We developed a monoclonal antibody, 1F5, which sensitizes human erythrocytes for hemolysis by homologous human complement. The antigenic molecule was identified to be HRF20 (20 kDa homologous restriction factor) which inhibit the reaction of C8 and C9 to form a membrane attack complex on C5b67, the trimolecular intermediate complement complex on cell membrane.
HRF20 was found to be deficient on the abnormal erythrocytes of patients suffering from paroxysmal nocturnal hemoglobinuria (PNH). Adsorption of purified HRF20 on to the abnormal PNH erythrocytes reconstituted their ability to resist against homologous complement attack. We encountered to find a patient who is hereditary-deficient in HRF20 expression. Analysis of HRF20 gene of the patient revealed that one base deletion was found at 2 sites of the patient's HRF20 gene.
Immunohistochemical analysis revealed that HRF20 is expressed not only on blood cells but also on endothelial cells, kidney cells, nerve Schwann cells, etc. HRF20 is also significantly expressed on fetal cells indicating that it would be playing a role to protect fetal, cells including nerve cell from membrane damage by complement.
We also found that HRF20 expression was decreased on CD8 bright T lymphocytes from HIV-infected patients even before appearance of AIDS or ARC symptoms.