PD-1 Imposes Qualitative Control of Cellular Transcriptomes in Response to T Cell Activation

Context-restricted PD-(L)1 checkpoint agonism by CTLA4-Ig therapies inhibits T cell activity

T cell surface CTLA4 sequesters the costimulatory ligands CD80 and CD86 on antigen-presenting cells (APCs) to prevent autoimmunity. Therapeutic immunosuppression by recombinant CTLA4-immunoglobulin (Ig) fusion proteins, including abatacept, is also attributed to CD80/CD86 blockade. Recent studies show that CTLA4-Ig binding to APC surface cis-CD80:PD-L1 complexes can release the inhibitory ligand PD-L1, but whether this contributes to T cell inhibition remains unclear. Here, we show that PD-L1 liberation by CTLA4-Ig is strictly limited, both in extent and context, relative to PD-L1-competing anti-CD80 antibodies. At APC surface CD80:PD-L1 ratios exceeding 2:1, CTLA4-Ig therapies fail to release PD-L1 regardless of their CD80 affinity. Additionally, introducing flexibility into CTLA4-Ig by modifying its rigid homodimer interface produces biologics that retain bivalent CD80 binding without dissociating cis-bound PD-L1. These findings demonstrate that CTLA4-Ig therapies liberate PD-L1 through a CD80 reorientation mechanism that imposes a strict context dependence to their PD-1 checkpoint agonism and resultant T cell inhibition.

Identification and characterization of circulating and adipose tissue infiltrated CD20+T cells from subjects with obesity that undergo bariatric surgery

T cells play critical roles in adipose tissue (AT) inflammation. The role of CD20+T cell in AT dysfunction and their contributing to insulin resistance (IR) and type 2 diabetes progression, is not known. The aim was to characterize CD20+T cells in omental (OAT), subcutaneous (SAT) and peripheral blood (PB) from subjects with obesity (OB, n = 42), by flow cytometry. Eight subjects were evaluated before (T1) and 12 months post (T2) bariatric/metabolic surgery (BMS). PB from subjects without obesity (nOB, n = 12) was also collected. Higher percentage of CD20+T cells was observed in OAT, compared to PB or SAT, in OB-T1. CD20 expression by PB CD4+T cells was inversely correlated with adiposity markers, while follicular-like CD20+T cells were positively correlated with impaired glucose tolerance (increased HbA1c). Notably, among OB-T1, IR establishment was marked by a lower percentage and absolute number of PB CD20+T cells, compared nOB. Obesity was associated with higher percentage of activated CD20+T cells; however, OAT-infiltrated CD20+T cells from OB-T1 with diabetes displayed the lowest activation. CD20+T cells infiltrating OAT from OB-T1 displayed a phenotype towards IFN-γ-producing Th1 and Tc1 cells. After BMS, the percentage of PB CD4+CD20+T cells increased, with reduced Th1 and increased Th17 phenotype. Whereas in OAT the percentage of CD20+T cells with Th1/17 and Tc1/17 phenotypes increased. Interestingly, OAT from OB pre/post BMS maintained higher frequency of effector memory CD20+T cells. In conclusion, CD20+T cells may play a prominent role in obesity-related AT inflammation.

Targeting PD-1/PD-L1 in tumor immunotherapy: Mechanisms and interactions with host growth regulatory pathways

Tumor immunotherapy has garnered considerable attention, emerging as a new standard of care in cancer treatment. The conventional targets, such as VEGF and EGFR, have been extended to others including BRAF and PD-1/PD-L1, which have shown significant potential in recent cancer treatments. This review aims to succinctly overview the impact and mechanisms of therapies that modulate PD-1/PD-L1 expression by targeting VEGF, EGFR, LAG-3, CTLA-4 and BRAF. We investigated how modulation of PD-1/PD-L1 expression impacts growth factor signaling, shedding light on the interplay between immunomodulatory pathways and growth factor networks within the tumor microenvironment. By elucidating these interactions, we aim to provide insights into novel potential synergistic therapeutic strategies for cancer immunotherapy.

Neoadjuvant PARPi or chemotherapy in ovarian cancer informs targeting effector Treg cells for homologous-recombination-deficient tumors

Homologous recombination deficiency (HRD) is prevalent in cancer, sensitizing tumor cells to poly (ADP-ribose) polymerase (PARP) inhibition. However, the impact of HRD and related therapies on the tumor microenvironment (TME) remains elusive. Our study generates single-cell gene expression and T cell receptor profiles, along with validatory multimodal datasets from >100 high-grade serous ovarian cancer (HGSOC) samples, primarily from a phase II clinical trial (NCT04507841). Neoadjuvant monotherapy with the PARP inhibitor (PARPi) niraparib achieves impressive 62.5% and 73.6% response rates per RECIST v.1.1 and GCIG CA125, respectively. We identify effector regulatory T cells (eTregs) as key responders to HRD and neoadjuvant therapies, co-occurring with other tumor-reactive T cells, particularly terminally exhausted CD8+ T cells (Tex). TME-wide interferon signaling correlates with cancer cells upregulating MHC class II and co-inhibitory ligands, potentially driving Treg and Tex fates. Depleting eTregs in HRD mouse models, with or without PARP inhibition, significantly suppresses tumor growth without observable toxicities, underscoring the potential of eTreg-focused therapeutics for HGSOC and other HRD-related tumors.

HIV/HBV coinfection remodels the immune landscape and natural killer cell ADCC functional responses

BACKGROUND AND AIMS

HBV and HIV coinfection is a common occurrence globally, with significant morbidity and mortality. Both viruses lead to immune dysregulation including changes in natural killer (NK) cells, a key component of antiviral defense and a promising target for HBV cure strategies. Here we used high-throughput single-cell analysis to explore the immune cell landscape in people with HBV mono-infection and HIV/HBV coinfection, on antiviral therapy, with emphasis on identifying the distinctive characteristics of NK cell subsets that can be therapeutically harnessed.

APPROACH AND RESULTS

Our data show striking differences in the transcriptional programs of NK cells. HIV/HBV coinfection was characterized by an over-representation of adaptive, KLRC2 -expressing NK cells, including a higher abundance of a chemokine-enriched ( CCL3/CCL4 ) adaptive cluster. The NK cell remodeling in HIV/HBV coinfection was reflected in enriched activation pathways, including CD3ζ phosphorylation and ZAP-70 translocation that can mediate stronger antibody-dependent cellular cytotoxicity responses and a bias toward chemokine/cytokine signaling. By contrast, HBV mono-infection imposed a stronger cytotoxic profile on NK cells and a more prominent signature of "exhaustion" with higher circulating levels of HBsAg. Phenotypic alterations in the NK cell pool in coinfection were consistent with increased "adaptiveness" and better capacity for antibody-dependent cellular cytotoxicity compared to HBV mono-infection. Overall, an adaptive NK cell signature correlated inversely with circulating levels of HBsAg and HBV-RNA in our cohort.

