Inducible MHC class II expression by mast cells supports effector and regulatory T cell activation.

In addition to their well-established role as regulators of allergic response, recent evidence supports a role for mast cells in influencing the outcome of physiologic and pathologic T cell responses. One mechanism by which mast cells (MCs) influence T cell function is indirectly through secretion of various cytokines. It remains unclear, however, whether MCs can directly activate T cells through Ag presentation, as the expression of MHC class II by MCs has been controversial. In this report, we demonstrate that in vitro stimulation of mouse MCs with LPS and IFN-gamma induces the expression of MHC class II and costimulatory molecules. Although freshly isolated peritoneal MCs do not express MHC class II, an in vivo inflammatory stimulus increases the number of MHC class II-positive MCs in situ. Expression of MHC class II granted MCs the ability to process and present Ags directly to T cells with preferential expansion of Ag-specific regulatory T cells over naive T cells. These data support the notion that, in the appropriate setting, MCs may regulate T cell responses through the direct presentation of Ag.