Christopher P. Locher

Baboons as an animal model for human immunodeficiency virus pathogenesis and vaccine development.

Summary: Baboons (Papio cynocephalus) provide a valuable animal model for the study of human immunodeficiency virus (HIV) pathogenesis because HIV‐2 infection of baboons causes a chronic viral disease that progresses over several years before clinical signs of acquired immunodeficiency syndrome (AIDS) appear. Since HIV‐2‐infected baboons develop a chronic viral infection, insights into the immuno‐biology of viral latency, clinical stages of disease, virus infection of lymphatic tissue and HIV transmission can be gained using this animal model. The development of an AIDS‐like disease in baboons is viral isolate and baboon subspecies dependent. Thus, viral virulence factors and host resistance can be studied as well as the mechanisms of innate and acquired immunity. The control of virus infection is dependent upon cytotoxic and non‐cytotoxic antiviral activity of CD8+ T cells. In this regard, some of the HIV‐2‐infected baboons develop potent antiviral cellular immune responses that have a similar magnitude to that found in HIV‐1‐infected long‐term survivors (or non‐progressors). In our laboratory, baboons have been used to study DNA vaccine strategies using new cationic liposome formulations and granulocyte macrophage–colony stimulating factor and B7–2 as genetic adjuvants. The results demonstrate the value of using baboons as an animal model of AIDS pathogenesis and vaccine development.

CD8+ cells from HIV-2-infected baboons control HIV replication

Objective:To analyze the CD8+ cell antiviral immune response in HIV-2-infected baboons. Design:Baboons were infected with clinical isolates of HIV-2. CD8+ cells were isolated from phytohemagglutinin (PHA)-stimulated baboon peripheral blood mononuclear cells (PBMC). These cells were cultured with PHA-stimulated CD4+ cells acutely infected with HIV-2 at several CD8+ : CD4+ cell ratios. Control of HIV-2 replication was determined by comparing peak levels of HIV-2 replication in fluids from CD8+ : CD4+ cell cocultures with those in fluids from infected CD4+ cells cultured alone. Results:CD8+ cells from HIV-2-infected baboons inhibited HIV-2 replication in acutely infected autologous CD4+ cells to a significantly greater extent than did CD8+ cells from uninfected baboons (P = 0.0001). At the beginning of the acute phase of HIV-2 infection, CD8+ cells showed either a transient reduction or loss in the antiviral activity. In some cases the CD8+ cell response enhanced HIV-2 replication. Subsequently, the strength of the CD8+ cell antiviral activity increased concomitant with a decrease in the HIV-2 load in the PBMC. Suppression of HIV replication could be demonstrated with filtered fluid from CD8+ cells. Other studies indicated that infected CD4+ cells are lost during coculture of CD8+ cells with infected CD4+ cells. Conclusions:CD8+ cells of HIV-2-infected baboons develop substantial anti-HIV-2 activity following HIV-2 infection, which may account in part for the low frequency of pathogenesis in HIV-2-infected baboons. Studies to elucidate the mechanism of this CD8+ cell antiviral activity suggest that it is mediated in part by a soluble antiviral factor, but primarily in association with the loss of infected CD4+ cells.

CD8+ cells from asymptomatic human immunodeficiency virus-infected individuals suppress superinfection of their peripheral blood mononuclear cells.

Most human immunodeficiency virus (HIV)-infected individuals show evidence of infection by only one strain of the virus despite possible frequent contact with multiple strains. The reason(s) for the emergence of a dominant strain of virus in HIV-infected people and the mechanism(s) which prevent other strains from establishing an infection is not known. In the present study, we demonstrate that peripheral blood mononuclear cells (PBMC) of asymptomatic HIV-infected individuals can resist productive infection by HIV-1 and HIV-2 strains. Although the PBMC of these individuals are resistant to superinfection, their CD4+ cells are susceptible to infection. Moreover, two weeks after infection of their PBMC in culture, the superinfecting virus can be recovered from isolated CD4+ cells. When CD8+ cells from asymptomatic individuals are added to the superinfected CD4+ cells, replication of the exogenously introduced virus is inhibited. In contrast, PBMC from individuals who have progressed to disease (Progressors) do not resist superinfection and their CD8+ cells do not showthe antiviral activity which controls productive HIV infection. These findings suggest that CD8+ cells suppressing HIV replication in infected individuals may be critical in preventing the establishment of infection by other strains of HIV by blocking virus replication.

