Comparison of simian immunodeficiency virus SIVagmVer replication and CD4+ T-cell dynamics in vervet and sabaeus African green monkeys

Abstract

The simian immunodeficiency viruses (SIV) naturally infect a wide range of African primates, including African green monkeys (AGM). Despite moderate to high levels of plasma viremia in naturally infected AGM, infection is not associated with immunodeficiency. We recently reported that SIVagmVer90 isolated from a naturally infected vervet AGM induced AIDS following experimental inoculation of pigtailed macaques. The goal of the present study was to evaluate the replication of this isolate in two species of AGM, sabaeus monkeys (Chlorocebus sabaeus) and vervets (C. pygerythrus). Inoculation of sabaeus AGM with SIVagmVer90 resulted in low and variable primary and set-point viremia (<10(2) to 10(4) copies/ml). In contrast, inoculation of vervet AGM with either SIVagmVer90 or blood from a naturally infected vervet (Ver1) resulted in high primary viremia and moderate plateau levels, similar to the range seen in naturally infected vervets from this cohort. CD4(+) T cells remained stable throughout infection, even in AGM with persistent high viremia. Despite the lack of measurable lymphadenopathy, infection was associated with an increased number of Ki-67(+) T cells in lymph node biopsies, consistent with an early antiviral immune response. The preferential replication of SIVagmVer in vervet versus sabaeus AGM shows that it is critical to match AGM species and SIV strains for experimental models of natural SIV infection.

FIG. 1.

AGM speciation based on mitochondrial DNA characterization. Cytochrome b gene sequences (267 bp) were amplified from vervet AGM PBMC and compared with sequences from different species of AGM as well as from other African primates, including Cercopithecus species, mandrills, and mangabeys. Sequences originating from AGM are labeled in bold, and sequences from vervets are highlighted in gray. The phylogenetic trees were estimated by the maximum parsimony method. The reliability was estimated from 1,000 bootstrap replicates; only bootstrap values relevant for lineage definition are shown. Bar, 0.01 nucleotide substitution.

FIG. 2.

Phylogenetic analysis of a portion of the envelopes of SIVagmVer viruses isolated from naturally infected vervet monkeys. A neighbor-joining tree of consensus nucleotide sequences of a 370-bp fragment of the transmembrane glycoprotein-encoding region of envelope amplified from virus isolates of seropositive Tanzanian AGM demonstrates that they cluster phylogenetically with other published vervet isolates (SIVagmVer90 and SIVagmVer155) and are distinct from the tantalus, sabaeus, and grivet clusters. Bar, 5.0 nucleotide substitutions.

FIG. 3.

Sequential plasma viral RNA levels in vervet and sabaeus AGM inoculated with SIVagm and comparisons of mean (SEM) viral RNA levels. Vervet AGM used for panel A and sabaeus AGM used for panel C were inoculated with the same stock of SIVagmVer90. Vervet AGM used for panel B were inoculated with 1 ml of heparinized peripheral blood from the naturally infected vervet Ver1. The limit of detection of the assay was 100 copies/ml. A real-time RT-PCR assay for quantitation of viral RNA in plasma was performed as previously described and has been shown to amplify divergent SIVagmVer isolates (11). Panel D shows the means and SEM of plasma viral RNA levels in the three cohorts of animals.

FIG. 4.

Absolute blood CD4⁺ T-cell numbers throughout 90 weeks of infection in vervets infected with SIVagmVer90 (A), vervets infected with SIVagmVer1 (B), and sabaeus monkeys infected with SIVagmVer90 (C). (D) Mean values with SEM. CD4⁺ T-cell numbers were stable throughout 90 weeks of infection, with the exception of transient declines during peak viremia in some animals.

FIG. 5.

Western blot reactivities of sequential plasma samples from SIVagmVer90-inoculated sabaeus and vervet AGM. The positions of the gp120, gp41, and p27 Gag antigens are indicated to the right of each blot. For the sabaeus AGM (A), samples were collected preinoculation and at 4, 8, 32, 56, and 90 weeks postinoculation (strips 1 to 6, respectively). For vervet monkeys (B), samples were analyzed from 0, 4, 8, 16, and 32 weeks postinoculation (strips 1 to 5).

FIG. 6.

SIV-specific ISH of sequential lymph node biopsies collected from AGM infected with SIVagmVer90. (A) Numbers of SIV-expressing cells per high-power field (HPF) in sequential lymph node biopsies are shown graphically for vervet (top) and sabaeus (bottom) monkeys inoculated with SIVagmVer90. (B) Representative fields of view for ISH of lymph nodes biopsied from vervet Ver22 at 1 and 2 weeks postinfection are shown, with numerous SIV-expressing cells (dark blue) at 1 week, coincident with the peak of plasma viremia, and a decrease in their number by 2 weeks postinoculation.

FIG. 7.

Immunohistochemical detection of Ki-67⁺ cells in sequential lymph node biopsies from SIVagmVer90-inoculated vervet AGM and PT macaques. (A) Graphical representation of mean numbers (SEM) of Ki-67⁺ cells in the paracortical regions of lymph nodes biopsied at 0, 1, 2, and 4 weeks postinoculation from four vervets inoculated with SIVagmVer90. HPF, high-power field. (B) Representative fields of view (×10 objective) of Ki-67-stained (brown), hematoxylin-counterstained lymph nodes at 0 and 2 weeks postinoculation. Preinoculation lymph nodes were quiescent, with some Ki-67 expression in germinal centers (top), and a relative increase in the number of Ki-67⁺ cells was observed at 2 weeks postinoculation in both germinal centers (GC) and paracortexes (P).