Characterisation of Mannan Binding Lectin associated serine proteases MASP-2, MASP-3, and of the MASP-2 related plasma protein MAp19 in the chicken : evidence for the absence of MASP-1 in birds

Very recently a novel pathway of complement activation, termed the lectin pathway, has been described. In this pathway, three serum components act as sugar recognition molecules, Mannan Binding Lectin (MBL) and two members of the ficolin family (H-ficolin and L-Ficolin). In human and rodents, these three molecules associate with serine proteases, the so-called MBL associated serine proteases (MASP-1, MASP-2 and MASP-3) and a MASP-2 related non-protease MAp19. Activation of complement via the lectin pathway occurs when MBL or ficolins interact with carbohydrate structures present on yeast, bacteria and viruses. This interaction is translated into the activation of the complement cascade by the conversion of MASP-2 proenzyme (it is not yet clear if and how MASP-1 participates in this process) into its enzymatically active form. The functions of MASP-3 and MAp19 are unknown. The present PhD project was aimed at the characterisation of chicken MASPs and MAp19. Complete MASP-3 specific cDNA and partial MASP-2 and MAp19 specific cDNAs were isolated from chicken liver by RT-PCR, cDNA library screening and RACE technique. Northern blot analyses on RNA prepared from livers from seven birds provided evidence for the absence of MASP-l, while MASP-3 is expressed. In situ hybridisation experiments on chicken embryos revealed that while MASP-2 mRNA is expressed only in the liver, MASP-3 mRNA is also synthesised in extra-hepatic locations e.g. organs of nervous system, urinary system and gastrointestinal tract. Finally C4 cleavage assays were done to measure the lectin pathway activity in sera of birds in response to microbial surfaces and microbial carbohydrates. To conclude, this project is the first description of the primary structure and site of synthesis of MASP-2, MAp19 and MASP-3 mRNA in sauropsides.