Activation of CD3/TCR negative human thymocytes via CD28 molecule.

We have observed that the CD28 molecule was present on the cell surface of a large fraction of resting CD3- thymocytes (40 to 100%). Interestingly, the majority (greater than 90%) of surface CD3-CD28-cells reacted in the cytoplasm with anti-CD28 (CK248, 9.3) and anti-CD3 epsilon chain mAbs (Leu4, OKT3). Along this line, we found that CD28 surface expression could be induced within 18 hr on CD3-CD28- thymocytes using very low doses of phorbol-13-myristate-12-acetate (PMA). This event was accompanied by the appearance of CD25 and CD69 activation antigens but not of CD3/TCR complex. These results were further confirmed by immunoprecipitation studies. It is noteworthy that the T-cell activation pathway initiated via the CD28 molecule is functional in resting CD3- thymocytes in the presence of PMA and/or IL2. Finally, stimulation of CD3- immature thymocytes via CD28 gave rise to a large fraction (about one-third) of CD3-CD8+ cells.