Thymocyte-dendritic cell interactions near sources of CCR7 ligands in the thymic cortex.

Little is known about the dynamics of the interactions between thymocytes and other cell types, as well as the spatiotemporal distribution of thymocytes during positive selection in the microenvironment of the cortex. We used two-photon laser scanning microscopy of the mouse thymus to visualize thymocytes and dendritic cells (DCs) and to characterize their interactions in the cortex. We show that thymocytes make frequent contacts with DCs in the thymic cortex and that these associations increase when thymocytes express T cell receptors that mediate positive selection. We also show that cortical DCs and the chemokine CCL21 expression are closely associated with capillaries throughout the cortex. The overexpression of the chemokine receptor CCR7 in thymocytes results in an increase in DC-thymocyte interactions, while the loss of CCR7 in the background of a positive-selecting TCR reduces the extent of DC-thymocyte interactions. These observations identify a vasculature-associated microenvironment within the thymic cortex that promotes interactions between DCs and thymocytes that are receiving positive selection signals.