An alternative pathway of mast cell activation in gastrointestinal inflammation.

Abstract

Mast cell activation through its high affinity IgE receptor has been extensively studied and is thought to play an important role in allergic diseases and parasite rejection. However, mast cells can release mediators in response to a variety of stimuli independent of IgE antibodies, including certain neuropeptides, and in particular substance P, suggesting a role for mast cells in neurogenic inflammation. We therefore took a combined in vitro and in vivo approach to elucidate the mediator production, gene expression, and biological significance of mast cell interactions with substance P. Using in vitro derived bone marrow mast cells, we found distinct activation profiles when we compared substance P and IgE/Ag stimulation. Specifically, different patterns of mediators release occurred, which were dependent on the nature of the stimulus, the phenotypic characteristics of the mast cells, and presence or absence of calcium in the incubation medium. We also examined the gene expression pattern in response to substance P using microarray technology. We found that calcium had a profound influence on the number of genes expressed in response to substance P. In the presence of calcium, more genes were upregulated, and of particular interest were several genes involved in signal transduction (Ramp2, rhoGAP1, Map2k3) and inflammatory response (MCPT-4, MCPT-7, IL-1 β, CCL3, CCL4, CCL7). We utilized normal (+/+), mast cell-deficient KitW/KitW-v , and mast cell-deficient KitW/KitW-v mice that had undergone local and selective mast cell reconstitution to investigate the role of mast cells and mast cell mediators in substance P-induced gastric inflammation in vivo. We found that substance P elicited mast cell-dependent neutrophil infiltration. Interestingly, unlike IgE and antigen responses in the stomach, substance Pelicited inflammation did not require TNF-α production by mast cells for the recruitment of neutrophils. However, we did demonstrate a role for leukotriene production by mast cells in this response. These studies provide the first in vivo evidence of differential mast cell mediator production. We also examined the gene expression pattern induced by substance P in mast cell deficient and normal mice. We found an upregulation of certain genes that were dependent on the presence of mast cells; including certain inflammatory response genes (Adora1, CXCL4, CXCL5, CXCR3) and signal transduction molecules (Rac1, Grk6) that could potentially play a role in substance P-induced gastric inflammation. Activation of mast cells by substance P determines a distinct activation profile, which is reflected in a specific pattern of in vitro mediator release, an in vivo characteristic reaction during gastric inflammation and distinctive patterns of gene expression in cultured mast cells and stomach tissues. Our findings suggest a complex and unique pattern of responses of mast cells to the neuropeptide substance P and offer insights into the unique contribution that mast cells may make to neurogenic inflammation.