Antigenic variation of primate lentiviruses in humans and experimentally infected macaques

N-terminal proteolytic processing by cathepsin G converts RANTES/CCL5 and related analogs into a truncated 4-68 variant

N-terminal proteolytic processing modulates the biological activity and receptor specificity of RANTES/CCL5. Previously, we showed that an unidentified protease associated with monocytes and neutrophils digests RANTES into a variant lacking three N-terminal residues (4-68 RANTES). This variant binds CCR5 but exhibits lower chemotactic and antiviral activities than unprocessed RANTES. In this study, we characterize cathepsin G as the enzyme responsible for this processing. Cell-mediated production of the 4-68 variant was abrogated by Eglin C, a leukocyte elastase and cathepsin G inhibitor, but not by the elastase inhibitor elastatinal. Further, anti-cathepsin G antibodies abrogated RANTES digestion in neutrophil cultures. In accordance, reagent cathepsin G specifically digested recombinant RANTES into the 4-68 variant. AOP-RANTES and Met-RANTES were also converted into the 4-68 variant upon exposure to cathepsin G or neutrophils, while PSC-RANTES was resistant to such cleavage. Similarly, macaque cervicovaginal lavage samples digested Met-RANTES and AOP-RANTES, but not PSC-RANTES, into the 4-68 variant and this processing was also inhibited by anti-cathepsin G antibodies. These findings suggest that cathepsin G mediates a novel pathway for regulating RANTES activity and may be relevant to the role of RANTES and its analogs in preventing HIV infection.

Some basic properties of immune selection

We analyze models for the evolutionary dynamics of viral or other infectious agents within a host. We study how the invasion of a new strain affects the composition and diversity of the viral population. We show that--under strain-specific immunity--the equilibrium abundance of uninfected cells declines during viral evolution. In addition, for cytotoxic immunity the absolute force of infection, and for non-cytotoxic immunity the absolute cellular virulence increases during viral evolution. We prove global stability by means of Lyapunov functions. These unidirectional trends of virus evolution under immune selection do not hold for general cross-reactive immune responses, which introduce frequency-dependent selection among viral strains. Therefore, appropriate cross-reactive immunity can lead to a viral evolution within a host which limits the extent of the disease.

Live, attenuated simian immunodeficiency virus vaccines elicit potent resistance against a challenge with a human immunodeficiency virus type 1 chimeric virus

Three rhesus macaques, previously immunized with SIVdelta3 or SIVdelta2, each an attenuated derivative of SIVmac239, and two naive monkeys were challenged with 30,000 50% tissue culture infective doses of SHIV, an SIV/human immunodeficiency virus type 1 (HIV-1) chimeric virus bearing the dual-tropic envelope of HIV-1DH12. By several criteria, including virus isolation, serological assays, and PCR (both DNA and reverse transcriptase), SHIV levels were reduced to barely detectable levels in the circulating blood of vaccinated animals. The resistant SIV-vaccinated macaques had no preexisting neutralizing antibodies directed against SHIV, nor did they produce neutralizing antibodies at any time over a 14-month observation period following SHIV challenge. Interestingly, SIV sequences, derived from the vaccine, could be amplified from numerous tissue samples collected at the conclusion of the experiment, 60 weeks postchallenge, but SHIV-specific sequences (viz., HIV-1 env) could not. These results demonstrate that live attenuated SIV vaccines provide strong long-term protection even against challenge strains with highly divergent envelope sequences.

Infected macaques that controlled replication of SIVmac or nonpathogenic SHIV developed sterilizing resistance against pathogenic SHIV(KU-1)

