Apoptosis of mouse mast cells is reciprocally regulated by the IgG receptors FcγRIIB and FcγRIIIA

Lung ILC2s are activated in BALB/c mice born to immunized mothers despite complete protection against respiratory syncytial virus

Activated lung ILC2s produce large quantities of IL-5 and IL-13 that contribute to eosinophilic inflammation and mucus production following respiratory syncytial virus infection (RSV). The current understanding of ILC2 activation during RSV infection, is that ILC2s are activated by alarmins, including IL-33, released from airway epithelial cells in response to viral-mediated damage. Thus, high levels of RSV neutralizing maternal antibody generated from maternal immunization would be expected to reduce IL-33 production and mitigate ILC2 activation. Here we report that lung ILC2s from mice born to RSV-immunized dams become activated despite undetectable RSV replication. We also report, for the first time, expression of activating and inhibitory Fcgamma receptors on ILC2s that are differentially expressed in offspring born to immunized versus unimmunized dams. Alternatively, ex vivo IL-33-mediated activation of ILC2s was mitigated following the addition of antibody: antigen immune complexes. Further studies are needed to confirm the role of Fcgamma receptor ligation by immune complexes as an alternative mechanism of ILC2 regulation in RSV-associated eosinophilic lung inflammation.

Human placental extract suppresses mast cell activation and induces mast cell apoptosis

BACKGROUND

Human placental extract (HPE) has been documented to facilitate the healing of certain disorders including allergy. However, the effects of HPE on the functionality of mast cells, a critical cell type in allergic diseases, have not been reported.

METHODS

To investigate the effects of HPE on the regulation of allergy with respect to the biological functions of mast cells, the mast cell line C57 or HMC-1 cells were treated with HPE followed by the assessment of cell proliferation, apoptosis, activation, chemotaxis and phagocytosis. Mouse peritoneal mast cells were also investigated for their responses to induction of apoptosis by HPE in vivo. Furthermore, the effect of HPE on mast cell degranulation was confirmed using the passive cutaneous anaphylaxis (PCA) assay, an acute allergy model.

RESULTS

HPE was capable of suppressing mast cell proliferation and inducing mast cell apoptosis. Mast cell degranulation in response to compound 48/80- or anti-DNP IgE and DNP-mediated activation was suppressed. In addition, treatment with HPE compromised the production of cytokines by mast cells and cell chemotaxis. These observations were consistent with the dampened passive cutaneous anaphylaxis (PCA) assay following treatment with HPE.

CONCLUSION

This study revealed a suppressive effect of HPE on overall mast cell activities, suggesting a potential regulatory role of HPE on the alleviation of allergic diseases through mast cells.

FcγR-dependent apoptosis regulates tissue persistence of mucosal and connective tissue mast cells

Rodent mast cells can be divided into two major subtypes: the mucosal mast cell (MMC) and the connective tissue mast cell (CTMC). A decade-old observation revealed a longer lifespan for CTMC compared with MMC. The precise mechanisms underlying such differential tissue persistence of mast cell subsets have not been described. In this study, we have discovered that mast cells expressing only one receptor, either FcγRIIB or FcγRIIIA, underwent caspase-independent apoptosis in response to IgG immune complex treatment. Lower frequencies of CTMC in mice that lacked either FcγRIIB or FcγRIIIA compared with WT mice were recorded, especially in aged mice. We proposed that this paradigm of FcγR-mediated mast cell apoptosis could account for the more robust persistence of CTMC, which express both FcγRIIB and FcγRIIIA, than MMC, which express only FcγRIIB. Importantly, we reproduced these results using a mast cell engraftment model, which ruled out possible confounding effects of mast cell recruitment or FcγR expression by other cells on mast cell number regulation. In conclusion, our work has uncovered an FcγR-dependent mast cell number regulation paradigm that might provide a mechanistic explanation for the long-observed differential mast cell subset persistence in tissues.

