Targeting the HIV Reservoir: Chimeric Antigen Receptor Therapy for HIV Cure
Despite the remarkable success of antiretroviral therapy (ART) in suppressing human immunodeficiency virus (HIV) replication, it remains incapable of eradicating the virus due to the persistence of viral reservoirs. These reservoirs, primarily composed of long-lived latently infected cells harboring replication-competent viral DNA, pose a significant barrier to achieving a functional cure for HIV. To address this challenge, innovative immunotherapies, particularly chimeric antigen receptor (CAR) therapy, have emerged as promising strategies. This review explores the design, development, and clinical applications of CAR therapy in targeting the HIV reservoir, with a focus on CAR T cells, CAR natural killer (NK) cells, and CAR-encoding hematopoietic stem/progenitor cells (HSPCs).
Introduction
ART has transformed HIV infection from a fatal disease to a manageable chronic condition by reducing viral load to undetectable levels. However, ART cannot eliminate HIV because the virus integrates into the host cell genome, forming latent reservoirs. These reservoirs remain dormant and evade immune detection, leading to viral rebound upon ART cessation. Strategies such as “shock and kill,” “block and lock,” and gene therapy have been explored to reduce the reservoir size, but none have achieved sustained viral suppression without ART. Elite controllers, rare individuals who spontaneously control HIV replication, have inspired therapeutic approaches combining reservoir reduction and enhanced anti-HIV immunity. Among these, CAR therapy has gained traction due to its success in treating hematological malignancies and its potential to redirect immune cells to target HIV-infected cells.
CAR Therapy for HIV Cure
CAR therapy involves engineering immune cells to express synthetic receptors that recognize specific antigens. CARs consist of an extracellular antigen-binding domain, a transmembrane domain, and intracellular signaling domains. The extracellular domain, typically derived from a monoclonal antibody (mAb), allows antigen recognition independent of major histocompatibility complex (MHC) molecules. Intracellular signaling domains, such as CD3ζ, CD28, 4-1BB, or OX40, enhance cell activation, proliferation, and cytotoxicity.
CAR T cells have demonstrated remarkable success in treating cancers like B-cell lymphomas, acute lymphoblastic leukemia, and multiple myeloma. Similarly, CAR therapy for HIV aims to enhance anti-HIV immunity and eliminate viral reservoirs. Early studies tested adoptive T-cell therapies in HIV-infected individuals, but their efficacy was limited. Recent advancements in CAR technology have renewed interest in its application for HIV cure.
CAR T Therapy for HIV Cure
CAR T cells for HIV are designed to recognize and kill cells expressing HIV antigens. Unlike conventional cytotoxic T lymphocytes (CTLs), which require MHC-I presentation, CAR T cells can directly bind to antigens, overcoming HIV’s immune evasion mechanisms. For example, HIV Nef protein downregulates MHC-I expression, enabling infected cells to evade CTL-mediated killing. CAR T cells bypass this limitation by targeting HIV antigens in an MHC-independent manner.
Autologous CAR T cell therapy involves isolating T cells from the patient, genetically modifying them to express CARs, expanding the modified cells in vitro, and reinfusing them into the patient. These CAR T cells can traffic to tissue reservoirs, such as lymph nodes and the gastrointestinal tract, where HIV persists. Studies have shown that CAR T cells can reduce viral load and protect CD4+ T cells from HIV-induced depletion in humanized mouse models.
Design of Anti-HIV CAR Constructs
The design of CAR constructs has evolved to improve their efficacy. First-generation CARs contained a single intracellular signaling domain (CD3ζ) but exhibited limited persistence and cytotoxicity. Second-generation CARs incorporated costimulatory domains (e.g., CD28 or 4-1BB), enhancing cell activation and survival. Third-generation CARs combined multiple costimulatory domains, further improving anti-HIV potency. For example, a third-generation CAR with 4-1BB and OX40 domains showed superior HIV suppression compared to earlier designs.
To enhance CAR T cell function, dual CAR constructs have been developed. These express two CARs with distinct signaling domains (e.g., 4-1BB/CD3ζ and CD28/CD3ζ), improving proliferation and effector functions. Dual CAR T cells coexpressing CXCR4 fusion inhibitors have shown enhanced survival and HIV suppression in humanized mouse models.
Bispecific and trispecific CARs target multiple conserved epitopes on the HIV envelope (Env) protein, reducing the risk of viral escape. For instance, a bispecific CAR combining CD4 and the 17b mAb demonstrated enhanced potency against diverse HIV isolates. Trispecific CARs, incorporating a third targeting moiety (e.g., CCR5), have shown promise in achieving durable ART-free remission.
