High-level HIV-1 viremia suppresses viral antigen-specific CD4(+) T cell proliferation

Abstract

In chronic viral infections of humans and experimental animals, virus-specific CD4(+) T cell function is believed to be critical for induction and maintenance of host immunity that mediates effective restriction of viral replication. Because in vitro proliferation of HIV-specific memory CD4(+) T cells is only rarely demonstrable in HIV-infected individuals, it is presumed that HIV-specific CD4(+) T cells are killed upon encountering the virus, and maintenance of CD4(+) T cell responses in some patients causes the restriction of virus replication. In this study, proliferative responses were absent in patients with poorly restricted virus replication although HIV-specific CD4(+) T cells capable of producing IFN-gamma were detected. In a separate cohort, interruption of antiretroviral therapy resulted in the rapid and complete abrogation of virus-specific proliferation although HIV-1-specific CD4(+) T cells were present. HIV-specific proliferation returned when therapy was resumed and virus replication was controlled. Further, HIV-specific CD4(+) T cells of viremic patients could be induced to proliferate in response to HIV antigens when costimulation was provided by anti-CD28 antibody in vitro. Thus, HIV-1-specific CD4(+) T cells persist but remain poorly responsive (produce IFN-gamma but do not proliferate) in viremic patients. Unrestricted virus replication causes diminished proliferation of virus-specific CD4(+) T cells. Suppression of proliferation of HIV-specific CD4(+) T cells in the context of high levels of antigen may be a mechanism by which HIV or other persistently replicating viruses limit the precursor frequency of virus-specific CD4(+) T cells and disrupt the development of effective virus-specific immune responses.

Figure 1

Detection of HIV-1 antigen-specific CD4⁺ T cells in patients lacking proliferative responses. The frequency of CD4⁺ T cells that produce IFN-γ (○) or proliferative responses in PBMC (▴) in response to HIV-1 p24 (A), HIV-1 p55 (B), or CMV lysate (C). Only CMV seropositive patients (n = 20) are shown in C. Bars represent median values.

Figure 2

HIV-specific proliferative responses are suppressed during high-level viremia. (A) Changes in peripheral blood CD4⁺ T cell count, plasma viral RNA, and proliferation in response to PHA or HIV-1 p24 before, during, and after interruption of therapy. The therapy interruption time point shown is just before resumption of therapy (35–111 days; median 45 days). (B) The frequency of CD4⁺ T cells in the peripheral blood that produce IFN-γ in response to HIV-1 p24 (gray symbols) or p55 (open symbols). A subset of patients (n = 5) with significant proliferative responses on whom flow cytometry on freshly isolated PBMC could be obtained is shown. HIV-1 p24-specific proliferation (closed symbols) is plotted for comparison.

Figure 3

IFN-γ⁺ CD4⁺ T cells in PBMC proliferate in response to antigen. PBMC were stimulated with the indicated antigen for 14 h, and IFN-γ-secreting cells were isolated. Either IFN-γ⁺ or IFN-γ⁻ cells were labeled with CFSE and recombined with the corresponding unlabeled fraction (see Methods) and incubated for 5 days.

Figure 4

CD4⁺ T cells specific for HIV-1 p24 proliferate in viremic patients when costimulation is provided through CD28. (A) PBMC labeled with CFSE and incubated for 5 days with or without p24 antigen or anti-CD28 mAb. (B) Summary data of the percent CD4⁺ T cells that are CFSE^low after 5 days under the indicated stimulation conditions for nine patients within group B. (C) Summary data of the percent proliferating CD4⁺ T cells of seven patients before interruption of therapy and during viremia when therapy was interrupted. Parallel experiments in 10 HIV seronegative donors are shown for comparison.