Loss of thymus cellularity is a common feature among inflammatory/infectious processes [24]. Moreover, it has been reported that when the cellularity of this organ is compromised, the number of peripheral cells infiltrating into the thymus considerably increases [4, 6, 18, 19]. Then, we speculated that available space could represent a crucial situation for cell migration to the thymus in inflammatory conditions. To test this hypothesis, we examined T. cruzi infected mice at two different times: before the parasitemia peak (BPP, between 9 and 11 days postinfection), where the part of resident thymocytes (especially double positive (DP) cells) are depleted, and during the parasitemia

peak (PP, between 12 and 14 days postinfection), when a larger number of thymocytes are depleted (Fig. 2A). As CD4+ and CD8+

cells are found selleck compound in the thymus as single positive thymocytes, it is difficult to discriminate between resident Fulvestrant and peripheral mature T cells; however, we and others have demonstrated that expression of CD44, an activation marker for T cells is preferentially expressed by mature T cells that enter the thymus [7, 17, 25] (Fig. 3A). Thus, we evaluated the percentage and the absolute number of CD44hi T cells present in the thymi of T. cruzi infected mice. As shown in Fig. 2, the percentage (Fig. 2B) as well as the absolute number (Fig. 2C) of CD44hi cells in the CD4+ or CD8+ single positive compartment significantly increase when the total cellularity of the thymus decreases (Fig. 2A) (compare Phospholipase D1 BPP and PP). Based on the high percentages of CFSE+ CD19+ cells that enter the thymus in the three inflammatory conditions evaluated (Fig. 1), we also analyzed the absolute number of B cells in the thymi of control or T. cruzi infected mice. Both the percentage and the absolute number of B cells increased (Fig. 2D) with the reduction in the cellularity of the organ (Fig. 2A). Interestingly, the kinetics of cell entry to the thymus varies depending upon the inflammatory/infection process being evaluated (after 3 days of LPS treatment, around

days 12–14 in T. cruzi infected mice and around days 6–7 in C. albicans infected mice). However, what they all have in common is the fact that cells enter the thymus when cellularity of this organ starts to diminish. Based on the later data, we speculated that any situation where the total thymocyte number is reduced would favor the entrance of peripheral cells to the thymus. To prove this hypothesis, we treated mice with dexamethasone (Dex) since it has been demonstrated that this hormone considerably decreases the cellularity of the thymus [26, 27]. We adoptively transferred CFSE splenocytes from LPS-treated mice into LPS- or Dex-treated recipient mice [26]. Even though the total cell number of thymocytes is highly diminished in both LPS- and Dex-treated mice, peripheral cells could enter the thymus only in LPS-treated mice (Fig. 2E).