Simian immunodeficiency virus replicates to high levels in naturally infected African green monkeys without inducing immunologic or neurologic disease

Abstract

African green monkeys can maintain long-term persistent infection with simian immunodeficiency viruses (SIVagm) without developing AIDS and thus provide an important model for understanding mechanisms of natural host resistance to disease. This study assessed the levels and anatomic distribution of SIVagm in healthy, naturally infected monkeys. Quantitative competitive reverse transcriptase PCR assays developed to measure SIVagm from two African green monkey subspecies demonstrated high levels of SIV RNA in plasma (>6 x 10(6) RNA copies/ml) in sabaeus and vervet monkeys. Infectious virus was readily recovered from plasma and peripheral blood mononuclear cells and shown to be highly cytopathic in human cell lines and macrophages. SIVagm DNA levels were highest in the gastrointestinal tract, suggesting that the gut is a major site for SIVagm replication in vivo. Appreciable levels of virus were also found within the brain parenchyma and the cerebrospinal fluid (CSF), with lower levels detected in peripheral blood cells and lymph nodes. Virus isolates from the CSF and brain parenchyma readily infected macrophages in culture, whereas lymph node isolates were more restricted to growth in human T-cell lines. Comparison of env V2-C4 sequences showed extensive amino acid diversity between SIVagm recovered from the central nervous system and that recovered from lymphoid tissues. Homology between brain and CSF viruses, macrophage tropism, and active replication suggest compartmentalization in the central nervous system without associated neuropathology in naturally infected monkeys. These studies provide evidence that the nonpathogenic nature of SIVagm in the natural host can be attributed neither to more effective host control over viral replication nor to differences in the tissue and cell tropism from those for human immunodeficiency virus type 1-infected humans or SIV-infected macaques.

FIG. 1

Levels of viral DNA present in peripheral blood cells from naturally infected green monkeys. Semiquantitative PCR followed by Southern blotting was performed with DNA from the PBMC of three vervets (6242, 9648, and 9649). The number designation for each animal represents the amount of total cellular DNA amplified (in nanograms). To determine the sensitivity of the assay, 10-fold dilutions of pSIVgag were amplified in the presence of 100 ng of cellular DNA. The numbers represent copy number based on the control pSIVgag plasmid.

FIG. 2

Immunoblot analysis of SIVagm virus isolates recovered from selected tissues of a naturally infected monkey (AGM 9315). Viruses were purified from culture supernatants of infected cell lines and reacted with serum from an SIVagm-infected rhesus macaque (MM7695). Lane B, SIVagm(tyo-1); lane C, SIVagm(sab-4br); lane D, SIVagm(sab-4CSF); lane E, SIVagm(sab-4ln). Lane A represents molecular mass markers. Viral proteins were identified as gp120(env), p34(pol), p24(gag), and p17(gag).

FIG. 3

In vitro growth characteristics of SIVagm variants in a human T-cell line and macrophages. SIVagm viruses recovered from brain (sab-4br), CSF (sab-4CSF), and LNs (sab-4ln) were normalized for p27 (nanograms per milliliter) and used to infect Molt4 clone 8 cells (A) and human macrophages (B). Well-characterized isolates [SIVagm(tyo-1), SIVagm(MJ8), and SIVagm(gri-1)] were run for comparison. HIV-1/Ba-L is not shown in panel B due to the very high virus peak at day 6 (1,200 ng/ml). Ag, antigen.

FIG. 4

Cytopathic effects of SIVagm and HIV-1 on human macrophages. Macrophages prepared from human PBMC were infected with medium alone (A), HIV-1/Ba-L (B), SIVagm(sab-4ln) (C), or SIVagm(sab-4br) (D). Cultures are shown at day 14 postinfection. Magnification, ×40.

FIG. 5

Deduced amino acid sequence alignment of the V2-C4 env gene region from SIVagm viruses isolated from the CSF, brain, and LNs of a naturally infected green monkey (AGM 9315). The dots above the sequence indicate potential N-linked glycosylation sites, and underlined sequences represent positional changes in these sites. The outlined regions indicate relative variable regions (V) for SIVagm strains. The dashes represent amino acid homology, and lowercase letters are nonsynonymous mutations.