Twenty macaques were used to evaluate the ability of nonpathogenic SIV(mac) or nonpathogenic chimeric SIV-HIV (SHIV) to induce protection in macaques against superinfection with a pathogenic variant of SHIV (SHIV(KU-1)) originally containing the tat, rev, vpu, and env of HIV-1 (strain HXB2) in a genetic background of SIV(mac)239. Specifically, three macaques inoculated with molecularly cloned, macrophage-tropic SIV(mac)LG1 developed an early systemic infection but recovered with only traces of SIV(mac) DNA in visceral lymphoid tissues. These animals were then inoculated parenterally with pathogenic SHIV(KU-1). All three animals resisted infection with SHIV(KU-1), as indicated by lack of virus recovery and absence of SHIV-specific env and vpu sequences in the visceral lymphoid tissues and multiple regions in the CNS. We also examined the ability of five macaques that had been inoculated with nonpathogenic SHIV (NP-SHIV) to withstand challenge with the pathogenic SHIV(KU-1). Like the SIV(mac)LG1-inoculated macaques, these animals also resisted SHIV(KU-1) challenge as judged by the inability to recover infectious virus, normal CD4+ T cell counts, and the absence of SHIV(KU-1) signature sequences in the lymph node tissue. Thus, eight of eight animals that developed control over primary lentivirus infections had also developed resistance to infection with pathogenic SHIV(KU-1). Three groups of macaques were used as controls for this study. The first group consisted of six macaques inoculated with SHIV(KU-1) alone. All animals developed viremia, showed severe loss of CD4+ T cells within 4 weeks, and succumbed to AIDS within 6 months. The second group of three macaques was inoculated first with SHIV(KU-1) and inoculated later with uncloned, neurovirulent SIV(mac)7F-Lu. A third group of three macaques was inoculated with SIV(mac)7F-Lu followed by inoculation with SHIV(KU-1). PCR analyses using oligonucleotide primers specific for the SIV or HIV env revealed that macaques from the last two groups had widespread infection with both SHIV(KU-1) and SIV(mac), indicating that animals that failed to control productive replication of either SHIV(KU-1) or SIV(mac)7F-Lu could not resist superinfection with the other virus. These data indicate that sterilizing immunity against the virulent SHIV could be induced in animals that had experienced an immunizing infection. Moreover, the divergence of the envelope glycoprotein of the protective avirulent and virulent challenge virus suggests that a single vaccine could protect against infection with a virus containing a different envelope glycoprotein.

Brucella abortus conjugated with a peptide derived from the V3 loop of human immunodeficiency virus (HIV) type 1 induces HIV-specific cytotoxic T-cell responses in normal and in CD4+ cell-depleted BALB/c mice

We have previously shown that immunization of mice with human immunodeficiency virus (HIV)-derived proteins or peptides conjugated to inactivated Brucella abortus induces the secretion of virus-neutralizing antibodies, predominantly of the immunoglobulin G2a (IgG2a) isotype. In addition, B. abortus activates human CD4+ and CD8+ cells to secrete gamma interferon. Since these are both characteristics of a Th1-type immune response, which is associated with the development of cell-mediated immunity, it was important to determine if B. abortus conjugates would also act as a carrier to induce a cytotoxic T-lymphocyte (CTL) response. To test this hypothesis, we conjugated an 18-amino-acid peptide from the V3 loop of the MN strain of HIV-1 gp120 that contains both B- and cytotoxic T-cell epitopes to B. abortus (B. abortus-MN 18-mer). A 10-amino-acid fragment of this peptide has been shown to be the minimal CTL determinant presented by murine H-2Dd. It was found that two in vivo immunizations with 10(8) organisms of B. abortus-MN 18-mer followed by in vitro stimulation with peptide induced a virus-specific CTL response. Conjugation to B. abortus was required for in vivo priming, since there was no induction of memory CTLs when B. abortus was only mixed with peptide. Targets pulsed with peptide as well as those infected with a vaccinia virus encoding HIV gp160 were killed, demonstrating recognition of naturally processed envelope. Also, major histocompatibility complex-incompatible L cells which were infected with vaccinia viruses that encoded H-2Dd, but not H-2Kd, and pulsed with peptide were lysed. This demonstrated the appropriate major histocompatibility complex class I restriction. Treatment of the mice with anti-L3T4 prior to immunization caused a severe depletion of CD4+ lymphocytes, yet it did not decrease the CTL priming. Thus, inactivated B. abortus can induce non-CD4+ cells to produce the cytokines required for CTL induction. We conclude that B. abortus stimulates a cellular as well as a humoral immune response, even in the relative absence of CD4+ helper cells. It may be a particularly useful vaccine carrier in HIV-1-infected individuals or others with impaired CD4+ T-cell function.

Can evolutionary principles help resolve the acquired immune deficiency syndrome crisis?

The population of retroviruses inside each infected host can quickly adapt its genes in response to survival threats. For this reason, therapeutics attempting to eliminate or inhibit the human immunodeficiency virus have been unsuccessful. However, it may be possible to apply evolutionary principles to the problem of human immunodeficiency virus infection/acquired immune deficiency syndrome. The rapid evolution capabilities of this pathogen might assist researchers in evolving human immunodeficiency virus toward the goal of virus-host coexistence. In this relationship, infecting viruses do not harm their host. Two different examples illustrate the theoretical application of evolutionary principles to this problem. First, by viewing the virus as a new gene of the immune system, it is possible to evolve this gene toward the goal of improving immune function. Second, the retroviral population of each host is maladapted for long-term survival. Retroviral genes could be evolved with respect to continued existence in the host.