Pig complement regulator factor H: molecular cloning and functional characterization.

Complement is an efficient defense mechanism of innate immunity. Factor H is the central complement regulator of the alternative pathway, acting in the fluid-phase and on self surfaces. Pigs are considered a suitable source for xenotransplantation and thus several membrane-bound pig complement regulators with importance for the acute rejection phase have been investigated. However, pig fluid-phase regulators have not been described so far. We report the cloning, expression and functional characterization of pig factor H. After constructing a pig liver cDNA library, a full-length factor H cDNA was isolated and sequenced. The predicted protein is organized in 20 short consensus repeat (SCR) domains and has an overall identity of 62% to the human protein. For functional characterization, three deletion constructs of pig factor H were expressed in insect cells. Pig factor H construct SCR 1-4 has cofactor activity for factor I-mediated cleavage of human C3b, which is similar to the human regulator. In addition, this N-terminal construct binds to human C3b, while a construct consisting of SCR 15-20 showed a weaker binding to human C3b/C3d. Pig factor H has two major binding sites for heparin, as the two constructs representing SCR 1-7 and SCR 15-20 proteins, but not the SCR 1-4 protein, bind heparin. The C-terminal construct is able to bind to human endothelial cells, as assayed by FACS. We show that pig and human factor H share functional characteristics in complement regulation and cell surface binding. Possible consequences of using pig livers for xenotransplantation are discussed.

The molecular basis for hereditary porcine membranoproliferative glomerulonephritis type II: point mutations in the factor H coding sequence block protein secretion.

Porcine membranoproliferative glomerulonephritis type II in piglets of the Norwegian Yorkshire breed is considered the first animal model of human dense deposit disease. Porcine dense deposit disease is caused by the absence of the complement regulator factor H in plasma. Here we report the molecular basis for this absence. Single nucleotide exchanges at position C1590G and T3610G in the coding region of the factor H gene result in amino acid exchanges at nonframework residues L493V and I1166R that are located within SCR 9 and SCR 20, respectively. Apparently the L493V mutation represents a polymorphism whereas the I1166R causes the physiological consequences a block in protein secretion. Expression analysis shows comparable mRNA levels for factor H in liver tissue derived from both affected and healthy animals. In affected piglets, factor H protein is detected in increased amounts in liver cells. Factor H accumulates inside the hepatocytes and is not released as shown by Western blot analysis and immunohistochemistry. These data demonstrate that single amino acid exchanges of two nonframework amino acids either alone or in combination block protein secretion of factor H. This observation is also of interest for other human diseases in which factor H is involved, such as human factor H-associated form of hemolytic uremic syndrome.