Complement-dependent cytotoxicity in the analysis of antigenic determinants on human hematopoietic progenitor cells with HLA-DR as a model.

Abstract

Complement-dependent cytotoxicity (CDC) assays using anti-Ia antisera have resulted in controversial conclusions about the expression of HLA-DR determinants on human hematopoietic progenitor cells (HPC). The expression of these antigens on CFU-E in particular could often not be demonstrated. Since this is contradictory to our own results, we decided to study the influence of both the antibody and complement concentrations in CDC assays using murine monoclonal and polyclonal anti-Ia (HLA-DR "backbone") antibodies and human anti-HLA-DR typing sera. We found that with use of the same anti-Ia antibody concentrations, elimination of CFU-E required significantly more complement than CFU-GM and BFU-E. In CDC assays with antipolymorphic HLA-DR antisera, complete kill of both CFU-E and BFU-E required significantly more complement than of CFU-GM. Insufficient complement concentration could be partly overcome by increasing the antibody concentration. Intrinsic insensitivity of CFU-E to low concentrations of complement could be excluded by experiments using monoclonal anti-HLA-A/B/C "backbone" antibodies. Furthermore, FACS experiments demonstrated that the density of HLA-DR determinants on CFU-E is lower than on BFU-E and CFU-GM. These data show that in CDC assays, antigens with low expression on HPC can easily be overlooked.