis-competent C57BL/6 Tetherin exhibited lower viremia and pathology at 7 d postinfection with Friend retrovirus (FV) compared with mice encoding endocytosis-defective NZW/LacJ Tetherin. Notably, antiretroviral protection correlated with stronger NK cell responses. In addition, Friend retrovirus infection levels were significantly lower in wild-type C57BL/6 mice than in Tetherin knockout mice at 2 wk postinfection, and antiretroviral protection correlated with stronger NK cell and virus-specific CD8+ T cell responses. The results demonstrate that Tetherin acts as a modulator of the cell-mediated immune response against retrovirus infection in vivo.

Differential requirements of cellular and humoral immune responses for Fv2-associated resistance to erythroleukemia and for regulation of retrovirus-induced myeloid leukemia development

To assess the possible contribution of host immune responses to the exertion of Fv2-associated resistance to Friend virus (FV)-induced disease development, we inoculated C57BL/6 (B6) mice that lacked various subsets of lymphocytes with FV containing no lactate dehydrogenase-elevating virus. Fv2(r) B6 mice lacking CD4(+) T cells developed early polycythemia and fatal erythroleukemia, while B6 mice lacking CD8(+) T cells remained resistant. Erythroid progenitor cells infected with spleen focus-forming virus (SFFV) were eliminated, and no polycythemia was observed in B cell-deficient B6 mice, but they later developed myeloid leukemia associated with oligoclonal integration of ecotropic Friend murine leukemia virus. Additional depletion of natural killer and/or CD8(+) T cells from B cell-deficient B6 mice resulted in the expansion of SFFV proviruses and the development of polycythemia, indicating that SFFV-infected erythroid cells are not only restricted in their growth but are actively eliminated in Fv2(r) mice through cellular immune responses.

Interferon-alpha subtype 11 activates NK cells and enables control of retroviral infection

The innate immune response mediated by cells such as natural killer (NK) cells is critical for the rapid containment of virus replication and spread during acute infection. Here, we show that subtype 11 of the type I interferon (IFN) family greatly potentiates the antiviral activity of NK cells during retroviral infection. Treatment of mice with IFN-α11 during Friend retrovirus infection (FV) significantly reduced viral loads and resulted in long-term protection from virus-induced leukemia. The effect of IFN-α11 on NK cells was direct and signaled through the type I IFN receptor. Furthermore, IFN-α11-mediated activation of NK cells enabled cytolytic killing of FV-infected target cells via the exocytosis pathway. Depletion and adoptive transfer experiments illustrated that NK cells played a major role in successful IFN-α11 therapy. Additional experiments with Mouse Cytomegalovirus infections demonstrated that the therapeutic effect of IFN-α11 is not restricted to retroviruses. The type I IFN subtypes 2 and 5, which bind the same receptor as IFN-α11, did not elicit similar antiviral effects. These results demonstrate a unique and subtype-specific activation of NK cells by IFN-α11.

The innate immune response mediated by cells such as natural killer (NK) cells can contribute to immunity against viral infections. NK cells can kill virus-infected cells and thus inhibit virus replication and spread during acute infection. However, in infections with retroviruses, like HIV, these cells are not sufficient to prevent pathology. Here, we describe a new strategy to augment natural killer cell responses during virus infections by using a subtype of the type I interferon family as antiviral drug. This therapy strongly activated NK cells and enabled them to control retrovirus as well as herpes virus infections in mice. The new approach might have great potential for the treatment of many infectious and tumor diseases in which natural killer cells play a significant role in immunity.

