Everett LA, Cleuren ACA, Khoriaty RN, et al. Murine coagulation factor VIII is synthesized in endothelial cells. Blood. 2014;123:3697-3705.

Fahs SA, Hille MT, Shi Q, et al A conditional knockout mouse model reveals endothelial cells as the principal and possibly exclusive source of plasma factor VIII. Blood. 2014;123:3706-3713.

As a medical student in 1974, I read in the 7th Edition of Wintrobe’s Clinical Hematology that the sites of production of factor VIII (FVIII) had yet to be defined.1  Roles for the liver, spleen, and reticuloendothelial system had been proposed. Transplantation of a normal liver into a hemophilic dog resulted in a 50 percent increase in FVIII, while transplanting a hemophiliac liver into a normal dog decreased the FVIII by 50 percent.2,3  These experiments suggested that extrahepatic cells also produce FVIII. In the 1980s, immunohistochemical assays showed FVIII antigen in guinea pig liver and spleen. FVIII antigen could be found in normal human liver sinusoidal endothelial cells, as well as in extracts of human lymph nodes, lung, liver, and spleen.4  The localization of antigen in tissues did not, however, distinguish sites of FVIII synthesis from those of storage, and such experiments were subject to misinterpretation due to entrapment of plasma FVIII in tissues. FVIII mRNA was detected in isolated human hepatocyte-rich extracts, as well as in the spleen, the lymph nodes, and the kidney, but not in white blood cells or cultured endothelial cells. More recently, cell sorting studies of microvascular endothelial cells have documented FVIII production.5  Von Willebrand Factor, which circulates with FVIII, is known to be synthesized in endothelial cells and megakaryocytes, but definitive proof of the endothelial origin of FVIII in vivo was lacking. Now, however, the results of two elegant studies published in Blood, one from David Ginsburg’s laboratory at the University of Michigan and the other from Robert Montgomery’s laboratory at the Blood Center of Wisconsin, demonstrate that endothelial cells are indeed the primary source of plasma FVIII.

The Michigan group took advantage of the special function of LMAN1, a cargo receptor that is responsible for efficient secretion of factors V and VIII into plasma. Lman1 mutations in mice reduced plasma concentrations of both factors to 10 to 15 percent of normal. Using Lman1 conditional murine knockouts, the investigators demonstrated that endothelial cells were the primary site of FVIII biosynthesis, while hepatocytes made no significant contributions to the FVIII plasma pool. Confirmatory studies using endothelial-specific mRNA isolation and PCR showed high level expression of FVIII by endothelial cells from multiple tissues.

The Wisconsin group utilized a Cre/lox-dependent conditional knockout mouse model in which exons 17 and 18 of FVIII are flanked by loxP sites, and when Cre-recombinase is expressed, the FVIII gene is inactivated. By breeding these mice with various tissue-specific Cre strains, hepatocyte-specific FVIII-knockout animals had similar FVIII levels as control mice, while endothelial-knockout mice had a severe hemophiliac phenotype with no detectable FVIII. And bone marrow transplant studies showed that hematopoietic cells do not contribute to FVIII production.

These investigators are to be congratulated for solving a long-standing mystery that I first learned of while reading Wintrobe 40 years ago. Upon reflection, it does make sense teleologically and clinically to have both FVIII and von Willebrand factor synthesized in the same cell. In different types of vascular beds, there is marked heterogeneity of expression of various hemostatic proteins, including von Willebrand factor. In certain vascular beds, it would be advantageous to have FVIII synthesized together with von Willebrand factor to assure prompt hemostasis. The findings of Dr. Everett and colleagues and Dr. Fahs and colleagues may also explain why FVIII levels are elevated in liver disease despite compromised synthetic function of other clotting factors.  Further, FVIII levels are elevated in pregnancy, suggesting that FVIII derived from placental endothelial cells may have a role in pre-eclampsia. Finally, by transducing blood origin endothelial cells with FVIII expressing vectors, gene therapy for hemophilia could target endothelial cells rather than hepatocytes.6 

1.
1.Wintrobe WM, Lee GR, Boggs DR, et al.
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Marchioro TL, Hougie C, Ragde H, et al.
Hemophilia: role of organ homografts.
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http://www.ncbi.nlm.nih.gov/pubmed/4930132
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Webster WP, Zukoski CF, Hutchin P, et al.
Plasma factor VIII synthesis and control as revealed by canine organ transplantation.
Am J Physiol.
1971;220:1147-1154.
http://www.ncbi.nlm.nih.gov/pubmed/4930132
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Wion KL, Kelly D, Summerfield JA, et al.
Distribution of factor VIII mRNA and antigen in human liver and other tissues.
Nature.
1985;317:726-729.
http://www.ncbi.nlm.nih.gov/pubmed/3932885
5.
5.Jacquemin M, Neyrinck A, Hermanns MI, et al.
FVIII production by human lung microvascular endothelial cells.
Blood.
2006;108:515-517.
http://bloodjournal.hematologylibrary.org/content/108/2/515.long?sso-checked=1
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6.Lin Y, Chang L, Solovey A, et al.
Use of blood outgrowth endothelial cells for gene therapy for hemophilia A.
Blood.
2002;99:457-462.
http://bloodjournal.hematologylibrary.org/content/99/2/457.long

Competing Interests

Dr. Vercellotti indicated no relevant conflicts of interest.