Although the exact mechanisms are not yet clarified, it seems that the control of immune responses differs between thymic and non-thymic Tregs. During infections caused by enteric bacteria Tregs with high CD44 expression inhibit T cell responses indirectly by IL10 secretion. Lack of this cytokine production leads to intestinal inflammation but not to systemic auto immunity in mice[13].A different strategy is used by Tregs in lymphoid tissues; they suppress T cell responses directly in a cell contact dependent manner ,by inhibiting effect or T cell binding to DCs. Treg express high levels of cytotoxic T lymphocyte antigen 4(CTLA4) molecules on their surface, which compete with CD28-expressing effector T cells in binding CD80 or CD86 expressing DCs. Defective CTLA4 expression by Tregs causes systemic autoimmunity in mice [14].