Co-stimulatory and co-inhibitory receptors have a pivotal role in T cell biology, as they determine the functional outcome of T cell receptor (TCR) signalling. The classic definition of T cell co-stimulation continues to evolve through the identification of new co-stimulatory and co-inhibitory receptors, the biochemical characterization of their downstream signalling events and the delineation of their immunological functions. Notably, it has been recently appreciated that co-stimulatory and co-inhibitory receptors display great diversity in expression, structure and function, and that their functions are largely context dependent. Here, we focus on some of these emerging concepts and review the mechanisms through which T cell activation, differentiation and function is controlled by co-stimulatory and co-inhibitory receptors.

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Molecular mechanisms of T cell co-stimulation and co-inhibition

Infiltration of macrophages in solid tumours is associated with poor prognosis and correlates with chemotherapy resistance in most cancers. In mouse models of cancer, macrophages promote cancer initiation and malignant progression by stimulating angiogenesis, increasing tumour cell migration, invasion and intravasation and suppressing antitumour immunity. At metastatic sites, macrophages promote tumour cell extravasation, survival and subsequent growth. Each of these pro-tumoural activities is promoted by a subpopulation of macrophages that express canonical markers but have unique transcriptional profiles, which makes tumour-associated macrophages (TAMs) good targets for anticancer therapy in humans through either their ablation or their re-differentiation away from pro-tumoural towards antitumoural states. In this Review, we evaluate the state of the art of TAM-targeting strategies, focusing on the limitations and potential side effects of the different therapies such as toxicity, rebound effects and compensatory mechanisms. We provide an extensive overview of the different types of therapy used in the clinic and their limitations in light of known macrophage biology and propose new strategies for targeting TAMs.

Targeting macrophages: therapeutic approaches in cancer.

CD8+ T Cells: Foot Soldiers of the Immune System

In T cells, the kinase mTOR (mammalian target of rapamycin) integrates immune signals and metabolic cues to control T cell maintenance and activation. This Review describes the role of mTOR in determining T cell fate decisions and the implications of targeting mTOR in the treatment of disease.

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Regulation and function of mTOR signalling in T cell fate decisions

Following immune attack, solid tumors upregulate coinhibitory ligands that bind to inhibitory receptors on T cells. This adaptive resistance compromises the efficacy of chimeric antigen receptor (CAR) T cell therapies, which redirect T cells to solid tumors. Here, we investigated whether programmed death-1-mediated (PD-1-mediated) T cell exhaustion affects mesothelin-targeted CAR T cells and explored cell-intrinsic strategies to overcome inhibition of CAR T cells. Using an orthotopic mouse model of pleural mesothelioma, we determined that relatively high doses of both CD28- and 4-1BB-based second-generation CAR T cells achieved tumor eradication. CAR-mediated CD28 and 4-1BB costimulation resulted in similar levels of T cell persistence in animals treated with low T cell doses; however, PD-1 upregulation within the tumor microenvironment inhibited T cell function. At lower doses, 4-1BB CAR T cells retained their cytotoxic and cytokine secretion functions longer than CD28 CAR T cells. The prolonged function of 4-1BB CAR T cells correlated with improved survival. PD-1/PD-1 ligand [PD-L1] pathway interference, through PD-1 antibody checkpoint blockade, cell-intrinsic PD-1 shRNA blockade, or a PD-1 dominant negative receptor, restored the effector function of CD28 CAR T cells. These findings provide mechanistic insights into human CAR T cell exhaustion in solid tumors and suggest that PD-1/PD-L1 blockade may be an effective strategy for improving the potency of CAR T cell therapies.

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Human CAR T cells with cell-intrinsic PD-1 checkpoint blockade resist tumor-mediated inhibition

Various TLR agonists are currently under investigation in clinical trials for their ability to orchestrate antitumor immunity. The antitumor responses are largely attributed to their aptitude to stimulate APCs such as DCs which in turn, activate tumor-specific T cell responses. However, there is a potential for TLR signaling to occur on cells other than professional APCs that could negate antitumor responses or even worse, promote tumor growth. The impetus for this review is twofold. First, there is accumulating data demonstrating that the engagement of TLRs on different T cell subsets and different cancer types could promote tumor growth or conversely, contribute to antitumor responses. Second, the efficacy of TLR agonists as monotherapies to treat cancer patients has been limited. In this review, we discuss how TLR signaling within different T cell subsets and cancer cells can potentially impact the generation of antitumor responses. Based on evidence from preclinical models and clinical trials, we draw attention to several criteria that we believe must be considered when selecting TLR agonists for developing effective immunotherapeutic strategies against cancer.

