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Organ‐specific contribution to circulating C7 levels by the bone marrow and liver in humans

Many cells types can produce complement component C7, although the major site of C7 synthesis is unknown. Conversion from recipient to donor allotype following organ transplantation has demonstrated the synthetic sites of several complement proteins, but in the case of C7 this was not possible until recently. A novel C7 polymorphism (C7 M/N) has been described based on the reactivity with the monoclonal antibody WU 4–15 which identifies in allotype of C7 (C7 M). Bone marrow and hepatic C7 production was quantified in bone marrow transplant and liver transplant recipients, respectively, where a mismatch for the C7 allotypes distinguished by the monoclonal antibody had occurred. In the bone marrow transplant group, one informative transplant was identified and donor‐derived C7 was detected by enzyme‐linked immunosorbent assay. It contributed to 18–27% of the total circulating C7 during the post‐transplant phase and was increased during episodes of inflammation. In the liver transplant group, the hepatic contribution to the C7 levels were 30% and 52%, respectively, in two patients identified prospectively. A further three informative liver transplant patients were identified retrospectively and in these individuals, 56–62% of the circulating C7 was liver‐derived. This study demonstrates that the majority of the circulating C7 is derived from the liver and bone marrow with a lesser contribution from other sources. These findings provide further support for the concept that locally secreted complement proteins have an important role in inflammation.