CONCLUSIONS

This study provides new insights into the differential signature and functional profile of NK cells in HBV and HIV/HBV coinfection, highlighting pathways that can be manipulated to tailor NK cell-focused approaches to advance HBV cure strategies in different patient groups.

Multifaceted roles of PD-1 in tumorigenesis: From immune checkpoint to tumor cell-intrinsic function

Programmed cell death 1 (PD-1), a key immune checkpoint receptor, has been extensively studied for its role in regulating immune responses in cancer. However, recent research has unveiled a complex and dual role for PD-1 in tumorigenesis. While PD-1 is traditionally associated with immune cells, this article explores its expression in various cancer cells and its impact on cancer progression. PD-1's functions extend beyond immune regulation, as it has been found to both promote and suppress tumor growth, depending on the cancer type. These findings have significant implications for the future of cancer treatment and our understanding of the immune response in the context of cancer. This article calls for further research into the multifaceted roles of PD-1 to optimize its therapeutic potential and improve patient outcomes in the fight against cancer.

Dostarlimab: A promising new PD-1 inhibitor for cancer immunotherapy

OBJECTIVE

Dostarlimab, a humanized monoclonal PD-1 blocking antibody, is being tested as a cancer therapy in this review. Specifically, it addresses mismatch repair failure in endometrial cancer and locally progressed rectal cancer patients.

DATA SOURCES

A thorough database search found Dostarlimab clinical trials and studies. Published publications and ongoing clinical trials on Dostarlimab's efficacy as a single therapy and in conjunction with other medicines across cancer types were searched.

DATA SUMMARY

The review recommends Dostarlimab for endometrial cancer mismatch repair failure, as supported by GARNET studies. The analysis also highlights locally advanced rectal cancer findings. In the evolving area of cancer therapy, immune checkpoint inhibitors including pembrolizumab, avelumab, atezolizumab, nivolumab, and durvalumab were discussed.

CONCLUSIONS

Locally advanced rectal cancer patients responded 100% to Dostarlimab. Many clinical trials, including ROSCAN, AMBER, IOLite, CITRINO, JASPER, OPAL, PRIME, PERLA, and others, are investigating Dostarlimab in combination treatment. This research sheds light on Dostarlimab's current and future possibilities, in improving cancer immunotherapy understanding.

BTLA and PD-1 signals attenuate TCR-mediated transcriptomic changes

T cell co-inhibitory immune checkpoints, such as PD-1 or BTLA, are bona fide targets in cancer therapy. We used a human T cell reporter line to measure transcriptomic changes mediated by PD-1- and BTLA-induced signaling. T cell receptor (TCR)-complex stimulation resulted in the upregulation of a large number of genes but also in repression of a similar number of genes. PD-1 and BTLA signals attenuated transcriptomic changes mediated by TCR-complex signaling: upregulated genes tended to be suppressed and the expression of a significant number of downregulated genes was higher during PD-1 or BTLA signaling. BTLA was a significantly stronger attenuator of TCR-complex-induced transcriptome changes than PD-1. A strong overlap between genes that were regulated indicated quantitative rather than qualitative differences between these receptors. In line with their function as attenuators of TCR-complex-mediated changes, we found strongly regulated genes to be prime targets of PD-1 and BTLA signaling.

PD-1 downregulation enhances CAR-T cell antitumor efficiency by preserving a cell memory phenotype and reducing exhaustion

BACKGROUND

Despite the encouraging outcome of chimeric antigen receptor T cell (CAR-T) targeting B cell maturation antigen (BCMA) in managing relapsed or refractory multiple myeloma (RRMM) patients, the therapeutic side effects and dysfunctions of CAR-T cells have limited the efficacy and clinical application of this promising approach.

METHODS

In this study, we incorporated a short hairpin RNA cassette targeting PD-1 into a BCMA-CAR with an OX-40 costimulatory domain. The transduced PD-1KD BCMA CAR-T cells were evaluated for surface CAR expression, T-cell proliferation, cytotoxicity, cytokine production, and subsets when they were exposed to a single or repetitive antigen stimulation. Safety and efficacy were initially observed in a phase I clinical trial for RRMM patients.

RESULTS

Compared with parental BCMA CAR-T cells, PD-1KD BCMA CAR-T cell therapy showed reduced T-cell exhaustion and increased percentage of memory T cells in vitro. Better antitumor activity in vivo was also observed in PD-1KD BCMA CAR-T group. In the phase I clinical trial of the CAR-T cell therapy for seven RRMM patients, safety and efficacy were initially observed in all seven patients, including four patients (4/7, 57.1%) with at least one extramedullary site and four patients (4/7, 57.1%) with high-risk cytogenetics. The overall response rate was 85.7% (6/7). Four patients had a stringent complete response (sCR), one patient had a CR, one patient had a partial response, and one patient had stable disease. Safety profile was also observed in these patients, with an incidence of manageable mild to moderate cytokine release syndrome and without the occurrence of neurological toxicity.

CONCLUSIONS

Our study demonstrates a design concept of CAR-T cells independent of antigen specificity and provides an alternative approach for improving the efficacy of CAR-T cell therapy.

Inhibitory CARs fail to protect from immediate T cell cytotoxicity

Chimeric antigen receptors (CARs) equipped with an inhibitory signaling domain (iCARs) have been proposed as strategy to increase on-tumor specificity of CAR-T cell therapies. iCARs inhibit T cell activation upon antigen recognition and thereby program a Boolean NOT gate within the CAR-T cell. If cancer cells do not express the iCAR target antigen while it is highly expressed on healthy tissue, CAR/iCAR coexpressing T cells are supposed to kill cancer cells but not healthy cells expressing the CAR antigen. In this study, we employed a well-established reporter cell system to demonstrate high potency of iCAR constructs harboring BTLA-derived signaling domains. We then created CAR/iCAR combinations for the clinically relevant antigen pairs B7-H3/CD45 and CD123/CD19 and show potent reporter cell suppression by iCARs targeting CD45 or CD19. In primary human T cells αCD19-iCARs were capable of suppressing T cell proliferation and cytokine production. Surprisingly, the iCAR failed to veto immediate CAR-mediated cytotoxicity. Likewise, T cells overexpressing PD-1 or BTLA did not show impaired cytotoxicity toward ligand-expressing target cells, indicating that inhibitory signaling by these receptors does not mediate protection against cytotoxicity by CAR-T cells. Future approaches employing iCAR-equipped CAR-T cells for cancer therapy should therefore monitor off-tumor reactivity and potential CAR/iCAR-T cell dysfunction.

