Virulence and reduced fitness of simian immunodeficiency virus with the M184V mutation in reverse transcriptase

Abstract

Drug-resistant mutants with a methionine-to-valine substitution at position 184 of reverse transcriptase (M184V) emerged within 5 weeks of initiation of therapy in four newborn macaques infected with simian immunodeficiency virus (SIVmac251) and treated with lamivudine (3TC) or emtricitabine [(-)-FTC] (two animals per drug). Thus, this animal model mimics the rapid emergence of M184V mutants of HIV-1 during 3TC therapy of human patients. One animal of each treatment group developed fatal immunodeficiency at 12 weeks of age, which is similar to the rapid disease course seen in most untreated SIVmac251-infected infant macaques. To further evaluate the effect of the M184V mutation on viral fitness and virulence, groups of juvenile macaques were inoculated with the molecular clone SIVmac239 with either the wild-type sequence (group A [n = 5]) or the M184V sequence (SIVmac239-184V; group B [n = 5] and group C [n = 2]). The two SIVmac239-184V-infected animals of group C did not receive any drug treatment, and in both animals the virus population reverted to predominantly wild type (184M) by 8 weeks after inoculation. The other five SIVmac239-184V-infected animals (group B) were treated with (-)-FTC to prevent reversion. Although virus levels 1 week after inoculation were lower in the SIVmac239-184V-infected macaques than in the SIVmac239-infected animals, no significant differences were observed from week 2 onwards. Two animals in each group developed AIDS and were euthanized, while all other animals were clinically stable at 46 weeks of infection. These data demonstrate that the M184V mutation in SIV conferred a slightly reduced fitness but did not affect disease outcome.

FIG. 1.

Virus levels in SIVmac251-infected infant macaques receiving 3TC or (−)-FTC treatment. All newborn macaques were inoculated at birth orally with SIVmac251. The dashed lines represent 14 untreated SIVmac251-infected infant macaques from a number of previous studies. Four animals were started on 3TC or FTC treatment at 3 weeks of age (arrow). Plasma RNA levels were measured by branched-DNA assay.

FIG. 2.

Virus levels in juvenile macaques infected with SIVmac239 or SIVmac239-184V. Plasma RNA levels, measured by real-time PCR (TaqMan), are presented for animals inoculated intravenously with SIVmac239 (A) or SIVmac239-184V (B and C). Some animals (B) also received (−)-FTC treatment to prevent reversion. +, euthanasia because of simian AIDS. (D) Averages for the different groups, including the average for group A when animal 31304 is excluded. There were no statistically significant differences in virus loads between the groups regardless of whether data from animal 31304 were included.

FIG. 3.

Changes in absolute CD4⁺-T-lymphocyte counts and CD4/CD8 T-lymphocyte ratios. Animals were inoculated intravenously with SIVmac239 or SIVmac239-184V. Five of the SIVmac239-184V-infected animals also received (−)-FTC treatment. Absolute CD4⁺ CD3⁺ T lymphocytes and the ratio of CD4⁺ CD3⁺ to CD8⁺ CD3⁺ T lymphocytes are presented for the first 10 weeks of infection.