PD-L1 on mast cells suppresses effector CD8+ T-cell activation in the skin in murine contact hypersensitivity

The mast cell-T lymphocyte axis impacts cancer: Friend or foe?

Crosstalk between mast cells (MCs) and T lymphocytes (TLs) releases specific signals that create an environment conducive to tumor development. Conversely, they can protect against cancer by targeting tumor cells for destruction. Although their role in immunity and cancer is complex, their potential in anticancer strategies is often underestimated. When peripheral MCs are activated, they can affect cancer development. Tumor-infiltrating TLs may malfunction and contribute to aggressive cancer and poor prognoses. One promising approach for cancer patients is TL-based immunotherapies. Recent reports suggest that MCs modulate TL activity in solid tumors and may be a potential therapeutic layer in multitargeting anticancer strategies. Pharmacologically modulating MC activity can enhance the anticancer cytotoxic TL response in tumors. By identifying tumor-specific targets, it has been possible to genetically alter patients' cells into fully humanized anticancer cellular therapies for autologous transplantation, including the engineering of TLs and MCs to target and kill cancer cells. Hence, recent scientific evidence provides a broader understanding of MC-TL activity in cancer.

Melanomas and mast cells: an ambiguous relationship

Mast cells (MCs) accumulate in a broad range of tumors, including melanomas. While MCs are potent initiators of immunity in infection, and in allergic inflammation, the function of MCs in anti-melanoma immunity is unclear. MCs have the potential to release tumoricidal cytokines and proteases, to activate antigen-presenting cells and to promote anti-tumor adaptive immunity. However, within the immunosuppressive tumor microenvironment (TME), MC activation may promote angiogenesis and contribute to tumor growth. In this review, the relationship between MCs and melanomas is discussed with a focus on the impact of the TME on MC activation.

Tumor immune escape: extracellular vesicles roles and therapeutics application

BACKGROUND

Immune escape, a process by which tumor cells evade immune surveillance, remains a challenge for cancer therapy. Tumor cells produce extracellular vesicles (EVs) that participate in immune escape by transferring bioactive molecules between cells. EVs refer to heterogeneous vesicles that participate in intercellular communication. EVs from tumor cells usually carry tumor antigens and have been considered a source of tumor antigens to induce anti-tumor immunity. However, evidence also suggests that these EVs can accelerate immune escape by carrying heat shock proteins (HSPs), programmed death-ligand 1 (PD-L1), etc. to immune cells, suppressing function and exhausting the immune cells pool. EVs are progressively being evaluated for therapeutic implementation in cancer therapies. EVs-based immunotherapies involve inhibiting EVs generation, using natural EVs, and harnessing engineering EVs. All approaches are associated with advantages and disadvantages. The EVs heterogeneity and diverse physicochemical properties are the main challenges to their clinical applications.

SHORT CONCLUSION

Although EVs are criminal; they can be useful for overcoming immune escape. This review discusses the latest knowledge on EVs population and sheds light on the function of tumor-derived EVs in immune escape. It also describes EVs-based immunotherapies with a focus on engineered EVs, followed by challenges that hinder the clinical translation of EVs that are essential to be addressed in future investigations. Video Abstract.

IFN-γ-dependent interactions between tissue-intrinsic γδ T cells and tissue-infiltrating CD8 T cells limit allergic contact dermatitis

BACKGROUND

Elicitation of allergic contact dermatitis (ACD), an inflammatory type 4 hypersensitivity disease, induces skin infiltration by polyclonal effector CD8 αβ T cells and precursors of tissue-resident memory T (TRM) cells. Because TRM have long-term potential to contribute to body-surface immunoprotection and immunopathology, their local regulation needs a fuller understanding.

OBJECTIVE

We sought to investigate how TRM-cell maturation might be influenced by innate-like T cells pre-existing within many epithelia.

METHODS

This study examined CD8+ TRM-cell maturation following hapten-induced ACD in wild-type mice and in strains harboring altered compartments of dendritic intraepidermal γδ T cells (DETCs), a prototypic tissue-intrinsic, innate-like T-cell compartment that reportedly regulates ACD, but by no elucidated mechanism.

RESULTS

In addition to eliciting CD8 TRM, ACD induced DETC activation and an intimate coregulatory association of the 2 cell types. This depended on DETC sensing IFN-γ produced by CD8 cells and involved programmed death-ligand 1 (PD-L1). Thus, in mice lacking DETC or lacking IFN-γ receptor solely on γδ cells, ACD-elicited CD8 T cells showed enhanced proliferative and effector potentials and reduced motility, collectively associated with exaggerated ACD pathology. Comparable dysregulation was elicited by PD-L1 blockade in vitro, and IFN-γ-regulated PD-L1 expression was a trait of human skin-homing and intraepithelial γδ T cells.