Negative selection by an endogenous retrovirus promotes a higher-avidity CD4+ T cell response to retroviral infection

Effective T cell responses can decisively influence the outcome of retroviral infection. However, what constitutes protective T cell responses or determines the ability of the host to mount such responses is incompletely understood. Here we studied the requirements for development and induction of CD4+ T cells that were essential for immunity to Friend virus (FV) infection of mice, according to their TCR avidity for an FV-derived epitope. We showed that a self peptide, encoded by an endogenous retrovirus, negatively selected a significant fraction of polyclonal FV-specific CD4+ T cells and diminished the response to FV infection. Surprisingly, however, CD4+ T cell-mediated antiviral activity was fully preserved. Detailed repertoire analysis revealed that clones with low avidity for FV-derived peptides were more cross-reactive with self peptides and were consequently preferentially deleted. Negative selection of low-avidity FV-reactive CD4+ T cells was responsible for the dominance of high-avidity clones in the response to FV infection, suggesting that protection against the primary infecting virus was mediated exclusively by high-avidity CD4+ T cells. Thus, although negative selection reduced the size and cross-reactivity of the available FV-reactive naïve CD4+ T cell repertoire, it increased the overall avidity of the repertoire that responded to infection. These findings demonstrate that self proteins expressed by replication-defective endogenous retroviruses can heavily influence the formation of the TCR repertoire reactive with exogenous retroviruses and determine the avidity of the response to retroviral infection. Given the overabundance of endogenous retroviruses in the human genome, these findings also suggest that endogenous retroviral proteins, presented by products of highly polymorphic HLA alleles, may shape the human TCR repertoire that reacts with exogenous retroviruses or other infecting pathogens, leading to interindividual heterogeneity.

Improved vaccine protection against retrovirus infection after co-administration of adenoviral vectors encoding viral antigens and type I interferon subtypes

BACKGROUND

Type I interferons (IFNs) exhibit direct antiviral effects, but also distinct immunomodulatory properties. In this study, we analyzed type I IFN subtypes for their effect on prophylactic adenovirus-based anti-retroviral vaccination of mice against Friend retrovirus (FV) or HIV.

RESULTS

Mice were vaccinated with adenoviral vectors encoding FV Env and Gag proteins alone or in combination with vectors encoding IFNα1, IFNα2, IFNα4, IFNα5, IFNα6, IFNα9 or IFNβ. Only the co-administration of adenoviral vectors encoding IFNα2, IFNα4, IFNα6 and IFNα9 resulted in strongly improved immune protection of vaccinated mice from subsequent FV challenge infection with high control over FV-induced splenomegaly and reduced viral loads. The level of protection correlated with augmented virus-specific CD4(+) T cell responses and enhanced antibody titers. Similar results were obtained when mice were vaccinated against HIV with adenoviral vectors encoding HIV Env and Gag-Pol in combination with various type I IFN encoding vectors. Here mainly CD4(+) T cell responses were enhanced by IFNα subtypes.

CONCLUSIONS

Our results indicate that certain IFNα subtypes have the potential to improve the protective effect of adenovirus-based vaccines against retroviruses. This correlated with augmented virus-specific CD4(+) T cell and antibody responses. Thus, co-expression of select type I IFNs may be a valuable tool for the development of anti-retroviral vaccines.

Distinct roles of CD4+ T cell subpopulations in retroviral immunity: lessons from the Friend virus mouse model

It is well established that CD4⁺ T cells play an important role in immunity to infections with retroviruses such as HIV. However, in recent years CD4⁺ T cells have been subdivided into several distinct populations that are differentially regulated and perform widely varying functions. Thus, it is important to delineate the separate roles of these subsets, which range from direct antiviral activities to potent immunosuppression. In this review, we discuss contributions from the major CD4⁺ T cell subpopulations to retroviral immunity. Fundamental concepts obtained from studies on numerous viral infections are presented along with a more detailed analysis of studies on murine Friend virus. The relevance of these studies to HIV immunology and immunotherapy is reviewed.

