Summary: We have demonstrated that mannose-binding lectin (MBL) recognizes various slow-growing, pathogenic mycobacteria [Mycobacterium tuberculosis (MTB), M. bovis, M. kansasii, M. gordonae] as well as non-pathogenic M. smegmatis. Recognition resulted in activation of the lectin pathway (LP) of complement and an enhancement of phagocytosis (shown for M. tuberculosis). Although MBL may be considered the main factor activating the LP upon recognition of mycobacteria, involvement of ficolins has also to be considered. Interaction of ficolin-3 with M. tuberculosis, M. bovis and M. kansasii, and ficolin-1 with M. tuberculosis and M. bovis was shown for the first time. Binding of recombinant MBL or ficolin-3 to MTB H37Rv led to the agglutination of bacteria and promoted their phagocytosis, but little effect was apparent with ficolin-1 or ficolin-2. Data from Western blots suggest mannosylated lipoarabinomannan (ManLAM) to be one of the main cell components of slow-growing mycobacteria, involved in LP activation. However, the LP was also activated by other cell fractions. Results presented here supplement considerably the data concerning the ability of complement-activating lectins to interact with mycobacteria. Ficolins (especially ficolin-3) might influence host response to infection and thus have clinical significance, at least as disease modifiers.

The Malpuech, Michels, Mingarelli, Carnevale (3MC) syndrome is a rare, autosomal recessive genetic- disorder associated with mutations in the MASP1/3, COLEC1,1 or COLEC10 genes. The number of 3MC patients with known mutations in these three genes reported so far remains very small. To date, 16 mutations in MASP-1/3, 12 mutations in COLEC11 and three in COLEC10 associated with 3MC syndrome have been identified. Their products play an essential role as factors involved in the activation of complement via the lectin or alternative (MASP-3) pathways. Recent data indicate that mannose-binding lectin-associated serine protease-1 (MASP-1), MASP-3, collectin kidney-1 (collectin-11) (CL-K1), and collectin liver-1 (collectin-10) (CL-L1) also participate in the correct migration of neural crest cells (NCC) during embryogenesis. This is supported by relationships between MASP1/3, COLEC10, and COLEC11 gene mutations and the incidence of 3MC syndrome, associated with craniofacial abnormalities such as radioulnar synostosis high-arched eyebrows, cleft lip/palate, hearing loss, and ptosis.

Ficolin-3 (also called H-ficolin or Hakata antigen) is a complement-activating pattern recognition molecule, possessing a fibrinogen-like domain involved in carbohydrate binding. Amongst human ficolins, Ficolin-3 has the highest concentration in serum and is the most potent lectin pathway activator in vitro. Evidence for its physiological function is sparse, although its deficiency has been suggested to increase susceptibility to infections. The specificity of Ficolin-3 is poorly characterized and currently few ligands are known. Here we report agglutination of Hafnia alvei, a Gram-negative enteric commensal bacterium and opportunist pathogen, in the presence of recombinant Ficolin-3 and calcium. Ficolin-3 also augmented phagocytosis of H. alvei by macrophages and displayed bactericidal activity. Additionally, Ficolin-3 inhibited host cells' response to TLR4/MD-2/CD14-LPS dependent NF-κB activation. This is the first demonstration of protective activity of Ficolin-3 against a human bacterial pathogen. Although human Ficolin-3 does not recognise and bind to common pathogenic bacteria, it could be an important component of innate immunity providing protection, for example, from commensal flora that can cause extraintestinal, opportunistic infections.