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T cell Ig domain and mucin domain protein 1 (TIM-1) is a costimulatory molecule that regulates immune responses by modulating CD[4.sup.+] T cell effector differentiation. However, the function of TIM-1 on other immune cell populations is unknown. Here, we show that in vivo in mice, TIM-1 is predominantly expressed on B rather than T cells. Importantly, TIM-1 was expressed by a large majority of IL-10-expressing regulatory B cells in all major B cell subpopulations, including transitional, marginal zone, and follicular B cells, as well as the B cell population characterized as CD1[d.sup.hi]CD[5.sup.+]. A low-affinity TIM-1-specific antibody that normally promotes tolerance in mice, actually accelerated (T cell-mediated) immune responsiveness in the absence of B cells. TIM-[1.sup.+] B cells were highly enriched for IL-4 and IL-10 expression, promoted Th2 responses, and could directly transfer allograft tolerance. Both cytokine expression and number of TIM-[1.sup.+] regulatory B cells (Bregs) were induced by TIM-1-specific antibody, and this was dependent on IL-4 signaling. Thus, TIM-1 is an inclusive marker for IL-[10.sup.+] Bregs that can be induced by TIM-1 ligation. These findings suggest that TIM-1 may be a novel therapeutic target for modulating the immune response and provide insight into the signals involved in the generation and induction of Bregs., Introduction T cell Ig domain and mucin domain (TIM) proteins constitute a family of costimulatory molecules that play an important role in effector differentiation of CD[4.sup.+] cells (1). The 8 [...]