Vesicular stomatitis virus matrix protein impairs CD1d-mediated antigen presentation through activation of the p38 MAPK pathway

Natural killer T (NKT) cells are unique T lymphocytes that recognize CD1d-bound lipid antigens and play an important role in both innate and acquired immune responses against infectious diseases and tumors. We have already shown that a vesicular stomatitis virus (VSV) infection results in the rapid inhibition of murine CD1d-mediated antigen presentation to NKT cells. In the present study, it was found that the VSV matrix (VSV-M) protein is an important element in this decrease in antigen presentation postinfection. The VSV-M protein altered the intracellular distribution of murine CD1d molecules, resulting in qualitative (but not quantitative) changes in cell surface CD1d expression. The M protein was distributed throughout the infected cell, and it was found to activate the mitogen-activated protein kinase (MAPK) p38 very early postinfection. Infection of CD1d⁺ cells with a temperature-sensitive VSV-M mutant at the nonpermissive temperature both substantially reversed the inhibition of antigen presentation by CD1d and delayed the activation of p38. Thus, the VSV-M protein plays an important role in permitting the virus to evade important components of the innate immune response by regulating specific MAPK pathways.     

CD4+ T cell responses to self- and mutated p53 determinants during tumorigenesis in mice

We analyzed CD4+ T helper responses to wild-type (wt) and mutated (mut) p53 protein in normal and tumor-bearing mice. In normal mice, we observed that although some self-p53 determinants induced negative selection of p53-reactive CD4+ T cells, other p53 determinants (cryptic) were immunogenic. Next, BALB/c mice were inoculated with J774 syngeneic tumor cell line expressing mut p53. BALB/c tumor-bearing mice mounted potent CD4+ T cell responses to two formerly cryptic peptides on self-p53. This response was characterized by massive production of IL-5, a Th2-type lymphokine. Interestingly, we found that T cell response was induced by different p53 peptides depending upon the stage of cancer. Mut p53 gene was shown to contain a single mutation resulting in the substitution of a tyrosine by a histidine at position 231 of the protein. Two peptides corresponding to wt and mutated sequences of this region were synthesized. Both peptides bound to the MHC class II-presenting molecule (Ed) with similar affinities. However, only mut p53.225-239 induced T cell responses in normal BALB/c mice, a result strongly suggesting that high-affinity wt p53.225-239 autoreactive T cells had been eliminated in these mice. Surprisingly, CD4+ T cell responses to both mut and wt p53.225-239 peptides were recorded in J774 tumor-bearing mice, a phenomenon attributed to the recruitment of low-avidity p53.225-239 self-reactive T cells.     

The stress-response factor SigH modulates the interaction between Mycobacterium tuberculosis and host phagocytes

The Mycobacterium tuberculosis stress response factor SigH plays a crucial role in modulating the pathogen's response to heat, oxidative-stress, envelope damage and hypoxia. We hypothesized that the lack of this key stress response factor would alter the interaction between the pathogen and its host cells. We compared the interaction of Mtb, Mtb:Δ-sigH and a strain where the mutation had been genetically complemented (Mtb: Δ-sigH:CO) with primary rhesus macaque bone marrow derived macrophages (Rh-BMDMs). The expression of numerous inducible and homeostatic (CCL) β-chemokines and several apoptotic markers was induced to higher levels in the cells infected with Mtb:Δ-sigH, relative to Mtb or the complemented strain. The differential expression of these genes manifested into functional differences in chemotaxis and apoptosis in cells infected with these two strains. The mutant strain also exhibited reduced late-stage survival in Rh-BMDMs. We hypothesize that the product of one or more SigH-dependent genes may modulate the innate interaction of Mtb with host cells, effectively reducing the chemokine-mediated recruitment of immune effector cells, apoptosis of infected monocytes and enhancing the long-term survival and replication of the pathogen in this milieu The significantly higher induction of Prostaglandin Synthetase 2 (PTGS2 or COX2) in Rh-BMDMs infected with Mtb relative to Mtb: Δ-sigH may explain reduced apoptosis in Mtb-infected cells, as PTGS2 is known to inhibit p53-dependent apoptosis.The SigH-regulon modulates the innate interaction of Mtb with host phagocytes, perhaps as part of a strategy to limit its clearance and prolong its survival. The SigH regulon appears to be required to modulate innate immune responses directed against Mtb.     

