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The adoptive transfer of T lymphocytes reprogrammed to target tumour cells has demonstrated potential for treatment of various cancers 1 – 7 . However, little is known about the long-term potential and clonal stability of the infused cells. Here we studied long-lasting CD19-redirected chimeric antigen receptor (CAR) T cells in two patients with chronic lymphocytic leukaemia 1 – 4 who achieved a complete remission in 2010. CAR T cells remained detectable more than ten years after infusion, with sustained remission in both patients. Notably, a highly activated CD4 + population emerged in both patients, dominating the CAR T cell population at the later time points. This transition was reflected in the stabilization of the clonal make-up of CAR T cells with a repertoire dominated by a small number of clones. Single-cell profiling demonstrated that these long-persisting CD4 + CAR T cells exhibited cytotoxic characteristics along with ongoing functional activation and proliferation. In addition, longitudinal profiling revealed a population of gamma delta CAR T cells that prominently expanded in one patient concomitant with CD8 + CAR T cells during the initial response phase. Our identification and characterization of these unexpected CAR T cell populations provide novel insight into the CAR T cell characteristics associated with anti-cancer response and long-term remission in leukaemia. Infusion of CD19-directed chimeric antigen receptor T cells into two patients with chronic lymphocytic leukaemia resulted in complete tumour remission and persistence of the infused cells more than ten years later.

T-cell receptor (TCR) diversity, a prerequisite for immune system recognition of the universe of foreign antigens, is generated in the first two decades of life in the thymus and then persists to an unknown extent through life via homeostatic proliferation of naïve T cells. We have used next-generation sequencing and nonparametric statistical analysis to estimate a lower bound for the total number of different TCR beta (TCRB) sequences in human repertoires. We arrived at surprisingly high minimal estimates of 100 million unique TCRB sequences in naïve CD4 and CD8 T-cell repertoires of young adults. Naïve repertoire richness modestly declined two-to fivefold in healthy elderly. Repertoire richness contraction with age was even less pronounced for memory CD4 and CD8 T cells. In contrast, age had a major impact on the inequality of donai sizes, as estimated by a modified Gini-Simpson index clonality score. In particular, large naïve T-cell clones that were distinct from memory clones were found in the repertoires of elderly individuals, indicating uneven homeostatic proliferation without development of a memory cell phenotype. Our results suggest that a highly diverse repertoire is maintained despite thymic involution; however, peripheral fitness selection of T cells leads to repertoire perturbations that can influence the immune response in the elderly.

Tumor cell-derived exosomes (TEX) suppress functions of immune cells. Here, changes in the gene profiles of primary human T lymphocytes exposed in vitro to exosomes were evaluated. CD4 + Tconv, CD8 + T or CD4 + CD39 + Treg were isolated from normal donors’ peripheral blood and co-incubated with TEX or exosomes isolated from supernatants of cultured dendritic cells (DEX). Expression levels of 24–27 immune response-related genes in these T cells were quantified by qRT-PCR. In activated T cells, TEX and DEX up-regulated mRNA expression levels of multiple genes. Multifactorial data analysis of ΔCt values identified T cell activation and the immune cell type, but not exosome source, as factors regulating gene expression by exosomes. Treg were more sensitive to TEX-mediated effects than other T cell subsets. In Treg, TEX-mediated down-regulation of genes regulating the adenosine pathway translated into high expression of CD39 and increased adenosine production. TEX also induced up-regulation of inhibitory genes in CD4 + Tconv, which translated into a loss of CD69 on their surface and a functional decline. Exosomes are not internalized by T cells, but signals they carry and deliver to cell surface receptors modulate gene expression and functions of human T lymphocytes.

