Mast cell activation enhances the TLR4-mediated antigen-specific CD4 T cell response

Mast cells are important immune mediators located at host environment interfaces and are most well-known as the major mediators of allergy and asthma. When activated, they have the ability to release prestored granules that contain immune modulating compounds, such as histamine and inflammatory cytokines. In skin, mast cell activation happens via crosslinking of the high-affinity FcɛRI receptor which is activated by the antibody IgE on the mast cell surface binding to a pathogen or allergen inducing intracellular signaling. While there is some evidence that mast cell activation alone does not induce expansion of endogenous antigen-specific CD4 helper T cells, it is not clear whether mast cells have the capacity to fine tune the expansion of T cells that are responding to other stimuli. To assess this, we focused on determining the effects of mast cell activation through FcɛRI alone or in combination with bacterial products on the generation of antigen specific CD4 T cells. Using an in vivo skin model of mast cell activation, combined with MHC class II tetramers, we show that crosslinking FcɛRI alone does not induce antigen-specific CD4 T cell immunity; however, cross-linking FcɛRI in the presence of the Toll-like receptor 4 (TLR4) agonist LPS provides a synergistic co-stimulatory signal that significantly increases T cell expansion. This effect is lost when stimulating with CpG, a TLR9 agonist. We also show that this synergy is lost when dendritic cells are ablated prior to activation. Further, we demonstrate that TLR4 expression by the mast cells themselves is essential to maintain this synergy. This study provides insights into the ability of mast cells to alter antigen-specific CD4 T cell responses in vivo in response to inflammatory stimuli.

Cutting Edge: Murine Mast Cells Rapidly Modulate Metabolic Pathways Essential for Distinct Effector Functions

There is growing appreciation that cellular metabolic and bioenergetic pathways do not play merely passive roles in activated leukocytes. Rather, metabolism has important roles in controlling cellular activation, differentiation, survival, and effector function. Much of this work has been performed in T cells; however, there is still very little information regarding mast cell metabolic reprogramming and its effect on cellular function. Mast cells perform important barrier functions and help control type 2 immune responses. In this study we show that murine bone marrow-derived mast cells rapidly alter their metabolism in response to stimulation through the FcεRI. We also demonstrate that specific metabolic pathways appear to be differentially required for the control of mast cell function. Manipulation of metabolic pathways may represent a novel point for the manipulation of mast cell activation.

Mast cell activation is enhanced by Tim1:Tim4 interaction but not by Tim-1 antibodies

Polymorphisms in the T cell (or transmembrane) immunoglobulin and mucin domain 1 ( TIM-1) gene, particularly in the mucin domain, have been associated with atopy and allergic diseases in mice and human. Genetic- and antibody-mediated studies revealed that Tim-1 functions as a positive regulator of Th2 responses, while certain antibodies to Tim-1 can exacerbate or reduce allergic lung inflammation. Tim-1 can also positively regulate the function of B cells, NKT cells, dendritic cells and mast cells. However, the precise molecular mechanisms by which Tim-1 modulates immune cell function are currently unknown. In this study, we have focused on defining Tim-1-mediated signaling pathways that enhance mast cell activation through the high affinity IgE receptor (FceRI). Using a Tim-1 mouse model lacking the mucin domain (Tim-1 ^Dmucin), we show for the first time that the polymorphic Tim-1 mucin region is dispensable for normal mast cell activation. We further show that Tim-4 cross-linking of Tim-1 enhances select signaling pathways downstream of FceRI in mast cells, including mTOR-dependent signaling, leading to increased cytokine production but without affecting degranulation.

T cell receptor-dependent activation of mTOR signaling in T cells is mediated by Carma1 and MALT1, but not Bcl10

Signaling to the mechanistic target of rapamycin (mTOR) regulates diverse cellular processes, including protein translation, cellular proliferation, metabolism, and autophagy. Most models place Akt upstream of the mTOR complex, mTORC1; however, in T cells, Akt may not be necessary for mTORC1 activation. We found that the adaptor protein Carma1 [caspase recruitment domain (CARD)-containing membrane-associated protein 1] and at least one of its associated proteins, the paracaspase MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1), were required for optimal activation of mTOR in T cells in response to stimulation of the T cell receptor (TCR) and the co-receptor CD28. However, Bcl10, which binds to Carma1 and MALT1 to form a complex that mediates signals from the TCR to the transcription factor NF-κB (nuclear factor κB), was not required. The catalytic activity of MALT1 was required for the proliferation of stimulated CD4+ T cells, but not for early TCR-dependent activation events. Consistent with an effect on mTOR, MALT1 activity was required for the increased metabolic flux in activated CD4+ T cells. Together, our data suggest that Carma1 and MALT1 play previously unappreciated roles in the activation of mTOR signaling in T cells after engagement of the TCR.

