Simian immunodeficiency virus SIVagm.sab infection of Caribbean African green monkeys: a new model for the study of SIV pathogenesis in natural hosts

Abstract

Caribbean-born African green monkeys (AGMs) were classified as Chlorocebus sabaeus by cytochrome b sequencing. Guided by these phylogenetic analyses, we developed a new model for the study of simian immunodeficiency virus (SIV) infection in natural hosts by inoculating Caribbean AGMs with their species-specific SIVagm.sab. SIVagm.sab replicated efficiently in Caribbean AGM peripheral blood mononuclear cells in vitro. During SIVagm.sab primary infection of six Caribbean AGMs, the virus replicated at high levels, with peak viral loads (VLs) of 10(7) to 10(8) copies/ml occurring by day 8 to 10 postinfection (p.i.). Set-point values of up to 2 x 10(5) copies/ml were reached by day 42 p.i. and maintained throughout follow-up (through day 450 p.i.). CD4(+) T-cell counts in the blood showed a transient depletion at the peak of VL, and then returned to near preinfection values by day 28 p.i. and remained relatively stable during the chronic infection. Preservation of CD4 T cells was also found in lymph nodes (LNs) of chronic SIVagm.sab-infected Caribbean AGMs. No activation of CD4(+) T cells was detected in the periphery in SIV-infected Caribbean AGMs. These virological and immunological profiles from peripheral blood and LNs were identical to those previously reported in African-born AGMs infected with the same viral strain (SIVagm.sab92018). Due to these similarities, we conclude that Caribbean AGMs are a useful alternative to AGMs of African origin as a model for the study of SIV infection in natural African hosts.

FIG. 1.

CD4 and CD8 expression in blood of Caribbean AGMs. Caribbean (St. Kitts) AGMs display two different phenotypes of CD4⁺ T cells; a representative example of each is shown here. In some animals, such as AGM EI49, very few CD4⁺ CD8α⁺ T cells are observed (a); AGM EI53 is representative of animals the majority of whose CD4⁺ T cells have a low expression of CD8α (b). When CD8⁺ T cells are defined with a CD8β monoclonal antibody, a similar pattern of staining is observed for all Caribbean AGMs, due to few CD4⁺ CD8β⁺ T cells (c and d).

FIG. 2.

Caribbean AGM speciation based on mitochondrial DNA characterization. Cytochrome b sequences (267 bp) were amplified from Caribbean AGMs and compared with those of different species of AGMs from Africa. Sequences originating from different species of AGMs of African origin are color coded: sabaeus in red, tantalus in blue, grivet in orange, and vervet in green. Sequences from Caribbean AGMs are shown in black. Monkeys coded EI are from St. Kitts island; monkeys coded A and B are from Barbados. Cytochrome b sequences from other species of African monkeys are shown 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.

FIG. 3.

Expression of CD4 before (a) and after stimulation with PHA (b) and IL-2 (c). An increase of CD4-positive cells was noted after 3 days of PHA stimulation (b). After 5 additional days of treatment with IL-2, a decrease of CD4 T⁺ cells was observed (c), but this reduction was insignificant compared with CD4 levels before stimulation (a).

FIG. 4.

Antibody responses (a) and p27 antigenemia (b) in Caribbean AGMs infected with SIVagm.sab. Anti-gp41 antibodies against SIVagm.sab were detected by an in-house peptide ELISA using specific peptides (51); all animals seroconverted between days 21 and 45 p.i. Positive P27 antigenemia was detected only during the acute infection (up to day 28 p.i.). Symbols for animals: no. EI45 (•), no. EI53 (○), no. EI52 (▾), no. EI42 (▿), no. EI51 (▪), and no. EI44 (□).

FIG. 5.

Viral RNA copy numbers in plasma of SIVagm.sab92018-infected Caribbean AGMs. The detection limit of the assay was 10³ copies/ml. The range of plasma VLs in Caribbean AGMs is in the same range as values observed in African AGMs infected with the same viral strain (42). Symbols for animals: no. EI45 (•), no. EI53 (○), no. EI52 (▾), no. EI42 (▿), no. EI51 (▪), and no. EI44 (□).

FIG. 6.

T- and B-cell dynamics in blood of SIVagm.sab-infected Caribbean AGMs. CD4⁺ T cells transiently decreased in the blood (a) of SIVagm.sab-infected AGMs at the peak of VLs. The levels rebounded to levels close to preinfection values during chronic SIVagm.sab infection. No significant modification was observed in the dynamics of CD8⁺ T cells in the peripheral blood. (b) A transient decrease in the numbers of CD20⁺ B cells was present in the blood of SIVagm.sab-infected AGMs (c). Symbols for animals: no. EI45 (•), no. EI53 (○), no. EI52 (▾), no. EI42 (▿), no. EI51 (▪), and no. EI44 (□).

FIG. 7.

CD4⁺ T-cell and B-cell dynamics in LNs in SIVagm.sab-infected Caribbean AGMs. A significant (P < 0.05) decrease in CD4⁺ T-cell percentages was observed in the LNs at day 28 post-SIVagm.sab inoculation. The levels of CD4 T⁺ cells remained decreased at day 200, but the difference was not significant compared with preinfection values (a). Concomitant relative increase and return to normal levels of CD20 cells in the same compartment (b). Symbols for animals: no. EI45 (•), no. EI53 (○), no. EI52 (▾), no. EI42 (▿), no. EI51 (▪), and no. EI44 (□).

FIG. 8.

Kinetic expression of percent HLA-DR upon CD4⁺ (•) and CD8⁺ (○) T lymphocytes in peripheral blood of SIVagm.sab92018-infected Caribbean AGMs. A significant but transitory activation of CD8 T cells can be observed during the primary infection. No modification of the activation status was observed for the CD4⁺ T cells in SIVagm.sab-infected Caribbean AGMs.