Co-inhibitory immune receptors can contribute to T cell dysfunction in patients with cancer 1 , 2 . Blocking antibodies against cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death 1 (PD-1) partially reverse this effect and are becoming standard of care in an increasing number of malignancies 3 . However, many of the other axes by which tumours become inhospitable to T cells are not fully understood. Here we report that V-domain immunoglobulin suppressor of T cell activation (VISTA) engages and suppresses T cells selectively at acidic pH such as that found in tumour microenvironments. Multiple histidine residues along the rim of the VISTA extracellular domain mediate binding to the adhesion and co-inhibitory receptor P-selectin glycoprotein ligand-1 (PSGL-1). Antibodies engineered to selectively bind and block this interaction in acidic environments were sufficient to reverse VISTA-mediated immune suppression in vivo. These findings identify a mechanism by which VISTA may engender resistance to anti-tumour immune responses, as well as an unexpectedly determinative role for pH in immune co-receptor engagement. V-domain immunoglobulin suppressor of T cell activation (VISTA) selectively engages P-selectin glycoprotein ligand-1 (PSGL-1) and suppresses T cells at acidic pH similar to those in tumour microenvironments, thereby mediating resistance to anti-tumour immune responses.

The expression of co-inhibitory receptors, such as CTLA-4 and PD-1, on effector T cells is a key mechanism for ensuring immune homeostasis. Dysregulated expression of co-inhibitory receptors on CD4 + T cells promotes autoimmunity, whereas sustained overexpression on CD8 + T cells promotes T cell dysfunction or exhaustion, leading to impaired ability to clear chronic viral infections and diseases such as cancer 1 , 2 . Here, using RNA and protein expression profiling at single-cell resolution in mouse cells, we identify a module of co-inhibitory receptors that includes not only several known co-inhibitory receptors (PD-1, TIM-3, LAG-3 and TIGIT) but also many new surface receptors. We functionally validated two new co-inhibitory receptors, activated protein C receptor (PROCR) and podoplanin (PDPN). The module of co-inhibitory receptors is co-expressed in both CD4 + and CD8 + T cells and is part of a larger co-inhibitory gene program that is shared by non-responsive T cells in several physiological contexts and is driven by the immunoregulatory cytokine IL-27. Computational analysis identified the transcription factors PRDM1 and c-MAF as cooperative regulators of the co-inhibitory module, and this was validated experimentally. This molecular circuit underlies the co-expression of co-inhibitory receptors in T cells and identifies regulators of T cell function with the potential to control autoimmunity and tumour immunity. A module of co-inhibitory T cell receptors, driven by the cytokine IL-27, is identified in mice that is regulated by the transcription factors PRDM1 and c-MAF.

The cross-talk between immune cells and blood vessel endothelial cells promotes pericyte coverage and decreases hypoxia in mouse tumour models, and correlative evidence suggests that these processes influence cancer prognosis in humans. Normalizing tumour vasculature Tumours often develop with abnormal vasculature, characterized among other things by lower pericyte coverage of blood vessels, as well as leaky vessels that result in a hypoxic environment. Abnormal vessels limit immune infiltration and CD4 T cells can regulate angiogenesis. Using mouse models, the authors further dissect this crosstalk between immune cells and blood vessels in cancer, and describe a role for immune cells in normalizing the vasculature of tumours. The crosstalk between CD4 T cells and endothelial cells promotes pericyte coverage and decreases hypoxia, and correlative evidence suggests that these processes influence cancer prognosis in humans. The authors postulate that interventions that foster CD4 T-cell function, such as immune checkpoint blockade, also have a beneficial effect by normalizing the tumour vasculature. Blockade of angiogenesis can retard tumour growth, but may also paradoxically increase metastasis 1 , 2 . This paradox may be resolved by vessel normalization 3 , which involves increased pericyte coverage, improved tumour vessel perfusion, reduced vascular permeability, and consequently mitigated hypoxia 3 . Although these processes alter tumour progression, their regulation is poorly understood. Here we show that type 1 T helper (T H 1) cells play a crucial role in vessel normalization. Bioinformatic analyses revealed that gene expression features related to vessel normalization correlate with immunostimulatory pathways, especially T lymphocyte infiltration or activity. To delineate the causal relationship, we used various mouse models with vessel normalization or T lymphocyte deficiencies. Although disruption of vessel normalization reduced T lymphocyte infiltration as expected 4 , reciprocal depletion or inactivation of CD4 + T lymphocytes decreased vessel normalization, indicating a mutually regulatory loop. In addition, activation of CD4 + T lymphocytes by immune checkpoint blockade increased vessel normalization. T H 1 cells that secrete interferon-γ are a major population of cells associated with vessel normalization. Patient-derived xenograft tumours growing in immunodeficient mice exhibited enhanced hypoxia compared to the original tumours in immunocompetent humans, and hypoxia was reduced by adoptive T H 1 transfer. Our findings elucidate an unexpected role of T H 1 cells in vasculature and immune reprogramming. T H 1 cells may be a marker and a determinant of both immune checkpoint blockade and anti-angiogenesis efficacy.

