Mannan-binding lectin exacerbates the severity of psoriasis by promoting plasmacytoid dendritic cell differentiation via the signal transducer and activator of transcription 3-interferon regulatory factor 8 axis.

Psoriasis is a chronic inflammatory skin disease mediated by host immune responses. Plasmacytoid dendritic cells (pDC) and interferon (IFN)-α secreted by pDC are involved in the initiation of psoriasis. Mannan-binding lectin (MBL), a vital component of the complement pathway, plays a critical role in innate immune defense and the inflammatory response. Our previous study found that MBL could exacerbate skin inflammation in psoriatic mice, but the effect of MBL on pDC remains unstudied. Herein, we revealed that the circulating level of MBL was elevated in patients with psoriasis compared with the healthy controls. Moreover, the MBL level was positively correlated with disease severity, relative inflammatory cytokine levels, and peripheral blood (PB) pDC frequency in psoriasis. An in vitro study determined that the MBL protein could promote the differentiation of human pDC and upregulate the production of relative inflammatory cytokines and chemokines. Additionally, MBL-deficient (MBL-/- ) mice exhibited decreased accumulation of pDC in lymph nodes, spleens, and skin lesions with reduced secretion of pDC-related cytokines compared with wild-type (WT) mice in the preliminary stage of psoriasis induced by imiquimod. Notably, the differentiation of pDC from bone marrow (BM) cells derived from MBL-/- mice was weakened compared with that from WT mice upon Fms-like tyrosine kinase 3 ligand (Flt3L) incubation. Mechanistic research indicated that the signal transducer and activator of transcription 3 (STAT3)-interferon regulatory factor 8 (IRF8) axis was responsible for MBL-modulated pDC differentiation. In summary, these results suggest that MBL exacerbates the severity of psoriasis by enhancing pDC differentiation and pDC-related cytokine secretion via the STAT3-IRF8 axis, thus providing a new target for psoriasis treatment.

Mannan-binding lectin mediates renal ischemia/reperfusion injury independent of complement activation.

Ischemia/reperfusion injury (IRI) remains a major problem in renal transplantation. Clinical studies have identified that high serum levels of Mannan-binding lectin (MBL), the initiator of the lectin pathway of complement activation, are associated with inferior renal allograft survival. Using a rat model, we identified an entirely novel role for MBL in mediating renal IRI. Therapeutic inhibition of MBL was protective against kidney dysfunction, tubular damage, neutrophil and macrophage accumulation, and expression of proinflammatory cytokines and chemokines. Following reperfusion, exposure of tubular epithelial cells to circulation-derived MBL resulted in internalization of MBL followed by the rapid induction of tubular epithelial cell death. Interestingly, this MBL-mediated tubular injury was completely independent of complement activation since attenuation of complement activation was not protective against renal IRI. Our identification that MBL-mediated cell death precedes complement activation strongly suggests that exposure of epithelial cells to MBL immediately following reperfusion is the primary culprit of tubular injury. In addition, also human tubular epithelial cells in vitro were shown to be susceptible to the cytotoxic effect of human MBL. Taken together, these data reveal a crucial role for MBL in the early pathophysiology of renal IRI and identify MBL as a novel therapeutic target in kidney transplantation.

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 immunocompromised 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 microg/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 < 0.05). This MBL-substitution study demonstrates that plasma-derived MBL infusions increase MBL/MASP-mediated C3 and C4 activation and opsonophagocytosis, but that higher circulating levels of plasma-derived MBL are required to achieve MBL-mediated complement activation comparable to healthy controls. Other patient cohorts should be considered to demonstrate clinical efficacy in phase II/III MBL-substitution studies, because we found a suboptimal recovery of (in vitro) biological activity upon MBL substitution in our neutropenic pediatric oncology cohort.

Safety and pharmacokinetics of plasma-derived mannose-binding lectin (MBL) substitution in children with chemotherapy-induced neutropaenia.

Mannose-binding lectin (MBL)-deficient children with cancer may benefit from substitution of the innate immune protein MBL during chemotherapy-induced neutropaenia. We determined the safety and pharmacokinetics of MBL substitution in a phase II study in MBL-deficient children. Twelve MBL-deficient children with cancer (aged 0-12 years) received infusions of plasma-derived MBL once, or twice weekly during a chemotherapy-induced neutropaenic episode (range: 1-4 weeks). Four patients participated multiple times. Target levels of 1.0 microg/ml were considered therapeutic. In total, 65 MBL infusions were given. No MBL-related adverse reactions were observed, and the observed trough level was 1.06 microg/ml (range: 0.66-2.05 microg/ml). Pharmacokinetics were not related to age after correction for body weight. The half-life of MBL, for a child of 25 kg, was 36.4h (range: 23.7-66.6h). No anti-MBL antibodies were measured 4 weeks after each MBL course. Substitution therapy with MBL-SSI twice weekly was safe and resulted in trough levels considered protective.

Infusion of plasma-derived mannan-binding lectin (MBL) into MBL-deficient humans.

Our first experience of mannan-binding lectin (MBL)-replacement therapy was with a patient experiencing recurrent erythema multiforme associated with reactivation of herpes simplex virus; his erythematous eruptions could be controlled with infusions of fresh frozen plasma containing MBL, but not with plasma lacking MBL. Some years later, we treated a young girl with recurrent, debilitating infections with purified MBL; this was also followed by a dramatic clinical improvement. We have now carried out a phase I clinical trial on 20 MBL-deficient, but healthy, adult volunteers. The MBL was prepared by the State Serum Institute in Copenhagen, Denmark, from blood donor plasma. Each volunteer received a total of 18 mg of MBL in three 6-mg doses given intravenously once a week over 3 weeks. The volunteers were monitored closely after each infusion and no adverse clinical or laboratory effects were observed. Laboratory parameters included C-reactive protein, various complement components, and antibodies to MBL, HIV and hepatitis viruses. C3a (the anaphylotoxin derived from complement component C3) was monitored for signs of complement activation, but no significant infusion-associated fluctuations were observed. Serum levels of MBL after each 6-mg infusion ranged between 1200 and 2500 ng/ml. The half-life of the infused MBL was about 70 h, or 3 days. It was concluded that infusion of purified MBL manufactured by the Danish State Serum Institute is a safe procedure. However, adults may have to be given 6 mg or more at least twice weekly to maintain protective plasma MBL levels in MBL-deficient individuals.