Clec9A+ Dendritic Cells Are Not Essential for Antitumor CD8+ T Cell Responses Induced by Poly I:C Immunotherapy

A review concerning the breast cancer-related tumour microenvironment

Breast cancer (BC) is currently the most common malignant tumour in women and one of the leading causes of their death around the world. New and increasingly personalised diagnostic and therapeutic tools have been introduced over the last few decades, along with significant advances regarding the study and knowledge related to BC. The tumour microenvironment (TME) refers to the tumour cell-associated cellular and molecular environment which can influence conditions affecting tumour development and progression. The TME is composed of immune cells, stromal cells, extracellular matrix (ECM) and signalling molecules secreted by these different cell types. Ever deeper understanding of TME composition changes during tumour development and progression will enable new and more innovative therapeutic strategies to become developed for targeting tumours during specific stages of its evolution. This review summarises the role of BC-related TME components and their influence on tumour progression and the development of resistance to therapy. In addition, an account on the modifications in BC-related TME components associated with therapy is given, and the completed or ongoing clinical trials related to this topic are presented.

MHC class I-dressing is mediated via phosphatidylserine recognition and is enhanced by polyI:C

In addition to cross-presentation, cross-dressing plays an important role in the induction of CD8+ T cell immunity. In the process of cross-dressing, conventional dendritic cells (DCs) acquire major histocompatibility complex class I (MHCI) from other cells and subsequently prime CD8+ T cells via the pre-formed antigen-MHCI complexes without antigen processing. However, the mechanisms underlying the cross-dressing pathway, as well as the relative contributions of cross-presentation and cross-dressing to CD8+ T cell priming are not fully understood. Here, we demonstrate that DCs rapidly acquire MHCI-containing membrane fragments from dead cells via the phosphatidylserine recognition-dependent mechanism for cross-dressing. The MHCI dressing is enhanced by a TLR3 ligand polyinosinic-polycytidylic acid (polyI:C). Further, polyI:C promotes not only cross-presentation but also cross-dressing in vivo. Taken together, these results suggest that cross-dressing as well as cross-presentation is involved in inflammatory diseases associated with cell death and type I IFN production.

Dendritic Cell-Based Vaccines Against Cancer: Challenges, Advances and Future Opportunities

As the most potent professional antigen presenting cells, dendritic cells (DCs) have the ability to activate both naive CD4 and CD8 T cells. Recognized for their exceptional ability to cross-present exogenous antigens to prime naive antigen-specific CD8 T cells, DCs play a critical role in generating CD8 T cell immunity, as well as mediating CD8 T cell tolerance to tumor antigens. Despite the ability to potentiate host CD8 T cell-mediated anti-tumor immunity, current DC-based cancer vaccines have not yet achieved the promised success clinically with the exception of FDA-approved Provenge. Interestingly, recent studies have shown that type 1 conventional DCs (cDC1s) play a critical role in cross-priming tumor-specific CD8 T cells and determining the anti-tumor efficacy of cancer immunotherapies including immune checkpoint blockade (ICB). Together with promising clinical results in neoantigen-based cancer vaccines, there is a great need for DC-based vaccines to be further developed and refined either as monotherapies or in combination with other immunotherapies. In this review, we will present a brief review of DC development and function, discuss recent progress, and provide a perspective on future directions to realize the promising potential of DC-based cancer vaccines.

Contribution of Dendritic Cell Subsets to T Cell-Dependent Responses in Mice

BATF3-deficient mice that lack CD8⁺ dendritic cells (DCs) showed an exacerbation of chronic graft-versus-host disease (cGVHD), including T follicular helper (Tfh) cell and autoantibody responses, whereas mice carrying the Sle2c2 lupus-suppressive locus with a mutation in the G-CSFR showed an expansion of CD8⁺ DCs and a poor mobilization of plasmacytoid DCs (pDCs) and responded poorly to cGVHD induction. Here, we investigated the contribution of CD8⁺ DCs and pDCs to the humoral response to protein immunization, where CD8^neg DCs are thought to represent the major inducers. Both BATF3^-/- and Sle2c2 mice had reduced humoral and germinal center (GC) responses compared with C57BL/6 (B6) controls. We showed that B6-derived CD4⁺ DCs are the major early producers of IL-6, followed by CD4^-CD8^- DCs. Surprisingly, IL-6 production and CD80 expression also increased in CD8⁺ DCs after immunization, and B6-derived CD8⁺ DCs rescued Ag-specific adaptive responses in BATF3^-/- mice. In addition, inflammatory pDCs (ipDCs) produced more IL-6 than all conventional DCs combined. Interestingly, G-CSFR is highly expressed on pDCs. G-CSF expanded pDC and CD8⁺ DC numbers and IL-6 production by ipDCs and CD4⁺ DCs, and it improved the quality of Ab response, increasing the localization of Ag-specific T cells to the GC. Finally, G-CSF activated STAT3 in early G-CSFR⁺ common lymphoid progenitors of cDCs/pDCs but not in mature cells. In conclusion, we showed a multilayered role of DC subsets in priming Tfh cells in protein immunization, and we unveiled the importance of G-CSFR signaling in the development and function pDCs.

