Mast cells are unique tissue-resident immune cells that express an array of receptors that can be activated by several extracellular cues, including antigen-immunoglobulin E (IgE) complexes, ...bacteria, viruses, cytokines, hormones, peptides, and drugs. Mast cells constitute a small population in tissues, but their extraordinary ability to respond rapidly by releasing granule-stored and newly made mediators underpins their importance in health and disease. In this review, we document the biology of mast cells and introduce new concepts and opinions regarding their role in human diseases beyond IgE-mediated allergic responses and antiparasitic functions. We bring to light recent discoveries and developments in mast cell research, including regulation of mast cell functions, differentiation, survival, and novel mouse models. Finally, we highlight the current and future opportunities for therapeutic intervention of mast cell functions in inflammatory diseases.
Summary
Granulocyte–macrophage colony‐stimulating factor (GM–CSF), interleukin‐3 (IL‐3), and IL‐5 are members of a discrete family of cytokines that regulates the growth, differentiation, migration ...and effector function activities of many hematopoietic cells and immunocytes. These cytokines are involved in normal responses to infectious agents, bridging innate and adaptive immunity. However, in certain cases, the overexpression of these cytokines or their receptors can lead to excessive or aberrant initiation of signaling resulting in pathological conditions, with chronic inflammatory diseases and myeloid leukemias the most notable examples. Recent crystal structures of the GM–CSF receptor ternary complex and the IL‐5 binary complex have revealed new paradigms of cytokine receptor activation. Together with a wealth of associated structure–function studies, they have significantly enhanced our understanding of how these receptors recognize cytokines and initiate signals across cell membranes. Importantly, these structures provide opportunities for structure‐based approaches for the discovery of novel and disease‐specific therapeutics. In addition, recent biochemical evidence has suggested that the GM–CSF/IL‐3/IL‐5 receptor family is capable of interacting productively with other membrane proteins at the cell surface. Such interactions may afford additional or unique biological activities and might be harnessed for selective modulation of the function of these receptors in disease.
Although circumstantial evidence supports enhanced Toll-like receptor 7 (TLR7) signalling as a mechanism of human systemic autoimmune disease
, evidence of lupus-causing TLR7 gene variants is ...lacking. Here we describe human systemic lupus erythematosus caused by a TLR7 gain-of-function variant. TLR7 is a sensor of viral RNA
,
and binds to guanosine
-
. We identified a de novo, previously undescribed missense TLR7
variant in a child with severe lupus and additional variants in other patients with lupus. The TLR7
variant selectively increased sensing of guanosine and 2',3'-cGMP
, and was sufficient to cause lupus when introduced into mice. We show that enhanced TLR7 signalling drives aberrant survival of B cell receptor (BCR)-activated B cells, and in a cell-intrinsic manner, accumulation of CD11c
age-associated B cells and germinal centre B cells. Follicular and extrafollicular helper T cells were also increased but these phenotypes were cell-extrinsic. Deficiency of MyD88 (an adaptor protein downstream of TLR7) rescued autoimmunity, aberrant B cell survival, and all cellular and serological phenotypes. Despite prominent spontaneous germinal-centre formation in Tlr7
mice, autoimmunity was not ameliorated by germinal-centre deficiency, suggesting an extrafollicular origin of pathogenic B cells. We establish the importance of TLR7 and guanosine-containing self-ligands for human lupus pathogenesis, which paves the way for therapeutic TLR7 or MyD88 inhibition.
Successful treatment of acute myeloid leukemia (AML) with chimeric antigen receptor (CAR) T cells is hampered by toxicity on normal hematopoietic progenitor cells and low CAR T cell persistence. ...Here, we develop third-generation anti-CD123 CAR T cells with a humanized CSL362-based ScFv and a CD28-OX40-CD3ζ intracellular signaling domain. This CAR demonstrates anti-AML activity without affecting the healthy hematopoietic system, or causing epithelial tissue damage in a xenograft model. CD123 expression on leukemia cells increases upon 5'-Azacitidine (AZA) treatment. AZA treatment of leukemia-bearing mice causes an increase in CTLA-4
anti-CD123 CAR T cell numbers following infusion. Functionally, the CTLA-4
anti-CD123 CAR T cells exhibit superior cytotoxicity against AML cells, accompanied by higher TNFα production and enhanced downstream phosphorylation of key T cell activation molecules. Our findings indicate that AZA increases the immunogenicity of AML cells, enhancing recognition and elimination of malignant cells by highly efficient CTLA-4
anti-CD123 CAR T cells.
