Human complement-activating immunoglobulin (Ig)G3 antibodies are essential for porcine endothelial cell activation

Complement-activating naturally occurring anti-porcine endothelial cell antibodies (Abs) are responsible for hyperacute rejection in porcine-to-primate transplantation, whereas the role of complement in acute vascular rejection, characterized by type II endothelial cell activation, is less well understood. We previously demonstrated a correlation between porcine type II endothelial cell activation, as detected by E-selectin expression, and human immunoglobulin (Ig)G3 anti-Gal alpha1-3Gal (Gal) Abs, which was not seen for IgG1, IgG2 or IgG4. The present study was undertaken to investigate whether there is a causal relationship between human anti-porcine IgG3 Abs and porcine endothelial cell activation.

IgG3 was isolated employing a Protein A column to 98.3% purity. Porcine endothelial cells were incubated with isolated human IgG3 or the combination of IgG1, IgG2 and IgG4. E-selectin expression and complement activation were investigated by flow cytometry and Western blotting, respectively.

Purified IgG3, in contrast to the other IgG subclasses, induced a substantial increase in E-selectin expression. This activation was accompanied by complement activation as detected by C3 cleavage, and was abolished by heat inactivation or by adding the complement inhibitor FUT-175. Depletion of anti-Gal Abs reduced E-selectin expression by 60%, consistent with the presence of complement-activating anti-porcine non-Gal Abs of the IgG3 subclass.

Collectively, these data strengthen the hypothesis that human anti-porcine endothelial cell Abs of the IgG3 subclass are essential for endothelial cell activation in porcine-to-human species grafts and demonstrate such activation to be partly independent of Gal epitopes.