Characterization and functional analysis of mouse Ficolin B

The innate immune system contributes day by day to the elimination of invading pathogens. The lectin pathway of the complement system is initiated when mannose-binding-lectin (MBL) or ficolins interact with MASP-2. The aim of the present work was to characterize the mouse FcnB and define its function in the innate immune system and especially its role in the complement activation. Results from this thesis revealed that immature macrophages and dendritic cells as well as PMN express mouse FcnB (immature myeloid cells). While inflammatory stimuli can clearly enhance the FcnB expression in PMN no further enhancement was observed in mature macrophages or dendritic cells from bone marrow cultures of wild type mice, but was also seen in dendritic cells derived from mice that lack TNFR2. Previous results from our group (Runza et al., 2006) were supported by demonstrating the presence of FcnB on protein level in lysates of PMN, bone marrow-derived macrophages and dendritic cells, using newly generated monoclonal antibodies specific for mouse FcnB. In native PMN from Balb/c mice FcnB was localized in lysosomal granules similarly as previously found for peritoneal macrophages (Runza et al., 2006). In addition, in serum from Balb/c mice FcnB was detected. Further, binding properties of FcnB to various ligands were investiagted in this thesis. Our results indicate that FcnB might act as a potent pattern recognition molecule by recognizing sialic acid (present in the glycoprotein fetuin), DNA (potentially a target in dying cells) and chitin (potentially a target in fungi). The fact that FcnB binds to DNA shown in this thesis might be of importance concerning the finding that PMN have stored FcnB and produce NET structures upon stimulation, which contain DNA. Furthermore, recombinant FcnB bound to various strains of GBS and to serotype T-5 of S. aureus. One major function of ficolins is their interaction with MASP-2 what subsequently leads to the activation of the lectin pathway. In this thesis it was demonstrated for the first time that also mouse FcnB is able to activate the lectin pathway. Contradictory results regarding the role of mouse FcnB within the complement system are probably due to different oligomerization forms of the used FcnB. All results obtained during this work lead to the conclusion that mouse FcnB plays a role in the innate immune system, serving as a pattern recognition molecule for the detection of microorganisms and activating the complement system by binding to MASP-2, like the other ficolins or MBL