Monoclonal antibodies, capable of detecting monomorphic epitopes on HLA class I polypeptides and beta-microglobulin (β2-M), have been used by a variety of techniques to ascertain the type of structure detected on red blood cells (RBCs). Hemagglutination with class I monoclonal antibodies confirmed the reported relationship between Bg blood groups and HLA. It also established that the expression of HLA on RBCs which do not have nuclei is not normally strong, but may be enhanced in patients, notably those with systemic lupus erythematosus (SLE). Estimates of the number of class I molecules on mature RBCs by a radioligand-binding assay have confirmed that all HLA-B7 (Bga) individuals have higher numbers but that SLE patients usually have the most (1240/RBC). Class I polypeptides were not elevated in the plasma of SLE patients and all RBCs lost molecules on aging in the circulation. These two facts suggest that HLA on RBCs is not acquired from plasma. When RBCs from SLE patients were immunoblotted with monoclonal antibodies, a complete 45 kDa intrinsic transmembrane heavy chain of HLA class I and a light chain of 11 kDa (β2-M) were detected. Chloroquine treatment and acid elution of RBCs did not remove HLA class I but only β2-M. As most antibodies recognize epitopes that depend on close association of class I with β2-M, the lost reactivity of treated RBCs may be understood.
