Functional characterization of blocking antibody after human renal transplantation

The site of action and specificity of blocking IgG have now been examined in five recipients one year after transplantation, using a panel of 30 responding and stimulating cell combinations, in which lymphocytes of each specific donor were included as well. The strongest blocking occurred using IgG specific for donor lymphocytes, but blocking also occurred reproducibly with certain other cell combinations. This did not correlate with HLA antigens in common between donor cells and these other cell combinations. Blocking IgG was not cytotoxic for donor or other lymphocytes, using cytotoxicity tests of magnified sensitivity, i.e., heterologous, antihuman globulin. Moreover, recipient-blocking IgG, incubated with donor cells, did not block cytotoxic reactions using known monospecific HLA antisera directed against HLA donor determinants not present in the recipient. Binding of blocking IgG could only minimally be demonstrated by immunofluorescence, possibly due to its low density and avidity on the cell membrane, as contrasted with anti-HLA antibody (strongly positive). Therefore, blocking IgG is not directed against HLA determinants but against other determinants found thus far at random in an outbred human population, i.e., phenotypes of either special MLC genotypes closely linked with HLA subloci, or "minor" histocompatibility genotypes heretofore unable to be serologically classified.