Comment on Jacquemin et al, page 515
Many years of study have been devoted to the seemingly simple question of where coagulation factor VIII is made, but the question has never been completely answered. There remains more to know about the origin of coagulation factor VIII in the circulation. Jacquemin and colleagues have shown in this issue of Blood that factor VIII is synthesized in the lung microvascular endothelium.
Liver transplantation in human patients with hemophilia A restores normal factor VIII levels.1 Reciprocal organ transplantation studies between normal dogs and those with hemophilia A have shown that the liver is the primary (but not the only) source of factor VIII, however.2 It has been shown in mice that hepatocytes and sinusoidal endothelial cells are the cellular sites of synthesis in the liver.3 However, hepatocyte-specific toxins eliminate almost all factor VIII from the circulation in this species.4 Northern blot analysis of factor VIII expression is technically difficult (since the transcript is large and not very abundant) and has yielded inconclusive results. More sensitive methods to detect factor VIII transcripts (eg, reverse transcriptase–polymerase chain reaction [RT-PCR]) have shown factor VIII mRNA in liver, spleen, and peripheral lymphocytes in humans and other species.4-7 Furthermore, in humans it is known that factor VIII levels remain normal or even elevated in patients with liver disease, even when other coagulation factors are greatly diminished. All of these findings point to hepatic as well as extrahepatic synthesis of factor VIII.
Jacquemin and colleagues have shown in this issue of Blood that factor VIII is synthesized in the lung microvascular endothelium. They did so by ventilating and perfusing human cadaveric lungs with a synthetic medium and measuring levels of factor VIII and other proteins during the course of perfusion. Significant amounts of factor VIII (and von Willebrand factor) appeared in the perfusate while factor V, fibrinogen, and immunoglobulin G (IgG) remained unchanged for the duration of the experiments. In vitro studies were performed on conditioned medium from cultivation of fibroblasts, epithelial cells, macrophages, smooth muscle cells, and microvascular endothelial cells. Factor VIII synthesis was observed only in the endothelial cells. The rate of factor VIII synthesis by these endothelial cells in vitro could be extrapolated to account for about 20% of the human factor VIII produced in vivo. Thus, the pulmonary endothelium makes a significant contribution to the factor VIII found in the circulation.
This kind of study could be profitably applied to other organs, and it would not be surprising if factor VIII synthesis could be demonstrated in other nonhepatic sites. Perhaps more of this work will appear in the pages of Blood, where much of this story has already been told.
This work is the opinion of the author and does not constitute US government policy. ▪