Tenofovir treatment augments anti-viral immunity against drug-resistant SIV challenge in chronically infected rhesus macaques

Abstract

Background: Emergence of drug-resistant strains of human immunodeficiency virus type 1 (HIV-1) is a major obstacle to successful antiretroviral therapy (ART) in HIV-infected patients. Whether antiviral immunity can augment ART by suppressing replication of drug-resistant HIV-1 in humans is not well understood, but can be explored in non-human primates infected with simian immunodeficiency virus (SIV). Rhesus macaques infected with live, attenuated SIV develop robust SIV-specific immune responses but remain viremic, often at low levels, for periods of months to years, thus providing a model in which to evaluate the contribution of antiviral immunity to drug efficacy. To investigate the extent to which SIV-specific immune responses augment suppression of drug-resistant SIV, rhesus macaques infected with live, attenuated SIVmac239Deltanef were treated with the reverse transcriptase (RT) inhibitor tenofovir, and then challenged with pathogenic SIVmac055, which has a five-fold reduced sensitivity to tenofovir.

Results: Replication of SIVmac055 was detected in untreated macaques infected with SIVmac239Deltanef, and in tenofovir-treated, naïve control macaques. The majority of macaques infected with SIVmac055 experienced high levels of plasma viremia, rapid CD4+ T cell loss and clinical disease progression. By comparison, macaques infected with SIVmac239Deltanef and treated with tenofovir showed no evidence of replicating SIVmac055 in plasma using allele-specific real-time PCR assays with a limit of sensitivity of 50 SIV RNA copies/ml plasma. These animals remained clinically healthy with stable CD4+ T cell counts during three years of follow-up. Both the tenofovir-treated and untreated macaques infected with SIVmac239Deltanef had antibody responses to SIV gp130 and p27 antigens and SIV-specific CD8+ T cell responses prior to SIVmac055 challenge, but only those animals receiving concurrent treatment with tenofovir resisted infection with SIVmac055.

Conclusion: These results support the concept that anti-viral immunity acts synergistically with ART to augment drug efficacy by suppressing replication of viral variants with reduced drug sensitivity. Treatment strategies that seek to combine immunotherapeutic intervention as an adjunct to antiretroviral drugs may therefore confer added benefit by controlling replication of HIV-1, and reducing the likelihood of treatment failure due to the emergence of drug-resistant virus, thereby preserving treatment options.

Figure 1

Pre-treatment of naïve rhesus macaques with tenofovir and subsequent infection with SIVmac055. Adult rhesus macaques were treated for 4 weeks with tenofovir at a dose of 30 mg/kg body weight, and then inoculated intravenously with SIVmac055 on day 28 (arrow). Tenofovir treatment was continued for an additional 2 weeks after SIVmac055 infection. Virus replication and CD4⁺ T cell counts were monitored for > 1 year of follow-up. (A) Plasma viral load was measured by real-time PCR with a sensitivity of 50 SIV RNA copies/ml, (B) CD4⁺ T-cell counts.

Figure 2

SIV-specific antibody responses in tenofovir-treated macaques infected with SIVmac055. Plasma antibody titers to (A) SIV gp130 and (B) SIV p27 were measured during tenofovir treatment (days 0–42) and after challenge with SIVmac055 (day 28, arrow). Data is expressed as the midpoint antibody titer based on serial titration of plasma and antibody detection by antigen-specific ELISA [16, 41].

Figure 3

Effect of tenofovir on plasma viral load in macaques infected with SIVmac251 or SIVmac239Δnef. Rhesus macaques with chronic SIV infection were treated for 6 weeks with tenofovir at a dose of 30 mg/kg body weight. The effect on SIV replication was determined by quantification of plasma SIV RNA by allele-specific real-time PCR. Plasma viral load is expressed as SIV RNA copies/ml and shown for macaques with replicating (A) SIVmac251 and (B) SIVmac239Δnef.

Figure 4

Replication of SIVmac055 in tenofovir-treated and untreated macaques infected with SIVmac239Δnef. Macaques chronically infected with SIVmac239Δnef were treated for 4 weeks with tenofovir (1488, 1514) or left untreated (1494, 1512). Both treated and untreated macaques were challenged with SIVmac055 on day 28 (arrow). Replication of SIV was measured by allele-specific PCR to discriminate between SIVmac055 (●) and SIVmac239Δnef (○). Data is expressed as SIV RNA copies/ml of plasma.

Figure 5

CD4⁺ T cell counts and SIV-specific antibody responses in tenofovir-treated and untreated macaques infected with SIVmac239Δnef. Tenofovir-treated (1488, 1514) and untreated (1494, 1512) macaques infected with SIVmac239Δnef were challenged intravenously with SIVmac055 and monitored for (A) CD4⁺ T cell counts, (B) SIV gp130 antibody responses, and (C) SIV p27 antibody responses for approximately one year. Tenofovir treatment was given on days 0–42. Intravenous inoculation of SIVmac055 occurred on day 28 (arrow).