Active and passive immunization against HIV type 1 infection in chimpanzees

Several vaccine strategies have been developed to prevent primary infection with human immunodeficiency virus (HIV), and some of the candidate vaccines have been tested in chimpanzees to determine their safety, efficacy, and to delineate immune correlates of protection. To date, more than 25 vaccines representing active and passive immunization strategies have been evaluated in the chimpanzee model. Efficacy of a given vaccine was based on protection against primary infection with HIV after intravenous or mucosal challenge with cell-free or cell-associated virus. Based on the results from a majority of the studies, neutralizing antibodies appear to play a major role in preventing primary infection with HIV.

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.

The consequence of passive administration of an anti-human immunodeficiency virus type 1 neutralizing monoclonal antibody before challenge of chimpanzees with a primary virus isolate

The anti-gp41 virus neutralizing monoclonal antibody 2F5 was infused into chimpanzees, which were then given an intravenous challenge with a primary human immunodeficiency virus type I (HIV-1) isolate. In two control animals, the infection was established immediately, as evidenced by positive cell-associated DNA PCR and serum RNA PCR tests within 1 week, seroconversion by 4 weeks, and development of lymphadenopathy in this acute phase. Serum RNA PCR tests were negative in one of the two antibody-infused animals until week 8 and in the other antibody-infused animal until week 12; both animals seroconverted at week 14. The peak of measurable virus-specific serum RNA was delayed until week 16 in one antibody-infused animal. Virus-specific RNA in the other animal did not reach levels comparable to those in the other animals through 1 year of follow-up studies. Virus was isolated from the week 16 blood sample from one infused animal. Virus was not isolated from peripheral blood of the second animal but was isolated from lymph node cells taken at week 36. The infection of untreated chimpanzees with this primary isolate appears robust. Use of this isolate should widen the scope of possible experiments in the chimpanzee model. This antibody infusion study indicates that neutralizing antibody, when present at the time of challenge, affects the timing and level of infection and remains influential after it can no longer be detected in the peripheral circulation. It is possible that preexisting, neutralizing antibodies (passively administered or actively elicited) affect the course of acute-phase virus replication in humans. It remains to be established whether these immunologically mediated early effects will influence the course of HIV-1 disease.

Titration of a vaccine stock preparation of human immunodeficiency virus type 1SF2 in cultured lymphocytes and in chimpanzees

A large stock preparation of the HIV-1SF2 isolate has been derived after serial passage in human peripheral blood mononuclear cells (PBMCs). This viral stock has a titer of 10(4.9) TCID50 in human PBMCs and 10(4.2) TCID50 in chimpanzee PBMCs. By inoculation into animals the 50% chimpanzee infectious dose titer was found to be about 10(2.3). Virus isolation from animals was achieved on most occasions within 1-4 weeks after inoculation and then became transient. Viral RNA and DNA PCR analyses confirmed the virus infection of the chimpanzees. Anti-HIV antibody levels in the inoculated animals ranged from 1:400 to 1:6400 as measured by ELISA. About 680 vials of this stock preparation, frozen at -190 degrees C, are available for future studies of vaccines and antiviral therapies.

Correlates of protective immunity against HIV-1 infection in immunized chimpanzees

Several experimental vaccination strategies have been developed to prevent primary infection with human immunodeficiency virus (HIV), and as immunotherapeutics for infected individuals. Many of the putative vaccines have been tested in chimpanzees (p. troglodytes) to determine their safety, efficacy, and to delineate immune correlates of protection. To date, approximately 25 candidate vaccines representing active or passive strategies have been evaluated in chimpanzees, and efficacy has been based on protection against primary infection following intravenous or mucosal challenge with cell-free or cell-associated virus. Active immunization has been attempted with whole inactivated virus, envelope depleted viral preparation, vaccinia vector expressing gp 160 in combination with other viral gene products, and subunit vaccines containing recombinant gp 120 derived from different isolates. Polyclonal and monoclonal antibodies with neutralizing activity have been utilized for pre- and post-exposure passive immunization to block primary infection with HIV.