https://jlb.onlinelibrary.wiley.com/doi/pdf/10.1189/jlb.1012501

TLR agonists: our best frenemy in cancer immunotherapy

Purpose: To develop an adaptable gene-based vector that will confer immune cell specificity to various cancer types. Experimental Design: Human and mouse T cells were genetically engineered to express a chimeric antigen receptor (CAR) that binds a fluorescein isothiocyanate (FITC) molecule, termed anti-FITC CAR T cells. Various antibodies (Ab) currently in clinical use including cetuximab (Ctx), trastuzumab (Her2), and rituximab (Rtx) were conjugated with FITC and tested for their ability to bind tumor cells, activate T cells, and induce antitumor effects in vitro and in vivo . Results: Anti-FITC CAR T cells recognize various cancer types when bound with FITC-labeled Abs resulting in efficient target lysis, T-cell proliferation, and cytokine/chemokine production. The treatment of immunocompromised mice with human anti-FITC CAR T cells plus FITC-labeled cetuximab (FITC-Ctx) delayed the growth of colon cancer but unexpectedly led to the outgrowth of EGF receptor (EGFR)-negative tumor cells. On the other hand, in a human pancreatic cancer cell line with uniform EGFR expression, anti-FITC CAR T cells plus FITC-Ctx eradicated preestablished late-stage tumors. In immunocompetent mice, anti-FITC CAR T cells exhibited potent antitumor activity against syngeneic mouse breast cancer expressing Her2 and B-cell lymphoma expressing CD20 by combining with FITC-Her2 and FITC-Rtx, respectively. In addition, the activity of anti-FITC CAR T cells could be attenuated by subsequent injections of nonspecific FITC-IgG. Conclusion: These studies highlight an applicability of anti-tag CAR technology to treat patients with different types of cancers and a possibility to regulate CAR T-cell functions with competing FITC molecules. Clin Cancer Res; 18(23); 6436–45. ©2012 AACR .

https://clincancerres.aacrjournals.org/content/clincanres/18/23/6436.full.pdf

Redirecting gene-modified T cells toward various cancer types using tagged antibodies.

Chronic inflammation has long been associated with the development of cancer. Amongst the various signaling pathways within cancer cells that can incite the expression of inflammatory molecules are those that activate IL-1 receptor associated kinases (IRAK). The IRAK family comprises of four family members, IRAK-1, IRAK-2, IRAK-3 (also known as IRAK-M), and IRAK-4, which play important roles in both positively and negatively regulating the expression of inflammatory molecules. The wide array of inflammatory molecules that are expressed in response to IRAK signaling within the tumor microenvironment regulate the production of factors that promote tumor growth, metastasis, immune suppression and chemotherapy resistance. Based on published reports and compelling preliminary data, we propose that the dysregulated activation of the IRAK signaling pathway in cancer cells contributes to disease progression by creating a highly inflammatory tumor environment. In this article, we present both theoretical arguments and experimental data in support of this hypothesis.

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IL-1 Receptor-Associated Kinase Signaling and Its Role in Inflammation, Cancer Progression, and Therapy Resistance

The present invention provides a universal, yet adaptable, anti-tag chimeric antigen receptor (AT-CAR) system which provides T cells with the ability and specificity to recognize and kill target cells, such as tumor cells, that have been marked by tagged antibodies. As an example, αFITC-CAR-expressing T cells have been developed that specifically recognize various human cancer cells when those cells are bound by cancer-reactive FITC-labeled antibodies. The activation of αFITC-CAR-expressing T cells is shown to induce efficient target lysis, T cell proliferation, and cytokine/chemokine production. The system can be used to treating subjects having cancer.

Universal anti-tag chimeric antigen receptor-expressing t cells and methods of treating cancer

Cancer metastasis and immune suppression are critical issues in cancer therapy. Here, we show that a β-galactoside–binding lectin [galectin-3 (gal3)] that recognizes the Thomsen-Friedenreich disaccharide (TFD, Galβ1,3GalNAc) present on the surface of most cancer cells is involved in promoting angiogenesis, tumor-endothelial cell adhesion, and metastasis of prostate cancer cells, as well as evading immune surveillance through killing of activated T cells. To block gal3-mediated interactions, we purified a glycopeptide from cod (designated TFD100) that binds gal3 with picomolar affinity. TFD100 blocks gal3-mediated angiogenesis, tumor-endothelial cell interactions, and metastasis of prostate cancer cells in mice at nanomolar levels. Moreover, apoptosis of activated T cells induced by either recombinant gal3 or prostate cancer patient serum-associated gal3 was inhibited at nanomolar concentration of TFD100. Because the gal3–TFD interaction is a key factor driving metastasis in most epithelial cancers, this high-affinity TFD100 should be a promising antimetastatic agent for the treatment of various cancers, including prostate adenocarcinoma.

https://www.pnas.org/content/pnas/110/13/5052.full.pdf

Eduardo Davila

Papers

TLR agonists: our best frenemy in cancer immunotherapy

Redirecting gene-modified T cells toward various cancer types using tagged antibodies.

IL-1 Receptor-Associated Kinase Signaling and Its Role in Inflammation, Cancer Progression, and Therapy Resistance

Universal anti-tag chimeric antigen receptor-expressing t cells and methods of treating cancer

Cod glycopeptide with picomolar affinity to galectin-3 suppresses T-cell apoptosis and prostate cancer metastasis