Short Communication Antibody and Cellular Immune Responses in Breakthrough Infection Subjects after HIV Type 1 Glycoprotein 120 Vaccination

HIV-specific antibodies and CD8+ T cell antiviral responses were evaluated in three human immunodeficiency virus 1 (HIV-1) gp120 vaccine recipients who later became infected with HIV-1. Titers of neutralizing antibody to the HIV-1(SF2) vaccine isolate were boosted, but titers of antibody to the autologous infecting viruses were never high and required at least 6 months after HIV infection to develop. Similarly, a marginal noncytotoxic CD8+ T cell antiviral response was observed only in one of the three vaccinees 3 months after HIV-1 infection. The infecting virus isolates had several amino acid substitutions in the HIV-1 envelope V3 region but were similar to other regional HIV-1 clade B isolates. Viral loads were similar to those of other HIV-1-infected individuals who had not been vaccinated and transient CD4+ T cell declines were observed in each person, suggesting that the vaccine was not effective at controlling these prognostic markers early in infection.

Increased Virus Replication and Virulence after Serial Passage of Human Immunodeficiency Virus Type 2 in Baboons

ABSTRACT Similar to human immunodeficiency virus type 1 (HIV-1) infection of humans, the natural history of HIV-2 infection in baboons (Papio cynocephalus) is a slow and chronic disease that generally takes several years before an AIDS-like condition develops. To shorten the amount of time to the development of disease, we performed five serial passages of HIV-2UC2 in baboons by using blood and bone marrow samples during the acute phase of infection when viral loads were at high levels. After these serial passages, virus levels in plasma, peripheral blood mononuclear cells (PBMC) and lymphatic tissues in the acutely infected baboons were increased. Within 1 year of the HIV-2 infection, all of the inoculated baboons showed specific signs of AIDS-related disease progression within the lymphatic tissues, such as vascular proliferation and lymphoid depletion. The HIV-2UC2 recovered after four serial passages showed increased kinetics of viral replication in baboon PBMC and cytopathicity. This study suggests that the HIV-2 isolate recovered after several serial passages in baboons will be useful in future studies of AIDS pathogenesis and vaccine development by using this animal model.

Superinfection with Human Immunodeficiency Virus Type 2 Can Reactivate Virus Production in Baboons but Is Contained by a CD8 T Cell Antiviral Response

An animal model was used to assess whether resistance to superinfection by human immunodeficiency virus (HIV) can exist in vivo. Asymptomatic baboons (Papio cynocephalus), previously infected with HIV-2, were first challenged with homologous virus (HIV-2UC2 or HIV-2UC14) and later with heterologous virus (HIV-2UC12). After both virus inoculations, either resistance to viral infection or a transient viremia was observed. The original virus was recovered in 3 baboons, suggesting that reactivation of a latent infection occurred on heterologous challenge and that HIV-2 superinfection is blocked by processes established during prior infection. Antibody titers measured by ELISA and virus neutralization remained at low levels. However, suppression of HIV-1 replication was observed with CD8 T cells and filtered cell culture supernatants. The soluble factor involved was not a beta-chemokine. This resistance to HIV superinfection appears to be mediated at least in part by CD8 T cells that suppress virus production.

Suppression of human immunodeficiency virus type 1 replication by a soluble factor produced by CD8+ lymphocytes from HIV-2-infected baboons.

