Semaphorin and Semaphorin Receptor Expression in the Mouse

Semaphorins function in the nervous system as axon guidance cues and in the immune system where they modulate immune cell activation and migration in the immune system periphery. Their roles in migration led us to hypothesize that semaphorins may be important in T-cell migration and/or activation and proliferation. Studies show that thymic T-cell migration depends on differential expression of chemokines/chemokine receptors in the different thymic regions. Likewise, the differential expression of semaphorins/semaphorin receptors in the thymic medulla and thymic cortex and on various subsets of thymus cells would suggest functional roles for these molecules in T-cell development. Reverse-transcriptase (RT)-PCR and irnmunohistochemistry results respectively show that Sema4A protein; is expressed by hematopoeitic thymic cells and is found in the thymic medulla, but not the thymic cortex. The thymic medulla contains dendritic cells (DCs) not usually found in the cortex. Moreover, Sema4A is expressed by DCs in the periphery, thus suggesting that the Sema4A-expressing cells in the medulla may be DCs. Thus, thymic DCs may express Sema4A to attractSema4A receptor-expressing thymocytes and facilitate the process of antigen presentation during negative selection in the medulla