Transmission of simian immunodeficiency virus SIVcpz and the evolution of infection in the presence and absence of concurrent human immunodeficiency virus type 1 infection in chimpanzees

Abstract

Current data suggest that the human immunodeficiency virus type 1 (HIV-1) epidemic arose by transmission of simian immunodeficiency virus (SIV) SIVcpz from a subspecies of common chimpanzees (Pan troglodytes troglodytes) to humans. SIVcpz of chimpanzees is itself a molecular chimera of SIVs from two or more different monkey species, suggesting that recombination was made possible by coinfection of one individual animal with different lentiviruses. However, very little is known about SIVcpz transmission and the susceptibility to lentivirus coinfection of its natural host, the chimpanzee. Here, it is revealed that either infected plasma or peripheral blood mononuclear cells readily confer infection when exposure occurs by the intravenous or mucosal route. Importantly, the presence of preexisting HIV-1 infection did not modify the kinetics of SIVcpz infection once it was established by different routes. Although humoral responses appeared as early as 4 weeks postinfection, neutralization to SIVcpz-ANT varied markedly between animals. Analysis of the SIVcpz env sequence over time revealed the emergence of genetic viral variants and persistent SIVcpz RNA levels of between 10(4) and 10(5) copies/ml plasma regardless of the presence or absence of concurrent HIV-1 infection. These unique data provide important insight into possible routes of transmission, the kinetics of acute SIVcpz infection, and how readily coinfection with SIVcpz and other lentiviruses may be established as necessary preconditions for potential recombination.

FIG. 1.

(A) Plasma p24 antigen (Ag) and virus load kinetics in acute versus chronic SIVcpz-ANT infection. Antigen capture of p24 over time was evaluated in all plasma samples, either directly or following acidic dissociation (AT) (bold lines), of antibody-bound antigen complexes. Plasma viral-RNA levels during the same period are shown plotted on the y axis and are represented by the upper two dotted lines. (B) Seroconversion profile of ch-Ni using HIV-1 Western blot strips (Diagnostics HIV Blot 2.2; Genelabs, Redwood City, CA). Sera of ch-Ni from consecutive time points pre- and postinfection were evaluated for the presence of antibodies to SIVcpz-ANT. As a positive control, serum of the naturally infected ch-No was used.

FIG. 2.

Absolute lymphocyte counts. Whole blood of ch-Ni was analyzed for specific lymphocyte subsets. □, CD4 T-cells; ▵, CD8 T-cells; •, NK cells; ▾, B cells.

FIG. 3.

Viral RNA in plasma. (A) SIVcpz-specific, (B) HIV-1-specific, and (C) cumulative lentiviral loads per ml of plasma. Column 1 (from left), i.v. exposure of naive P. t. schweinfurthii (ch-Ni); column 2, i.v. exposure of two HIV-1 carriers; column 3, i.r. exposure of two HIV-1 carriers; column 4, i.vag. exposure of two HIV-1 carriers and one naive animal (X284).

FIG. 4.

Absolute CD4 T-cell counts (A) and absolute CD8 T-cell counts (B) in the three different groups of animals used in the SIVcpz transmission/superinfection study. I, intravenous exposure; II, intrarectal exposure; III, intravaginal exposure.

FIG. 5.

Neutralization of SIVcpz by plasma from HIV-1-infected chimpanzees (closed symbols) and a control chimpanzee (open symbols) prior to (Pre) and following infection with SIVcpz. Means with standard-error bars of antigen concentrations released into culture supernatants expressed in log₁₀ arbitrary units are displayed. P, statistical significance of the F ratio derived from analysis of variance; asterisks indicate statistically significant reduction in antigen release relative to the mean from plasmas of two control chimpanzees. The dotted lines indicate 50% and 90% reductions in antigen release.

FIG.6.

Quasispecies analysis of challenge inocula and virus populations in SIVcpz-ANT-infected chimpanzees. (A) Maximum likelihood analysis using the General Time Reversible plus Gamma model of nucleotide sequences obtained from the ch-No inoculum (light-green boxes) and from the 2-week postexposure ch-Ni sample (dark-green boxes), encompassing the gp120 V1-V5 regions (nucleotides 5813 to 7161 relative to the SIVcpz-ANT published sequence). Bootstrap values (as percentages of 1,000 resamplings) are indicated. The tree was rooted with the published SIVcpz-ANT sequence derived from ch-No 5 years earlier (GenBank accession no. U42720). (B) Phylogenetic analysis of the SIVcpz quasispecies after superinfection of HIV-1-infected chimpanzees. The ML tree was estimated using the Hasegawa-Kishino-Yano plus Gamma model as suggested by FINDMODEL. The analysis was based on the alignment of the gp120 V1-V5 regions (nucleotides 5987 to 6828). Viral variants found after infection via different routes are indicated by color coding. Pink boxes, intravenous; yellow boxes, intrarectal; blue boxes, intravaginal. Sequences derived from the ch-Ni inoculum are indicated in green. Different individuals receiving SIV via the same route are indicated by different shades of the same color.

FIG.6.

Quasispecies analysis of challenge inocula and virus populations in SIVcpz-ANT-infected chimpanzees. (A) Maximum likelihood analysis using the General Time Reversible plus Gamma model of nucleotide sequences obtained from the ch-No inoculum (light-green boxes) and from the 2-week postexposure ch-Ni sample (dark-green boxes), encompassing the gp120 V1-V5 regions (nucleotides 5813 to 7161 relative to the SIVcpz-ANT published sequence). Bootstrap values (as percentages of 1,000 resamplings) are indicated. The tree was rooted with the published SIVcpz-ANT sequence derived from ch-No 5 years earlier (GenBank accession no. U42720). (B) Phylogenetic analysis of the SIVcpz quasispecies after superinfection of HIV-1-infected chimpanzees. The ML tree was estimated using the Hasegawa-Kishino-Yano plus Gamma model as suggested by FINDMODEL. The analysis was based on the alignment of the gp120 V1-V5 regions (nucleotides 5987 to 6828). Viral variants found after infection via different routes are indicated by color coding. Pink boxes, intravenous; yellow boxes, intrarectal; blue boxes, intravaginal. Sequences derived from the ch-Ni inoculum are indicated in green. Different individuals receiving SIV via the same route are indicated by different shades of the same color.