CPPLS-MLP: a method for constructing cell-cell communication networks and identifying related highly variable genes based on single-cell sequencing and spatial transcriptomics data

In the growth and development of multicellular organisms, the immune processes of the immune system and the maintenance of the organism's internal environment, cell communication plays a crucial role. It exerts a significant influence on regulating internal cellular states such as gene expression and cell functionality. Currently, the mainstream methods for studying intercellular communication are focused on exploring the ligand-receptor-transcription factor and ligand-receptor-subunit scales. However, there is relatively limited research on the association between intercellular communication and highly variable genes (HVGs). As some HVGs are closely related to cell communication, accurately identifying these HVGs can enhance the accuracy of constructing cell communication networks. The rapid development of single-cell sequencing (scRNA-seq) and spatial transcriptomics technologies provides a data foundation for exploring the relationship between intercellular communication and HVGs. Therefore, we propose CPPLS-MLP, which can identify HVGs closely related to intercellular communication and further analyze the impact of Multiple Input Multiple Output cellular communication on the differential expression of these HVGs. By comparing with the commonly used method CCPLS for constructing intercellular communication networks, we validated the superior performance of our method in identifying cell-type-specific HVGs and effectively analyzing the influence of neighboring cell types on HVG expression regulation. Source codes for the CPPLS_MLP R, python packages and the related scripts are available at 'CPPLS_MLP Github [https://github.com/wuzhenao/CPPLS-MLP]'.

Pathogenicity of functionally activated PD-1+CD8+ cells and counterattacks by muscular PD-L1 through IFNγ in myositis

Programmed-cell-death 1 (PD-1) expression is associated not only with T-cell activation but with exhaustion. Specifically, PD-1+ T cells present an exhausted phenotype in conditions of chronic antigen exposure, such as tumor microenvironments and chronic viral infection. However, the immune status regarding exhaustion of PD-1+CD8+ T cells in chronic autoimmune diseases including idiopathic inflammatory myopathies (IIMs) remains unclear. We aimed to clarify the role of PD-1+CD8+ T cells and PD-1 ligand (PD-L1) in IIMs. We showed that PD-1+ cells infiltrated into PD-L1-expressing muscles in patients with IIMs and immune checkpoint inhibitor-related myopathy. According to the peripheral blood immunophenotyping, the PD-1+CD8+ cell proportions were comparable between the active and inactive patients. Of note, PD-1+CD8+ cells in the active patients highly expressed cytolytic molecules, indicating their activation, while PD-1-CD8+ cells expressed low levels of cytolytic molecules in the active and inactive patients. A part of PD-1+CD8+ cells expressed the HMG-box transcription factor TOX highly and presented the exhausted phenotype in the active patients. Among PD-1+CD4+ T cells, PD-1highCXCR5-CD45RO+CD4+ peripheral helper T cells were increased in the active patients. PD-L1-deficient mice developed severer C-protein-induced myositis (CIM), a model of polymyositis, with abundant infiltration of PD-1+CD8+ cells expressing cytolytic molecules than wild-type mice, indicating pathogenicity of the PD-1+CD8+ cells and the protective role of PD-L1. The deficiency of IFNγ, a general PD-L1-inducer, impaired muscular PD-L1 expression and exacerbated CIM, indicating IFNγ-dependent muscular PD-L1 regulation. IFNγ-induced PD-L1 on myotubes was protective in an established muscle injury model. In conclusion, PD-1+CD8+ T cells rather than PD-1-CD8+ T cells were a pathogenic subset of IIMs. Muscular PD-L1 was regulated by IFNγ and exerted protective properties in IIMs.

Immune checkpoint inhibitors therapy as the game-changing approach for pediatric lymphoma: A brief landscape

Lymphoma is known as the third most common malignancy in children, and its prevalence and mortality are increasing. Common treatments, including chemotherapy, radiotherapy, and also surgery, despite their efficacy, have many side effects and, have a high chance of disease relapse. Immune Checkpoint Inhibitors (ICIs) offer a promising alternative with potentially fewer risks of relapse and toxicity. This review article aims to investigate the efficacy and safety of ICIs, either as monotherapy or in combination, for pediatric lymphoma patients. ICIs have revolutionized cancer treatment in recent years and have shown remarkable results in several adult cancers. However, their efficacy in treating pediatrics requires further investigation. Nevertheless, some ICIs, including nivolumab, pembrolizumab, and ipilimumab, have demonstrated encouraging outcomes. ICIs therapy is not without risks and can cause side effects, including rash, itching, vitiligo, abdominal pain, diarrhea, dysphagia, epigastric pain, nausea, vomiting, thyroid, and pituitary dysfunction. Overall, this review article highlights the potential benefits and risks of ICIs in treating pediatric lymphoma.

TCR ligand potency differentially impacts PD-1 inhibitory effects on diverse signaling pathways

Checkpoint blockade revolutionized cancer therapy, but we still lack a quantitative, mechanistic understanding of how inhibitory receptors affect diverse signaling pathways. To address this issue, we developed and applied a fluorescent intracellular live multiplex signal transduction activity reporter (FILMSTAR) system to analyze PD-1-induced suppressive effects. These studies identified pathways triggered solely by TCR or requiring both TCR and CD28 inputs. Using presenting cells differing in PD-L1 and CD80 expression while displaying TCR ligands of distinct potency, we found that PD-1-mediated inhibition primarily targets TCR-linked signals in a manner highly sensitive to peptide ligand quality. These findings help resolve discrepancies in existing data about the site(s) of PD-1 inhibition in T cells while emphasizing the importance of neoantigen potency in controlling the effects of checkpoint therapy.

T-cell response to checkpoint blockade immunotherapies: from fundamental mechanisms to treatment signatures

Immune checkpoint immunotherapies act to block inhibitory receptors on the surface of T cells and other cells of the immune system. This can increase activation of immune cells and promote tumour clearance. Whilst this is very effective in some types of cancer, significant proportions of patients do not respond to single-agent immunotherapy. To improve patient outcomes, we must first mechanistically understand what drives therapy resistance. Many studies have utilised genetic, transcriptional, and histological signatures to find correlates of effective responses to treatment. It is key that we understand pretreatment predictors of response, but also to understand how the immune system becomes treatment resistant during therapy. Here, we review our understanding of the T-cell signatures that are critical for response, how these immune signatures change during treatment, and how this information can be used to rationally design therapeutic strategies. We highlight how chronic antigen recognition drives heterogeneous T-cell exhaustion and the role of T-cell receptor (TCR) signal strength in exhausted T-cell differentiation and molecular response to therapy. We explore how dynamic changes in negative feedback pathways can promote resistance to single-agent therapy. We speculate that this resistance may be circumvented in the future through identifying the most effective combinations of immunotherapies to promote sustained and durable antitumour responses.

