Immune checkpoint expression on HIV-specific CD4+ T cells and response to their blockade are dependent on lineage and function
Authors: Brunet-Ratnasingham Elsa , Morou Antigoni , Dubé Mathieu , Niessl Julia , Baxter Amy E. , Tastet Olivier , Brassard Nathalie , Ortega-Delgado Gloria , Charlebois Roxanne , Freeman Gordon J. , Tremblay Cécile , Routy Jean-Pierre , Kaufmann Daniel E. 
Published: September 2021

AbstractBackgroundAntigen-specific T cell impairment is observed in chronic infections. CD4+ T cells are diverse in phenotype and function; how their different lineages are impacted by inhibitory immune checkpoints (IC) is unknown.MethodsWe examined IC expression and function in HIV-specific CD4+ T cells of viremic individuals prior to ART initiation and persons with spontaneous or therapy-induced viral suppression. We investigated IC patterns associated with exhaustion-related transcription factors and chemokine receptors using cytokine-independent activation-induced marker assays. We determined effector functions representative of TFH, TH1 and TH17/TH22 using ultra-sensitive RNA flow cytometric fluorescence in situ hybridization (FISH), and their response to IC blockade.FindingsThe dysfunction-related transcription factor TOX was elevated in HIV-specific CD4+ T cells of viremic patients, and its expression was associated with lineage differentiation. We observed a hierarchy of PD-1, TIGIT and CD200 expression associated with both infection status and effector profile.In vitroresponsiveness to PD-L1 blockade varied with defined CD4+ T cell functions rather than IC expression levels: frequencies of cells with TH1- and TH17/TH22-, but not TFH-related functions, increased. Response to PD-L1 blockade was strongest in viremic participants and reduced after ART initiation.InterpretationOur data highlight a polarization-specific regulation of IC expression and differing sensitivities of antigen-specific Thelper subsets to PD-1-mediated inhibition. This heterogeneity may direct ICB efficacy on CD4+ T cells in HIV infection.FundingThis work was supported by the National Institutes of Health, the Canadian Institutes for Health Research, the Canada Foundation for Innovation and the Fonds de Recherche du Québec-Santé.Research in ContextEvidence before this studyCombination antiretroviral therapy (ART) is highly effective in controlling HIV but requires life-long medication due to the latent viral reservoir, and does not restore suppressive immune responses. In particular, there is no generation of effective HIV-specific T cell responses, which are thought to play an important role in controlling HIV in the rare individuals who can spontaneously control the virus. Inhibitory immune checkpoints (IC) such as PD-1 contribute to T cell dysfunction and failure to control viral infections, including HIV, and IC blockade (ICB) represents a potential adjuvant to ART through restoration of T cell functions. While most studies have focused on CD8+ T cells, increasing evidence shows that the remarkable impact of ICB therapy in a subset of cancer patients is enhanced by functional CD4+ T cell help, which can be directly affected by ICB. While effective virus-specific CD4+ T cell responses are also thought to be important for immune control of HIV, these cells are highly heterogenous. How IC expression and function differs across CD4+ T cell lineages and the consequences of this diversity for IC blockade (ICB) strategies are still poorly understood.Added value of this studyTo compare various stages of immune dysfunction, we examined people living with HIV (PLWH) with different levels of viral control pre-ART (including elite controllers who spontaneously control virus) and followed a cohort longitudinally post-ART. We used a panel of assays to characterize HIV-specific CD4+ T cell subsets, including activation-induced marker (AIM) assays and flow cytometric detection of mRNAs coding for a wider variety of HIV-specific CD4+ T cell functions than what is detected by standard procedures. Our experiments indicate a hierarchy of IC (PD-1, TIGIT, CD200) expression on blood HIV-specific CD4+ T cells that depends not only on the person’s infection status but also on expression of lineage differentiation markers and effector functions representative of CD4+ T cell subsets critical for antiviral responses (TFH, TH1 and TH17/TH22 cells). This hierarchy was also present in the putatively functional cells of elite controllers. We characterized the expression of the dysfunction-related transcription factor TOX, and saw that its association with the key IC PD-1 in the setting of viremia varied across CD4+ T cell polarizations. Response to blockade of the PD-1 pathway resulted in increased antiviral and mucosal-protective functions, but did not affect TFH-related functions. Response to ICB was most prominent in viremic patients, and subdued but not fully abrogated in the setting of viral suppression.Implications of all available evidenceThese results highlight a previously unrecognized impact of ICB on mucosal immunity-related CD4+ functions, which are known to be depleted upon HIV infection and not restored by ART, and strong links between IC expression patterns and HIV-specific CD4+ T cell differentiation. The impact of ICB on CD4+ T cells in HIV infection has primarily been studied in the context of viral reservoir reactivation, which are preferentially harbored in IC+ cells. Our work emphasizes the importance of considering the differentiation profile of the virus-specific CD4+ T cells in studies of ICB blockade, as it may direct ICB efficacy in HIV infection. This data may also have implications for CD4+ T cell help in other infectious and non-infectious chronic human diseases.