Peripheral T cells in the thymus: have they just lost their way or do they do something?

In young adult mice, the thymus produces about a million newly formed T cells every day that colonize peripheral lymphoid tissues. Mostly regarded as a primary lymphoid organ only, the relationship between the thymus and peripheral lymphoid organs is considered unidirectional. However, this perception has been challenged by reports showing that peripheral lymphocytes, mostly T cells, can migrate back into the thymus. The presence of recirculating T cells in the thymus is rather incongruous and raises the question: is the presence of 'peripheral' T cells in the thymus superfluous or do these cells fulfill some relevant physiologic functions? There is now evidence that cells of the hematopoietic lineage, including T cells, can play an active role during thymocyte selection, a role generally considered the exclusive property of thymic epithelial cells and dendritic cells. Although, on a per cell basis, peripheral T cells in the thymus may be less efficient than thymus epithelial cells or dendritic cells at thymocyte positive and negative selection, they may nevertheless contribute to selection by influencing the selectable TCR repertoire and post-selection T cell functionality. Here, peripheral lymphocytes re-entering the thymus may be envisioned as Trojan horses as these cells may introduce antigens necessary for both positive and negative selection of T cells.