CONCLUSIONS

The size and quality of the tissue-infiltrating CD8 T-cell response during ACD can be profoundly regulated by local innate-like T cells responding to IFN-γ and involving PD-L1. Thus, interindividual and tissue-specific variations in tissue-intrinsic lymphocytes may influence responses to allergens and other challenges and may underpin inflammatory pathologies such as those repeatedly observed in γδ T-cell-deficient settings.

Role of Innate Immunity in Allergic Contact Dermatitis: An Update

Our understanding of allergic contact dermatitis mechanisms has progressed over the past decade. Innate immune cells that are involved in the pathogenesis of allergic contact dermatitis include Langerhans cells, dermal dendritic cells, macrophages, mast cells, innate lymphoid cells (ILCs), neutrophils, eosinophils, and basophils. ILCs can be subcategorized as group 1 (natural killer cells; ILC1) in association with Th1, group 2 (ILC2) in association with Th2, and group 3 (lymphoid tissue-inducer cells; ILC3) in association with Th17. Pattern recognition receptors (PRRs) including toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) in innate immune cells recognize damage-associated molecular patterns (DAMPs) and cascade the signal to produce several cytokines and chemokines including tumor necrosis factor (TNF)-α, interferon (IFN)-α, IFN-γ, interleukin (IL)-1β, IL-4, IL-6, IL-12, IL-13, IL-17, IL-18, and IL-23. Here we discuss the recent findings showing the roles of the innate immune system in allergic contact dermatitis during the sensitization and elicitation phases.

Mast cells in type 2 skin inflammation: Maintenance and function

Mast cells (MCs) are immune cells residing in tissues and playing indispensable roles in maintaining homeostasis and inflammatory states. Skin lesions associated with atopic dermatitis (AD) and type 2 skin inflammation display an increment in MCs, which have both pro- and anti-inflammatory effects. The direct and indirect activations of skin MCs by environmental factors such as Staphylococcus aureus can instigate type 2 skin inflammation in AD with poorly understood mechanisms. Furthermore, both IgE-dependent and -independent degranulation of MCs contribute to pruritus in AD. Conversely, MCs suppress type 2 skin inflammation by promoting Treg expansion through IL-2 secretion in the spleen. Moreover, skin MCs can upregulate gene expression involved in skin barrier function, thus mitigating AD-like inflammation. These functional variances of MCs in AD could stem from differences in experimental systems, their localization, and origins. In this review, we will focus on how MCs are maintained in the skin under homeostatic and inflammatory conditions, and how they are involved in the pathogenesis of type 2 skin inflammation.

Virus-infected mast cells activate virus-specific CD8+ T cells

Efficient anti-viral responses of CD8+ T cells require signals that promote their effector cell differentiation, that are mainly provided by dendritic cells (DCs). Mast cells (MCs) are key drivers of DC maturation, but also influence their migration and antigen presenting properties and therefore indirectly mediate CD8+ T cell activation. MCs initiate innate immune responses at pathogen entry sites, promote the development of adaptive immune responses after infection, and release mediators including chemokines that recruit and activate immune cells including T cells during viral infections. However, whether MCs can directly activate virus-specific CD8+ T cells remains largely unknown. Here, we used an in vitro viral infection model with lymphocytic choriomeningitis virus (LCMV)-infected MCs or DCs co-cultured with either LCMV-specific CD8+ T cells or with WT (unspecific) CD8+ T cells. Similar to LCMV-infected DCs, LCMV-infected MCs clustered with virus-specific CD8+ T cells and induced their activation and production of antiviral cytokines. In addition, the co-stimulatory molecules CD86 and OX40L, but not CD80, were upregulated on MCs and an increased production of IL-6 and type I interferons after LCMV infection was shown. Our findings suggest that MCs can promote CD8+ T cell activation during viral infections. MC-mediated CD8+ T cell activation might be especially important within infected tissues where direct cellular interaction can take place. A better understanding of anti-viral functions of MCs may help developing new strategies to better treat viral infections.