Tumor microenvironment signaling and therapeutics in cancer progression

Tumor development and metastasis are facilitated by the complex interactions between cancer cells and their microenvironment, which comprises stromal cells and extracellular matrix (ECM) components, among other factors. Stromal cells can adopt new phenotypes to promote tumor cell invasion. A deep understanding of the signaling pathways involved in cell-to-cell and cell-to-ECM interactions is needed to design effective intervention strategies that might interrupt these interactions. In this review, we describe the tumor microenvironment (TME) components and associated therapeutics. We discuss the clinical advances in the prevalent and newly discovered signaling pathways in the TME, the immune checkpoints and immunosuppressive chemokines, and currently used inhibitors targeting these pathways. These include both intrinsic and non-autonomous tumor cell signaling pathways in the TME: protein kinase C (PKC) signaling, Notch, and transforming growth factor (TGF-β) signaling, Endoplasmic Reticulum (ER) stress response, lactate signaling, Metabolic reprogramming, cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) and Siglec signaling pathways. We also discuss the recent advances in Programmed Cell Death Protein 1 (PD-1), Cytotoxic T-Lymphocyte Associated Protein 4 (CTLA4), T-cell immunoglobulin mucin-3 (TIM-3) and Lymphocyte Activating Gene 3 (LAG3) immune checkpoint inhibitors along with the C-C chemokine receptor 4 (CCR4)- C-C class chemokines 22 (CCL22)/ and 17 (CCL17), C-C chemokine receptor type 2 (CCR2)- chemokine (C-C motif) ligand 2 (CCL2), C-C chemokine receptor type 5 (CCR5)- chemokine (C-C motif) ligand 3 (CCL3) chemokine signaling axis in the TME. In addition, this review provides a holistic understanding of the TME as we discuss the three-dimensional and microfluidic models of the TME, which are believed to recapitulate the original characteristics of the patient tumor and hence may be used as a platform to study new mechanisms and screen for various anti-cancer therapies. We further discuss the systemic influences of gut microbiota in TME reprogramming and treatment response. Overall, this review provides a comprehensive analysis of the diverse and most critical signaling pathways in the TME, highlighting the associated newest and critical preclinical and clinical studies along with their underlying biology. We highlight the importance of the most recent technologies of microfluidics and lab-on-chip models for TME research and also present an overview of extrinsic factors, such as the inhabitant human microbiome, which have the potential to modulate TME biology and drug responses.

Promise of dostarlimab in cancer therapy: Advancements and cross-talk considerations

In recent years, immunotherapy for cancer treatment using monoclonal antibodies has shown clinical success, particularly with programmed cell death protein 1 (PD-1) and its ligand programmed death-ligand 1 (PD-L1). Dostarlimab, an immune checkpoint inhibitor, interacts with adaptive immunity by binding to human PD-1, inhibiting PD-L1 and PD-L2 interactions, and cross-talk with adaptive immunity. Recent clinical trials have shown that dostarlimab is effective in treating mismatch repair deficiency (dMMR) in endometrial cancer patients, leading to its approval in the United States and the European Union in 2021. This article provides a comprehensive overview of dostarlimab, its therapeutic ability, and the different indications for which it is being used. Dostarlimab could serve as a potential alternative to many cancer treatments that frequently have severe consequences on patients' quality of life. Teaser The comprehensive story behind dostarlimab is how it cured all 18 cancer patients who took part in the experimental clinical trial, ultimately leading to its approval by the US FDA.

Human PD-1 agonist treatment alleviates neutrophilic asthma by reprogramming T cells

BACKGROUND

Neutrophilic asthma is associated with disease severity and corticosteroid insensitivity. Novel therapies are required to manage this life-threatening asthma phenotype. Programmed cell death protein-1 (PD-1) is a key homeostatic modulator of the immune response for T-cell effector functions.

OBJECTIVE

We sought to investigate the role of PD-1 in the regulation of acute neutrophilic inflammation in a murine model of airway hyperreactivity (AHR).

METHODS

House dust mite was used to induce and compare neutrophilic AHR in wild-type and PD-1 knockout mice. Then, the therapeutic potential of a human PD-1 agonist was tested in a humanized mouse model in which the PD-1 extracellular domain is entirely humanized. Single-cell RNA sequencing and flow cytometry were mainly used to investigate molecular and cellular mechanisms.

RESULTS

PD-1 was highly induced on pulmonary T cells in our inflammatory model. PD-1 deficiency was associated with an increased neutrophilic AHR and high recruitment of inflammatory cells to the lungs. Consistently, PD-1 agonist treatment dampened AHR, decreased neutrophil recruitment, and modulated cytokine production in a humanized PD-1 mouse model. Mechanistically, we demonstrated at the transcriptional and protein levels that the inhibitory effect of PD-1 agonist is associated with the reprogramming of pulmonary effector T cells that showed decreased number and activation.

CONCLUSIONS

PD-1 agonist treatment is efficient in dampening neutrophilic AHR and lung inflammation in a preclinical humanized mouse model.

An integrative systems biology view of host-pathogen interactions: The regulation of immunity and homeostasis is concomitant, flexible, and smart

The systemic bio-organization of humans and other mammals is essentially "preprogrammed", and the basic interacting units, the cells, can be crudely mapped into discrete sets of developmental lineages and maturation states. Over several decades, however, and focusing on the immune system, we and others invoked evidence - now overwhelming - suggesting dynamic acquisition of cellular properties and functions, through tuning, re-networking, chromatin remodeling, and adaptive differentiation. The genetically encoded "algorithms" that govern the integration of signals and the computation of new states are not fully understood but are believed to be "smart", designed to enable the cells and the system to discriminate meaningful perturbations from each other and from "noise". Cellular sensory and response properties are shaped in part by recurring temporal patterns, or features, of the signaling environment. We compared this phenomenon to associative brain learning. We proposed that interactive cell learning is subject to selective pressures geared to performance, allowing the response of immune cells to injury or infection to be progressively coordinated with that of other cell types across tissues and organs. This in turn is comparable to supervised brain learning. Guided by feedback from both the tissue itself and the neural system, resident or recruited antigen-specific and innate immune cells can eradicate a pathogen while simultaneously sustaining functional homeostasis. As informative memories of immune responses are imprinted both systemically and within the targeted tissues, it is desirable to enhance tissue preparedness by incorporating attenuated-pathogen vaccines and informed choice of tissue-centered immunomodulators in vaccination schemes. Fortunately, much of the "training" that a living system requires to survive and function in the face of disturbances from outside or within is already incorporated into its design, so it does not need to deep-learn how to face a new challenge each time from scratch. Instead, the system learns from experience how to efficiently select a built-in strategy, or a combination of those, and can then use tuning to refine its organization and responses. Efforts to identify and therapeutically augment such strategies can take advantage of existing integrative modeling approaches. One recently explored strategy is boosting the flux of uninfected cells into and throughout an infected tissue to rinse and replace the infected cells.