Matthieu Perreau and colleagues show that HIV-infected lymph node PD-1 + and follicular helper T cells account for the major source of replication-competent HIV-1 in individuals who have been treated with antiretroviral drugs. The mechanisms responsible for the persistence of HIV-1 after many years of suppressive antiretroviral therapy (ART) have been only partially elucidated. Most of the studies investigating HIV-1 persistence have been performed with blood, although it is well known that germinal centers (GCs) within lymph nodes (LNs) serve as primary sites for HIV-1 replication. We sought to identify the memory CD4 T cell populations in blood and LNs that are responsible for the production of replication-competent and infectious HIV-1, as well as for active and persistent virus transcription in ART-treated (for 1.5–14.0 years), aviremic (<50 HIV RNA copies/ml) HIV-infected individuals. We demonstrate that LN CD4 T cells that express programmed cell death 1 (PDCD1; also known as PD-1), which are composed of about 65% T follicular helper cells as defined by the expression of the cell surface receptors CXCR5 and PD-1, are the major source of replication-competent HIV-1 and of infectious virus, as compared to any other (CXCR5 − PD-1 − and CXCR5 + PD-1 − ) blood or LN memory CD4 T cell populations. LN PD-1 + cells accounted for 46% and 96% of the total pools of memory CD4 T cells containing inducible replication-competent or infectious virus, respectively. Notably, higher levels of cell-associated HIV-1 RNA were present in LN PD-1 + cells after long-term (up to 12 years) ART than in other memory CD4 T cell subpopulations. These results indicate that LN PD-1 + cells are the major CD4 T cell compartment in the blood and LNs for the production of replication-competent and infectious HIV-1, and for active and persistent virus transcription in long-term-ART-treated aviremic individuals. Thus, these cells may represent a major obstacle to finding a functional cure for HIV-1 infection.

Most clinically applied cancer immunotherapies rely on the ability of CD8 + cytolytic T cells to directly recognize and kill tumour cells 1 – 3 . These strategies are limited by the emergence of major histocompatibility complex (MHC)-deficient tumour cells and the formation of an immunosuppressive tumour microenvironment 4 – 6 . The ability of CD4 + effector cells to contribute to antitumour immunity independently of CD8 + T cells is increasingly recognized, but strategies to unleash their full potential remain to be identified 7 – 10 . Here, we describe a mechanism whereby a small number of CD4 + T cells is sufficient to eradicate MHC-deficient tumours that escape direct CD8 + T cell targeting. The CD4 + effector T cells preferentially cluster at tumour invasive margins where they interact with MHC-II + CD11c + antigen-presenting cells. We show that T helper type 1 cell-directed CD4 + T cells and innate immune stimulation reprogramme the tumour-associated myeloid cell network towards interferon-activated antigen-presenting and iNOS-expressing tumouricidal effector phenotypes. Together, CD4 + T cells and tumouricidal myeloid cells orchestrate the induction of remote inflammatory cell death that indirectly eradicates interferon-unresponsive and MHC-deficient tumours. These results warrant the clinical exploitation of this ability of CD4 + T cells and innate immune stimulators in a strategy to complement the direct cytolytic activity of CD8 + T cells and natural killer cells and advance cancer immunotherapies. This article describes a mechanism through which CD4 + T cells can eradicate MHC-deficient tumours that escape direct CD8 + T cell targeting and thereby complement the activity of CD8 + T cells and natural killer cells to advance cancer immunotherapies.