Natural killer cells recognize friend retrovirus-infected erythroid progenitor cells through NKG2D-RAE-1 interactions In Vivo

Natural killer (NK) cells function as early effector cells in the innate immune defense against viral infections and also participate in the regulation of normal and malignant hematopoiesis. NK cell activities have been associated with early clearance of viremia in experimental simian immunodeficiency virus and clinical human immunodeficiency virus type 1 (HIV-1) infections. We have previously shown that NK cells function as major cytotoxic effector cells in vaccine-induced immune protection against Friend virus (FV)-induced leukemia, and NK cell depletion totally abrogates the above protective immunity. However, how NK cells recognize retrovirus-infected cells remains largely unclear. The present study demonstrates a correlation between the expression of the products of retinoic acid early transcript-1 (RAE-1) genes in target cells and their susceptibility to killing by NK cells isolated from FV-infected animals. This killing was abrogated by antibodies blocking the NKG2D receptor in vitro. Further, the expression of RAE-1 proteins on erythroblast surfaces increased early after FV inoculation, and administration of an RAE-1-blocking antibody resulted in increased spleen infectious centers and exaggerated pathology, indicating that FV-infected erythroid cells are recognized by NK cells mainly through the NKG2D-RAE-1 interactions in vivo. Enhanced retroviral replication due to host gene-targeting resulted in markedly increased RAE-1 expression in the absence of massive erythroid cell proliferation, indicating a direct role of retroviral replication in RAE-1 upregulation.

Antileukemia and antitumor effects of the graft-versus-host disease: a new immunovirological approach

In leukemic mice, the native host's explicit and well-defined immune reactions to the leukemia virus (a strong exogenous antigen) and to leukemia cells (pretending in their native hosts to be protected "self" elements) are extinguished and replaced in GvHD (graft-versus-host disease) by those of the immunocompetent donor cells. In many cases, the GvHD-inducer donors display genetically encoded resistance to the leukemia virus. In human patients only antileukemia and anti-tumor cell immune reactions are mobilized; thus, patients are deprived of immune reactions to a strong exogenous antigen (the elusive human leukemia-sarcoma retroviruses). The innate and adaptive immune systems of mice have to sustain the immunosuppressive effects of leukemia-inducing retroviruses. Human patients due to the lack of leukemiainducing retroviral pathogens (if they exist, they have not as yet been discovered), escape such immunological downgrading. After studying leukemogenic retroviruses in murine and feline (and other mammalian) hosts, it is very difficult to dismiss retroviral etiology for human leukemias and sarcomas. Since no characterized and thus recognized leukemogenic-sarcomagenic retroviral agents are being isolated from the vast majority of human leukemias-sarcomas, the treatment for these conditions in mice and in human patients vastly differ. It is immunological and biological modalities (alpha interferons; vaccines; adoptive lymphocyte therapy) that dominate the treatment of murine leukemias, whereas combination chemotherapy remains the main remission-inducing agent in human leukemias-lymphomas and sarcomas (as humanized monoclonal antibodies and immunotoxins move in). Yet, in this apparently different backgrounds in Mus and Homo, GvHD, as a treatment modality, appears to work well in both hosts, by replacing the hosts' anti-leukemia and anti-tumor immune faculties with those of the donor. The clinical application of GvHD in the treatment of human leukemias-lymphomas and malignant solid tumors remains a force worthy of pursuit, refinement and strengthening. Graft engineering and modifications of the inner immunological environment of the recipient host by the activation or administration of tumor memory T cells, selected Treg cells and natural killer (NKT) cell classes and cytokines, and the improved pharmacotherapy of GvHD without reducing its antitumor efficacy, will raise the value of GvHD to the higher ranks of the effective antitumor immunotherapeutical measures. Clinical interventions of HCT/HSCT (hematopoietic cell/stem cell transplants) are now applicable to an extended spectrum of malignant diseases in human patients, being available to elderly patients, who receive non-myeloablative conditioning, are re-enforced by post-transplant donor lymphocyte (NK cell and immune T cell) infusions and post-transplant vaccinations, and the donor cells may derive from engineered grafts, or from cord blood with reduced GvHD, but increased GvL/GvT-inducing capabilities (graft-versus leukemia/tumor). Post-transplant T cell transfusions are possible only if selected leukemia antigen-specific T cell clones are available. In verbatim quotation: "Ultimately, advances in separation of GvT from GvHD will further enhance the potential of allogeneic HCT as a curative treatment for hematological malignancies" (Rezvani, A.R. and Storb, R.F., Journal of Autoimmunity 30:172-179, 2008 (see in the text)). It may be added: for cure, a combination of the GvL/T effects with new targeted therapeutic modalities, as elaborated on in this article, will be necessary.