Discovery of highly immunogenic spleen-resident FCGR3+CD103+ cDC1s differentiated by IL-33-primed ST2+ basophils

Recombinant interleukin-33 (IL-33) inhibits tumor growth, but the detailed immunological mechanism is still unknown. IL-33-mediated tumor suppression did not occur in Batf3^-/- mice, indicating that conventional type 1 dendritic cells (cDC1s) play a key role in IL-33-mediated antitumor immunity. A population of CD103⁺ cDC1s, which were barely detectable in the spleens of normal mice, increased significantly in the spleens of IL-33-treated mice. The newly emerged splenic CD103⁺ cDC1s were distinct from conventional splenic cDC1s based on their spleen residency, robust effector T-cell priming ability, and surface expression of FCGR3. DCs and DC precursors did not express Suppressor of Tumorigenicity 2 (ST2). However, recombinant IL-33 induced spleen-resident FCGR3⁺CD103⁺ cDC1s, which were found to be differentiated from DC precursors by bystander ST2⁺ immune cells. Through immune cell fractionation and depletion assays, we found that IL-33-primed ST2⁺ basophils play a crucial role in the development of FCGR3⁺CD103⁺ cDC1s by secreting IL-33-driven extrinsic factors. Recombinant GM-CSF also induced the population of CD103⁺ cDC1s, but the population neither expressed FCGR3 nor induced any discernable antitumor immunity. The population of FCGR3⁺CD103⁺ cDC1s was also generated in vitro culture of Flt3L-mediated bone marrow-derived DCs (FL-BMDCs) when IL-33 was added in a pre-DC stage of culture. FL-BMDCs generated in the presence of IL-33 (FL-33-DCs) offered more potent tumor immunotherapy than control Flt3L-BMDCs (FL-DCs). Human monocyte-derived DCs were also more immunogenic when exposed to IL-33-induced factors. Our findings suggest that recombinant IL-33 or an IL-33-mediated DC vaccine could be an attractive protocol for better tumor immunotherapy.

Targeting the RhoA-GEF-H1 pathway of mast cells attenuates experimental airway allergy

Mast cells are the major effector cells in allergic diseases. RhoA and its downstream pathway is associated with the pathogenesis of airway allergy. The objective of this study is to test a hypothesis that modulating the RhoA-GEF-H1 axis in mast cells can attenuate airway allergy. An airway allergic disorder (AAD) mouse model was employed. Mast cells were isolated from AAD mouse airway tissues to be analyzed by RNA sequencing. We observed that mast cells isolated from the respiratory tract of AAD mice were resistant to apoptosis. Mast cell mediator levels in nasal lavage fluid were correlated with apoptosis resistance in AAD mice. Activation of RhoA in AAD mast cells was related to resistance to apoptosis. Mast cells isolated from the airway tissues in AAD mouse exhibited strong RhoA-GEF-H1 expression. The RhoA-GEF-H1 axis was associated with the lower FasL expression in AAD mast cells. Activation of the RhoA-GEF-H1 axis promoted the production of mediators in mast cells. Inhibition of GEF-H1 facilitated the SIT-induced mast cell apoptosis and enhanced the therapeutic efficacy of AAD. In conclusion, RhoA-GEF-H1 activities are associated with resistance to apoptosis in mast cells isolated from sites of allergic lesions. The state of apoptosis resistance in mast cells is associated with the state of AAD disease. Inhibition of GEF-H1 restores the sensitivity of mast cells to apoptosis inducers, and alleviates experimental AAD in mice.

Thymic Stromal Lymphopoietin Interferes with the Apoptosis of Human Skin Mast Cells by a Dual Strategy Involving STAT5/Mcl-1 and JNK/Bcl-xL

Mast cells (MCs) play critical roles in allergic and inflammatory reactions and contribute to multiple pathologies in the skin, in which they show increased numbers, which frequently correlates with severity. It remains ill-defined how MC accumulation is established by the cutaneous microenvironment, in part because research on human MCs rarely employs MCs matured in the tissue, and extrapolations from other MC subsets have limitations, considering the high level of MC heterogeneity. Thymic stromal lymphopoietin (TSLP)—released by epithelial cells, like keratinocytes, following disturbed homeostasis and inflammation—has attracted much attention, but its impact on skin MCs remains undefined, despite the vast expression of the TSLP receptor by these cells. Using several methods, each detecting a distinct component of the apoptotic process (membrane alterations, DNA degradation, and caspase-3 activity), our study pinpoints TSLP as a novel survival factor of dermal MCs. TSLP confers apoptosis resistance via concomitant activation of the TSLP/ signal transducer and activator of transcription (STAT)-5 / myeloid cell leukemia (Mcl)-1 route and a newly uncovered TSLP/ c-Jun-N-terminal kinase (JNK)/ B-cell lymphoma (Bcl)-x_L axis, as evidenced by RNA interference and pharmacological inhibition. Our findings highlight the potential contribution of TSLP to the MC supportive niche of the skin and, vice versa, highlight MCs as crucial responders to TSLP in the context of TSLP-driven disorders.