Broadly Neutralizing Antibody-Based CARs
Broadly neutralizing antibodies (bNAbs) that target conserved Env epitopes have been integrated into CAR designs. bNAb-based CARs, such as those derived from 10E8, 3BNC117, and VRC01, exhibit potent cytolytic activity against HIV-infected cells. These CARs can recognize and kill cells reactivated from latency in ART-treated individuals, highlighting their potential for reservoir reduction.
Multispecific duoCARs, targeting multiple Env sites, have shown broad antiviral activity in humanized mouse models. These duoCAR T cells effectively eliminate peripheral blood mononuclear cells infected with bNAb-resistant HIV strains and suppress viral replication in anatomical sites like the spleen. Additionally, they can sense and kill HIV-infected monocytes and macrophages, which contribute to the latent reservoir.
Universal CAR T Cell Platforms
To address HIV diversity and minimize resistance, universal CAR T cell platforms have been developed. One such platform, convertible CAR™ (cCAR™), uses an engineered NKG2D receptor fused to 4-1BB and CD3ζ signaling domains. The receptor binds to a mutant MIC ligand fused to HIV bNAbs (MicAbody), enabling adaptable targeting of HIV-infected cells. cCAR™ T cells have demonstrated effective killing of reactivated reservoir cells in ART-treated individuals.
CAR NK Cells for HIV Cure
CAR NK cells offer several advantages over CAR T cells, including reduced risk of graft-versus-host disease (GVHD) and cytokine release syndrome (CRS). NK cells play a crucial role in antiviral immunity, but their function is compromised during HIV infection. CAR NK cells, engineered to express HIV-specific CARs, can restore antiviral activity.
The NK-92 cell line is commonly used for CAR NK cell studies. For example, CD4ζ-expressing NK cells have shown enhanced HIV suppression in vitro. Universal CAR NK cells, targeting multiple Env epitopes via adaptor molecules, have demonstrated broad antiviral activity against HIV subtypes B and C. These cells, combined with diverse antibodies, offer a promising platform for overcoming HIV diversity and eliminating reservoirs.
CAR-Encoding HSPCs for HIV Cure
HSPC-based CAR therapy involves modifying hematopoietic stem cells to express anti-HIV CARs, which differentiate into functional T and NK cells. These cells can traffic to tissue reservoirs and suppress HIV replication in vivo. For example, HSPC-derived CD4 CAR cells have shown long-term engraftment and persistence in lymphoid tissues, the brain, and the gastrointestinal tract of nonhuman primate models.
Second-generation CD4-based CARs, optimized for HSPC differentiation, have demonstrated improved HIV suppression and reduced off-target effects. HSPC-based CAR therapy represents a powerful approach for achieving durable viral control and functional cure.
Clinical Trials of CAR T Cell Therapy in HIV Infection
Clinical trials of CAR T cell therapy for HIV date back to the 1990s. Early studies using CD4ζ-modified T cells demonstrated safety but limited efficacy. Recent trials have focused on improved CAR designs, such as dual CARs and bNAb-based CARs. For example, a phase I trial of VRC01-based CAR T cells showed reduced cell-associated viral RNA and intact provirus in ART-treated individuals, despite eventual viral rebound.
Ongoing trials are evaluating the safety and efficacy of novel CAR T cell therapies, including duoCAR T cells and CXCR5-expressing CAR T cells targeting follicular helper T cell reservoirs. These studies aim to achieve durable viral suppression and reduce the HIV reservoir.
Prospects and Challenges in CAR Therapy for HIV Infection
Despite its promise, CAR therapy faces several challenges. First, ensuring CAR T cell persistence and trafficking to tissue reservoirs is critical for effective reservoir elimination. Second, susceptibility of CD4-based CARs to HIV infection necessitates strategies like coreceptor knockout or bNAb-based CARs to protect engineered cells. Third, large-scale production and safety of CAR therapies require further optimization.
Conclusion
CAR therapy represents a transformative approach for targeting the HIV reservoir and achieving a functional cure. Advances in CAR design, including bispecific, trispecific, and universal CARs, have enhanced anti-HIV potency and breadth. CAR T cells, CAR NK cells, and CAR-encoding HSPCs offer complementary strategies for eliminating HIV reservoirs in diverse anatomical sites. Ongoing clinical trials are exploring the safety and efficacy of these therapies, with the ultimate goal of achieving durable ART-free remission. Continued innovation and collaboration will be essential to overcome remaining challenges and realize the full potential of CAR therapy for HIV cure.
doi.org/10.1097/CM9.0000000000002904
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