Current and future concepts for the generation and application of genetically engineered CAR-T and TCR-T cells

Adoptive cell therapy (ACT) has seen a steep rise of new therapeutic approaches in its immune-oncology pipeline over the last years. This is in great part due to the recent approvals of chimeric antigen receptor (CAR)-T cell therapies and their remarkable efficacy in certain soluble tumors. A big focus of ACT lies on T cells and how to genetically modify them to target and kill tumor cells. Genetically modified T cells that are currently utilized are either equipped with an engineered CAR or a T cell receptor (TCR) for this purpose. Both strategies have their advantages and limitations. While CAR-T cell therapies are already used in the clinic, these therapies face challenges when it comes to the treatment of solid tumors. New designs of next-generation CAR-T cells might be able to overcome these hurdles. Moreover, CARs are restricted to surface antigens. Genetically engineered TCR-T cells targeting intracellular antigens might provide necessary qualities for the treatment of solid tumors. In this review, we will summarize the major advancements of the CAR-T and TCR-T cell technology. Moreover, we will cover ongoing clinical trials, discuss current challenges, and provide an assessment of future directions within the field.

Evaluation of cutaneous immune response in a controlled human in vivo model of mosquito bites

Mosquito-borne viruses are a growing global threat. Initial viral inoculation occurs in the skin via the mosquito 'bite', eliciting immune responses that shape the establishment of infection and pathogenesis. Here we assess the cutaneous innate and adaptive immune responses to controlled Aedes aegypti feedings in humans living in Aedes-endemic areas. In this single-arm, cross-sectional interventional study (trial registration #NCT04350905), we enroll 30 healthy adult participants aged 18 to 45 years of age from Cambodia between October 2020 and January 2021. We perform 3-mm skin biopsies at baseline as well as 30 min, 4 h, and 48 h after a controlled feeding by uninfected Aedes aegypti mosquitos. The primary endpoints are measurement of changes in early and late innate responses in bitten vs unbitten skin by gene expression profiling, immunophenotyping, and cytokine profiling. The results reveal induction of neutrophil degranulation and recruitment of skin-resident dendritic cells and M2 macrophages. As the immune reaction progresses T cell priming and regulatory pathways are upregulated along with a shift to Th2-driven responses and CD8+ T cell activation. Stimulation of participants' bitten skin cells with Aedes aegypti salivary gland extract results in reduced pro-inflammatory cytokine production. These results identify key immune genes, cell types, and pathways in the human response to mosquito bites and can be leveraged to inform and develop novel therapeutics and vector-targeted vaccine candidates to interfere with vector-mediated disease.

CCPLS reveals cell-type-specific spatial dependence of transcriptomes in single cells

MOTIVATION

Cell-cell communications regulate internal cellular states, e.g. gene expression and cell functions, and play pivotal roles in normal development and disease states. Furthermore, single-cell RNA sequencing methods have revealed cell-to-cell expression variability of highly variable genes (HVGs), which is also crucial. Nevertheless, the regulation of cell-to-cell expression variability of HVGs via cell-cell communications is still largely unexplored. The recent advent of spatial transcriptome methods has linked gene expression profiles to the spatial context of single cells, which has provided opportunities to reveal those regulations. The existing computational methods extract genes with expression levels influenced by neighboring cell types. However, limitations remain in the quantitativeness and interpretability: they neither focus on HVGs nor consider the effects of multiple neighboring cell types.

RESULTS

Here, we propose CCPLS (Cell-Cell communications analysis by Partial Least Square regression modeling), which is a statistical framework for identifying cell-cell communications as the effects of multiple neighboring cell types on cell-to-cell expression variability of HVGs, based on the spatial transcriptome data. For each cell type, CCPLS performs PLS regression modeling and reports coefficients as the quantitative index of the cell-cell communications. Evaluation using simulated data showed our method accurately estimated the effects of multiple neighboring cell types on HVGs. Furthermore, applications to the two real datasets demonstrate that CCPLS can extract biologically interpretable insights from the inferred cell-cell communications.

AVAILABILITY AND IMPLEMENTATION

The R package is available at https://github.com/bioinfo-tsukuba/CCPLS. The data are available at https://github.com/bioinfo-tsukuba/CCPLS_paper.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Memory CD4+ T cells that co-express PD1 and CTLA4 have reduced response to activating stimuli facilitating HIV latency

Programmed cell death 1 (PD1) and cytotoxic T lymphocyte-associated protein 4 (CTLA4) suppress CD4⁺ T cell activation and may promote latent HIV infection. By performing leukapheresis (n = 21) and lymph node biopsies (n = 8) in people with HIV on antiretroviral therapy (ART) and sorting memory CD4⁺ T cells into subsets based on PD1/CTLA4 expression, we investigate the role of PD1 and CTLA 4 in HIV persistence. We show that double-positive (PD1⁺CTLA4⁺) cells in blood contain more HIV DNA compared with double-negative (PD1⁻CTLA4⁻) cells but still have a lower proportion of cells producing multiply spliced HIV RNA after stimulation as well as reduced upregulation of T cell activation and proliferation markers. Transcriptomics analyses identify differential expression of key genes regulating T cell activation and proliferation with MAF, KLRB1, and TIGIT being upregulated in double-positive compared with double-negative cells, whereas FOS is downregulated. We conclude that, in addition to being enriched for HIV DNA, double-positive cells are characterized by negative signaling and a reduced capacity to respond to stimulation, favoring HIV latency.

