Characterizing Thymocyte Migration in the Medulla Using Two-Photon Microscopy (138.20)

Abstract To establish tolerance, a developing thymocyte must be screened for self-reactivity before exiting the thymus. However, the mechanism by which a thymocyte searches the medulla and antigen-presenting cells for self-antigens is not clear. We have developed a cut thymic lobe preparation that allows us to document thymocyte migration within the medulla by two-photon laser scanning microscopy. Medullary thymocytes migrate rapidly through confined random walk making short and dynamic contacts with dendritic cells. In response to negative selection signals, thymocyte migration is significantly slower and constrained to smaller areas. In addition, self-reactive thymocytes engage in long lasting contacts with dendritic cells. Polyclonal medullary thymocytes consist of a small but significant population of relatively slow thymocytes with more constrained migration that may correspond to auto-reactive thymocytes undergoing negative selection. Taken together, our data defines scanning strategies used by medullary thymocytes during negative selection and suggests that a significant proportion of medullary thymocytes are actively receiving negative selecting signals.