Dynamics of complex formation between thymocytes and thymic medullary epithelial cells.

Direct cell contact is an intrinsic part of several differentiation processes. A case in point is the formation of complexes between thymic lymphocytes and stromal cells, which are essential for T-cell maturation. The objective of the present work was to gain an insight into the mechanisms underlying the formation and dissociation of such lymphostromal complexes. Using an in vitro system, we show that the number of thymocytes adhering to a thymic medullary epithelial cell line (E-5) increases with time and reaches a plateau, after which some thymocytes spontaneously detach, while the rest remain attached to epithelium. The detached and forcibly removed thymocytes were analysed for their expression of L3T4 and Ly-2 antigens. The detached thymocytes showed a markedly lowered expression of both antigens. Both of these subtypes were shown to be totally refractory to form further complexes after a first encounter with E-5 cells. We also show that, once a mean adherence level of one thymocyte per E-5 cell was reached, (a) the level of further adherence increased exponentially, and (b) during this phase, binding of thymocytes to E-5 cells occurred in clusters, indicating pre-existing polarity or contact-induced polarization of the receptors at the E-5 cell surface.