Subsets of thymopoietic rat thymocytes defined by expression of the CD2 antigen and the MRC OX-22 determinant of the leukocyte-common antigen CD45.

The MRC OX-22 monoclonal antibody recognizes a restricted determinant of the rat leukocyte-common antigen (L-CA, CD45), which is expressed on most peripheral T cells and all B cells. In contrast only 2%-3% of thymocytes are OX-22+ and these are now shown to be mostly of the immature CD4-CD8- (double-negative, DN), phenotype with very few of the mature phenotype cells being OX-22+. Analysis of immunoperoxidase sections suggests that the DN OX-22+ cells are located in the cortex. Among the DN cells about 60% are OX-22+ and a similar percentage are positive for CD2 antigen. Double staining showed that OX-22+CD2-, OX-22+CD2+, and OX-22-CD2+ populations can be defined and that these three sets account for approximately 95% of the DN cells. Measurement of the thymopoietic activity of DN subsets showed that OX-22+CD2- and OX-22+CD2+ cells have regenerative capacity whilst OX-22-CD2+ cells do not.