Mast Cell Changes the Phenotype of Microglia via Histamine and ATP

Microglia are the dynamic motile phagocytes of the brain considered the first line of defense against threats or disturbances to the Central Nervous System (CNS). Microglia help orchestrate the immunological response by interacting with others immune cells. Mast cells (MCs) are effector cells of the innate immune system distributed in all organs and vascularized tissues, brain included. Several molecular mechanisms for potential interactions between MCs and microglia have been determined. However, the effect of MCs on regulated exocytosis and phagocytic clearance in microglia has not been explored.

Cocktails of MCs mediators (MCM) obtained at 37°C and 53°C were used to induce microglia activation. Changes in intracellular calcium [Ca²⁺]_i and ATP release were studied by calcium and quinacrine fluorescence imaging. Fluorescent latex beads were used to assay phagocytosis in microglia after MCM treatment and compared to that measured in the presence of histamine, ATP and lipopolysaccharide (LPS). Iba-1 expression and area were quantified by immunofluorescence and histamine levels evaluated by ELISA techniques.

Local application onto microglia of the MC mediator cocktail elicited Ca²⁺ transients and exocytotic release associated with quinacrine dye de-staining. Ca²⁺ signals were mimicked by histamine and blocked by the H1 receptor (H1R) antagonist, cetirizine. Hydrolysis of ATP by apyrase also affected Ca²⁺ transients to a lesser extent. Iba-1 fluorescence, cell area and phagocytosis were enhanced by histamine through H1R. However, ATP prevented iba-1 expression and microglial phagocytosis. MCM showed combined effects of histamine and ATP, increasing the number of internalized microbeads per cell and area without raising iba1 expression.

Our results highlight the relevance of MC-derived histamine and ATP in the modulation of secretory and phagocytic activities that would explain the heterogeneity of microglial responses in different pathological contexts.