IFNβ Is a Potent Adjuvant for Cancer Vaccination Strategies

Cancer vaccination drives the generation of anti-tumor T cell immunity and can be enhanced by the inclusion of effective immune adjuvants such as type I interferons (IFNs). Whilst type I IFNs have been shown to promote cross-priming of T cells, the role of individual subtypes remains unclear. Here we systematically compared the capacity of distinct type I IFN subtypes to enhance T cell responses to a whole-cell vaccination strategy in a pre-clinical murine model. We show that vaccination in combination with IFNβ induces significantly greater expansion of tumor-specific CD8+ T cells than the other type I IFN subtypes tested. Optimal expansion was dependent on the presence of XCR1+ dendritic cells, CD4+ T cells, and CD40/CD40L signaling. Therapeutically, vaccination with IFNβ delayed tumor progression when compared to vaccination without IFN. When vaccinated in combination with anti-PD-L1 checkpoint blockade therapy (CPB), the inclusion of IFNβ associated with more mice experiencing complete regression and a trend in increased overall survival. This work demonstrates the potent adjuvant activity of IFNβ, highlighting its potential to enhance cancer vaccination strategies alone and in combination with CPB.

Secreted gelsolin inhibits DNGR-1-dependent cross-presentation and cancer immunity

Cross-presentation of antigens from dead tumor cells by type 1 conventional dendritic cells (cDC1s) is thought to underlie priming of anti-cancer CD8+ T cells. cDC1 express high levels of DNGR-1 (a.k.a. CLEC9A), a receptor that binds to F-actin exposed by dead cell debris and promotes cross-presentation of associated antigens. Here, we show that secreted gelsolin (sGSN), an extracellular protein, decreases DNGR-1 binding to F-actin and cross-presentation of dead cell-associated antigens by cDC1s. Mice deficient in sGsn display increased DNGR-1-dependent resistance to transplantable tumors, especially ones expressing neoantigens associated with the actin cytoskeleton, and exhibit greater responsiveness to cancer immunotherapy. In human cancers, lower levels of intratumoral sGSN transcripts, as well as presence of mutations in proteins associated with the actin cytoskeleton, are associated with signatures of anti-cancer immunity and increased patient survival. Our results reveal a natural barrier to cross-presentation of cancer antigens that dampens anti-tumor CD8+ T cell responses.

The Flt3L/Flt3 Axis in Dendritic Cell Biology and Cancer Immunotherapy

Dendritic cells (DCs) prime anti-tumor T cell responses in tumor-draining lymph nodes and can restimulate T effector responses in the tumor site. Thus, in addition to unleashing T cell effector activity, current immunotherapies should be directed to boost DC function. Herein, we review the potential function of Flt3L as a tool for cancer immunotherapy. Flt3L is a growth factor that acts in Flt3-expressing multipotent progenitors and common lymphoid progenitors. Despite the broad expression of Flt3 in the hematopoietic progenitors, the main effect of the Flt3/Flt3L axis, revealed by the characterization of mice deficient in these genes, is the generation of conventional DCs (cDCs) and plasmacytoid DCs (pDCs). However, Flt3 signaling through PI3K and mTOR may also affect the function of mature DCs. We recapitulate the use of Flt3L in preclinical studies either as a single agent or in combination with other cancer therapies. We also analyze the use of Flt3L in clinical trials. The strong correlation between type 1 cDC (cDC1) infiltration of human cancers with overall survival in many cancer types suggests the potential use of Flt3L to boost expansion of this DC subset. However, this may need the combination of Flt3L with other immunomodulatory agents to boost cancer immunotherapy.

The role of dendritic cells in cancer and anti-tumor immunity

Dendritic cells (DC) are key sentinels of the host immune response with an important role in linking innate and adaptive immunity and maintaining tolerance. There is increasing recognition that DC are critical determinants of initiating and sustaining effective T-cell-mediated anti-tumor immune responses. Recent progress in immuno-oncology has led to the evolving insight that the presence and function of DC within the tumor microenvironment (TME) may dictate efficacy of cancer immunotherapies as well as conventional cancer therapies, including immune checkpoint blockade, radiotherapy and chemotherapy. As such, improved understanding of dendritic cell immunobiology specifically focusing on their role in T-cell priming, migration into tissues and TME, and the coordinated in vivo responses of functionally specialized DC subsets will facilitate a better mechanistic understanding of how tumor-immune surveillance can be leveraged to improve patient outcomes and to develop novel DC-targeted therapeutic approaches.

