A low T regulatory cell response may contribute to both viral control and generalized immune activation in HIV controllers

Abstract

HIV-infected individuals maintaining undetectable viremia in the absence of therapy (HIV controllers) often maintain high HIV-specific T cell responses, which has spurred the development of vaccines eliciting HIV-specific T cell responses. However, controllers also often have abnormally high T cell activation levels, potentially contributing to T cell dysfunction, CD4+ T cell depletion, and non-AIDS morbidity. We hypothesized that a weak T regulatory cell (Treg) response might contribute to the control of viral replication in HIV controllers, but might also contribute to generalized immune activation, contributing to CD4+ T cell loss. To address these hypotheses, we measured frequencies of activated (CD38+ HLA-DR+), regulatory (CD4+CD25+CD127(dim)), HIV-specific, and CMV-specific T cells among HIV controllers and 3 control populations: HIV-infected individuals with treatment-mediated viral suppression (ART-suppressed), untreated HIV-infected "non-controllers" with high levels of viremia, and HIV-uninfected individuals. Despite abnormally high T cell activation levels, controllers had lower Treg frequencies than HIV-uninfected controls (P = 0.014). Supporting the propensity for an unusually low Treg response to viral infection in HIV controllers, we observed unusually high CMV-specific CD4+ T cell frequencies and a strong correlation between HIV-specific CD4+ T cell responses and generalized CD8+ T cell activation levels in HIV controllers (P ≤ 0.001). These data support a model in which low frequencies of Tregs in HIV controllers may contribute to an effective adaptive immune response, but may also contribute to generalized immune activation, potentially contributing to CD4 depletion.

Figure 1. HIV Controllers Have Abnormally Low Treg Frequencies Despite Abnormally High T Cell Activation.

The frequency of activated (CD38+ HLA-DR+) CD8+ T cells (A) and CD4+ T cells (B) in fresh whole blood was compared between 52 HIV-infected untreated HIV controllers, 37 HIV-uninfected participants, 176 HIV-infected participants with plasma HIV RNA levels <75 copies/ml on antiretroviral therapy, and 64 untreated HIV-infected participants with plasma HIV RNA levels >10,000 copies/ml. Cryopreserved PBMC from 34 healthy HIV-uninfected participants in ACTG 5015 (HIV-), 20 HIV controllers, 20 antiretroviral therapy (ART)-treated participants with plasma HIV RNA levels <75 copies/ml and 20 untreated HIV-infected participants with plasma HIV RNA levels >10,000 copies/ml were also evaluated for the frequency of CD4+ Tregs (CD25+CD127^dim). PBMC preparations were first gated on lymphocytes based on their forward and side scatter properties, then gated for CD4⁺ lymphocytes, then CD4+ lymphocytes positive for CD25 and only dimly expressing CD127, results expressed as a percentage of the parent CD4⁺ population (C). HIV controllers had lower frequencies of Tregs than HIV-uninfected controls and both other HIV-infected groups (D). Among HIV controllers, higher frequencies of activated CD8+ T cells were associated with higher frequencies of Tregs (E). The curve represents the best-fit linear regression model.

Figure 2. Relationship between Adaptive HIV-specific Immune Responses and CD8+ T Cell Activation in HIV Controllers.

The association between the frequency of activated (CD38+ HLA-DR+) CD8+ T cells and the frequency of CD4+ T cells producing both IFN-γ and IL-2 after stimulation with overlapping HIV Gag (A) or HIV Env peptides (B), CD8+ T cells producing only IFN-γ after stimulation with overlapping Nef peptides (C), and plasma HIV-specific antibody levels (as assessed by de-tuned ELISA, D) were assessed among HIV Controllers. The curves in each plot represent best-fit linear or quadratic regression models using untransformed data.

Figure 3. HIV Controllers Have High Frequencies of CMV-specific T Cells.

(A) The frequency of CD4+ T cells producing IFN-γ after incubation with CMV pp65 peptides in vitro (representative flow plot depicted in Figure 1 from reference [85]) was assessed in HIV-uninfected but CMV-seropositive controls, HIV controllers, and untreated HIV-infected participants with varying degrees of detectable viremia (75–2000, 2001–10,000, and >10,000 copies/ml). The HIV controllers had higher CMV pp65-specific IFN-bright CD4+ T cell responses than HIV-uninfected controls and HIV-infected participants with high levels of viremia. HIV controllers with higher CMV pp65-specific CD4+ T cell responses also had higher HIV Gag-specific CD4+ T cell responses (curve represents linear regression model on untransformed data, B).

Figure 4. Theoretical Model to Describe Positive and Negative Consequences of Low Treg Frequencies in HIV Controllers.

A theoretical model to describe the potential positive and negative consequences of low Treg frequencies in HIV controllers is presented. While a low Treg response might increase HIV-specific T cell responses, contributing to the clearance of HIV-infected cells and the maintenance of extremely low levels of viral replication, a low Treg response might also increase generalized T cell activation, contributing to CD4+ T cell decline and other inflammation-associated comorbidities even in the presence of very low levels of viral replication.