Because most patients with cancer are aged and because immunological functions are altered during aging, it is important to account for aging-associated immunological alterations in the design of new cancer immunotherapies. We thus compared immune populations in young and aged mice and found that B7-DC+ (PD-L2/CD273) B cells, a minor population in young mice, were significantly increased in aged mice. Induction of both Th1 and Th17 cells was significantly augmented by B7-DC+ B cells from aged mice, and this effect was blocked with anti-B7-DC antibodies in vitro and in vivo. Moreover, retardation of tumor growth in aged mice was largely B7-DC dependent. Tumor growth in young mice was significantly inhibited by immunization with B7-DC+ B cells from aged mice owing to increased induction of tumor antigen-specific cytotoxic T lymphocytes. These data indicate that B7-DC+ B cells could play an important role in aging-associated cancer immunopathology as well as in other aging-associated diseases and further suggest that B7-DC+ B cells have potential for future cancer immunotherapy.