Background Mast cells have gained notoriety based on their detrimental contributions to IgE-mediated allergic disorders. Although mast cells express the vitamin D receptor (VDR), it is not clear to ...what extent 1α,25-dihydroxyvitamin D3 (1α,25OH2 D3 ) or its predominant inactive precursor metabolite in the circulation, 25-hydroxyvitamin D3 (25OHD3 ), can influence IgE-mediated mast cell activation and passive cutaneous anaphylaxis (PCA) in vivo. Objective We sought to assess whether the vitamin D3 metabolites 25OHD3 and 1α,25(OH)2 D3 can repress IgE-dependent mast cell activation through mast cell–25-hydroxyvitamin D-1α-hydroxylase (CYP27B1) and mast cell–VDR activity. Methods We measured the extent of vitamin D3 suppression of IgE-mediated mast cell degranulation and mediator production in vitro , as well as the vitamin D3 –induced curtailment of PCA responses in WBB6F1 - KitW/W-v or C57BL/6J- KitW-sh/W-sh mice engrafted with mast cells that did or did not express VDR or CYP27B1. Results Here we show that mouse and human mast cells can convert 25OHD3 to 1α,25(OH)2 D3 through CYP27B1 activity and that both of these vitamin D3 metabolites suppressed IgE-induced mast cell–derived proinflammatory and vasodilatory mediator production in a VDR-dependent manner in vitro . Furthermore, epicutaneously applied vitamin D3 metabolites significantly reduced the magnitude of skin swelling associated with IgE-mediated PCA reactions in vivo ; a response that required functional mast cell–VDRs and mast cell–CYP27B1. Conclusion Taken together, our findings provide a mechanistic explanation for the anti-inflammatory effects of vitamin D3 on mast cell function by demonstrating that mast cells can actively metabolize 25OHD3 to dampen IgE-mediated mast cell activation in vitro and in vivo.
Already 20 years have passed since the cloning of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor α-chain, the first member of the GM-CSF/interleukin (IL)–3/IL-5 family of ...hemopoietic cytokine receptors to be molecularly characterized. The intervening 2 decades have uncovered a plethora of biologic functions transduced by the GM-CSF receptor (pleiotropy) and revealed distinct signaling networks that couple the receptor to biologic outcomes. Unlike other hemopoietin receptors, the GM-CSF receptor has a significant nonredundant role in myeloid hematologic malignancies, macrophage-mediated acute and chronic inflammation, pulmonary homeostasis, and allergic disease. The molecular mechanisms underlying GM-CSF receptor activation have recently been revealed by the crystal structure of the GM-CSF receptor complexed to GM-CSF, which shows an unexpected higher order assembly. Emerging evidence also suggests the existence of intracellular signosomes that are recruited in a concentration-dependent fashion to selectively control cell survival, proliferation, and differentiation by GM-CSF. These findings begin to unravel the mystery of cytokine receptor pleiotropy and are likely to also apply to the related IL-3 and IL-5 receptors as well as other heterodimeric cytokine receptors. The new insights in GM-CSF receptor activation have clinical significance as the structural and signaling nuances can be harnessed for the development of new treatments for malignant and inflammatory diseases.
The GM-CSF, IL-3, and IL-5 receptors constitute the βc family, playing important roles in inflammation, autoimmunity, and cancer. Typical of heterodimeric type I cytokine receptors, signaling ...requires recruitment of the shared subunit to the initial cytokine:α subunit binary complex through an affinity conversion mechanism. This critical process is poorly understood due to the paucity of crystal structures of both binary and ternary receptor complexes for the same cytokine. We have now solved the structure of the binary GM-CSF:GMRα complex at 2.8-Å resolution and compared it with the structure of the ternary complex, revealing distinct conformational changes. Guided by these differences we performed mutational and functional studies that, importantly, show GMRα interactions playing a major role in receptor signaling while βc interactions control high-affinity binding. These results support the notion that conformational changes underlie the mechanism of GM-CSF receptor activation and also suggest how related type I cytokine receptors signal.
