Thymic nurse cells: a school for alloreactive and autoreactive cortical thymocytes?

The possibilities for functional analysis of thymic nurse cells (TNC) have so far been hampered by the low yield of these cells by means of gradient centrifugation and their fragility preventing purification by a fluorescence-activated cell sorter. Furthermore, they are extremely unstable in cell culture. Here we present functional data obtained by the analysis of single TNC in an avian system: the chicken with its extramaternal embryonic development prompted us to use the chorioallantoic membrane (CAM) pock formation assay to investigate the possible graft-vs.-host reactivity (GVHR) of TNC lymphocytes (TNC-L). The CAM assay proved to be an optimal and unique "cell culture" system for the investigation of the functional characteristics of TNC. Our data show that TNC-L, unexpectedly, are able to exert a GVHR on allogeneic CAM with the very high efficiency of 1/13 as compared to thymocytes of the same donors showing a GVHR efficiency of 1/350. TNC-L are even able to induce GVH-like reactions in syngeneic combinations, while thymocytes and peripheral blood lymphocytes from the same donors always remain negative.