Tyrosyl kinases acquired from anchorage-independent cells by a membrane-enveloped virus

Tyrosyl kinase activity in vesicular stomatitis virus (VSV) acquired from host cells that differ in morphology was investigated. VSV grown in baby hamster kidney (BHK) cells with rounded morphology and a high efficiency of colony formation in soft agar (Rous sarcoma virus [RSV]- transformed and suspension BHK cells) was compared with VSV grown in BHK cells with a flattened morphology and lower efficiency of colony formation in soft agar (RSV-infected revertant and control BHK cells). Tyrosyl kinase activity measured with the substrates angiotensin II peptide or casein was found at 7-10-fold higher levels in virus released from the anchorage-independent BHK cells. Most of the VSV- associated tyrosyl kinases acquired from the RSV-transformed BHK cells reacted with antiserum to pp60src, whereas the activity acquired from the suspension BHK cells was unaffected by anti-src serum. The overall levels of tyrosyl kinase in subcellular fractions of the host BHK cells were also measured. Like the VSV released from them, the RSV- transformed cell extracts contained high levels. The suspension cells, however, contained the same low levels of tyrosyl kinase as was found in the control BHK cell extracts. Therefore, tyrosyl kinase was concentrated and acquired by VSV from the anchorage-independent suspension BHK cells. VSV-associated protein kinases acquired from other cell types followed a similar pattern. Tyrosyl kinase levels were high in VSV released from suspension cultures (Chinese hamster ovary and HeLa) and from virally transformed cells (Kirsten murine sarcoma virus-transformed rat kidney cells) and low in VSV released from an anchorage-dependent primary cell culture (chick embryo fibroblasts).     

Elimination of tumor by CD47/PD-L1 dual-targeting fusion protein that engages innate and adaptive immune responses

The host immune system generally serves as a barrier against tumor formation. Programmed death-ligand 1 (PD-L1) is a critical “don't find me” signal to the adaptive immune system, whereas CD47 transmits an anti-phagocytic signal, known as the “don't eat me” signal, to the innate immune system. These and similar immune checkpoints are often overexpressed on human tumors. Thus, dual targeting both innate and adaptive immune checkpoints would likely maximize anti-tumor therapeutic effect and elicit more durable responses. Herein, based on the variable region of atezolizumab and consensus variant 1 (CV1) monomer, we constructed a dual-targeting fusion protein targeting both CD47 and PD-L1 using “Knobs-into-holes” technology, denoted as IAB. It was effective in inducing phagocytosis of tumor cells, stimulating T-cell activation and mediating antibody-dependent cell-mediated cytotoxicity in vitro. No obvious sign of hematological toxicity was observed in mice administered IAB at a dose of 100 mg/kg, and IAB exhibited potent antitumor activity in an immune-competent mouse model of MC38. Additionally, the anti-tumor effect of IAB was impaired by anti-CD8 antibody or clodronate liposomes, which implied that both CD8+ T cells and macrophages were required for the anti-tumor efficacy of IAB and IAB plays an essential role in the engagement of innate and adaptive immune responses. Collectively, these results demonstrate the capacity of an elicited endogenous immune response against tumors and elucidate essential characteristics of synergistic innate and adaptive immune response, and indicate dual blockade of CD47 and PD-L1 by IAB may be a synergistic therapy that activates both innate and adaptive immune response against tumors.     

Generalized selection to overcome innate immunity selects for host breadth in an RNA virus

Virus‐host coevolution has selected for generalized host defense against viruses, exemplified by interferon production/signaling and other innate immune function in eukaryotes such as humans. Although cell‐surface binding primarily limits virus infection success, generalized adaptation to counteract innate immunity across disparate hosts may contribute to RNA virus emergence potential. We examined this idea using vesicular stomatitis virus (VSV) populations previously evolved on strictly immune‐deficient (HeLa) cells, strictly immune competent (MDCK) cells, or on alternating deficient/competent cells. By measuring viral fitness in unselected human cancer cells of differing innate immunity, we confirmed that HeLa‐adapted populations were specialized for innate immune‐deficient hosts, whereas MDCK‐adapted populations were relatively more generalized for fitness on hosts of differing innate immune capacity and of different species origin. We also confirmed that HeLa‐evolved populations maintained fitness in immune‐deficient nonhuman primate cells. These results suggest that innate immunity is more prominent than host species in determining viral fitness at the host‐cell level. Finally, our prediction was inexact that selection on alternating deficient/competent hosts should produce innate viral generalists. Rather, fitness differences among alternating host‐evolved VSV populations indicated variable capacities to evade innate immunity. Our results suggest that the evolutionary history of innate immune selection can affect whether RNA viruses evolve greater host‐breadth.     

