Nonhuman Primate Testing of the Impact of Different Regulatory T Cell Depletion Strategies on Reactivation and Clearance of Latent Simian Immunodeficiency Virus

Abstract

Regulatory T cells (Tregs) may be key contributors to the HIV/SIV latent reservoir, as they harbor high levels of HIV/SIV; reverse CD4+ T cell immune activation status, increasing the pool of resting CD4+ T cells; impair CD8+ T cell function, favoring HIV persistence. We tested the hypothesis that Treg depletion is a valid intervention towards an HIV cure by depleted Tregs in fourteen SIVsab-infected RM controllers through different strategies: administration of an anti-CCR4 immunotoxin; two doses of an anti-CD25 immunotoxin (IL-2 with diphtheria toxin: IL-2-DT); or two combinations of both. All these treatments resulted in significant depletion of the circulating Tregs (>70%) and their partial depletion in the gut (25%) and lymph nodes (>50%). The fractions of CD4+ T cells expressing Ki-67 increased up-to 80% in experiments containing IL-2-DT and only 30% in anti-CCR4–treated RMs, paralleled by increases in the inflammatory cytokines. In the absence of ART, plasma virus rebounded to 103 vRNA copies/mL by day 10 post-IL-2-DT administration. A large but transient boost of the SIV-specific CD8+ T cell responses occurred in IL-2-DT–treated RMs. Such increases were minimal in the RMs receiving anti-CCR4-based regimens. Five RMs received IL-2DT on ART, but treatment was discontinued because of high toxicity and lymphopenia. As such, while all treatments depleted a significant proportion of Tregs, the side effects in the presence of ART prevent their clinical use and call for different Treg depletion approaches. Thus, based on our data, Treg targeting as a strategy for HIV cure cannot be discarded. IMPORTANCE Regulatory T cells (Tregs) can decisively contribute to the establishment and persistence of the HIV reservoir, as they harbor high levels of HIV/SIV, increase the pool of resting CD4+ T cells by reversing their immune activation status, and impair CD8+ T cell function, favoring HIV persistence. We tested multiple Treg depletion strategies and showed that all of them are at least partially successful in depleting Tregs. As such, Treg depletion appears to be a valid intervention towards an HIV cure, reducing the size of the reservoir, reactivating the virus, and boosting cell-mediated immune responses. Yet, when Treg depletion was attempted in ART-suppressed animals, the treatment had to be discontinued due to high toxicity and lymphopenia. Therefore, while Treg targeting as a strategy for HIV cure cannot be discarded, the methodology for Treg depletion has to be revisited.

So Pathogenic or So What?—A Brief Overview of SIV Pathogenesis with an Emphasis on Cure Research

“…Numerous strategies have been proposed for the elimination of the viral reservoir: (i) ART intensification [428][429][430][431]; (ii) "block and lock" permanent transcriptional silencing [307,432]; (iii) gene editing of CCR5 [433]; (iv) chimeric antigen receptor (CAR) T cells [322,434,435]; (v) apoptosis promotion and viral cytopathic effect enhancement [436,437]; (vi) bone marrow transplantation [344]; (vii) broadly neutralizing antibodies [438,439]; (viii) vaccines and therapeutic vaccines [440]; (ix) regulatory T cell (Treg) manipulation and depletion [338,441,442]; (x) use of checkpoint inhibitors to enhance HIV-specific immune responses [443][444][445][446]; and (xi) "shock and kill" (Figure 3). However, these have been met with limited success.…”

“…Other cure strategies also have issues with efficiency, be it the lack of reducing residual viremia or reservoir size with ART intensification [455], insufficient virus reactivation with Treg manipulation [441] and additional toxicity when combined with ART [442], lack of viral clearance in response to checkpoint inhibitors [443,444], inherent resistance and escape mutations against broadly neutralizing antibodies [456], lack of great enough efficiency and long-term stability of gene therapies and CAR T cells [322,457], or safety concerns of bone marrow transplants coupled with the lack of success [344,458]. As such, after more than a decade of intensive research, the end is still not in sight.…”

“…(viii) vaccines and therapeutic vaccines [440]; (ix) regulatory T cell (Treg) manipulation and depletion [338,441,442]; (x) use of checkpoint inhibitors to enhance HIV-specific immune responses [443][444][445][446]; and (xi) "shock and kill" (Figure 3). However, these have been met with limited success.…”

The Hitchhiker Guide to CD4+ T-Cell Depletion in Lentiviral Infection. A Critical Review of the Dynamics of the CD4+ T Cells in SIV and HIV Infection

“…Since Treg are usually accumulating throughout chronic HIV/ SIV infection, are frequently infected and suppress HIV/SIVspecific cytotoxic T cell responses (231), different Treg-specific depletion strategies, targeting either CD25 (IL-2 receptor subunit) (232)(233)(234) or CCR4 (234,235), have been investigated. Despite achieving only partial Treg depletion with maximal effect in blood and lymph nodes, and minimal depletion in GI tract, this usually led to increased SIV-specific T cell responses (232,234), and increased immune activation (232)(233)(234). Another strategy aimed at blocking CTLA-4 also resulted in increased SIV-specific T cell responses (236).…”

“…Another strategy aimed at blocking CTLA-4 also resulted in increased SIV-specific T cell responses ( 236 ). Viral reactivation occurred in most NHPs in which Treg functions were blocked by anti-CTLA-4 monoclonal antibodies, or in which Treg were depleted ( 232 – 234 , 237 , 238 ). Higher viral loads in mucosal tissues and greater loss of CCR5 + CD4 + T cells in the rectal mucosa have been reported to occur in the NHPs receiving an anti-CTLA-4 blocking monoclonal antibody ( 237 ).…”