Protection of MN-rgp120-immunized chimpanzees from heterologous infection with a primary isolate of human immunodeficiency virus type 1

Three chimpanzees immunized with recombinant gp120 from human immunodeficiency virus type 1 (HIV-1) strain MN and 1 control animal were challenged intravenously with a primary isolate of HIV-1SF2. Viral infection was detected in the control animal by viral culture, polymerase chain reaction, and multiple serologic assays beginning 2 weeks after infection. Markers of HIV-1 infection were not detected in any of the gp120-vaccinated animals during 12 months of follow-up. Antisera from the gp120-immunized chimpanzees were unable to neutralize the challenge virus cultured in peripheral blood mononuclear cells (PBMC). These studies demonstrate that immunization with recombinant gp120 derived from a T cell-adapted isolate prevented infection by a heterologous primary isolate of HIV-1. The results suggest that in vitro virus neutralization assays utilizing primary isolates cultured in PBMC may be imperfect indicators of protection in vivo.

Resistance of chimpanzees immunized with recombinant gp120SF2 to challenge by HIV-1SF2

OBJECTIVE

To determine whether vaccination with recombinant HIV-1SF2 gp120 in a novel oil-in-water adjuvant emulsion, MF59, protects chimpanzees against challenge with HIV-1SF2, the homologous virus isolate.

METHODS

Two vaccinated chimpanzees and two control animals were challenged with 25-50 animal infectious doses of a stock of HIV-1SF2 that had been grown in mitogen-activated human peripheral blood mononuclear cells (PBMC). The animals were monitored by a series of serologic [enzyme-linked immunosorbent assay (ELISA), Western blot, and neutralization assays] and virologic [virus culture, RNA and DNA polymerase chain reaction (PCR)] assays for infection.

RESULTS

Both control animals showed evidence of seroconversion in ELISA and Western blot assays. In addition, virus was detected in the early, acute phase of infection of both control animals by (1) plasma RNA PCR, (2) virus culture, and (3) PBMC DNA PCR assays. One vaccinated animal showed no serologic or virologic evidence of infection. The other vaccinated animal has not seroconverted, and there was no evidence of plasma viremia. However, virus was detected at early timepoints in this animal's PBMC, and transient lymphoproliferation to HIV-1 proteins not in the vaccine was observed. These observations suggest that the former animal was protected from challenge while the latter may have experienced a transient or curtailed infection.

CONCLUSION

Two types of vaccine-induced protective immune responses were observed when chimpanzees immunized with rgp120SF2 were challenged with the homologous virus isolate: a response consistent with the 'sterilizing immune response' documented in the chimpanzee model in previous studies, as well as one that did not completely protect from infection, showing curtailment of the acute phase and a failure of the animal to seroconvert.

Isolation, cloning, and sequencing of simian foamy viruses from chimpanzees (SFVcpz): high homology to human foamy virus (HFV)

Several independent isolates of simian foamy viruses (SFV) were recovered from chimpanzee B-cell lines. One isolate, designated SFVcpz, was molecularly cloned and sequenced. In addition, the genome of SFV type 6 (SFV-6), an independent chimpanzee foamy virus isolate, was partially cloned. The SFVcpz provirus is 13,246 base pairs (bp) long. It is flanked by long terminal repeats (LTRs) and encodes the genes gag, pol, env, the transcriptional transactivator taf, and a second 3' open reading frame (orf-2). DNA sequences of molecular clones derived from the pol, env, and orf-2 genes of SFV-6 are almost identical to those of SFVcpz. DNA and deduced protein sequences of SFVcpz show high homologies to human foamy virus (HFV), whereas both SFV-1 from a rhesus macaque and SFV-3 from an African green monkey are phylogenetically further distant viruses. Amino acid homologies between corresponding genes of SFVcpz and HFV range between 86% for the taf gene and 95% for the pol gene. Comparisons of taf and pol of SFVcpz with SFV-1 and SFV-3 show 40 and 78% homology, respectively. The SFVcpz LTR consists of 1760 bp and is in the same size range as the LTRs of SFV-1 and -3, but significantly larger than the known HFV LTR. These comparisons reveal that a region approximately 500 bp long is missing in the HFV LTR. We also isolated and sequenced an LTR of a wild-type HFV provirus which aligns with high homology to the SFVcpz LTR without major gaps. Based on sequence comparisons in this report, primate foamy viruses may be arranged into different clusters with SFVcpz and HFV forming one cluster and SFV-1 and SFV-3 as prototypes for two unique clusters.