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CD4⁺ T cells co-expressing PD1 and CTLA4 (double positive [DP]) are enriched for HIV DNA

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DP cells contain virus that is more resistant to stimulation

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DP cells display differential expression of genes regulating T cell activation

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These features favor persistence of HIV latency in cells co-expressing PD1 and CTLA4

Immune Checkpoint Receptors Signaling in T Cells

The characterization of the receptors negatively modulating lymphocyte function is rapidly advancing, driven by success in tumor immunotherapy. As a result, the number of immune checkpoint receptors characterized from a functional perspective and targeted by innovative drugs continues to expand. This review focuses on the less explored area of the signaling mechanisms of these receptors, of those expressed in T cells. Studies conducted mainly on PD-1, CTLA-4, and BTLA have evidenced that the extracellular parts of some of the receptors act as decoy receptors for activating ligands, but in all instances, the tyrosine phosphorylation of their cytoplasmatic tail drives a crucial inhibitory signal. This negative signal is mediated by a few key signal transducers, such as tyrosine phosphatase, inositol phosphatase, and diacylglycerol kinase, which allows them to counteract TCR-mediated activation. The characterization of these signaling pathways is of great interest in the development of therapies for counteracting tumor-infiltrating lymphocyte exhaustion/anergy independently from the receptors involved.

PD-1 agonism by anti-CD80 inhibits T cell activation and alleviates autoimmunity

Targeted blockade of the checkpoint molecule programmed cell death 1 (PD-1) can activate tumor-specific T cells to destroy tumors, whereas targeted potentiation of PD-1 is expected to suppress autoreactive T cells and alleviate autoimmune diseases. However, the development of methods to potentiate PD-1 remains challenging. Here we succeeded in eliciting PD-1 function by targeting the cis-PD-L1-CD80 duplex, formed by binding of CD80 to the PD-1 ligand PD-L1, that attenuates PD-L1-PD-1 binding and abrogates PD-1 function. By generating anti-CD80 antibodies that detach CD80 from the cis-PD-L1-CD80 duplex and enable PD-L1 to engage PD-1 in the presence of CD80, we demonstrate that the targeted dissociation of cis-PD-L1-CD80 duplex elicits PD-1 function in the condition where PD-1 function is otherwise restricted. We demonstrate using murine models that the removal of PD-1 restriction is effective in alleviating autoimmune disease symptoms. Our findings establish a method to potentiate PD-1 function and propose the removal of restraining mechanisms as an efficient strategy to potentiate the function of inhibitory molecules.

T-cell-intrinsic and -extrinsic regulation of PD-1 function

Cancer immunotherapies that target PD-1 (programmed cell death 1) aim to destroy tumors by activating tumor-specific T cells that are otherwise inactivated by PD-1. Although these therapies have significantly improved the outcomes of patients with diverse cancer types and have revolutionized cancer treatment, only a limited proportion of patients benefits from the therapies currently. Therefore, there is a continued need to decipher the complex biology of PD-1 to improve therapeutic efficacies as well as to prevent immune-related adverse events. Especially, the spaciotemporal context in which PD-1 functions and the properties of T cells that are restrained by PD-1 are only vaguely understood. We have recently revealed that PD-1 function is strictly restricted at the activation phase of T-cell responses by the cis-interactions of PD-L1 and CD80 on antigen-presenting cells, which is critical for the induction of optimal T-cell responses. We also found that the sensitivity to the effects of PD-1 in T cells is essentially determined by T-cell-intrinsic factors. In T cells bearing T-cell antigen-receptors (TCRs) with lower affinity to antigenic peptides, PD-1 inhibits the expression of TCR-inducible genes more efficiently; thereby PD-1 preferentially suppresses low-affinity T cells. Thus, PD-1 function is coordinately regulated by various T-cell-intrinsic and -extrinsic factors that alter the responsiveness of T cells and the availability of PD-1 ligands. Precise and deeper understanding of the regulatory mechanisms of PD-1 is expected to facilitate the rational development of effective and safe immunotherapies.

Antigen and checkpoint receptor engagement recalibrates T cell receptor signal strength

How T cell receptor (TCR) signal strength modulates T cell function and to what extent this is modified by immune checkpoint blockade (ICB) are key questions in immunology. Using Nr4a3-Tocky mice, we characterized early quantitative and qualitative changes that occur in CD4+ T cells in relation to TCR signaling strength. We captured how dose- and time-dependent programming of distinct co-inhibitory receptors rapidly recalibrates T cell activation thresholds and visualized the immediate effects of ICB on T cell re-activation. Our findings reveal that anti-PD1 immunotherapy leads to an increased TCR signal strength. We defined a strong TCR signal metric of five genes upregulated by anti-PD1 in T cells (TCR.strong), which was superior to a canonical T cell activation gene signature in stratifying melanoma patient outcomes to anti-PD1 therapy. Our study therefore reveals how analysis of TCR signal strength-and its manipulation-can provide powerful metrics for monitoring outcomes to immunotherapy.

PD-1 preferentially inhibits the activation of low-affinity T cells

Anti-PD-1 therapies can activate tumor-specific T cells to destroy tumors. However, whether and how T cells with different antigen specificity and affinity are differentially regulated by PD-1 remain vaguely understood. Upon antigen stimulation, a variety of genes is induced in T cells. Recently, we found that T cell receptor (TCR) signal strength required for the induction of genes varies across different genes and PD-1 preferentially inhibits the induction of genes that require stronger TCR signal. As each T cell has its own response characteristics, inducibility of genes likely differs across different T cells. Accordingly, the inhibitory effects of PD-1 are also expected to differ across different T cells. In the current study, we investigated whether and how factors that modulate T cell responsiveness to antigenic stimuli influence PD-1 function. By analyzing TCRs with different affinities to peptide-MHC complexes (pMHC) and pMHCs with different affinities to TCR, we demonstrated that PD-1 inhibits the expression of TCR-inducible genes efficiently when TCR:pMHC affinity is low. In contrast, affinities of peptides to MHC and MHC expression levels did not affect PD-1 sensitivity of TCR-inducible genes although they markedly altered the dose responsiveness of T cells by changing the efficiency of pMHC formation, suggesting that the strength of individual TCR signal is the key determinant of PD-1 sensitivity. Accordingly, we observed a preferential expansion of T cells with low-affinity to tumor-antigen in PD-1-deficient mice upon inoculation of tumor cells. These results demonstrate that PD-1 imposes qualitative control of T cell responses by preferentially suppressing low-affinity T cells.