Analysis and Augmentation of the Immunologic Bystander Effects of CAR T Cell Therapy in a Syngeneic Mouse Cancer Model

The therapeutic efficacy of adoptive transfer of T cells transduced with chimeric antigen receptors (CARs) has been limited in the treatment of solid cancers, partly due to tumor antigen heterogeneity. Overcoming lack of universal tumor antigen expression would be achieved if CAR T cells could induce bystander effects. To study this process, we developed a system where CAR T cells targeting mesothelin could cure tumors containing 100% antigen-positive cells in immunocompetent mice. Using this model, we found that the CAR T cells were unable to cure tumors, even when only 10% of the tumor cells were mesothelin negative. A bystander effect was not induced by co-administration of anti-PD-1, anti-CTLA-4, or anti-TGF-β (transforming growth factor β) antibodies; agonistic CD40 antibodies; or an IDO (indoleamine 2,3-dioxygenase) inhibitor. However, pretreatment with a non-lymphodepleting dose of cyclophosphamide (CTX) prior to CAR T cells resulted in cures of tumors with up to 25% mesothelin-negative cells. The mechanism was dependent on endogenous CD8 T cells but not on basic leucine zipper transcription factor ATF-like 3 (BATF3)-dependent dendritic cells. These data suggest that CAR T cell therapy of solid tumors, in which the targeted antigen is not expressed by the vast majority of tumor cells, will not likely be successful unless combination strategies to enhance bystander effects are used.

Dendritic Cells and Their Role in Immunotherapy

Despite significant advances in the field of cancer immunotherapy, the majority of patients still do not benefit from treatment and must rely on traditional therapies. Dendritic cells have long been a focus of cancer immunotherapy due to their role in inducing protective adaptive immunity, but cancer vaccines have shown limited efficacy in the past. With the advent of immune checkpoint blockade and the ability to identify patient-specific neoantigens, new vaccines, and combinatorial therapies are being evaluated in the clinic. Dendritic cells are also emerging as critical regulators of the immune response within tumors. Understanding how to augment the function of these intratumoral dendritic cells could offer new approaches to enhance immunotherapy, in addition to improving the cytotoxic and targeted therapies that are partially dependent upon a robust immune response for their efficacy. Here we will discuss the role of specific dendritic cell subsets in regulating the anti-tumor immune response, as well as the current status of dendritic cell-based immunotherapies, in order to provide an overview for future lines of research and clinical trials.

Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses

Dendritic cells are powerful antigen-presenting cells that are essential for the priming of T cell responses. In addition to providing T-cell-receptor ligands and co-stimulatory molecules for naive T cell activation and expansion, dendritic cells are thought to also provide signals for the differentiation of CD4+ T cells into effector T cell populations. The mechanisms by which dendritic cells are able to adapt and respond to the great variety of infectious stimuli they are confronted with, and prime an appropriate CD4+ T cell response, are only partly understood. It is known that in the steady-state dendritic cells are highly heterogenous both in phenotype and transcriptional profile, and that this variability is dependent on developmental lineage, maturation stage, and the tissue environment in which dendritic cells are located. Exposure to infectious agents interfaces with this pre-existing heterogeneity by providing ligands for pattern-recognition and toll-like receptors that are variably expressed on different dendritic cell subsets, and elicit production of cytokines and chemokines to support innate cell activation and drive T cell differentiation. Here we review current information on dendritic cell biology, their heterogeneity, and the properties of different dendritic cell subsets. We then consider the signals required for the development of different types of Th immune responses, and the cellular and molecular evidence implicating different subsets of dendritic cells in providing such signals. We outline how dendritic cell subsets tailor their response according to the infectious agent, and how such transcriptional plasticity enables them to drive different types of immune responses.

A Proposal for Nomenclature in Myeloid C-Type Lectin Receptors

Myeloid C-type lectin receptors (CLRs) comprise a family of receptors expressed by immune myeloid cells that share homologous C-type lectin domains. The implication of these CLRs in the regulation of homeostasis and activation of myeloid cells has generated a buoyant growth in the number of studies involving these receptors. Since their first description, diverse nomenclature has been used to refer to each of them, ranging from systematic classifications, such as gene name or cluster of differentiation, to non-systematic ones that include terminology based on gene expression patterns or function. In this review, we aim to summarize the different names used for the main myeloid CLRs and analyze which of them have been more frequently used in the literature. In addition, we have examined the evolution of the terminology applied to these myeloid CLRs over time. Based on this analysis, we propose a consensus alias for each of those myeloid CLRs. However, we acknowledge that systematicity is required beyond this terminology based on use frequency. Therefore, we have included gene names as the standardization tool to gather the maximum agreement. We suggest that a standard nomenclature consisting of both gene names and consensus alias should be included at least in scientific abstracts, which would help to identify relevant literature, saving time and effort and fostering the research in this field in a more systematic manner.

