The Therapeutic Potential of Complement Inhibitors in Xenoreactive Human Complement-Dependent Cytotoxicity in Vitro

Complement activation is critical in xenograft rejection, but the relative efficacy of pharmacological complement inhibitors remains unclear.

Peripheral blood mononuclear cells (PBMCs) from wild-type (WT) pigs were exposed to human serum in the presence or absence of complement inhibitors. Complement-dependent cytotoxicity (CDC) and complement deposition were measured by flow cytometry. A C1s inhibitor (sutimlimab, 12.5-800 µg/mL), a C1-esterase inhibitor (berinert, 1.25-10U/mL), a C3/C3b inhibitor (pegcetacoplan, 0.25-4.0 mg/mL), and a C5 inhibitor (tesidolumab, 500-800 µg/mL) were applied to dose-response assays.

Only pegcetacoplan (C3/C3b inhibitor) achieved complete inhibition of CDC with minimal C3b/iC3b and C5b-9 deposition. Sutimlimab (C1s inhibitor) produced partial inhibition of CDC (hitting a plateau) and complement deposition, while C1-esterase inhibitor had limited CDC effect and did not reduce C3b/iC3b. Tesidolumab (C5 inhibitor) suppressed C5b-9 but failed to prevent CDC or upstream opsonization.

Although these results might reflect experiments conducted at concentrations lower than clinical doses, our data suggest that, of the limited number of agents tested, pegcetacoplan may be the most effective in inhibiting xenoreactive human complement activation in this experimental setting (i.e., CDC of human complement in the pooled-serum against WT pig PBMCs). However, the results we obtained are puzzling because the C5 inhibitor, eculizumab, has been proven to be highly effective in inhibiting human complement activation in the in vitro CDC model. Furthermore, tesidolumab was reported to be effective in an in vivo NHP model. To obtain the additional information required for clinical application, further investigation would be needed. We would need to (i) ensure that the tesidolumab we were using had not deteriorated in any way and lost its efficacy, and (ii) compared pegcetacoplan with other C5 inhibitors (e.g., ravulizumab, eculizumab).