PD-1/PD-L1 Axis as a Potential Therapeutic Target for Multiple Sclerosis: A T Cell Perspective

The programmed cell death protein-1/programmed death ligand-1 (PD-1/PD-L1) axis is a widely studied immune checkpoint that modulates signaling pathways related to T cell activation. The use of PD-1/PD-L1 inhibitors is a promising immune therapy strategy for cancer patients. However, individuals treated with PD-1/PD-L1 inhibitors may develop immune-related adverse events due to excessive immune reactions. Multiple sclerosis (MS) is a chronic demyelinating and neurodegenerative disease of the central nervous system. T cells and the PD-1/PD-L1 axis play vital roles in the pathogenesis of MS. A better understanding of the complex relationship between the PD-1/PD-L1 axis and T cells may extend our knowledge of the molecular mechanisms and therapeutic approaches for MS. In this review, we summarize the most recent findings regarding the role of the PD-1/PD-L1 axis in MS and discuss the potential therapeutic strategies to modulate the expression of PD-1/PD-L1 in MS.

Cyclic AMP Response Element Modulator-α Suppresses PD-1 Expression and Promotes Effector CD4+ T Cells in Psoriasis

Effector CD4⁺ T lymphocytes contribute to inflammation and tissue damage in psoriasis, but the underlying molecular mechanisms remain poorly understood. The transcription factor CREMα controls effector T cell function in people with systemic autoimmune diseases. The inhibitory surface coreceptor PD-1 plays a key role in the control of effector T cell function and its therapeutic inhibition in patients with cancer can cause psoriasis. In this study, we show that CD4⁺ T cells from patients with psoriasis and psoriatic arthritis exhibit increased production of IL-17 but decreased expression of IL-2 and PD-1. In genetically modified mice and Jurkat T cells CREMα expression was linked to low PD-1 levels. We demonstrate that CREMα is recruited to the proximal promoter of PDCD1 in which it trans-represses gene expression and corecruits DNMT3a-mediating DNA methylation. As keratinocytes limit inflammation by PD-1 ligand expression and, in this study, reported reduced expression of PD-1 on CD4⁺ T cells is linked to low IL-2 and high IL-17A production, our studies reveal a molecular pathway in T cells from people with psoriasis that can deserve clinical exploitation.

Chromatin Priming Renders T Cell Tolerance-Associated Genes Sensitive to Activation below the Signaling Threshold for Immune Response Genes

Immunological homeostasis in T cells is maintained by a tightly regulated signaling and transcriptional network. Full engagement of effector T cells occurs only when signaling exceeds a critical threshold that enables induction of immune response genes carrying an epigenetic memory of prior activation. Here we investigate the underlying mechanisms causing the suppression of normal immune responses when T cells are rendered anergic by tolerance induction. By performing an integrated analysis of signaling, epigenetic modifications, and gene expression, we demonstrate that immunological tolerance is established when both signaling to and chromatin priming of immune response genes are weakened. In parallel, chromatin priming of immune-repressive genes becomes boosted, rendering them sensitive to low levels of signaling below the threshold needed to activate immune response genes. Our study reveals how repeated exposure to antigens causes an altered epigenetic state leading to T cell anergy and tolerance, representing a basis for treating auto-immune diseases.

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Activation of immune response genes is suppressed in tolerant T cells

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Epigenetic priming of repressive genes is boosted when tolerance is established

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Inhibitory receptor genes have a lower threshold of activation in tolerant cells

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Induction of tolerance by peptides points toward a therapy for multiple sclerosis

Nr4a1 and Nr4a3 Reporter Mice Are Differentially Sensitive to T Cell Receptor Signal Strength and Duration

Nr4a receptors are activated by T cell receptor (TCR) signaling and play key roles in T cell differentiation. Which TCR signaling pathways regulate Nr4a receptors and their sensitivities to TCR signal strength and duration remains unclear. Using Nr4a1/Nur77-GFP and Nr4a3-Timer of cell kinetics and activity (Tocky) mice, we elucidate the signaling pathways governing Nr4a receptor expression. We reveal that Nr4a1–Nr4a3 are Src family kinase dependent. Moreover, Nr4a2 and Nr4a3 are attenuated by calcineurin inhibitors and bind nuclear factor of activated T cells 1 (NFAT1), highlighting a necessary and sufficient role for NFAT1 in the control of Nr4a2 and Nr4a3, but redundancy for Nr4a1. Nr4a1-GFP is activated by tonic and cognate signals during T cell development, whereas Nr4a3-Tocky requires cognate peptide:major histocompatibility complex (MHC) interactions for expression. Compared to Nr4a3-Tocky, Nr4a1-GFP is approximately 2- to 3-fold more sensitive to TCR signaling and is detectable by shorter periods of TCR signaling. These findings suggest that TCR signal duration may be an underappreciated aspect influencing the developmental fate of T cells in vivo.

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Nr4a1 and Nr4a3 show differential dependency on the calcineurin/NFAT pathway

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Nr4a1-GFP is expressed in developing Tcon and Treg within the thymus

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Nr4a3-Timer expression is largely restricted to thymic and peripheral CD25⁺ Treg

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Nr4a3-Timer requires a stronger and/or longer TCR signal for its expression

Signaling Dynamics Regulating Crosstalks between T-Cell Activation and Immune Checkpoints

Immune checkpoint inhibitors (ICIs) targeting cytotoxic T lymphocyte-associated protein-4 (CTLA-4) and programmed cell death protein-1 (PD-1) have been hailed as major advances in cancer therapeutics; however, in many cancers response rates remain low. Extensive research efforts are underway to improve the efficacy of ICIs. The signaling pathways regulated by immune checkpoints (ICs) may be an important lever as they interfere with T-cell activation when activated by ICIs. Here, we review the current understanding of T-cell receptor signaling and their intersection with IC signaling pathways. As these signaling processes are highly dynamic and controlled by intricate spatiotemporal mechanisms, we focus on aspects of kinetic regulation that are modulated by ICs. Recent advances in computational modeling and experimental methods that can resolve spatiotemporal dynamics provide insights that reveal molecular mechanisms and new potential approaches for improving the design and application of ICIs.

Nr4a1 and Nr4a3 Reporter Mice Are Differentially Sensitive to T Cell Receptor Signal Strength and Duration

Nr4a receptors are activated by T cell receptor (TCR) signaling and play key roles in T cell differentiation. Which TCR signaling pathways regulate Nr4a receptors and their sensitivities to TCR signal strength and duration remains unclear. Using Nr4a1/Nur77-GFP and Nr4a3-Timer of cell kinetics and activity (Tocky) mice, we elucidate the signaling pathways governing Nr4a receptor expression. We reveal that Nr4a1-Nr4a3 are Src family kinase dependent. Moreover, Nr4a2 and Nr4a3 are attenuated by calcineurin inhibitors and bind nuclear factor of activated T cells 1 (NFAT1), highlighting a necessary and sufficient role for NFAT1 in the control of Nr4a2 and Nr4a3, but redundancy for Nr4a1. Nr4a1-GFP is activated by tonic and cognate signals during T cell development, whereas Nr4a3-Tocky requires cognate peptide:major histocompatibility complex (MHC) interactions for expression. Compared to Nr4a3-Tocky, Nr4a1-GFP is approximately 2- to 3-fold more sensitive to TCR signaling and is detectable by shorter periods of TCR signaling. These findings suggest that TCR signal duration may be an underappreciated aspect influencing the developmental fate of T cells in vivo.