Vaccination with an adenoviral vector that encodes and displays a retroviral antigen induces improved neutralizing antibody and CD4+ T-cell responses and confers enhanced protection

We present a new type of adenoviral vector that both encodes and displays a vaccine antigen on the capsid, thus combining in itself gene-based and protein vaccination; this vector resulted in an improved vaccination outcome in the Friend virus (FV) model. For presentation of the envelope protein gp70 of Friend murine leukemia virus on the adenoviral capsid, gp70 was fused to the adenovirus capsid protein IX. When compared to vaccination with conventional FV Env- and Gag-encoding adenoviral vectors, vaccination with the adenoviral vector that encodes and displays pIX-gp70 combined with an FV Gag-encoding vector resulted in significantly improved protection against systemic FV challenge infection, with highly controlled viral loads in plasma and spleen. This improved protection correlated with improved neutralizing antibody titers and stronger CD4(+) T-cell responses. Using a vector that displays gp70 without encoding it, we found that while the antigen display on the capsid alone was sufficient to induce high levels of binding antibodies, in vivo expression was necessary for the induction of neutralizing antibodies. This new type of adenovirus-based vaccine could be a valuable tool for vaccination.

Race between retroviral spread and CD4+ T-cell response determines the outcome of acute Friend virus infection

Retroviruses can establish persistent infection despite induction of a multipartite antiviral immune response. Whether collective failure of all parts of the immune response or selective deficiency in one crucial part underlies the inability of the host to clear retroviral infections is currently uncertain. We examine here the contribution of virus-specific CD4(+) T cells in resistance against Friend virus (FV) infection in the murine host. We show that the magnitude and duration of the FV-specific CD4(+) T-cell response is directly proportional to resistance against acute FV infection and subsequent disease. Notably, significant protection against FV-induced disease is afforded by FV-specific CD4(+) T cells in the absence of a virus-specific CD8(+) T-cell or B-cell response. Enhanced spread of FV infection in hosts with increased genetic susceptibility or coinfection with Lactate dehydrogenase-elevating virus (LDV) causes a proportional increase in the number of FV-specific CD4(+) T cells required to control FV-induced disease. Furthermore, ultimate failure of FV/LDV coinfected hosts to control FV-induced disease is accompanied by accelerated contraction of the FV-specific CD4(+) T-cell response. Conversely, an increased frequency or continuous supply of FV-specific CD4(+) T cells is both necessary and sufficient to effectively contain acute infection and prevent disease, even in the presence of coinfection. Thus, these results suggest that FV-specific CD4(+) T cells provide significant direct protection against acute FV infection, the extent of which critically depends on the ratio of FV-infected cells to FV-specific CD4(+) T cells.

Effects of acute and chronic murine norovirus infections on immune responses and recovery from Friend retrovirus infection

Murine norovirus (MNV) is a highly infectious but generally nonpathogenic agent that is commonly found in research mouse colonies in both North America and Europe. In the present study, the effects of acute and chronic infections with MNV on immune responses and recovery from concurrent Friend virus (FV) infections were investigated. No significant differences in T-cell or NK-cell responses, FV-neutralizing antibody responses, or long-term recovery from FV infection were observed. We conclude that concurrent MNV infections had no major impacts on FV infections.

