Mannose-Binding Lectin (MBL) Substitution: Recovery of Opsonic Function In Vivo Lags behind MBL Serum Levels

Mannose-binding lectin (MBL) deficiency is often associated with an increased risk of infection or worse prognosis in immuno-compromised patients. MBL substitution in these patients might diminish these risks. We therefore performed an open, uncontrolled safety and pharmacokinetic MBL-substitution study in 12 pediatric oncology patients with chemotherapy-induced neutropenia. Twice weekly MBL infusions with plasma-derived MBL yielded MBL trough levels > 1.0 mu g/ml. We tested whether MBL substitution in vivo increased MBL-dependent complement activation and opsonophagocytosis of zymosan in vitro. Upon MBL substitution, opsonophagocytosis by control neutrophils increased significantly (p <0.001) but remained suboptimal, although repeated MBL infusions resulted in improvement over time. The MBL-dependent MBL-associated serine protease (MASP)mediated complement C3 and C4 activation also showed a suboptimal increase. To explain these results, complement activation was studied in detail. We found that in the presence of normal MASP-2 blood levels, MASP-2 activity (p <0.0001) was reduced as well as the alternative pathway of complement activation (p