CD19 and type 1 diabetes mellitus: To investigate a potential immunoregulatory defect in the B cell compartment of T1D patients, we next assessed the frequency of IL-10+ B cells by intracellular immunostaining in magnetically purified CD19+ B cells isolated from the same donors, and cultured in the presence of IL-21 for 3 days to induce the differentiation of IL-10-secreting CD19+CD27−CD24hiCD38hi B cells, as described previously 20.