Anti-retroviral effects of type I IFN subtypes in vivo

Type I IFN play a very important role in immunity against viral infections. Murine type I IFN belongs to a multigene family including 14 IFN-alpha subtypes but the biological functions of IFN-alpha subtypes in retroviral infections are unknown. We have used the Friend retrovirus model to determine the anti-viral effects of IFN-alpha subtypes in vitro and in vivo. IFN-alpha subtypes alpha1, alpha4, alpha6 or alpha9 suppressed Friend virus (FV) replication in vitro, but differed greatly in their anti-viral efficacy in vivo. Treatment of FV-infected mice with the IFN-alpha subtypes alpha1, alpha4 or alpha9, but not alpha6 led to a significant reduction in viral loads. Decreased splenic viral load after IFN-alpha1 treatment correlated with an expansion of activated FV-specific CD8(+) T cells and NK cells into the spleen, whereas in IFN-alpha4- and -alpha9-treated mice it exclusively correlated with the activation of NK cells. The results demonstrate the distinct anti-retroviral effects of different IFN-alpha subtypes, which may be relevant for new therapeutic approaches.

Differential regulation of GM1 and asialo-GM1 expression by T cells and natural killer (NK) cells in respiratory syncytial virus infection

We previously reported that respiratory syncytial virus (RSV) infection increases lung CD8(+) T cell GM1 expression. The related lipid asialo-GM1 (ASGM1) is expressed by T cells in viral infection and by natural killer (NK) cells. The in vivo co-expression of GM1 and ASGM1 by immune cells is not defined. Here we analyzed lung lymphocyte GM1 and ASGM1 expression in RSV-infected mice. GM1 and ASGM1 were coordinately upregulated by activated CD8(+) T cells in RSV-infected BALB/c and C57BL/6 mice. In contrast, RSV infection had no effect on constitutively high NK cell GM1 expression, while increasing NK cell ASGM1 expression. GM1 and ASGM1 co-localized in lipid raft structures in NK and CD8(+) T cells sorted from the lungs of RSV-infected mice. Anti-ASGM1 Ab treatment of RSV-infected BALB/c mice depleted GM1/ASGM1-expressing NK cells and GM1/ASGM1-expressing T cells, reduced lung IFN-gamma levels, increased viral load, delayed viral clearance, and reduced illness. STAT1(-/-) mice are more susceptible to RSV replication and disease than wild-type mice. In RSV-infected STAT1(-/-) mice, anti-ASGM1 Ab altered cytokine levels, but in contrast to BALB/c mice, antibody treatment had no effect on viral load or illness. Taken together, GM1 and ASGM1 expression are differentially regulated by T and NK cells in RSV infection. Also, GM1/ASGM1-expressing cells are important for control of RSV in BALB/c mice, whereas STAT1(-/-) mice clear RSV by an alternative pathway.

Mouse APOBEC3 restricts friend leukemia virus infection and pathogenesis in vivo

Several members of the apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like complex 3 (APOBEC3) family in primates act as potent inhibitors of retroviral replication. However, lentiviruses have evolved mechanisms to specifically evade host APOBEC3. Likewise, murine leukemia viruses (MuLV) exclude mouse APOBEC3 from the virions and cleave virion-incorporated APOBEC3. Although the betaretrovirus mouse mammary tumor virus has been shown to be susceptible to mouse APOBEC3, it is not known if APOBEC3 has a physiological role in restricting more widely distributed and long-coevolved mouse gammaretroviruses. The pathogenicity of Friend MuLV (F-MuLV) is influenced by several host genes: some directly restrict the cell entry or integration of the virus, while others influence the host immune responses. Among the latter, the Rfv3 gene has been mapped to chromosome 15 in the vicinity of the APOBEC3 locus. Here we have shown that polymorphisms at the mouse APOBEC3 locus indeed influence F-MuLV replication and pathogenesis: the APOBEC3 alleles of F-MuLV-resistant C57BL/6 and -susceptible BALB/c mice differ in their sequences and expression levels in the hematopoietic tissues and in their abilities to restrict F-MuLV replication both in vitro and in vivo. Furthermore, upon infection with the pathogenic Friend virus complex, (BALB/c x C57BL/6)F(1) mice displayed an exacerbated erythroid cell proliferation when the mice carried a targeted disruption of the C57BL/6-derived APOBEC3 allele. These results indicate, for the first time, that mouse APOBEC3 is a physiologically functioning restriction factor to mouse gammaretroviruses.

