Conformational changes in CD45 upon monoclonal antibody crosslinking

The CD45 protein tyrosine phosphatase is expressed in different isoforms 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 CD45 that is recognized by the MAb BL-TSub/2. Crossblocking studies showed that BL-TSub/2 and UCHL1 (CD45RO) binding sites are partially overlapping. However, in marked contrast to the CD45RO epitope, protease treatment of cells strongly diminished BL-TSub/2 binding. Similar to the UCHL1 epitope, the BL-TSub/2 binding site involves carbohydrate moieties, since neuraminidase treatment abrogated the reactivity of the MAb. Markedly, preincubation of cells with both CD45 common and CD45RA MAb induced a pronounced increase of BL-TSub/2 binding. This latter finding suggests that crosslinking of the CD45 molecule leads to conformational changes that could influence association of the molecule with putative ligands.