Activation-induced changes in the histamine content of murine mast cells and their adhesion to NIH-3T3 fibroblasts

Mast cells are immune effectors that are localized in tissues, proximal to sites of entry for allergens and other external stimuli. The mediators produced by mast cells are involved in innate immunity against bacteria and parasites, and are key effectors of allergic and asthmatic responses. Mast cell mediators also contribute to fibrosis in asthma, pulmonary fibrosis, scleroderma, and rheumatoid arthritis. The aim of this study was to test the hypothesis that mast cell interactions with fibroblasts are affected by their activation state, in a way that could contribute to the progression of fibrosis. Bone marrow mast cells (BMMC) were derived in vitro from bone marrow progenitors and connective tissue mast cells (CTMC) were harvested from mouse peritoneal cavity. An ELISA was used to measure histamine in the supernatants and cell lysates of activated BMMC and CTMC. Following activation by crosslinking the high affinity IgE receptor, histamine was released from BMMC and CTMC. Starting 24 hours after activation, the total cellular histamine concentration of BMMC was 755+/-14 ng/106 cells, and exceeded the amount in resting BMMC (214+/-9 ng/106 cells). The over-accumulation of histamine peaked 72 hours after activation (3136+/-394 ng/106 cells), and was transient, returning toward the amount in resting BMMC by 96 hours (1787+/-280 ng/106 cells). The increase in histamine was accompanied by an increase in the expression of histidine decarboxylase mRNA, and a 70 kD form of the protein encoded by this mRNA. Similar changes in histamine concentration did not occur in CTMC, indicating that histamine over-accumulation is associated with mast cell phenotype.; To study changes in mast cell adhesion to fibroblasts following activation, an assay was developed to quantify adhesion between mast cells and the NIH-3T3 fibroblast cell line. Resting or activated BMMC or CTMC were cocultured with the fibroblasts for various times to allow adhesion to occur. Nonadherent cells were washed away and adherent mast cells were quantified based on recovery of histamine from the adherent cells. Following activation, adhesion of both BMMC and CTMC to fibroblasts was increased three to six fold. This increased adhesion was not due to increased integrin expression, cell proliferation, or inflammatory mediators released from the mast cells. These findings offer new insights into the role of activated mast cells in fibrotic diseases.