Development of a real-time quantitative reverse transcriptase PCR assay for detection of the Friend leukemia virus load in murine plasma

Friend leukemia virus (FLV), a murine retrovirus, has been used as a model for elucidation of human immunodeficiency virus (HIV) immunopathogenesis and evaluation of anti-HIV drug effects for several decades. However, no method for direct detection of the plasma viral load has yet been reported. In this study, a TaqMan real-time quantitative reverse transcriptase PCR (qRT-PCR) assay was established for the rapid detection and quantitation of FLV. Measurement of the absolute FLV load was achieved through synthesis of a standard RNA from within the FLV envelope gene for generation of a standard curve. The assay allows quantitation over a range from 20 to 2 x 10(8) RNA copies per reaction in a two-step real-time quantitative reverse transcriptase PCR protocol. The relationships between the initially injected FLV dose and the plasma FLV load and spleen index were explored. Following this, the in vivo effects of zidovudine, adefovir dipivoxil, and entecavir on mice infected with FLV were evaluated. The results showed that the plasma FLV load was not proportional to the spleen index over the same FLV injection dosage series, although a trend was observed. When evaluated using plasma viral load, high dose (15 mg/(kg d)) adefovir dipivoxil was capable of significant inhibition of FLV replication in mice. The qRT-PCR assay described here allows specific, sensitive and direct detection of FLV and may also provide more precise measurement of FLV load.

Host genetic factors that control immune responses to retrovirus infections

Several host genes control retroviral replication and pathogenesis. These include genes that directly affect the replication of retroviruses in target cells and those that control the host immune responses to the viral antigens. Host genetic factors that affect retroviral replication and immune responses to the viral antigens have been best studied in mouse models of Friend leukemia virus (FV) infection. Several genes located within the major histocompatibility complex (MHC), along with a separate gene not linked to the MHC, influence the host immune responses to FV antigens. The latter, the Rfv3, regulates the production of virus-neutralizing antibodies, and thus affects the duration of viremia. T-cell responses to the viral epitopes are controlled by MHC class I and class II genotypes, and both CD8(+) and CD4(+) T-cells are required for spontaneous immune resistance to FV infection. When CD4(+) T-helper cells are efficiently primed with a viral epitope, however, CD8(+) T-cells are not required for immune protection against FV infection, while B cells are absolutely required. There are individuals who possess human immunodeficiency virus type 1 (HIV-1)-reactive IgA antibodies in their mucosal secretions and show strong T-cell responses to HIV-1 antigens, even though they are negative for HIV-1 genome and HIV-1-reactive serum IgG. These HIV-1-exposed but uninfected individuals rarely possess resistance-associated alleles at known AIDS-restricting loci such as CCR5Delta32. Recent genetic analyses have indicated that a large proportion of such exposed but uninfected individuals may share a common genetic background.

Evaluation of the Friend Virus model for the development of improved adenovirus-vectored anti-retroviral vaccination strategies

We evaluated the suitability of the Friend Virus (FV) model for the development of improved adenovirus vectors for anti-retroviral vaccination using two types of adenovirus vectors, encoding F-MuLV Env and Gag, which differed only in their fiber genes (Ad5 and Ad5F35). Genetically FV-resistant C57BL/6 mice and highly susceptible CB6F1 hybrid mice were vaccinated by either homologous or heterologous prime-boost regimen. After FV challenge, viral loads in the spleens of C57BL/6 mice were reduced approximately 250-fold and were below the detection threshold in >50% of the mice. Vaccination outcome was critically influenced by the route of vector administration. In CB6F1 mice, vaccination resulted in reduced viremia, delayed onset of splenomegaly, and induction of FV-specific T cells as assessed by tetramer staining. Heterologous prime-boost vaccination resulted in significantly higher neutralizing antibody titers, translating into improved immune protection, in contrast to coexpression of cytokines. Our results suggest that the FV model can provide insight into the development of improved adenovirus vectors for HIV-1 vaccination.