Human immunodeficiency virus type 2 (HIV-2)-infected baboons (Papio cynocephalus) provide a valuable animal model for the study of acquired immunodefidency syndrome (AIDS) pathogenesis since many features of disease progression resemble HIV-1-infection of humans. In some HIV-2-infected baboons that are clinically healthy, a CD8+ cell antiviral response, that is partly mediated by a soluble factor, controls viral replication in vitro. In the present study, we demonstrate that CD8+ cells derived from HIV-2-infected baboon peripheral blood, lymph nodes, adenoids and tonsils had antiviral activity in co-cultures of CD8+ and CD4+ cells that inversely correlates with viral load. A soluble factor was found to be active against the chemokine-resistant, syncytium-inducing HIV-1SF2 and HIV-1SF33 isolates and was relatively heat stable at 100 degrees C for 10 min. Moreover, inhibition of the transcription from the long terminal repeat of HIV-1 was observed in 1G5 cells after activation with phorbol 12-myristate 13-acetate. Therefore, the soluble suppressing activity of CD8+ cells in HIV-2-infected baboons may be analogous to the CD8+ cell antiviral factor described in human HIV-infected asymptomatic people.

Evaluation of genetic immunization adjuvants to improve the effectiveness of a human immunodeficiency virus type 2 (HIV-2) envelope DNA vaccine.

In an effort to develop a more effective genetic immunization strategy for HIV, we developed an HIV-2 env DNA vaccine and evaluated three adjuvant formulations. The gp140 gene from HIV-2(UC2 )was synthesized using mammalian codons and cloned into a plasmid vector that expresses eukaryotic genes at high levels. We found that after three immunizations in mice, a novel cationic liposome formulation (Vaxfectin) was superior at inducing systemic and mucosal antibody responses compared to a naked DNA, a controlled release device (an Alzet minipump) and polysaccharide microparticles made from chitosan (P = 0.027). Vaxfectin also induced higher levels of systemic antibodies for each isotype and IgG subclass as well as levels of HIV-2-specific mucosal IgA (P = 0.034). When different routes of immunization were used with the Vaxfectin formulation, gp140-specific systemic antibody responses were highest by the intradermal route, mucosal antibody responses were highest by the intramuscular route, while the intranasal route was the least effective. These results suggest that this cationic liposome formulation is an important adjuvant to improve the effectiveness of genetic immunization strategies for AIDS, and that multiple routes of immunization should be employed for optimal efficacy for HIV vaccine candidates.

Immune responses in baboons vaccinated with HIV‐2 genetic expression libraries

Abstract: Immunization using genetic expression libraries may be an improvement over conventional DNA immunization using a single gene because more epitopes are simultaneously presented to the immune system. In this study, we evaluated the effectiveness of an HIV‐2 vaccine made from a genomic expression library in baboons. We found that HIV‐2 expression library immunization induced HIV‐2‐specific memory responses but low levels of CD8+ cell anti‐viral responses and neutralizing antibodies. After intravenous virus challenge using a homologous pathogenic variant, HIV‐2UC2/9429, viral loads were similar in the HIV‐2‐immunized and control baboons. We conclude that although immunization using HIV‐2 expression libraries induces immune responses, this approach does not provide protection in baboons against intravenous challenge with HIV‐2.

Enhancement of antibody responses to an HIV-2 DNA envelope vaccine using an expression vector containing a constitutive transport element.

Because immune responses to DNA vaccines in humans remains suboptimal, strategies need to be devised to facilitate expression of the vaccine in vivo. One method to improve response to a DNA vaccine is to construct plasmid vectors with leader sequences and post-transcriptional elements that facilitate export of transcribed RNA. In this study, we sought to determine if a mammalian expression vector (pND-14) containing a tissue plasminogen activator (TPA) leader sequence and a constitutive transport element (CTE) from simian retrovirus was superior to other mammalian expression vectors containing a post-transcriptional regulatory element (PRE) from hepatitis B virus (pCMV-link) or a minimal mammalian expression vector (pVAX1). Toward this objective, we evaluated protein expression of the HIV-2 envelope gene (gp140) in vitro and immune responses in immunized mice. We found that pVAX1 produced three- to fourfold lower levels of gp140 in vitro (5 ng/ml) in contrast to the pCMV-link and pND-14 vectors. When we immunized groups of mice intradermally with two of the HIV-2 gp140 DNA vaccine constructs, we found that pND-14 induced higher levels of envelope-specific systemic and mucosal antibodies than pCMV-link. We conclude that expression vectors for DNA vaccines should contain TPA and CTE sequences to facilitate immune responses.