Regulatory B cells (tBreg) are disclosed herein. These regulatory B cells express CD25 (CD25+) a pan B cell marker such as B220 (B220+), and also express CD19 (CD19+). These regulatory B cells suppress resting and activated T cells in cell contact-dependent manner. Methods for generating these regulatory B cells are also disclosed herein, as are methods for using these regulatory B cells to produce regulatory T cells (Treg). In some embodiments, methods for treating an immune-mediated disorder, such as an autoimmune disease, transplant rejection, graft-versus-host disease or inflammation, are disclosed. These methods include increasing regulatory B cell number or activity and/or by administering autologous regulatory B cells. Methods for treating cancer are also disclosed herein. These methods include decreasing regulatory B cell activity and/or number.