Enhanced natural-killer cell and erythropoietic activities in VEGF-A-overexpressing mice delay F-MuLV-induced erythroleukemia

We have previously reported that VEGF-A, in combination with MCP-5, contributes to leukemia progression within the splenic microenvironment of mice infected with F-MuLV. To study the influence of constitutively elevated VEGF-A levels on the progression of erythroleukemia, mice heterozygous for a VEGF-A "hypermorphic" allele (Vegfhi/+) were inoculated with F-MuLV. Unexpectedly, a significant delay in erythroleukemia was observed in Vegfhi/+ mice when compared with wild-type controls. These results suggested an altered physiologic response arising from elevated VEGF-A levels that decelerated erythroleukemic progression. Characterization of hematopoiesis in Vegfhi/+ spleens showed a higher natural killer cell activity, elevated B cells, and a decrease in T-cell number. Furthermore, higher erythroid progenitors (ie, CD34+, CD36+, and Ter119+ cells) were evident in the bone marrow, spleen, and peripheral blood of Vegfhi/+ mice. The CFU-E levels were significantly elevated in Vegfhi/+ bone marrow cultures, and this elevation was blocked by a neutralizing antibody to VEGF-A receptor (VEGFR-2). Moreover, erythroleukemic mice were treated with recombinant erythropoietin and, similar to diseased Vegfhi/+ mice, showed a delay in disease progression. We propose that a compensatory erythropoietic response combined with increased natural killer (NK) cell activity account for the extended survival of erythroleukemic, Vegfhi/+ mice.

Peptide-induced immune protection of CD8+ T cell-deficient mice against Friend retrovirus-induced disease

CD8+ CTLs and virus-neutralizing antibodies have been associated with spontaneous and vaccine-induced immune control of retroviral infections. We previously showed that a single immunization with an env gene-encoded CD4+ T cell epitope protected mice against fatal Friend retrovirus infection. Here, we analyzed immune cell components required for the peptide-induced anti-retroviral protection. Mice lacking CD8+ T cells were nevertheless protected against Friend virus infection, while mice lacking B cells were not. Virus-producing cells both in the spleen and bone marrow decreased rapidly in their number and became undetectable by 4 weeks after infection in the majority of the peptide-immunized animals even in the absence of CD8+ T cells. In the vaccinated animals the production and class switching of virus-neutralizing and anti-leukemia cell antibodies were facilitated; however, virus-induced erythroid cell expansion was suppressed before neutralizing antibodies became detectable in the serum. Further, the numbers of virus-producing cells in the spleen and bone marrow in the early stage of the infection were smaller in the peptide-immunized than in unimmunized control mice in the absence of B cells. Thus, peptide immunization facilitates both early cellular and late humoral immune responses that lead to the effective control of the retrovirus-induced disease, but CD8+ T cells are not crucial for the elimination of virus-infected cells in the peptide-primed animals.

Identification of a protective CD4+ T-cell epitope in p15gag of Friend murine leukemia virus and role of the MA protein targeting the plasma membrane in immunogenicity

Recent studies have demonstrated an essential role of Gag-specific CD4+ T-cell responses for viral control in individuals infected with human immunodeficiency virus type 1. However, little is known about epitope specificities and functional roles of the Gag-specific helper T-cell responses in terms of vaccine-induced protection against a pathogenic retroviral challenge. We have previously demonstrated that immunization with Friend murine leukemia virus (F-MuLV) Gag proteins protects mice against the fatal Friend retrovirus (FV) infection. We report here the structure of a protective T helper cell (Th) epitope, (I)VTWEAIAVDPPP, identified in the p15 (MA) region of F-MuLV Gag. In mice immunized with the Th epitope-harboring peptide or a vaccinia virus-expressed native full-length MA protein, FV-induced early splenomegaly regressed rapidly. In these mice, FV-infected cells were eliminated within 4 weeks and the production of virus-neutralizing antibodies was induced rapidly after FV challenge, resulting in strong protection against the virus infection. Interestingly, mice immunized with the whole MA mounted strong CD4+ T-cell responses to the identified Th epitope, whereas mice immunized with mutant MA proteins that were not bound to the plasma membrane failed to mount efficient CD4+ T-cell responses, despite the presence of the Th epitope. These mutant MA proteins also failed to induce strong protection against FV challenge. These data indicate the importance of the properly processible MA molecule for CD4+ T-cell priming and for the resultant induction of an effective immune response against retrovirus infections.

