Expression of activation antigens on thymocytes in the 'common thymocyte' stage of differentiation.

The authors analysed 118 monoclonal antibodies in the Activation Antigen Section of the 4th International Workshop and Conference on Human Leucocyte Differentiation Antigens for their reactivity to thymus tissue sections. Nineteen samples gave a strong labelling of cells in the subcapsular area, located in a discontinuous layer or in small foci. Positive cells also occurred incidentally in the cortex and medulla. The staining was especially prominent in thymus specimens from newborns and early childhood. This thymus staining pattern was not related to staining for activation markers in peripheral lymphoid organs or skin. After decoding and clustering, the antibodies with this thymus labelling were in various clusters including those detecting early activation antigens (CD69, Activation Inducer Molecule) and intermediate activation antigens (CD25, interleukin-2 receptor: CD30, Ki-1 group: CDw70, Ki-24 group: and CD71, transferrin receptor). But, not all antibodies in these given clusters gave thymus labelling. In two-colour analysis, cells labelled were negative for keratin and HLA-DR, and showed positivity for CD1, CD2, CD3, CD4, CD7 and CD8. This indicates that the cells are in the common thymocyte stage of intrathymic T-cell differentiation, in which the majority of thymocytes in the normal cortex locate. Activation antigen expression may be related to intrathymic processing of precursor T cells with positive selection and expansion of relevant clones after surface expression of T-cell receptors. If so, the present data indicate that this process occurs already in the subcapsular area, but first after acquisition of common cortical thymocyte surface markers.