Impact of viral factors on very early in vivo replication profiles in simian immunodeficiency virus SIVagm-infected African green monkeys

Abstract

To better understand which factors govern the levels of viral loads in early lentiviral infections of primates, we developed a model that allows distinguishing between the influences of host and viral factors on viremia. Herein we report that two species of African green monkeys (Chlorocebus sabaeus and C. pygerythrus) infected with their respective wild-type simian immunodeficiency virus SIVagm viruses (SIVagm.sab92018 and SIVagm.ver644) consistently showed reproducible differences in viremia during primary infection but not at later stages of infection. Cross-infections of SIVagm.sab92018 and SIVagm.ver644 into, respectively, C. pygerythrus and C. sabaeus revealed that the dynamics of viral replication during primary infection were dependent on the viral strain used for the infection but not on the host. Hence, the kinetics of SIVagm.sab92018 and SIVagm.ver644 were similar in both sabaeus and vervet animals, indicating that the difference in viremia levels between the two groups during the early phase of infection was not associated with the host. Coreceptor usage for these two strains showed a larger coreceptor repertoire for SIVagm.sab92018, which is able to efficiently use CXCR4 in addition to CCR5, than for SIVagm.ver644, which showed a classical CCR5 coreceptor usage pattern. These differences could not be explained by different charges of the V3 loop for SIVagm.sab92018 and for SIVagm.ver644. In conclusion, our study showed that the extent of virus replication during the primary infection is primarily dependent on viral determinants.

FIG. 1.

CD4⁺ T-cell numbers (/mm³) before and after infection in 11 sabaeus infected with SIVagm.sab92018 (a) and 4 vervets infected with SIVagm.ver644 (b). Only SIVagm.sab92018-infected sabaeus showed constant significant declines in CD4 T-cell counts at the peak of plasma viral load. The CD4⁺ T cell counts rebounded to normal values after the viremia set point was reached.

FIG. 2.

Viral RNA copy numbers in plasma samples from SIVagm.sab92018- and SIVagm.ver644-infected AGMs. The detection limit of the assays was 10³ copies/ml and is indicated by the dashed horizontal lines. Values below the detection limit were arbitrarily set to 5.10² copies. (a) Plasma RNA load in SIVagm.sab92018-infected AGMs. The solid lines stand for sabaeus monkeys and the dashed lines for vervet monkeys infected with SIVagm.sab92018. RNA levels in sabaeus monkeys 96001, 96008, 96011, and 96023 are not shown since they have already been published elsewhere (18). (b) Plasma RNA levels in SIVagm.ver644-infected AGMs. The solid lines stand for vervets and the dashed lines for sabaeus monkeys infected with SIVagm.ver644. In animal 96029, viral RNA was detected only once at day 360 p.i. (7 × 10³ RNA copies/ml).

FIG. 3.

Alignment of partial envelope protein sequences from multiple SIVagm isolates and from SIVagm.sab92018, SIVagm.sabD46, and SIVagm.ver644. PCR-derived env nucleotide sequences from SIVagm.ver644-infected vervet, and SIVagm.sab (92018 and D46)-infected sabaeus were translated, aligned with previously reported SIVagm env sequences (http://www.hiv.lanl.gov/content/hiv-db), and compared with a consensus sequence available on Los Alamos HIV Sequence Database. Dashes represent gaps and dots denote sequence identity to the SIVagm.sab consensus sequence. V3 designates a previously described hypervariable SIVagm env domain (51). NXT(S) (with X representing any amino acid except tryptophan [W]) and C represent putative N-linked glycosylation sites and cysteine residues, respectively. The N glycosylation sites are boxed.