Activation of human T cells through CD6: functional effects of a novel anti-CD6 monoclonal antibody and definition of four epitopes of the CD6 glycoprotein

The CD6 glycoprotein is expressed primarily on lymphocytes andconveys co-activating signals to T cells, but its exact functionand ligand(s) are unknown. A novel mAb, termed UMCD6, was demonstratedto recognize CD6 by immunoprecipitation, Western blotting, andreactivity with COS cells transfected with CD6 cDNA. UMCD6 wasmitogenic for T cells and was strongly synergistic with phorbolester in inducing T cell activation. UMCD6 enhanced the autologousmixed lymphocyte reaction as previously observed with anotheranti-CD6 mAb, anti-T12. The activating effects of UMCD6 weremore striking than those of other anti-CD6 mAbs and encompassedall of the diverse stimulatory properties previously reportedfor other anti-CD6 reagents. However, neither UMCD6 nor otheranti-CD6 antibodies alone or in combination with phorbol esteror IL-2 were able to induce thymocytes to proliferate. Stimulationby UMCD6 is dependent on accessory cell function in a mannernot accounted for simply by antibody crosslinking. UMCD6 didnot induce an increase in cytoplasmic free Ca²⁺, but the CD6activation pathway appears to involve protein kinase C. UMCD6and a panel of seven other anti-CD6 mAbs were used in a seriesof experiments to define four discrete epitopes of CD6 usingthe criteria of antibody cross-blocking, reactivity on reducedWestern blots, and resistance to controlled V8 protease digestion.The functional mAbs UMCD6, 2H1, and antl-T12 each recognizeda different epitope. Taken together, the results of these studiesstrongly reinforce the hypothesis that CD6 plays a significantand distinct role in T cell activation, and suggest that multipleregions of CD6 may be functionally active.