PTPN22 inhibition resets defective human central B cell tolerance.

The 1858T protein tyrosine phosphatase nonreceptor type 22 (PTPN22 T) allele is one of the main risk factors associated with many autoimmune diseases and correlates with a defective removal of developing autoreactive B cells in humans. To determine whether inhibiting PTPN22 favors the elimination of autoreactive B cells, we first demonstrated that the PTPN22 T allele interfered with the establishment of central B cell tolerance using NOD-scid-common γ chain knockout (NSG) mice engrafted with human hematopoietic stem cells expressing this allele. In contrast, the inhibition of either PTPN22 enzymatic activity or its expression by RNA interference restored defective central B cell tolerance in this model. Thus, PTPN22 blockade may represent a therapeutic strategy for the prevention or treatment of autoimmunity.

CD39 is a negative regulator of P2X7-mediated inflammatory cell death in mast cells.

BACKGROUND: Mast cells (MCs) are major contributors to an inflammatory milieu. One of the most potent drivers of inflammation is the cytokine IL-1ß, which is produced in the cytoplasm in response to danger signals like LPS. Several controlling mechanisms have been reported which limit the release of IL-1ß. Central to this regulation is the NLRP3 inflammasome, activation of which requires a second danger signal with the capacity to subvert the homeostasis of lysosomes and mitochondria. High concentrations of extracellular ATP have the capability to perturb the plasma membrane by activation of P2X7 channels and serve as such a danger signal. In this study we investigate the role of P2X7 channels and the ecto-5'-nucleotidase CD39 in ATP-triggered release of IL-1ß from LPS-treated mast cells. RESULTS: We report that in MCs CD39 sets an activation threshold for the P2X7-dependent inflammatory cell death and concomitant IL-1ß release. Knock-out of CD39 or stimulation with non-hydrolysable ATP led to a lower activation threshold for P2X7-dependent responses. We found that stimulation of LPS-primed MCs with high doses of ATP readily induced inflammatory cell death. Yet, cell death-dependent release of IL-1ß yielded only minute amounts of IL-1ß. Intriguingly, stimulation with low ATP concentrations augmented the production of IL-1ß in LPS-primed MCs in a P2X7-independent but caspase-1-dependent manner. CONCLUSION: Our study demonstrates that the fine-tuned interplay between ATP and different surface molecules recognizing or modifying ATP can control inflammatory and cell death decisions.

Regulation of FcεRI signaling by lipid phosphatases.

Mast cells (MCs) are tissue-resident sentinels of hematopoietic origin that play a prominent role in allergic diseases. They express the high-affinity receptor for IgE (FcεRI), which when cross-linked by multivalent antigens triggers the release of preformed mediators, generation of arachidonic acid metabolites, and the synthesis of cytokines and chemokines. Stimulation of the FcεRI with increasing antigen concentrations follows a characteristic bell-shaped dose-responses curve. At high antigen concentrations, the so-called supra-optimal conditions, repression of FcεRI-induced responses is facilitated by activation and incorporation of negative signaling regulators. In this context, the SH2-containing inositol-5'-phosphatase, SHIP1, has been demonstrated to be of particular importance. SHIP1 with its catalytic and multiple protein interaction sites provides several layers of control for FcεRI signaling. Regulation of SHIP1 function occurs on various levels, e.g., protein expression, receptor and membrane recruitment, competition for protein-protein interaction sites, and activating modifications enhancing the phosphatase function. Apart from FcεRI-mediated signaling, SHIP1 can be activated by diverse unrelated receptor systems indicating its involvement in the regulation of antigen-dependent cellular responses by autocrine feedback mechanisms or tissue-specific and/or (patho-) physiologically determined factors. Thus, pharmacologic engagement of SHIP1 may represent a beneficial strategy for patients suffering from acute or chronic inflammation or allergies.

The 1,4-benzodiazepine Ro5-4864 (4-chlorodiazepam) suppresses multiple pro-inflammatory mast cell effector functions.

Activation of mast cells (MCs) can be achieved by the high-affinity receptor for IgE (FcεRI) as well as by additional receptors such as the lipopolysaccharide (LPS) receptor and the receptor tyrosine kinase Kit (stem cell factor [SCF] receptor). Thus, pharmacological interventions which stabilize MCs in response to different receptors would be preferable in diseases with pathological systemic MC activation such as systemic mastocytosis. 1,4-Benzodiazepines (BDZs) have been reported to suppress MC effector functions. In the present study, our aim was to analyze molecularly the effects of BDZs on MC activation by comparison of the effects of the two BDZs Ro5-4864 and clonazepam, which markedly differ in their affinities for the archetypical BDZ recognition sites, i.e., the GABAA receptor and TSPO (previously termed peripheral-type BDZ receptor). Ro5-4864 is a selective agonist at TSPO, whereas clonazepam is a selective agonist at the GABAA receptor. Ro5-4864 suppressed pro-inflammatory MC effector functions in response to antigen (Ag) (degranulation/cytokine production) and LPS and SCF (cytokine production), whereas clonazepam was inactive. Signaling pathway analyses revealed inhibitory effects of Ro5-4864 on Ag-triggered production of reactive oxygen species, calcium mobilization and activation of different downstream kinases. The initial activation of Src family kinases was attenuated by Ro5-4864 offering a molecular explanation for the observed impacts on various downstream signaling elements. In conclusion, BDZs structurally related to Ro5-4864 might serve as multifunctional MC stabilizers without the sedative effect of GABAA receptor-interacting BDZs.

Secretory lysosomes of mouse mast cells store and exocytose active caspase-3 in a strictly granzyme B dependent manner.