Conventional type 1 dendritic cells (cDC1) 1 are thought to perform antigen cross-presentation, which is required to prime CD8 + T cells 2 , 3 , whereas cDC2 are specialized for priming CD4 + T cells 4 , 5 . CD4 + T cells are also considered to help CD8 + T cell responses through a variety of mechanisms 6 – 11 , including a process whereby CD4 + T cells ‘license’ cDC1 for CD8 + T cell priming 12 . However, this model has not been directly tested in vivo or in the setting of help-dependent tumour rejection. Here we generated an Xcr1 Cre mouse strain to evaluate the cellular interactions that mediate tumour rejection in a model requiring CD4 + and CD8 + T cells. As expected, tumour rejection required cDC1 and CD8 + T cell priming required the expression of major histocompatibility class I molecules by cDC1. Unexpectedly, early priming of CD4 + T cells against tumour-derived antigens also required cDC1, and this was not simply because they transport antigens to lymph nodes for processing by cDC2, as selective deletion of major histocompatibility class II molecules in cDC1 also prevented early CD4 + T cell priming. Furthermore, deletion of either major histocompatibility class II or CD40 in cDC1 impaired tumour rejection, consistent with a role for cognate CD4 + T cell interactions and CD40 signalling in cDC1 licensing. Finally, CD40 signalling in cDC1 was critical not only for CD8 + T cell priming, but also for initial CD4 + T cell activation. Thus, in the setting of tumour-derived antigens, cDC1 function as an autonomous platform capable of antigen processing and priming for both CD4 + and CD8 + T cells and of the direct orchestration of their cross-talk that is required for optimal anti-tumour immunity. Conventional type 1 dendritic cells perform antigen processing and priming of CD4 + and CD8 + T cells against tumour antigens, orchestrating their cross-talk to effect anti-tumour immunity.

Successful pregnancy requires immune tolerance against paternal antigens expressed by the fetus; here pregnancy is shown to stimulate the selective accumulation of maternal immune-suppressive regulatory T cells with fetal specificity that are retained post-partum, which may explain the protective benefits of prior pregnancy against pre-eclampsia and other complications in subsequent pregnancy. Immune defence of the fetus Successful pregnancy requires the development of effective tolerance mechanisms for fetus antigens inherited from the father, which evoke an immune response because they are considered ‘non-self’ by the maternal immune system. Jared Rowe et al . show here that this is accomplished by the generation of maternal regulatory T cells with specificity for fetal antigens. Fetal-specific regulatory T cells re-accumulate with accelerated kinetics during secondary pregnancy and may explain why partner-specific secondary pregnancies show reduced rates of pre-eclampsia and other complications. Pregnancy is an intricately orchestrated process where immune effector cells with fetal specificity are selectively silenced. This requires the sustained expansion of immune-suppressive maternal FOXP3 + regulatory T cells (T reg cells), because even transient partial ablation triggers fetal-specific effector T-cell activation and pregnancy loss 1 , 2 . In turn, many idiopathic pregnancy complications proposed to originate from disrupted fetal tolerance are associated with blunted maternal T reg expansion 3 , 4 , 5 . Importantly, however, the antigen specificity and cellular origin of maternal T reg cells that accumulate during gestation remain incompletely defined. Here we show that pregnancy selectively stimulates the accumulation of maternal FOXP3 + CD4 cells with fetal specificity using tetramer-based enrichment that allows the identification of rare endogenous T cells 6 . Interestingly, after delivery, fetal-specific T reg cells persist at elevated levels, maintain tolerance to pre-existing fetal antigen, and rapidly re-accumulate during subsequent pregnancy. The accelerated expansion of T reg cells during secondary pregnancy was driven almost exclusively by proliferation of fetal-specific FOXP3 + cells retained from prior pregnancy, whereas induced FOXP3 expression and proliferation of pre-existing FOXP3 + cells each contribute to T reg expansion during primary pregnancy. Furthermore, fetal resorption in secondary compared with primary pregnancy becomes more resilient to partial maternal FOXP3 + cell ablation. Thus, pregnancy imprints FOXP3 + CD4 cells that sustain protective regulatory memory to fetal antigen. We anticipate that these findings will spark further investigation on maternal regulatory T-cell specificity that unlocks new strategies for improving pregnancy outcomes and novel approaches for therapeutically exploiting T reg cell memory.