p53 serves as a host antiviral factor that enhances innate and adaptive immune responses to influenza A virus

Several direct target genes of the p53 tumor suppressor have been identified within pathways involved in viral sensing, cytokine production, and inflammation, suggesting a potential role of p53 in antiviral immunity. The increasing need to identify immune factors to devise host-targeted therapies against pandemic influenza A virus (IAV) led us to investigate the role of endogenous wild-type p53 on the immune response to IAV. We observed that the absence of p53 resulted in delayed cytokine and antiviral gene responses in lung and bone marrow, decreased dendritic cell activation, and reduced IAV-specific CD8(+) T cell immunity. Consequently, p53(-/-) mice showed a more severe IAV-induced disease compared with their wild-type counterparts. These findings establish that p53 influences the antiviral response to IAV, affecting both innate and adaptive immunity. Thus, in addition to its established functions as a tumor suppressor gene, p53 serves as an IAV host antiviral factor that might be modulated to improve anti-IAV therapy and vaccines.     

Vesicular stomatitis virus as an oncolytic agent against pancreatic ductal adenocarcinoma

Vesicular stomatitis virus (VSV) is a promising oncolytic agent against a variety of cancers. However, it has never been tested in any pancreatic cancer model. Pancreatic ductal adenocarcinoma (PDA) is the most common and aggressive form of pancreatic cancer. In this study, the oncolytic potentials of several VSV variants were analyzed in a panel of 13 clinically relevant human PDA cell lines and compared to conditionally replicative adenoviruses (CRAds), Sendai virus and respiratory syncytial virus. VSV variants showed oncolytic abilities superior to those of other viruses, and some cell lines that exhibited resistance to other viruses were successfully killed by VSV. However, PDA cells were highly heterogeneous in their susceptibility to virus-induced oncolysis, and several cell lines were resistant to all tested viruses. Resistant cells showed low levels of very early VSV RNA synthesis, indicating possible defects at initial stages of infection. In addition, unlike permissive PDA cell lines, most of the resistant cell lines were able to both produce and respond to interferon, suggesting that intact type I interferon responses contributed to their resistance phenotype. Four cell lines that varied in their permissiveness to VSV-ΔM51 and CRAd dl1520 were tested in mice, and the in vivo results closely mimicked those in vitro. While our results demonstrate that VSV is a promising oncolytic agent against PDA, further studies are needed to better understand the molecular mechanisms of resistance of some PDAs to oncolytic virotherapy.     

Ambivalent role of the innate immune response in rabies virus pathogenesis

The neurotropic rabies virus (RABV) has developed several evasive strategies, including immunoevasion, to successfully infect the nervous system (NS) and trigger a fatal encephalomyelitis. Here we show that expression of LGP2, a protein known as either a positive or negative regulator of the RIG-I-mediated innate immune response, is restricted in the NS. We used a new transgenic mouse model (LGP2 TG) overexpressing LGP2 to impair the innate immune response to RABV and thus revealed the role of the RIG-I-mediated innate immune response in RABV pathogenesis. After infection, LGP2 TG mice exhibited reduced expression of inflammatory/chemoattractive molecules, beta interferon (IFN-β), and IFN-stimulated genes in their NS compared to wild-type (WT) mice, demonstrating the inhibitory function of LGP2 in the innate immune response to RABV. Surprisingly, LGP2 TG mice showed more viral clearance in the brain and lower morbidity than WT mice, indicating that the host innate immune response, paradoxically, favors RABV neuroinvasiveness and morbidity. LGP2 TG mice exhibited similar neutralizing antibodies and microglia activation to those of WT mice but showed a reduction of infiltrating CD4(+) T cells and less disappearance of infiltrating CD8(+) T cells. This occurred concomitantly with reduced neural expression of the IFN-inducible protein B7-H1, an immunoevasive protein involved in the elimination of infiltrated CD8(+) T cells. Our study shows that the host innate immune response favors the infiltration of T cells and, at the same time, promotes CD8(+) T cell elimination. Thus, to a certain extent, RABV exploits the innate immune response to develop its immunoevasive strategy.     