The role of dendritic cells in cancer

Cancer immunotherapy harnesses the ability of the immune system to recognize and eliminate cancer. The potent ability of dendritic cells (DCs) to initiate and regulate adaptive immune responses underpins the successful generation of anti-tumor immune responses. DCs are a heterogeneous leukocyte population comprised of distinct subsets that drive specific types of immune responses. Understanding how DCs induce tumor immune responses and the mechanisms adopted by tumors to evade DC surveillance is essential to render immunotherapies more effective. This review discusses current knowledge of the roles played by different DC subsets in human cancer and how these might be manipulated as new immunotherapeutics to improve CD8⁺ T cell-mediated immune responses, with a particular focus on the conventional type 1 DCs (cDC1).

Are Conventional Type 1 Dendritic Cells Critical for Protective Antitumor Immunity and How?

Dendritic cells (DCs) are endowed with a unique potency to prime T cells, as well as to orchestrate their expansion, functional polarization and effector activity in non-lymphoid tissues or in their draining lymph nodes. The concept of harnessing DC immunogenicity to induce protective responses in cancer patients was put forward about 25 years ago and has led to a multitude of DC-based vaccine trials. However, until very recently, objective clinical responses were below expectations. Conventional type 1 DCs (cDC1) excel in the activation of cytotoxic lymphocytes including CD8⁺ T cells (CTLs), natural killer (NK) cells, and NKT cells, which are all critical effector cell types in antitumor immunity. Efforts to investigate whether cDC1 might orchestrate immune defenses against cancer are ongoing, thanks to the recent blossoming of tools allowing their manipulation in vivo. Here we are reporting on these studies. We discuss the mouse models used to genetically deplete or manipulate cDC1, and their main caveats. We present current knowledge on the role of cDC1 in the spontaneous immune rejection of tumors engrafted in syngeneic mouse recipients, as a surrogate model to cancer immunosurveillance, and how this process is promoted by type I interferon (IFN-I) effects on cDC1. We also discuss cDC1 implication in promoting the protective effects of immunotherapies in mouse preclinical models, especially for adoptive cell transfer (ACT) and immune checkpoint blockers (ICB). We elaborate on how to improve this process by in vivo reprogramming of certain cDC1 functions with off-the-shelf compounds. We also summarize and discuss basic research and clinical data supporting the hypothesis that the protective antitumor functions of cDC1 inferred from mouse preclinical models are conserved in humans. This analysis supports potential applicability to cancer patients of the cDC1-targeting adjuvant immunotherapies showing promising results in mouse models. Nonetheless, further investigations on cDC1 and their implications in anti-cancer mechanisms are needed to determine whether they are the missing key that will ultimately help switching cold tumors into therapeutically responsive hot tumors, and how precisely they mediate their protective effects.

The Role of Type 1 Conventional Dendritic Cells in Cancer Immunity

Dendritic cells (DCs) are key orchestrators of immune responses. A specific DC subset, conventional type 1 DCs (cDC1s), has been recently associated with human cancer patient survival and, in preclinical models, is critical for the spontaneous rejection of immunogenic cancers and for the success of T cell-based immunotherapies. The unique role of cDC1 reflects the ability to initiate de novo T cell responses after migrating to tumor-draining lymph nodes, as well as to attract T cells, secrete cytokines, and present tumor antigens within the tumor microenvironment, enhancing local cytotoxic T cell function. Strategies aimed at increasing cDC1 abundance in tumors and enhancing their functionality provide attractive new avenues to boost anti-tumor immunity and overcome resistance to cancer immunotherapies.

Collateral Damage: What Effect Does Anti-CD4 and Anti-CD8α Antibody-Mediated Depletion Have on Leukocyte Populations?

Anti-CD4 or anti-CD8α Ab-mediated depletion strategies are widely used to determine the role of T cell subsets. However, surface expression of CD4 and CD8α is not limited to T cells and occurs on other leukocyte populations as well. Using both unbiased t-distributed stochastic neighbor embedding of flow cytometry data and conventional gating strategies, we assessed the impact of anti-CD4 and anti-CD8α Ab-mediated depletion on non-T cell populations in mice. Our results show that anti-CD4 and anti-CD8α Ab injections not only resulted in depletion of T cells but also led to depletion of specific dendritic cell subsets in a dose-dependent manner. Importantly, the extent of this effect varied between mock- and virus-infected mice. We also demonstrate the importance of using a second, noncompeting Ab (clone CT-CD8α) to detect CD8α⁺ cells following depletion with anti-CD8α Ab clone 2.43. Our study provides a necessary caution to carefully consider the effects on nontarget cells when using Ab injections for leukocyte depletion in all experimental conditions.