Frenemies in the Microenvironment: Harnessing Mast Cells for Cancer Immunotherapy

Tumor development, progression, and resistance to therapies are influenced by the interactions between tumor cells and the surrounding microenvironment, comprising fibroblasts, immune cells, and extracellular matrix proteins. In this context, mast cells (MCs) have recently emerged as important players. Yet, their role is still controversial, as MCs can exert pro- or anti-tumor functions in different tumor types depending on their location within or around the tumor mass and their interaction with other components of the tumor microenvironment. In this review, we describe the main aspects of MC biology and the different contribution of MCs in promoting or inhibiting cancer growth. We then discuss possible therapeutic strategies aimed at targeting MCs for cancer immunotherapy, which include: (1) targeting c-Kit signaling; (2) stabilizing MC degranulation; (3) triggering activating/inhibiting receptors; (4) modulating MC recruitment; (5) harnessing MC mediators; (6) adoptive transferring of MCs. Such strategies should aim to either restrain or sustain MC activity according to specific contexts. Further investigation would allow us to better dissect the multifaceted roles of MCs in cancer and tailor novel approaches for an "MC-guided" personalized medicine to be used in combination with conventional anti-cancer therapies.

Inflammation-Regulated Nanodrug Sensitizes Hepatocellular Carcinoma to Checkpoint Blockade Therapy by Reprogramming the Tumor Microenvironment

Immune checkpoint blockade (ICB) utilizing programmed death ligand-1 (PD-L1) antibody is a promising treatment strategy in solid tumors. However, in fact, more than half of hepatocellular carcinoma (HCC) patients are unresponsive to PD-L1-based ICB treatment due to multiple immune evasion mechanisms such as the hyperactivation of inflammation pathway, excessive tumor-associated macrophages (TAMs) infiltration, and insufficient infiltration of T cells. Herein, an inflammation-regulated nanodrug was designed to codeliver NF-κB inhibitor curcumin and PD-L1 antibody to reprogram the tumor microenvironment (TME) and activate antitumor immunity. The nanodrug accumulated in TME by an enhanced permeability and retention effect, where it left antibody to block PD-L1 on the membrane of tumor cells and TAMs due to pH-responsiveness. Simultaneously, a new curcumin-encapsulated nanodrug was generated, which was easily absorbed by either tumor cells or TAMs to inhibit the nuclear factor kappa-B (NF-κB) signal and related immunosuppressive genes. The inflammation-regulated nanodrug possessed good biocompatibility. Simultaneously, it reprogrammed TME effectively and exhibited an effective anticancer effect in immunocompetent mice. Overall, this study provided a potent strategy to improve the efficiency of ICB-based treatment for HCC.

Response to immune checkpoint blockade improved in pre-clinical model of breast cancer after bariatric surgery

Bariatric surgery is a sustainable weight loss approach, including vertical sleeve gastrectomy (VSG). Obesity exacerbates tumor growth, while diet-induced weight loss impairs progression. It remains unknown how bariatric surgery-induced weight loss impacts cancer progression or alters response to therapy. Using a pre-clinical model of obesity followed by VSG or diet-induced weight loss, breast cancer progression and immune checkpoint blockade therapy were investigated. Weight loss by VSG or weight-matched dietary intervention before tumor engraftment protected against obesity-exacerbated tumor progression. However, VSG was not as effective as diet in reducing tumor burden despite achieving similar weight and adiposity loss. Leptin did not associate with changes in tumor burden; however, circulating IL-6 was elevated in VSG mice. Uniquely, VSG tumors displayed elevated inflammation and immune checkpoint ligand PD-L1+ myeloid and non-immune cells. VSG tumors also had reduced T lymphocytes and markers of cytolysis, suggesting an ineffective anti-tumor microenvironment which prompted investigation of immune checkpoint blockade. While obese mice were resistant to immune checkpoint blockade, anti-PD-L1 potently impaired tumor progression after VSG through improved anti-tumor immunity. Thus, in formerly obese mice, surgical weight loss followed by immunotherapy reduced breast cancer burden. Finally, we compared transcriptomic changes in adipose tissue after bariatric surgery from patients and mouse models. A conserved bariatric surgery-associated weight loss signature (BSAS) was identified which significantly associated with decreased tumor volume. Findings demonstrate conserved impacts of obesity and bariatric surgery-induced weight loss pathways associated with breast cancer progression.

As the number of people classified as obese rises globally, so do obesity-related health risks. Studies show that people diagnosed with obesity have inflammation that contributes to tumor growth and their immune system is worse at detecting cancer cells. But weight loss is not currently used as a strategy for preventing or treating cancer.

Surgical procedures for weight loss, also known as ‘bariatric surgeries’, are becoming increasingly popular. Recent studies have shown that individuals who lose weight after these treatments have a reduced risk of developing tumors. But how bariatric surgery directly impacts cancer progression has not been well studied: does it slow tumor growth or boost the anti-tumor immune response?

