MLC and CML studies in the family of a pair of HLA haploidentical chimeric twins.

A pair of dizygotic twins of opposite sex were incidentally found to be erythrocyte chimeras. The chimerism was found to include the lymphocyte population as well, since the twins differed for the maternal while sharing one paternal HLA haplotype. In addition to lymphocytes carrying antigens controlled by the inherited maternal haplotype, each of the twins had a small fraction--about 15%--of lymphocytes of the other twin's type. Both twins are healthy and have normal children. Investigations of the family by mixed lymphocyte culture (MLC) and cell-mediated lympholysis (CML) tests showed that the two haplotypes by which the twins differed carried different HLA-D determinants; even so the lymphocytes of each twin were completely nonresponsive in MLC and CML to those of the other twin. No 'blocking' factors could be demonstrated in the serum of the twins. These results support Burnet's theory of clonal selection in fetal life.