CONFORMATIONAL-CHANGES IN CD45 UPON MONOCLONAL-ANTIBODY CROSS-LINKING

The CD45 protein tyrosine phosphatase is expressed in different isofor ms that result from alternative splicing of three exons (A, B, and C) encoding regions near the N-terminus of the extracellular part of the molecule, We describe here a novel epitope on the N-terminal end of CD 45 that is recognized by the MAb BL-TSub/2. Crossblocking studies show ed that BL-TSub/2 and UCHL1 (CD45RO) binding sites are partially overl apping. However, in marked contrast to the CD45RO epitope, protease tr eatment of cells strongly diminished BL-TSub/2 binding, Similar to the UCHL1 epitope, the BL-TSub/2 binding site involves carbohydrate moiet ies, since neuraminidase treatment abrogated the reactivity of the MAb , Markedly, preincubation of cells with both CD45 common and CD45RA MA b induced a pronounced increase of BL-TSub/2 binding, This latter find ing suggests that crosslinking of the CD45 molecule leads to conformat ional changes that could influence association of the molecule with pu tative ligands.