To answer these questions, Sipe et al. compared breast tumor growth in groups of laboratory mice that were obese due to being fed a high fat diet. The first group of mice lost weight after undergoing a bariatric surgery in which part of their stomach was removed. The second lost the same amount of weight but after receiving a restricted diet, and the third underwent a fake surgery and did not lose any weight. The experiments found that surgical weight loss cuts breast cancer tumor growth in half compared with obese mice. But mice who lost the same amount of weight through dietary restrictions had even less tumor growth than surgically treated mice.

The surgically treated mice who lost weight had more inflammation than mice in the two other groups, and had increased amounts of proteins and cells that block the immune response to tumors. Giving the surgically treated mice a drug that enhances the immune system’s ability to detect and destroy cancer cells reduced inflammation and helped shrink the mice’s tumors. Finally, Sipe et al. identified 54 genes which were turned on or off after bariatric surgery in both mice and humans, 11 of which were linked with tumor size.

These findings provide crucial new information about how bariatric surgery can impact cancer progression. Future studies could potentially use the conserved genes identified by Sipe et al. to develop new ways to stimulate the anti-cancer benefits of weight loss without surgery.

Mechanisms underlying immune-related adverse events during checkpoint immunotherapy

Immune checkpoint (IC) proteins are some of the most important factors that tumor cells hijack to escape immune surveillance, and inhibiting ICs to enhance or relieve antitumor immunity has been proven efficient in tumor treatment. Immune checkpoint blockade (ICB) agents such as antibodies blocking programmed death (PD) 1, PD-1 ligand (PD-L) 1, and cytotoxic T lymphocyte-associated antigen (CTLA)-4 have been approved by the U.S. Food and Drug Administration (FDA) to treat several types of cancers. Although ICB agents have shown outstanding clinical success, and their application has continued to expand to additional tumor types in the past decade, immune-related adverse events (irAEs) have been observed in a wide range of patients who receive ICB treatment. Numerous studies have focused on the clinical manifestations and pathology of ICB-related irAEs, but the detailed mechanisms underlying irAEs remain largely unknown. Owing to the wide expression of IC molecules on distinct immune cell subpopulations and the fact that ICB agents generally affect IC-expressing cells, the influences of ICB agents on immune cells in irAEs need to be determined. Here, we discuss the expression and functions of IC proteins on distinct immune cells and the potential mechanism(s) related to ICB-targeted immune cell subsets in irAEs.

Novel Insights Into the Immune-Regulatory Functions of Mast Cells in the Cutaneous Immune Response

Skin is a frontline organ that is continuously exposed to external stimuli, including pathogens. Various immune cells reside in the skin under physiological conditions and protect the body from the entry of pathogens/antigens by interacting with each other and orchestrating diverse cutaneous immune responses. To avoid unnecessary inflammation and tissue damage during the elimination of external pathogens and antigens, skin possesses regulatory systems that fine-tune these immune reactions. Mast cells (MCs) are one of the skin-resident immune cell populations that play both effector and regulatory functions in the cutaneous immune response. So far, the interleukin-10-mediated mechanisms have mostly been investigated as the regulatory mechanisms of MCs. Recent studies have elucidated other regulatory mechanisms of MCs, such as the maintenance of regulatory T/B cells and the programmed cell death protein-1/programmed cell death-ligand 1-mediated inhibitory pathway. These regulatory pathways of MCs have been suggested to play important roles in limiting the excessive inflammation in inflammatory skin diseases, such as contact and atopic dermatitis. The regulatory functions of MCs may also be involved in the escape mechanisms of antitumor responses in skin cancers, such as melanoma. Understanding and controlling the regulatory functions of skin MCs may lead to novel therapeutic strategies for inflammatory skin diseases and skin cancers.

Immunomodulatory Properties of Immune Checkpoint Inhibitors-More than Boosting T-Cell Responses?

The approval of immune checkpoint inhibitors (ICI) that serve to enhance effector T-cell anti-tumor responses has strongly improved success rates in the treatment of metastatic melanoma and other tumor types. The currently approved ICI constitute monoclonal antibodies blocking cytotoxic T-lymphocyte-associated protein (CTLA)-4 and anti-programmed cell death (PD)-1. By this, the T-cell-inhibitory CTLA-4/CD80/86 and PD-1/PD-1L/2L signaling axes are inhibited. This leads to sustained effector T-cell activity and circumvents the immune evasion of tumor cells, which frequently upregulate PD-L1 expression and modulate immune checkpoint molecule expression on leukocytes. As a result, profound clinical responses are observed in 40-60% of metastatic melanoma patients. Despite the pivotal role of T effector cells for triggering anti-tumor immunity, mounting evidence indicates that ICI efficacy may also be attributable to other cell types than T effector cells. In particular, emerging research has shown that ICI also impacts innate immune cells, such as myeloid cells, natural killer cells and innate lymphoid cells, which may amplify tumoricidal functions beyond triggering T effector cells, and thus improves clinical efficacy. Effects of ICI on non-T cells may additionally explain, in part, the character and extent of adverse effects associated with treatment. Deeper knowledge of these effects is required to further develop ICI treatment in terms of responsiveness of patients to treatment, to overcome resistance to ICI and to alleviate adverse effects. In this review we give an overview into the currently known immunomodulatory effects of ICI treatment in immune cell types other than the T cell compartment.