Regulation of the pleiotropic effects of tissue-resident mast cells

Mast cells (MCs), which are best known for their detrimental role in patients with allergic diseases, act in a diverse array of physiologic and pathologic functions made possible by the plurality of MC types. Their various developmental avenues and distinct sensitivity to (micro-) environmental conditions convey extensive heterogeneity, resulting in diverse functions. We briefly summarize this heterogeneity, elaborate on molecular determinants that allow MCs to communicate with their environment to fulfill their tasks, discuss the protease repertoire stored in secretory lysosomes, and consider different aspects of MC signaling. Furthermore, we describe key MC governance mechanisms (ie, the high-affinity receptor for IgE [FcεRI]), the stem cell factor receptor KIT, the IL-4 system, and both Ca²⁺- and phosphatase-dependent mechanisms. Finally, we focus on distinct physiologic functions, such as chemotaxis, phagocytosis, host defense, and the regulation of MC functions at the mucosal barriers of the lung, gastrointestinal tract, and skin. A deeper knowledge of the pleiotropic functions of MC mediators, as well as the molecular processes of MC regulation and communication, should enable us to promote beneficial MC traits in physiology and suppress detrimental MC functions in patients with disease.

Antigen/IgG immune complex-primed mucosal mast cells mediate antigen-specific activation of co-cultured T cells

Mast cells are proposed to be one of the targets for mucosal vaccine adjuvants. We previously demonstrated that mucosal adjuvants containing IgG immune complexes could activate connective tissue mast cells enhancing immune responses. Here we suggest that mucosal mast cells (MMC) may also contribute to augmentation of antigen-specific immune responses following treatment with antigens complexed with IgG. We demonstrated that both bone marrow-derived cultured MMC and tissue resident MMC incorporated ovalbumin (OVA) at a greater level in the presence of anti-OVA IgG. Co-culture of OVA/IgG-pulsed bone marrow-derived MMC with splenocytes from OT-II mice promoted OVA-specific activation and proliferation of T cells, a process known as cross-presentation. Furthermore, bone marrow-derived cultured MMC underwent apoptosis following treatment with IgG immune complexes, a feature that has been described as favouring phagocytosis of mast cells by professional antigen-presenting cells.

Mast cell activation: a complex interplay of positive and negative signaling pathways

Mast cells regulate the immunological responses causing allergy and autoimmunity, and contribute to the tumor microenvironment through generation and secretion of a broad array of preformed, granule-stored and de novo synthesized bioactive compounds. The release and production of mast cell mediators is the result of a coordinated signaling machinery, followed by the FcεRI and FcγR antigen ligation. In this review, we present the latest understanding of FcεRI and FcγR signaling, required for the canonical mast cell activation during allergic responses and anaphylaxis. We then describe the cooperation between the signaling of FcR and other recently characterized membrane-bound receptors (i.e., IL-33R and thymic stromal lymphopoietin receptor) and their role in the chronic settings, where mast cell activation is crucial for the development and the sustainment of chronic diseases, such as asthma or airway inflammation. Finally, we report how the FcR activation could be used as a therapeutic approach to treat allergic and atopic diseases by mast cell inactivation. Understanding the magnitude and the complexity of mast cell signaling is necessary to identify the mechanisms underlying the potential effector and regulatory roles of mast cells in the biology and pathology of those disease settings in which mast cells are activated.

Apoptosis and other immune biomarkers predict influenza vaccine responsiveness

Despite the importance of the immune system in many diseases, there are currently no objective benchmarks of immunological health. In an effort to identifying such markers, we used influenza vaccination in 30 young (20-30 years) and 59 older subjects (60 to >89 years) as models for strong and weak immune responses, respectively, and assayed their serological responses to influenza strains as well as a wide variety of other parameters, including gene expression, antibodies to hemagglutinin peptides, serum cytokines, cell subset phenotypes and in vitro cytokine stimulation. Using machine learning, we identified nine variables that predict the antibody response with 84% accuracy. Two of these variables are involved in apoptosis, which positively associated with the response to vaccination and was confirmed to be a contributor to vaccine responsiveness in mice. The identification of these biomarkers provides new insights into what immune features may be most important for immune health.