A subset of CD4+ T cells, the CD4+ CD25+ regulatory T (Treg) cells in the lymphoid organs and peripheral blood are known to possess suppressive function. Previous in vitro and in vivo studies have indicated that T cell receptor (TCR) signal is required for development of such 'natural regulatory (Treg) cells' and for activation of the effector function of CD4+ CD25+ regulatory T cells. CD5 is a cell surface molecule present on all T cells and a subtype of B lymphocytes, the B-1 cells, primarily localized to coelomic cavities, Peyer's patches, tonsils and spleen. CD5 acts as a negative regulator of T cell and B cell signaling via recruitment of SHP-1. Here, we demonstrate that Treg cells obtained from CD5-/- mice are more potent than those from wild type mice in suppressing the in vitro cell proliferation of anti-CD3 stimulated CD4+ CD25- responder T cells. This phenomenon was cell contact and GITR dependent. Lack of CD5 expression on Treg cells (from spleen, lymph node and thymus) did not affect the intracellular levels of Foxp3. However, CD5-/- Treg thymocytes were able to elicit a higher Ca2+ response to TCR + co-stimulatory signals than the wild type cells. CD5-/- mice expressed more Foxp3 mRNA in the colon than wild type mice, and additionally, the severity of the dextran sulfate sodium (DSS)-induced colitis in CD5-/- mice was less than the wild type strain. We suggest that manipulation of CD5 expression or the downstream signaling components of CD4+ CD25+ Treg cells as a potential strategy for therapeutic intervention in cases of auto-immune disorders. © 2008 Elsevier B.V. All rights reserved.