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Accumulating evidence suggests that regulatory T cells play a crucial role in preventing autoimmunity. Recently, a naturally occurring CD4+CD25+ T-cell subset that is anergic and also suppressive has been shown to suppress autoimmunity in several animal models. We used proteoglycan-induced arthritis (PGIA) as a study model to investigate the role of the CD4+CD25+ regulatory T cells in autoimmune arthritis. There was no significant change in the percentage of CD4+CD25+ T cells during the immunization period when proteoglycan- or ovalbumin-immunized BALB/c and C57BL/6 mice were compared. An adoptive transfer study showed that the CD4+CD25+ T cells did not protect severe combined immunodeficient mice from arthritis when they were cotransferred with splenocytes from arthritic animals. Similarly, depletion of the CD4+CD25+ T cells did not enhance the onset of the disease or disease severity in severe combined immunodeficient mice. Moreover, CD28-deficient mice, which have very few CD4+CD25+ T cells, were highly resistant to PGIA. These findings indicate that the CD4+CD25+ regulatory T cells may not play a critical role in controlling PGIA.