The limited immunocompetence of thymocytes within murine thymic nurse cells.

Thymic nurse cells, cortical epithelial cells enclosing 20-200 lymphocytes, were prepared from mouse thymus by enzyme digestion and repetitive sedimentation. Individual nurse cells were then isolated free of any exogenous thymocytes by micromanipulation, and the endogeneous thymocytes released from inside the nurse cells by a brief period of culture. The thymocytes from within individual nurse cells were tested, at the one cell/well level, for their capacity to proliferate in high cloning efficiency mitogen-stimulated limiting dilution cultures. The resultant clones were tested for their cytolytic capacity in a lectin-mediated isotype-release assay. Most intra-nurse cell thymocytes were unresponsive, like typical cortical thymocytes, but an average of 1/30, or around 2-6 lymphocytes/nurse cell, were able to proliferate in response to concanavalin A. The clones produced were of a relatively small size, similar to those characteristic of helper-lineage T cells. No cytolytic clones at all were obtained, despite stringent positive controls showing an efficient cytolytic response from known sources of cytolytic precursor cells. This finding disagrees with earlier studies on nurse cell lymphocytes, where there may have been a possibility of contamination with exogenous thymocytes. These results suggest either that the nurse cell represents a selective environment for helper-lineage T cell differentiation, or that further steps after the nurse cell stage are needed to produce mature cytolytic-lineage T cells.