SUMOylation of p53 mediates interferon activities

There is growing evidence that many host proteins involved in innate and intrinsic immunity are regulated by SUMOylation, and that SUMO contributes to the regulatory process that governs the initiation of the type I interferon (IFN) response. The tumor suppressor p53 is a modulator of the IFN response that plays a role in virus-induced apoptosis and in IFN-induced senescence. Here we demonstrate that IFN treatment increases the levels of SUMOylated p53 and induces cellular senescence through a process that is partially dependent upon SUMOylation of p53. Similarly, we show that vesicular stomatitis virus (VSV) infection induces p53 SUMOylation, and that this modification favors the control of VSV replication. Thus, our study provides evidence that IFN signaling induces p53 SUMOylation, which results in the activation of a cellular senescence program and contributes to the antiviral functions of interferon.     

Split T cell tolerance against a self/tumor antigen: spontaneous CD4+ but not CD8+ T cell responses against p53 in cancer patients and healthy donors

Analyses of NY-ESO-1-specific spontaneous immune responses in cancer patients revealed that antibody and both CD4(+) and CD8(+) T cell responses were induced together in cancer patients. To explore whether such integrated immune responses are also spontaneously induced for other tumor antigens, we have evaluated antibody and T cell responses against self/tumor antigen p53 in ovarian cancer patients and healthy individuals. We found that 21% (64/298) of ovarian cancer patients but no healthy donors showed specific IgG responses against wild-type p53 protein. While none of 12 patients with high titer p53 antibody showed spontaneous p53-specific CD8(+) T cell responses following a single in vitro sensitization, significant p53-specific IFN-γ producing CD4(+) T cells were detected in 6 patients. Surprisingly, similar levels of p53-specific CD4(+) T cells but not CD8(+) T cells were also detected in 5/10 seronegative cancer patients and 9/12 healthy donors. Importantly, p53-specific CD4(+) T cells in healthy donors originated from a CD45RA(-) antigen-experienced T cell population and recognized naturally processed wild-type p53 protein. These results raise the possibility that p53-specific CD4(+) T cells reflect abnormalities in p53 occurring in normal individuals and that they may play a role in processes of immunosurveillance or immunoregulation of p53-related neoplastic events.     

p53 Degradation by a Coronavirus Papain-like Protease Suppresses Type I Interferon Signaling

Infection by human coronaviruses is usually characterized by rampant viral replication and severe immunopathology in host cells. Recently, the coronavirus papain-like proteases (PLPs) have been identified as suppressors of the innate immune response. However, the molecular mechanism of this inhibition remains unclear. Here, we provide evidence that PLP2, a catalytic domain of the nonstructural protein 3 of human coronavirus NL63 (HCoV-NL63), deubiquitinates and stabilizes the cellular oncoprotein MDM2 and induces the proteasomal degradation of p53. Meanwhile, we identify IRF7 (interferon regulatory factor 7) as a bona fide target gene of p53 to mediate the p53-directed production of type I interferon and the innate immune response. By promoting p53 degradation, PLP2 inhibits the p53-mediated antiviral response and apoptosis to ensure viral growth in infected cells. Thus, our study reveals that coronavirus engages PLPs to escape from the innate antiviral response of the host by inhibiting p53-IRF7-IFNβ signaling.     

Persistent antigen presentation after acute vesicular stomatitis virus infection

Long-term antigen expression is believed to play an important role in modulation of T-cell responses to chronic virus infections. However, recent studies suggest that immune responses may occur late after apparently acute infections. We have now analyzed the CD8 T-cell response to vesicular stomatitis virus (VSV), which is thought to cause to an infection characterized by rapid virus clearance by innate and adaptive immune system components. Unexpectedly, virus-encoded antigen was detectable more than 6 weeks after intranasal VSV infection in both draining and nondraining lymph nodes by adoptively transferred CD8 T cells. Infection with Listeria monocytogenes expressing the same antigen did not result in prolonged antigen presentation. Weeks after VSV infection, discrete T-cell clustering with dendritic cells within the lymph node was observed after transfer of antigen-specific CD8 T cells. Moreover, memory CD8 T cells as defined by phenotype and function were generated from na?ve CD8 T cells entering the response late after infection. These findings suggested that protracted antigen presentation after an apparently acute virus infection may contribute to an ongoing antiviral immune response.     