Protection of chimpanzees from infection by HIV-1 after vaccination with recombinant glycoprotein gp120 but not gp160

The development of a vaccine to provide protective immunity to human immunodeficiency virus type 1 (HIV-1), the virus causing AIDS, would be the most practical method to control its spread. Subunit vaccines consisting of virus envelope glycoproteins, produced by recombinant DNA technology, are effective in preventing viral infections. We have now used this approach in the development of a candidate AIDS vaccine. Chimpanzees were immunized with recombinant forms of the HIV-1 glycoproteins gp120 and gp160 produced in Chinese hamster ovary cells, and then challenged with HIV-1. The control and the two animals immunized with the gp160 variant became infected within 7 weeks of challenge. The two animals immunized with the gp120 variant have shown no signs of infection after more than 6 months. These studies demonstrate that recombinant gp120, formulated in an adjuvant approved for human use, can elicit protective immunity against a homologous strain of HIV-1.

Isolation and characterization of simian immunodeficiency viruses from two subspecies of African green monkeys

Cercopithecus aethiops (African Green monkey), a nonhuman primate species distributed throughout subsaharan Africa, has been shown to have high seroprevalence rates of antibodies to simian immunodeficiency virus (SIV), and therefore, has been proposed as a natural reservoir for immunodeficiency viruses. Our laboratories have isolated SIV-like viruses from two East African subspecies of C. aethiops designated grivet and vervet monkeys. Analysis of the structural proteins based on the molecular weights and immunologic cross-reactivity to the prototypic SIV(MAC), HIV-1, and HIV-2 isolates suggests that these viruses are distinctly different. Heterogeneity was observed in the molecular weights of the gag, pol, and env gene products between SIV isolates from vervets [SIV(AGM(VER))] and grivets [SIV(AGM(GRI))]. Phenotypically, SIV(AGM(VER)) isolates were distinguishable from SIV(AGM(GRI)) isolates by the apparent size difference of the major core antigen p24. All SIV(AGM(GRI)) and SIV(AGM(VER)) isolates were found to encode a transmembrane protein of approximately 40 kD (gp40) in contrast to gp32 of SIV(MAC). Furthermore, the transmembrane protein was shown to be encoded by the entire env open reading frame, unlike gp32 of SIC(MAC) or gp36 of SIV(AGM(TYO-1)). These data indicate that viruses from C. aethiops share common features with SIV(MAC) and HIV-1, but represent diverse SIV-like viruses which may vary according to subspecies and geographic location.

In vitro reactivity of human lymphocytes in chronic uraemia: analysis and interpretation

Altered cellular immunity complicating chronic uraemia includes lymphocytopenia, thymic atrophy, impaired allograft rejection and delayed hypersensitivity in skin tests, diminished appearances of lymphocytes on skin windows, and shortened in vitro survival of uraemic lymphocytes. Studies were undertaken to further characterize these lymphocyte defects.

Lymphocytes separated from peripheral blood of twenty-four uraemic patients were compared with those from fifty-one normal persons in their rates of RNA and DNA synthesis in both PHA-stimulated and unstimulated cultures, as determined by incorporation of radiolabelled precursors. Synthesis, expressed as absolute incorporation/10⁶ viable lymphocytes, was accelerated in the majority of both stimulated and unstimulated uraemic cultures. Serial studies over several months of twenty-five uraemic patients (fifteen maintained by haemodialysis, ten by renal allotransplantation), compared with nineteen normal controls, showed these differences to be consistent and persistent. While normal lymphocytes exhibited stability, uraemic cells fluctuated widely in their serial synthesis rates. In 25% of such cultures, unstimulated synthesis exceeded that induced by PHA. Increased synthesis without PHA, reflecting enhanced spontaneous blastogenesis, is compatible with decreased survival, numbers, and functional capacity of uraemic lymphocytes.

Reports by others of diminished PHA-responsiveness of uraemic lymphocytes are based upon whole-culture incorporation and/or expressed as ratios between stimulated and unstimulated cultures. Both shortened survival and accelerated spontaneous nucleic acid synthesis by uraemic lymphocytes cause such ratios to be misleadingly low. Such factors as cell numbers and viability at harvest, counting efficiency, and culture sterility are essential to avoid misinterpretations of such data based on incorporation of radiolabelled precursors to the nucleic acids.