Deciphering new mechanisms on T‐cell costimulation by human mast cells

It is well established that full activation of T cells to recognize a specific antigen requires additional signals. These secondary signals are generated by the interaction of costimulatory molecules expressed on APCs. Classical APCs include DCs, macrophages, Langerhans cells, and B cells. However, in recent years, several haematopoietic and nonhaematopoietic cells have been described to express MHC class II antigens and, in appropriate conditions, costimulatory molecules. In this issue, Suurmond et al. Eur. J. Immunol. 2016. 46: 1132–1141] show, for the first time, that human mast cells not only express costimulatory molecules of the TNF‐receptor and CD28 families, but can also costimulate T cells through a yet‐to‐be‐defined CD28‐independent interaction.