Antibody MT3 is reactive with a novel exon B-associated 190-kDa sialic acid-dependent epitope of the leukocyte common antigen complex.

MT3 is a new antibody reactive with a restricted isoform of the leukocyte common Ag (CD45) family. The Ag is mainly expressed on T lymphocytes and thymocytes. It is differentially expressed on B cell subpopulations, with no staining of the majority of small follicular mantle zone B cells but positive staining of the majority of marginal zone B cells of spleen. Like most CD45 antibodies, reactivity can be demonstrated in fresh frozen as well as in formalin-fixed, paraffin-embedded tissues. This reactivity clearly differs from all other published anti-CD45 antibodies. In immunoprecipitation and Western blot procedures, the antibody reacts with a major band with a molecular mass of 190 kDa and weak bands with molecular masses of 205 and 220 kDa. Compared to antibody PD7 that reacts with exon B-encoded sequences, the reactivity with the 205 and 220 bands is much weaker. This is reflected in MT3 reactivity with leukocyte common Ag transfectants that include exon B-encoded sequences, such as AB and B, but not with those that also include C-encoded sequences, such as ABC or BC. It can be concluded that MT3 recognizes additional heterogeneity in the leukocyte common Ag complex, that is based on the differential expression of sialic acid-dependent determinants associated with exon B-encoded sequences.