Figure 1.

Relationship of thymically and extrathymically generated regulatory cells. T cells emerge from the thymus as either regulatory cells (T_R) or conventional naive T cells (Th0). The pathological responses of autoreactive effector cells (T_E) can be suppressed by the action of both thymically (T_R) and peripherally (Tr1/Th3) generated regulatory cells. The developmental relationship between the two classes of regulatory cells still needs to be fully clarified but some evidence suggests that Th0 are able to differentiate into T_R cells in the periphery under special conditions.