This effect is dependent on,

but not exclusive of, the available space in the thymus. Our data also demonstrate that MCP-1/CCR2 (where MCP-1 is monocyte chemoattractant protein-1) interaction is responsible for the infiltration of peripheral cells to the thymus in these Th1-inflammatory/infectious situations. Finally, systemic expression of IL-12 and IL-18 produced during the inflammatory process is ultimately responsible for these migratory events. The thymus is the primary source of T cells for peripheral lymphoid organs. T cells MK2206 produced in the thymus migrate to the spleen and lymph nodes (LNs), especially early in life. The reverse pathway, that is, mature T cells migrating from the periphery back into the thymus is less often considered although some studies have shown that this is a common pathway in healthy animals [1-5]. Moreover, it has been suggested that this pathway might preferentially be used by activated T cells [4, 6-8]. For example, it was shown that activated T cells homed to the thymus, and ATM inhibitor represented approximately 0.4% of mature T thymocytes [6]. Others have shown that, as compared with naive CD4+

T cells, there is a preferential accumulation of antigen-experienced T cells in the rat thymus [9]. Interestingly, the rate of homing was greatly increased when thymocyte depletion occurred after host irradiation [6]. In any case, Liothyronine Sodium accumulation of peripheral T cells within the thymus is largely restricted to the medulla [6,

10]. Although a small number of mature B cells can be found in a healthy thymus, the migration of peripheral B cells to the thymic medulla could increase several fold in certain pathological situations such as thymic lymphoma [11] and certain autoimmune diseases murine models [12]. The functional consequences of cellular migration of both T and B cells back to the thymus have been addressed by several investigators. For example, it has been proposed that B cells enter the thymus in order to achieve T-cell tolerance to immunoglobulins and to other B-cell-specific antigens [13]. Moreover, it has also been proposed that B cells found in the thymus could participate in negative selection by acting as Ag-presenting cells [14]. As for T cells, it has been proposed that the thymus can function as a repository of memory T cells [15], while others have demonstrated an important role of peripheral mature T cells in central tolerance during the processes of positive and negative selection in the thymus [10, 16]. It has also been proposed that migrating lymphocytes can participate in transplantation tolerance [17] and that mature T cells in the thymus are important in maintaining medullary epithelial cells [18]. Whereas naïve syngeneic T cells preferentially home to the peripheral lymphoid organs, they rarely reenter the thymus.