IDENTIFICATION OF MIC-11 ANTIGEN AS AN EPITOPE OF THE CD59 MOLECULE

The MIG 11 antigen is expressed on human cells and is characterized by reaction with a monoclonal antibody (mAb), 16.3A5. The gene controlli ng MIC 11 was recently mapped to the p13 region of chromosome 11 withi n 500 kb of the gene encoding CD59, a complement regulatory protein. T he present report investigates the antigenic relationship between thes e cell-membrane determinants and sets out evidence that MIC 11 and CD5 9 are encoded by the same gene. Western blotting of human erythrocyte membrane proteins and purified membrane CD59 showed that 16.3A5 anti-M IG 11 antibody bound to a 19-24 000 MW band with the characteristic ap pearance of CD59 protein, and gave staining patterns identical to thos e obtained with the CD59 antibody, BRIC 229. The binding of 16.3A5 mon oclonal IgG to purified urine-derived CD59 in enzyme-linked immunosorb ent assay (ELISA) was inhibited by YTH 53.1 rat CD59 antibody, indicat ing that the MIC 11 epitope is the same as, or close to, that recogniz ed by CD59 antibodies such as YTH 53.1, BRIC 229 and 2/24. Prior expos ure of erythrocytes to 16.3A5 anti-MIC 11 also reduced the ability of the CD59 antibodies, BRIC 229 and YTH 53.1: to block the complement-in hibiting function of membrane CD59. Anti-MIG 11 antibody alone, howeve r, had no inhibitory effect on CD59 function. This may be due to its r elatively low binding affinity or to some slight difference in epitope specificity, Further studies using immunofluorescence showed that the MIC 11 epitope, like CD59, is absent from EBV-B cells lacking GPI-anc hored proteins and from a B-cell line specifically deficient in CD59 p rotein. Overall, the results provide strong evidence that MIC 11 is a determinant on the CD59 molecule.