Extracellular vesicle PD-L1 in reshaping tumor immune microenvironment: biological function and potential therapy strategies

Programmed cell death 1 ligand 1 (PD-L1) is the ligand for programmed death protein-1 (PD-1), is associated with immunosuppression. Signaling via PD-1/PD-L1 will transmits negative regulatory signals to T cells, inducing T-cell inhibition, reducing CD8⁺ T-cell proliferation, or promoting T-cell apoptosis, which effectively reduces the immune response and leads to large-scale tumor growth. Accordingly, many antibody preparations targeting PD-1 or PD-L1 have been designed to block the binding of these two proteins and restore T-cell proliferation and cytotoxicity of T cells. However, these drugs are ineffective in clinical practice. Recently, numerous of studies have shown that, in addition to the surface of tumor cells, PD-L1 is also found on the surface of extracellular vesicles secreted by these cells. Extracellular vesicle PD-L1 can also interact with PD-1 on the surface of T cells, leading to immunosuppression, and has been proposed as a potential mechanism underlying PD-1/PD-L1-targeted drug resistance. Therefore, it is important to explore the production, regulation and tumor immunosuppression of PD-L1 on the surface of tumor cells and extracellular vesicles, as well as the potential clinical application of extracellular vesicle PD-L1 as tumor biomarkers and therapeutic targets.

Two Sides of the Coin: Mast Cells as a Key Regulator of Allergy and Acute/Chronic Inflammation

It is well known that mast cells (MCs) initiate type I allergic reactions and inflammation in a quick response to the various stimulants, including—but not limited to—allergens, pathogen-associated molecular patterns (PAMPs), and damage-associated molecular patterns (DAMPs). MCs highly express receptors of these ligands and proteases (e.g., tryptase, chymase) and cytokines (TNF), and other granular components (e.g., histamine and serotonin) and aggravate the allergic reaction and inflammation. On the other hand, accumulated evidence has revealed that MCs also possess immune-regulatory functions, suppressing chronic inflammation and allergic reactions on some occasions. IL-2 and IL-10 released from MCs inhibit excessive immune responses. Recently, it has been revealed that allergen immunotherapy modulates the function of MCs from their allergic function to their regulatory function to suppress allergic reactions. This evidence suggests the possibility that manipulation of MCs functions will result in a novel approach to the treatment of various MCs-mediated diseases.

Contact dermatitis

Contact dermatitis (CD) is among the most common inflammatory dermatological conditions and includes allergic CD, photoallergic CD, irritant CD, photoirritant CD (also called phototoxic CD) and protein CD. Occupational CD can be of any type and is the most prevalent occupational skin disease. Each CD type is characterized by different immunological mechanisms and/or requisite exposures. Clinical manifestations of CD vary widely and multiple subtypes may occur simultaneously. The diagnosis relies on clinical presentation, thorough exposure assessment and evaluation with techniques such as patch testing and skin-prick testing. Management is based on patient education, avoidance strategies of specific substances, and topical treatments; in severe or recalcitrant cases, which can negatively affect the quality of life of patients, systemic medications may be needed.

Mast Cells in the Skin: Defenders of Integrity or Offenders in Inflammation?

Mast cells (MCs) are best-known as key effector cells of immediate-type allergic reactions that may even culminate in life-threatening anaphylactic shock syndromes. However, strategically positioned at the host–environment interfaces and equipped with a plethora of receptors, MCs also play an important role in the first-line defense against pathogens. Their main characteristic, the huge amount of preformed proinflammatory mediators embedded in secretory granules, allows for a rapid response and initiation of further immune effector cell recruitment. The same mechanism, however, may account for detrimental overshooting responses. MCs are not only detrimental in MC-driven diseases but also responsible for disease exacerbation in other inflammatory disorders. Focusing on the skin as the largest immune organ, we herein review both beneficial and detrimental functions of skin MCs, from skin barrier integrity via host defense mechanisms to MC-driven inflammatory skin disorders. Moreover, we emphasize the importance of IgE-independent pathways of MC activation and their role in sustained chronic skin inflammation and disease exacerbation.