Both T and non-T cells with proliferating potentials are effective in inducing suppression of allograft responses by alloantigen-specific intravenous presensitization combined with suboptimal doses of 15-deoxyspergualin

In an MHC class I-disparate combination of mouse strains, a single intravenous injection of donor spleen cells combined with 10 suboptimal doses of 15-deoxyspergualin (DSG) administration was effective in inducing donor-specific suppression of cytotoxic T-lymphocyte (CTL) responses and prolonged survival of the relevant skin allograft. Proliferative potentials of the donor spleen cells were requirement for the induction of suppressed allospecific responses, but both highly purified T cells and non-T cells were equally effective to induce the suppression of CTL responses by intravenous injection. These results have shown that, although working on different mechanisms, DSG is as effective as FK506 or rapamycin in inducing allograft tolerance when used at suboptimal doses along with the donor-specific intravenous presensitization, and an immune mechanism other than well-characterized veto T cells is working in this model in suppressing alloreactive CTL precursors.

Effective postexposure treatment of retrovirus-induced disease with immunostimulatory DNA containing CpG motifs

Therapeutic strategies for the treatment of acute retroviral infections have relied mainly on antiviral drugs. In this study we used the Friend virus model system to demonstrate that enhancement of the immune system can also have dramatic therapeutic effects. Since resistance to Friend virus-induced leukemia in mice is associated with T helper cell type 1 (Th1) immune responses, we enhanced these responses in susceptible mice by treatment with synthetic oligodeoxynucleotides containing unmethylated CpG motifs (CpG-ODN). Treatments begun at 4 days postinfection increased recovery from 6% in the control group to 74% in the CpG-treated group. CpG-mediated recovery was associated with a significant reduction of viral loads in the blood and spleens of treated mice compared to those of control animals. The treatment promoted Th1-type cytokine production by splenocytes of Friend virus-infected mice and augmented Friend virus-specific cytotoxic T-cell responses, but no influence on the virus-specific neutralizing antibody response was observed. Friend virus-specific CD8(+) T cells were critical for effective treatment with CpG-ODN, since in vivo depletion of these cells from treated mice prevented their recovery. Our results demonstrate that CpG-ODN therapy can significantly enhance virus-specific cellular immune responses and prevent retrovirus-induced disease. These findings may have implications for antiviral therapy in general.

Temporal effects of gamma interferon deficiency on the course of Friend retrovirus infection in mice

The current studies demonstrate complex and seemingly contradictory effects by gamma interferon (IFN-gamma) on Friend virus (FV) infection. Both temporal and tissue-specific effects were observed. During the first week of infection, IFN-gamma-deficiency caused increased levels of FV infection in multiple tissues. Surprisingly, however, by 2 weeks postinfection, IFN-gamma-deficient mice had significantly lower levels of infection in both the spleen and bone marrow compared to wild-type mice. The rapid reduction of virus in the IFN-gamma-deficient mice correlated with a more rapid virus-neutralizing antibody response than was observed in the wild-type mice. Furthermore, the virus-neutralizing antibody response in wild-type mice could be accelerated by ablation of their IFN-gamma response. Although the IFN-gamma-deficient mice developed an accelerated virus-neutralizing antibody response, they did not class-switch to immunoglobulin G class immunoglobulins nor could they maintain long-term virus-neutralizing antibody titers. Eventually, all of the IFN-gamma-deficient mice failed to keep persistent virus in check and developed fatal FV-induced erythroleukemia.