Immunotherapy in Hematologic Malignancies: Emerging Therapies and Novel Approaches

Immunotherapy is extensively investigated for almost all types of hematologic tumors, from preleukemic to relapse/refractory malignancies. Due to the emergence of technologies for target cell characterization, antibody design and manufacturing, as well as genome editing, immunotherapies including gene and cell therapies are becoming increasingly elaborate and diversified. Understanding the tumor immune microenvironment of the target disease is critical, as is reducing toxicity. Although there have been many successes and newly FDA-approved immunotherapies for hematologic malignancies, we have learned that insufficient efficacy due to disease relapse following treatment is one of the key obstacles for developing successful therapeutic regimens. Thus, combination therapies are also being explored. In this review, immunotherapies for each type of hematologic malignancy will be introduced, and novel targets that are under investigation will be described.

Targeting immune checkpoints in hematological malignancies

Immune checkpoint blockade (ICB) therapies such as anti-programmed death 1 (PD-1) and anti-CTLA-4 (cytotoxic T lymphocyte-associated protein 4) have dramatically transformed treatment in solid tumor oncology. While immunotherapeutic approaches such as stem cell transplantation and anti-cancer monoclonal antibodies have made critical contributions to improve outcomes in hematological malignancies, clinical benefits of ICB are observed in only limited tumor types that are particularly characterized by a high infiltration of immune cells. Importantly, even patients that initially respond to ICB are unable to achieve long-term disease control using these therapies. Indeed, primary and acquired resistance mechanisms are differentially orchestrated in hematological malignancies depending on tumor types and/or genotypes, and thus, an in-depth understanding of the disease-specific immune microenvironments will be essential in improving efficacy. In addition to PD-1 and CTLA-4, various T cell immune checkpoint molecules have been characterized that regulate T cell responses in a non-redundant manner. Several lines of evidence suggest that these T cell checkpoint molecules might play unique roles in hematological malignancies, highlighting their potential as therapeutic targets. Targeting innate checkpoint molecules on natural killer cells and/or macrophages has also emerged as a rational approach against tumors that are resistant to T cell-mediated immunity. Given that various monoclonal antibodies against tumor surface proteins have been clinically approved in hematological malignancies, innate checkpoint blockade might play a key role to augment antibody-mediated cellular cytotoxicity and phagocytosis. In this review, we discuss recent advances and emerging roles of immune checkpoint blockade in hematological malignancies.

Metabolic optimization of adoptive T cell transfer cancer immunotherapy: A historical overview

After prolonged extracorporeal multiplication in physiological culture media, there can be curative infusions of a cancer patient's own cytotoxic T cells (adoptive T cell transfer; ACT), which must achieve efficient activation in potentially adverse tumour microenvironments. With spectacular, yet irregular, success, improvements are needed. Developing lymphoid cells are biologically selected, not only for 'near-self' reactivity (positive selection), but also to avoid self-reactivity (negative selection). Thus, success requires harnessing near-self cells while avoiding extreme autoimmune phenomena. Abrupt metabolic changes accompanying T cell activation to leave the G₀ stage and enter the G₁ stage of the cell cycle (eg enhanced glycolysis) are accompanied by increased transcription of the G0S9 gene that mediates salvage synthesis of NAD⁺ from nicotinamide; the latter has recently been shown to increase the efficiency of ACT. Despite theoretical and experimental advances, there has not been parallel progress in simulating in vivo conditions with culture media that were initially formulated for their positive benefits for tumour cell lines (cell survival and proliferation). Yet for lymphoid cells, inhibition or death (ie immunological tolerance) is as important as their activation and proliferation (immunological response). Thus, use of media optimized for the latter may mask the former. The resilience of established culture protocols may have been partly politically driven. However, unphysiological conditions have sometimes yielded fortuitous insights. Optimization of culture media for specific tissues must consider the nature of problems addressed in research settings and the need to avoid mishaps in clinical settings.

Immune checkpoint signaling and cancer immunotherapy

Immune checkpoint blockade therapy has become a major weapon in fighting cancer. Antibody drugs, such as anti-PD-1 and anti-PD-L1, demonstrate obvious advantages such as broad applicability across cancer types and durable clinical response when treatment is effective. However, the overall response rates are still unsatisfying, especially for cancers with low mutational burden. Moreover, adverse effects, such as autoimmune symptoms and tumor hyperprogression, present a significant downside in some clinical applications. These challenges reflect the urgent need to fully understand the basic biology of immune checkpoints. In this review, we discuss regulation of immune checkpoint signaling at multiple levels to provide an overview of our current understanding of checkpoint biology. Topics include the regulation of surface expression levels for known immune checkpoint proteins via surface delivery, internalization, recycling, and degradation. Upon reaching the surface, checkpoints engage in both conventional trans and also cis interactions with ligands to induce signaling and regulate immune responses. Novel therapeutic strategies targeting these pathways in addition to classical checkpoint blockade have recently emerged and been tested in preclinical models, providing new avenues for developing next-generation immunotherapies.

Proteasomal and lysosomal degradation for specific and durable suppression of immunotherapeutic targets

Cancer immunotherapy harness the body’s immune system to eliminate cancer, by using a broad panel of soluble and membrane proteins as therapeutic targets. Immunosuppression signaling mediated by ligand-receptor interaction may be blocked by monoclonal antibodies, but because of repopulation of the membrane via intracellular organelles, targets must be eliminated in whole cells. Targeted protein degradation, as exemplified in proteolysis targeting chimera (PROTAC) studies, is a promising strategy for selective inhibition of target proteins. The recently reported use of lysosomal targeting molecules to eliminate immune checkpoint proteins has paved the way for targeted degradation of membrane proteins as crucial anti-cancer targets. Further studies on these molecules’ modes of action, target-binding “warheads”, lysosomal sorting signals, and linker design should facilitate their rational design. Modifications and derivatives may improve their cell-penetrating ability and the in vivo stability of these pro-drugs. These studies suggest the promise of alternative strategies for cancer immunotherapy, with the aim of achieving more potent and durable suppression of tumor growth. Here, the successes and limitations of antibody inhibitors in cancer immunotherapy, as well as research progress on PROTAC- and lysosomal-dependent degradation of target proteins, are reviewed.