Modulation of mast cell function by TIM-3 signaling (HYP3P.351)

Transmembrane, immunoglobulin and mucin domain-3 (Tim-3) is an inducible negative regulator of Th1 function and anti-tumor immune responses. Recent studies found that Tim-3 is constitutively expressed on mast cells and that an anti-Tim-3 pAb enhanced secretion of pro-inflammatory cytokines by antigen-stimulated bone marrow-derived mast cells (BMMC). However, the mechanism(s) by which Tim-3 modulates mast cell function are unknown. Here we show that Tim-3 enhances FcεRI-derived signals to co-stimulate NF-AT, AP1, NF-κB, and IL-6 promoter transcriptional activation. This activity requires specific tyrosines in the Tim-3 cytoplasmic tail. Furthermore, various antibodies to Tim-3 significantly enhanced IgE+Ag-stimulated tyrosine phosphorylation and up-regulated BMMC cytokine secretion. Conversely, we observed impaired IgE+Ag-dependent activation of mast cells in which Tim-3 expression was knocked down. Using BMMC generated from Nur77-GFP reporter mice, we found that cross-linking Tim-3 synergizes with the antigen receptor FcεRI to enhance Nur77-GFP expression. Our work suggests that Tim-3 acts proximal to the antigen receptor, leading to enhancement of the NF-κB, NF-AT, and AP-1 pathways. Thus, these studies are consistent with Tim-3 being a positive regulator of mast cell function. Mast cells contribute to both innate and adaptive immune responses, so Tim-3 may serve as an important modulator of mast cell effector functions in autoimmune and allergic diseases.

Modulation of mast cell function by TIM-1 signaling (177.4)

Transmembrane, immunoglobulin and mucin domain (TIM)-1 belongs to a family of transmembrane proteins with emerging roles in immune modulation. As a putative atopy susceptibility gene, polymorphisms in human and mouse TIM-1 are associated with atopic diseases, including asthma and atopic dermatitis. Antibodies targeting different extracellular domains of Tim-1 either attenuated or exacerbated airway inflammation in a mouse model of asthma. Recent studies found that Tim-1 is expressed on mast cells. Tim-1 crosslinking by its ligand Tim-4 enhanced secretion of pro-inflammatory cytokines in IgE and Ag-activated bone marrow-derived mast cells (BMMCs). However, the mechanism(s) by which Tim-1 modulates mast cell function and how Tim-1 activity may be altered by antibody treatment is unknown. We hypothesize that Tim-1 engagement provides a co-stimulatory signal to enhance mast cell degranulation and mediator release in allergic inflammation. We found that Tim-1 requires a tyrosine phosphorylation motif in its cytoplasmic tail to co-stimulate NF-AT/ AP1, NF-κB, and IL-6 promoter transcriptional activation as well as IL-6 cytokine secretion. Furthermore, IgE-Ag and Tim-1 costimulation of BMMCs using a panel of Tim-1 antibodies and its ligand Tim-4 demonstrate that IL-6 and TNF-alpha secretion can be modulated by activity of agonistic and blocking Tim-1 antibodies. These studies suggest that Tim-1 may serve as an important modulator of mast cell effector functions in atopic diseases.

Akt fine-tunes NF-κB-dependent gene expression during T cell activation

Activation of the NF-κB signaling pathway is critical for leukocyte activation and development. Although previous studies suggested a role for the Akt kinase in coupling the T cell antigen receptor and CD28 to NF-κB activation in T cells, the nature of the role of Akt in this pathway is still unclear. Using a targeted gene profiling approach, we found that a subset of NF-κB-dependent genes required Akt for optimal up-regulation during T cell activation. The selective effects of Akt were manifest at the level of mRNA transcription and p65/RelA binding to upstream promoters and appear to be due to altered formation of the Carma1-Bcl10 complex. The proinflammatory cytokine TNF-α was found to be particularly sensitive to Akt inhibition or knockdown, including in primary human blood T cells and a murine model of rheumatoid arthritis. Our findings are consistent with a hierarchy in the expression of NF-κB-dependent genes, controlled by the strength and/or duration of NF-κB signaling. More broadly, our results suggest that defining the more graded effects of signaling, such as those demonstrated here for Akt and the NF-κB pathway, is important to understanding how cells can fine-tune signaling responses for optimal sensitivity and specificity.