OPG, which as has been noted is a soluble decoy receptor for RANKL, is also expressed by mTECs 19. OPG-deficient mice exhibit an increased number of mTECs and enlarged thymic medulla containing many Aire-expressing mTECs 19. Thus, RANKL

plays a major role in promoting the proliferation of mTECs, and OPG expressed by mTECs fine-tunes the RANKL-mediated mTEC proliferation and thymic medulla formation. In addition to RANKL, two other TNFSF cytokines are known to be involved in the formation of the thymic medulla. Using transgenic mice, the effect of CD40L (CD154, TNFSF5) in the thymus was first noted in that the forced expression of CD40L induced the formation of an enlarged thymic medulla 37, 38. In those CD40L-transgenic mice, T-cell development was perturbed selleck compound and lethal wasting disease with mononuclear infiltrates accumulating in multiple organs induced 37. On the other hand, mice deficient for CD40 exhibit only a mild

decrease in mTEC cellularity 19, 20. Unlike T cells from RANKL-deficient mice, the transfer of T cells from CD40-deficient mice does not induce autoimmune symptoms in the recipient nude mice 20. Interestingly, mice deficient for both RANKL and CD40 exhibit a more severe decrease in mTEC cellularity than RANKL-deficient mice, and T cells from RANKL and CD40 doubly deficient mice induce severe autoimmune symptoms 20. Thus, like RANKL, CD40L affects the cellularity of mTECs; however, unlike the major contribution of RANKL, the involvement CHIR-99021 nmr of CD40L in mTECs and the thymic medulla is minor, although RANKL and CD40 cooperate to optimize thymic medulla formation. It has also been reported that autoantigen-specific interactions between CD4+CD8− SP thymocytes and mTECs control mature mTEC cellularity through CD40L–CD40 signals 39. The role of LT in the thymus was first noted by the

analysis of mice deficient for the LT-β receptor (LTβR, TNFRSF3). LTβR-deficient mice exhibit aberrant differentiation of mTECs and autoimmune phenomena 40, 41. LTβR Idoxuridine is expressed by both mTECs and cTECs 19, whereas LT-α (TNFSF1) and LT-β (TNFSF3), which together form the ligand for LTβR, are strongly expressed by positively selected SP thymocytes 19, 40, 42. LIGHT (CD258, TNFSF14), another ligand for LTβR, is not clearly detected by DP or SP thymocytes 19 and seems to play a minor, if any, role in mTEC development 40. LTβR regulates the Aire-independent expression of promiscuously expressed genes and chemokine genes in mTECs 43–46. A recent study has shown that the LT-LTβR interaction is involved in the terminal differentiation of mTECs to form involucrin-expressing Hassall’s corpuscle-like structures, whereas RANKL-RANK interaction regulates the initial phase of development of mTECs to become Aire-expressing mTECs 21.