Foxp3 expression was initially thought to be restricted to the CD4⁺CD25⁺ regulatory T-cell population. However, recent studies suggest that forkhead box P3 (Foxp3) is expressed in CD4⁺CD25⁻ T cells in aged mice. In the present study in B-cell non-Hodgkin lymphoma (NHL), we found that a subset of intratumoral but not peripheral blood CD4⁺CD25⁻ T cells, comprising about 15% of intratumoral CD4⁺ T cells, express Foxp3 and are capable of suppressing the proliferation of autologous infiltrating CD8⁺ T cells. In vitro activation with OKT3/anti-CD28 antibody (Ab) or dendritic cells (DCs) induced Foxp3 expression in a subset of these CD4⁺CD25⁻Foxp3⁻ T cells. We found that the presence of lymphoma B cells during activation augmented activation-induced Foxp3 expression in CD4⁺CD25⁻ T cells. We also found that CD70⁺ lymphoma B cells significantly contributed to the activation-induced Foxp3 expression in intratumoral CD4⁺CD25⁻ T cells. Furthermore, the blockade of CD27-CD70 interaction by anti-CD70 Ab abrogated lymphoma B-cell–mediated induction of Foxp3 expression in intratumoral CD4⁺CD25⁻ T cells. Taken together, these studies reveal a novel role for NHL B cells in the development of intratumoral regulatory T cells.