Signaling Pathways in Murine Dendritic Cells That Regulate the Response to Vesicular Stomatitis Virus Vectors That Express Flagellin

Vesicular stomatitis virus (VSV) vectors that express heterologous antigens have shown promise as vaccines in preclinical studies. The efficacy of VSV-based vaccines can be improved by engineering vectors that enhance innate immune responses. We previously generated a VSV vaccine vector that incorporates two enhancing strategies: an M protein mutation (M51R) that prevents the virus from suppressing host antiviral responses and a gene encoding bacterial flagellin (M51R-F vector). The rationale was that intracellular expression of flagellin would activate innate immune pathways in addition to those activated by virus alone. This was tested with dendritic cells (DCs) from mice containing deletions in key signaling molecules. Infection of DC with either M51R or M51R-F vector induced the production of interleukin-12 (IL-12) and IL-6 and increased surface expression of T cell costimulatory molecules. These responses were dramatically reduced in DCs from IPS-1^−/− mice. Infection with M51R-F vector also induced the production of IL-1β. In addition, in approximately half of the DCs, M51R-F vector induced pyroptosis, a proinflammatory-type of cell death. These responses to flagellin were ablated in DCs from NLRC4^−/− mice but not Toll-like receptor 5-deficient (TLR5^−/−) mice, indicating that they resulted from inflammasome activation. These results demonstrate that flagellin induces additional innate immune mechanisms over those induced by VSV alone.     

Natural killer cells promote early CD8 T cell responses against cytomegalovirus

Understanding the mechanisms that help promote protective immune responses to pathogens is a major challenge in biomedical research and an important goal for the design of innovative therapeutic or vaccination strategies. While natural killer (NK) cells can directly contribute to the control of viral replication, whether, and how, they may help orchestrate global antiviral defense is largely unknown. To address this question, we took advantage of the well-defined molecular interactions involved in the recognition of mouse cytomegalovirus (MCMV) by NK cells. By using congenic or mutant mice and wild-type versus genetically engineered viruses, we examined the consequences on antiviral CD8 T cell responses of specific defects in the ability of the NK cells to control MCMV. This system allowed us to demonstrate, to our knowledge for the first time, that NK cells accelerate CD8 T cell responses against a viral infection in vivo. Moreover, we identify the underlying mechanism as the ability of NK cells to limit IFN-alpha/beta production to levels not immunosuppressive to the host. This is achieved through the early control of cytomegalovirus, which dramatically reduces the activation of plasmacytoid dendritic cells (pDCs) for cytokine production, preserves the conventional dendritic cell (cDC) compartment, and accelerates antiviral CD8 T cell responses. Conversely, exogenous IFN-alpha administration in resistant animals ablates cDCs and delays CD8 T cell activation in the face of NK cell control of viral replication. Collectively, our data demonstrate that the ability of NK cells to respond very early to cytomegalovirus infection critically contributes to balance the intensity of other innate immune responses, which dampens early immunopathology and promotes optimal initiation of antiviral CD8 T cell responses. Thus, the extent to which NK cell responses benefit the host goes beyond their direct antiviral effects and extends to the prevention of innate cytokine shock and to the promotion of adaptive immunity.     

Xenogeneic Human p53 DNA Vaccination by Electroporation Breaks Immune Tolerance to Control Murine Tumors Expressing Mouse p53