In this study, we report that cytoplasmic granules from in vivo and in vitro derived mouse mast cells (MCs) contain active granzyme B (gzmB) and caspase-3, which is consistent with recent findings. Studying WT and gzmB-deficient mice, we observed that BM-derived MCs (BMMCs) from both strains contain cytosolic pro-caspase-3, but only WT BMMCs expressed active caspase-3 limited to their secretory lysosomes. Confocal microscopy revealed colocalization of active caspase-3 and gzmB in these cytoplasmic granules. The combined data demonstrate that the generation and storage of active caspase-3 is gzmB-dependent. The finding that BMMCs secrete caspase-3 and gzmB after Ag stimulation suggests that both proteases contribute to extracellular MC-mediated proteolytic events. Although the extracellular function of MC-derived caspase-3 remains unclear, we show that BMMC-secreted caspase-3 cleaves IL-33, a cytokine that contributes to the development of asthma and arthritis. We also show that an in vitro propagated cytolytic T-lymphocyte line constitutively expresses gzmB together with active caspase-3, suggesting a novel interaction of these proteases in the execution of multiple innate and adaptive immune responses.

Activation of the PI3K pathway increases TLR-induced TNF-α and IL-6 but reduces IL-1ß production in mast cells.

Recognition of bacterial constituents by mast cells (MCs) is dependent on the presence of pattern recognition receptors, such as Toll-like receptors (TLRs). The final cellular response, however, depends on the influence of multiple environmental factors. In the current study we tested the hypothesis that the PI3K-activating ligands insulin-like growth factor-1 (IGF-1), insulin, antigen, and Steel Factor (SF) are able to modulate the TLR4-mediated production of proinflammatory cytokines in murine MCs. Costimulation with any of these ligands caused increased LPS-triggered secretion of IL-6 and TNF-α, but attenuated the production of IL-1ß, though all three cytokines were produced in an NFκB-dependent manner. The pan-specific PI3K-inhibitor Wortmannin reverted the altered production of these cytokines. In agreement, MCs deficient for SHIP1, a negative regulator of the PI3K pathway, showed augmented secretion of IL-6/TNF-α and reduced production of IL-1ß in response to LPS alone. The differential effects of IGF-1 on TLR4-mediated cytokine production were also observed in the context of TLR2 and IL-33 receptor-mediated MC activation. Importantly, these effects were seen in both bone marrow-derived and peritoneal MCs, suggesting general relevance for MCs. Using pharmacological and genetic tools, we could show that the p110δ isoform of PI3K is strongly implicated in SF-triggered suppression of LPS-induced IL-1ß production. Costimulation with antigen was affected to a lesser extent. In conclusion, NFκB-dependent production of proinflammatory cytokines in MCs is differentially controlled by PI3K-activating ligand/receptor systems.

Steel factor enhances supraoptimal antigen-induced IL-6 production from mast cells via activation of protein kinase C-beta.

Ag-triggered mast cell (MC) activation follows a bell-shaped dose-response curve. Reduced activation in response to supraoptimal Ag concentrations is thought to be due to preferential engagement of inhibitory-acting proteins like SHIP1, Lyn, and protein kinase C (PKC)-delta. We show in this study that short-term prestimulation with Steel factor (SF) prevents supraoptimal Ag inhibition, resulting in synergistic MC degranulation and IL-6 secretion. These events are preceded by synergistic phosphorylation/activation of numerous signaling proteins, e.g., Erk, p38, and LAT. However, these effects of prestimulation with SF appear not to be due to reduced engagement of the attenuator SHIP1. Pharmacological analyses suggest that the activation of conventional PKCs is important for this synergy. Specifically, although we found that the conventional PKC inhibitor, Gö6976, likely has some PKC-independent targets in MCs, it led us to further studies that established SF plus Ag-induced IL-6 secretion was severely impaired in PKC-beta(-/-) MCs, but not PKC-alpha(-/-) MCs. Thus, PKC-beta joins PI3K and Btk as important players in this synergistic MC activation.

Requirement for CD45 in fine-tuning mast cell responses mediated by different ligand-receptor systems.

The receptor-like protein tyrosine phosphatase CD45, the most abundant cell surface phosphatase on all nucleated hemopoietic cells, is a critical regulator of the activation status of Src family kinases (SFKs). To study the impact of CD45 on mast cell function, we compare bone marrow-derived mast cells (BMMCs) from CD45-deficient mice and from mice expressing an activating point mutation (E613R) in the juxtamembrane wedge of CD45. In response to Ag-triggered FcepsilonR1-mediated activation, CD45-deficient BMMCs exhibit increased inhibitory Lyn phosphorylation and drastically reduced effector functions (degranulation and cytokine secretion). In contrast, CD45 E613R BMMCs show stronger effector functions after Ag-triggering than wild-type (WT) BMMCs. Despite these dichotomous phenotypes, phosphorylation of the inhibitory tyrosine in the SFK Lyn of CD45 E613R BMMCs is comparable to CD45-deficient BMMCs. This unexpected phenotype most likely is due to attenuated interaction between CD45 E613R and Lyn and a hyper-activation of the Fyn-regulated phosphatidylinositol-3-kinase pathway. Interestingly, depending on the receptor system addressed, CD45-deficient and CD45 E613R BMMCs show uniform phenotypes as well. Proliferation of both cell types in response to IL-3 and/or SF is enhanced compared to WT BMMCs. Together, the data indicate that CD45 plays a complex and essential role in fine-tuning mast cell responses mediated by different ligand-receptor systems.