Differential expression and regulation of CD6 on T-cell subsets revealed by monoclonal antibody (MAb) CH11

A monoclonal antibody (MAb), CH11, was developed by immunizing mice with CD4⁺ γ δ T-cell receptor (TCR)⁺ cells. It recognized an antigen expressed in the surface membrane of T-cell lines, but not of U937, lymphoblastoid B cells (LBC), K562, Raji or Daudi cells, indicating selectivity for the T-cell lineage. In addition, it labelled 70-80% of normal peripheral blood mononuclear cells (PBMC), with high expression on the erythrocyte rosetting (E⁺) fraction, and low/absent expression on E^- cells. However, CD4⁺ T cells expressed higher levels of reactivity than CD8⁺ or γ δ⁺ T-cell receptor (TCR)⁺ lymphocytes in PB. Furthermore, in 7 of 10 individuals tested, 7.34 ± 63.88% of unselected PBMC were CH11^- CD3⁺ and were relatively enriched in CD8⁺ and in γ δ TCR⁺ -cells. In addition, thymic γ δ T cells, and γ δ lymphoproliferations from two patients were nonreactive or weakly reactive with the MAb. Activation of E⁺ cells with phorbol-12-myristate-13-acetate (PMA) enhanced CH11 expression uniformly, whereas activation with phytohemagglutinin (PHA) selectively down-regulated expression of the antigen on the CD8⁺ subset. In Western blots performed in nonreducing (NR) conditions, MAb CH11 detected a 100 kDa molecule in PBMC and Jurkat T-cell lysates. Preincubation of T cells with MAb CH11 specifically abrogated their subsequent reactivity with MAb to CD6, suggesting that MAb CH11 is recognizing an epitope of CD6. Given its function as a receptor for ligands on thymic epithelium, activated leukocytes and synoviocytes, this newly defined heterogeneity of expression and regulation of the CD6 molecule on subsets of T cells may help determine their functional repertoirein vivo.