Display omitted
•Crystal structure of human GM-CSF receptor binary complex•Improved structure of human GM-CSF receptor ternary complex•Mutational and functional studies reveal major interactions for receptor activation•Observed conformational changes underlie mechanism of receptor signaling
GM-CSF receptor signaling is triggered in response to invading pathogens, but excessive GM-CSF signaling can lead to leukemia and inflammatory diseases. Broughton et al. report on the structural basis for cytokine recognition by the receptor in the binary and ternary states that reveal conformational changes leading to receptor activation.
Mast cells (MC)s are evolutionarily conserved, tissue-resident immune cells with diverse roles in allergy, cancer, and protection from infection by helminths and microorganisms. The significant ...diversity in MC development and tissue-specific functional characteristics has recently begun to be understood. Exciting developments in single-cell-based RNA, protein, and chromatin profiling technologies offer new opportunities to characterize MC heterogeneity and to uncover novel MC functions and subtypes; these developments might lead to new and clinically effective therapies for certain pathologies. In this review, we provide an overview of the current understanding of MC development and heterogeneity and discuss new insights gained from single-cell-based studies that may lead to future research directions and therapeutic opportunities.
Bulk proteomic and transcriptomic studies show that mouse and human mast cells are unique in the immune system. Numerous proteins, typically expressed in non-immune cells, are expressed in different tissue-resident mast cells under basal conditions or upon stimulation.Single-cell RNA-sequencing studies are uncovering the developmental and functional heterogeneity of mast cells in different mouse and human tissues, as well as disease conditions.Recent findings indicate that mast cells have a dual developmental origin in mice and humans. In mice, bone marrow-derived and yolk sac-derived mast cells populate distinct tissues and have both common and distinct transcriptional profiles.Recent studies are revealing how the chromatin landscape of human mast cells undergoes a genome-wide reorganization upon activation with different stimuli. Several chromatin-associated proteins regulate mast cell differentiation and proliferation in mice and humans.
Basophils are recognized as immune modulators through their ability to produce IL-4, a key cytokine required for Th2 immunity. It has also recently been reported that basophils are transiently ...recruited into the draining lymph node (LN) after allergen immunization and that the recruited basophils promote the differentiation of naive CD4 T cells into Th2 effector cells. Using IL-3(-/-) and IL-3Rbeta(-/-) mice, we report in this study that the IL-3/IL-3R system is absolutely required to recruit circulating basophils into the draining LN following helminth infection. Unexpectedly, the absence of IL-3 or of basophil LN recruitment played little role in helminth-induced Th2 immune responses. Moreover, basophil depletion in infected mice did not diminish the development of IL-4-producing CD4 T cells. Our results reveal a previously unknown role of IL-3 in recruiting basophils to the LN and demonstrate that basophils are not necessarily associated with the development of Th2 immunity during parasite infection.
Acute myeloid leukemia (AML) is an aggressive malignancy where despite improvements in conventional chemotherapy and bone marrow transplantation, overall survival remains poor. Sphingosine kinase 1 ...(SPHK1) generates the bioactive lipid sphingosine 1-phosphate (S1P) and has established roles in tumor initiation, progression, and chemotherapy resistance in a wide range of cancers. The role and targeting of SPHK1 in primary AML, however, has not been previously investigated. Here we show that SPHK1 is overexpressed and constitutively activated in primary AML patient blasts but not in normal mononuclear cells. Subsequent targeting of SPHK1 induced caspase-dependent cell death in AML cell lines, primary AML patient blasts, and isolated AML patient leukemic progenitor/stem cells, with negligible effects on normal bone marrow CD34+ progenitors from healthy donors. Furthermore, administration of SPHK1 inhibitors to orthotopic AML patient–derived xenografts reduced tumor burden and prolonged overall survival without affecting murine hematopoiesis. SPHK1 inhibition was associated with reduced survival signaling from S1P receptor 2, resulting in selective downregulation of the prosurvival protein MCL1. Subsequent analysis showed that the combination of BH3 mimetics with either SPHK1 inhibition or S1P receptor 2 antagonism triggered synergistic AML cell death. These results support the notion that SPHK1 is a bona fide therapeutic target for the treatment of AML.
•Inhibition of SPHK1 in human AML cells induces MCL1 degradation and caspase-dependent cell death.•SPHK1 inhibitors reduce leukemic burden and prolong survival in orthotopic patient-derived xenografts of AML.
PDF