The pivotal role of p53 as a tumor suppressor protein is illustrated by the fact that this protein is found mutated in more than 50% of human cancers. In most cases, mutations in p53 greatly increase the otherwise short half-life of this protein in normal tissue and cause it to accumulate in the cytoplasm of tumors. The overexpression of mutated p53 in tumor cells makes p53 a potentially desirable target for the development of cancer immunotherapy. However, p53 protein represents an endogenous tumor-associated antigen (TAA). Immunization against a self-antigen is challenging because an antigen-specific immune response likely generates only low affinity antigen-specific CD8⁺ T-cells. This represents a bottleneck of tumor immunotherapy when targeting endogenous TAAs expressed by tumors. The objective of the current study is to develop a safe cancer immunotherapy using a naked DNA vaccine. The vaccine employs a xenogeneic p53 gene to break immune tolerance resulting in a potent therapeutic antitumor effect against tumors expressing mutated p53. Our study assessed the therapeutic antitumor effect after immunization with DNA encoding human p53 (hp53) or mouse p53 (mp53). Mice immunized with xenogeneic full length hp53 DNA plasmid intramuscularly followed by electroporation were protected against challenge with murine colon cancer MC38 while those immunized with mp53 DNA were not. In a therapeutic model, established MC38 tumors were also well controlled by treatment with hp53 DNA therapy in tumor bearing mice compared to mp53 DNA. Mice vaccinated with hp53 DNA plasmid also exhibited an increase in mp53-specific CD8⁺ T-cell precursors compared to vaccination with mp53 DNA. Antibody depletion experiments also demonstrated that CD8⁺ T-cells play crucial roles in the antitumor effects. This study showed intramuscular vaccination with xenogeneic p53 DNA vaccine followed by electroporation is capable of inducing potent antitumor effects against tumors expressing mutated p53 through CD8⁺ T cells.     

The emerging role of platelets in adaptive immunity

Platelets' foremost role in survival is hemostasis. However, a significant quantity of research has demonstrated that platelets are an integral part of inflammation and can also be potent effector cells of the innate immune response. CD154, a molecule of vital importance to adaptive immune responses, is expressed by activated platelets and has been implicated in platelet-mediated modulation of innate immunity and inflammatory disease states. Recent studies in mice extend the role of platelet CD154 to the adaptive immune response demonstrating that platelets can enhance antigen presentation, improve CD8 T cell responses, and play a critical function in normal T-dependent humoral immunity. The latter studies suggest that the current paradigm for the B cell germinal center response should be modified to include a role for platelets.     

The pathogenesis of schistosomiasis is controlled by cooperating IL-10-producing innate effector and regulatory T cells

IL-10 reduces immunopathology in many persistent infections, yet the contribution of IL-10 from distinct cellular sources remains poorly defined. We generated IL-10/recombination-activating gene (RAG)2-deficient mice and dissected the role of T cell- and non-T cell-derived IL-10 in schistosomiasis by performing adoptive transfers. In this study, we show that IL-10 is generated by both the innate and adaptive immune response following infection, with both sources regulating the development of type-2 immunity, immune-mediated pathology, and survival of the infected host. Importantly, most of the CD4(+) T cell-produced IL-10 was confined to a subset of T cells expressing CD25. These cells were isolated from egg-induced granulomas and exhibited potent suppressive activity in vitro. Nevertheless, when naive, naturally occurring CD4(+)CD25(+) cells were depleted in adoptive transfers, recipient IL-10/RAG2-deficient animals were more susceptible than RAG2-deficient mice, confirming an additional host-protective role for non-T cell-derived IL-10. Thus, innate effectors and regulatory T cells producing IL-10 cooperate to reduce morbidity and prolong survival in schistosomiasis.     

A single-cycle vaccine vector based on vesicular stomatitis virus can induce immune responses comparable to those generated by a replication-competent vector

Live attenuated vaccine vectors based on recombinant vesicular stomatitis virus (VSV) are effective in several viral disease models. In this study, we asked if a VSV vector capable of only a single cycle of replication might be an effective alternative to replication-competent VSV vectors. We compared the cellular immune responses to human immunodeficiency virus (HIV) envelope protein (Env) expressed by replication-competent and single-cycle VSV vectors and also examined the antibody response to Env. The single-cycle vector was grown by complementation with VSV G protein and then tested initially for immunogenicity when given by four different routes. When given by the intramuscular route in mice, we found that the single-cycle vector was equivalent to the replication-competent VSV vector in generating high-level primary and memory CD8 T-cell responses as well as antibody responses to Env. Cellular responses were analyzed using major histocompatibility complex class I tetramers and direct measurement of cytotoxic T-lymphocyte activity in vivo. We also found that the recall responses after boosting were equivalent in animals vaccinated with replication-competent or single-cycle vectors. Additionally, we observed recall and heightened memory responses after boosting animals with a single-cycle vector complemented with G protein from a different vesiculovirus. Because expression of HIV Env by G-deleted VSV might allow replication in human cells expressing CD4, we generated a single-cycle VSV recombinant expressing a secreted form of the HIV Env protein. This virus was just as effective as the recombinant expressing the membrane-anchored Env protein at producing CD8 T cells and antibody responses.