The Epitopic Dissection of the CD2 Defined Molecule: Relationship of the Second Workshop Antibodies in Terms of Reactivities with Leukocytes, Rosette Blocking Properties, Induction of Positive Modulation of the Molecule, and Triggering T Cell Activation

Before the era of monoclonal antibodies (mAbs), several reports indicated that the T cell surface molecule responsible for rosette formation with sheep erythrocytes (E) is a 50-Kd molecule (1,2). This was confirmed and extended by obtention of mAbs directed against this molecule (3–5) and the First International Workshop on Human Leucocyte Differentiation Antigens could define the typical clusters of differentiation as CD2 [T, gp50] with mAbs 9.6, 35.1, and T111 (6). We have also described another mAb, termed D66, reactive with the same molecule, but to a distinct epitope, since we observed no cross-blocking between the binding of D66 and the binding of 9.6/T111 antibodies. Several of our observations indicated that remarkable features characterize this epitope (4): the density of D66 is high on immature and activated T cells, whereas T-PBLs display only a low density of D66 epitopes; the low density of D66 on T-PBL can be explained, to a large extent, by their coverage with sialic acid residues (as such, D66 could not be classified, strictly speaking, as a CD2 mAb since the definition of CD by the First Workshop relied both on tissue distribution and M.W. determinations). In addition, D66 would not block T cell rosettes with E pretreated with AET or neuraminidase while it would efficiently block rosettes with untreated E, either at 4 or at 37°C. Moreover we observed a cell metabolism-independent positive modulation phenomenon of the CD2 defined molecule through the D66 epitope: binding of a second Ab, even an Fab fragment, on D66 mAb can totally reverse inhibition of E-rosette formation, and evidence that this is due to emergence of initially hidden D66 epitopes was provided by cytofluorometric studies.