Editorial: Updates on the role of surfactant proteins A and D in innate immune responses

International audienc

The Human C1q Globular Domain: Structure and Recognition of Non-Immune Self Ligands

C1q, the ligand-binding unit of the C1 complex of complement, is a pattern recognition molecule with the unique ability to sense an amazing variety of targets, including a number of altered structures from self, such as apoptotic cells. The three-dimensional structure of its C-terminal globular domain, responsible for its recognition function, has been solved by X-ray crystallography, revealing a tightly packed heterotrimeric assembly with marked differences in the surface patterns of the subunits, and yielding insights into its versatile binding properties. In conjunction with other approaches, this same technique has been used recently to decipher the mechanisms that allow this domain to interact with various non-immune self ligands, including molecules known to provide eat-me signals on apoptotic cells, such as phosphatidylserine and DNA. These investigations provide evidence for a common binding area for these ligands located in subunit C of the C1q globular domain, and suggest that ligand recognition through this area down-regulates C1 activation, hence contributing to the control of the inflammatory reaction. The purpose of this article is to give an overview of these advances which represent a first step toward understanding the recognition mechanisms of C1q and their biological implications

Interaction of C1q With Pentraxin 3 and IgM Revisited: Mutational Studies With Recombinant C1q Variants

Pentraxins and complement defense collagens are soluble recognition proteins that sense pathogens and altered-self elements, and trigger immune responses including complement activation. PTX3 has been shown to interact with the globular recognition domains (gC1q) of the C1q protein of the classical complement pathway, thereby modulating complement activity. The C1q-PTX3 interaction has been characterized previously by site-specific mutagenesis using individual gC1q domains of each of the three C1q chains. The present study is aimed at revisiting this knowledge taking advantage of full-length recombinant C1q. Four mutations targeting exposed amino acid residues in the gC1q domain of each of the C1q chains (LysA200Asp-LysA201Asp, ArgB108Asp-ArgB109Glu, TyrB175Leu, and LysC170Glu) were introduced in recombinant C1q and the interaction properties of the mutants were analyzed using surface plasmon resonance. All C1q mutants retained binding to C1r and C1s proteases and mannose-binding lectin-associated serine proteases, indicating that the mutations did not affect the function of the collagen-like regions of C1q. The effect of these mutations on the interaction of C1q with PTX3 and IgM, and both the PTX3- and IgM-mediated activation of the classical complement pathway were investigated. The LysA200Asp-LysA201Asp and LysC170Glu mutants retained partial interaction with PTX3 and IgM, however they triggered efficient complement activation. In contrast, the ArgB108Asp-ArgB109Glu mutation abolished C1q binding to PTX3 and IgM, and significantly decreased complement activation. The TyrB175Leu mutant exhibited decreased PTX3- and IgM-dependent complement activation. Therefore, we provided evidence that, in the context of the full length C1q protein, a key contribution to the interaction with both PTX3 and IgM is given by the B chain Arg residues that line the side of the gC1q heterotrimer, with a minor participation of a Lys residue located at the apex of gC1q. Furthermore, we generated recombinant forms of the human PTX3 protein bearing either D or A at position 48, a polymorphic site of clinical relevance in a number of infections, and observed that both allelic variants equally recognized C1q

The serine protease domain of MASP-3: enzymatic properties and crystal structure in complex with ecotin.

International audienceMannan-binding lectin (MBL), ficolins and collectin-11 are known to associate with three homologous modular proteases, the MBL-Associated Serine Proteases (MASPs). The crystal structures of the catalytic domains of MASP-1 and MASP-2 have been solved, but the structure of the corresponding domain of MASP-3 remains unknown. A link between mutations in the MASP1/3 gene and the rare autosomal recessive 3MC (Mingarelli, Malpuech, Michels and Carnevale,) syndrome, characterized by various developmental disorders, was discovered recently, revealing an unexpected important role of MASP-3 in early developmental processes. To gain a first insight into the enzymatic and structural properties of MASP-3, a recombinant form of its serine protease (SP) domain was produced and characterized. The amidolytic activity of this domain on fluorescent peptidyl-aminomethylcoumarin substrates was shown to be considerably lower than that of other members of the C1r/C1s/MASP family. The E. coli protease inhibitor ecotin bound to the SP domains of MASP-3 and MASP-2, whereas no significant interaction was detected with MASP-1, C1r and C1s. A tetrameric complex comprising an ecotin dimer and two MASP-3 SP domains was isolated and its crystal structure was solved and refined to 3.2 Å. Analysis of the ecotin/MASP-3 interfaces allows a better understanding of the differential reactivity of the C1r/C1s/MASP protease family members towards ecotin, and comparison of the MASP-3 SP domain structure with those of other trypsin-like proteases yields novel hypotheses accounting for its zymogen-like properties in vitro

State-of-the-Art Research on C1q and the Classical Complement Pathway

C1q is the target recognition protein of the classical complement pathway and a major connecting link between innate and acquired immunity. As a charge pattern recognition molecule of innate immunity, C1q can engage a broad range of ligands derived from self, non-self and altered self via its heterotrimeric globular (gC1q) domain and thus trigger the classical complement pathway. The trimeric gC1q signature domain has been identified in a variety of non-complement proteins that can be grouped together as a C1q family. C1q circulates in serum as part of the C1 complex, in association with a catalytic tetrameric assembly of two homologous yet distinct serine proteases, C1r and C1s. Binding of C1q to appropriate targets leads to sequential activation of C1r and C1s, the latter being able to cleave complement components C4 and C2 thereby triggering the complement cascade. Activation of the classical pathway plays an important role in innate immune protection against pathogens and damaged elements from self. However, its involvement has been shown in various pathologies including ischemia-reperfusion injury and hereditary angioedema. Unexpected roles for the classical pathway have also been discovered recently, linked to both physiological and pathological aspects of development, including brain and cancer cells. These new perspectives should arouse renewed interest in a search for specific inhibitors of the classical pathway. In addition, C1q has recently been shown to have a number of functions that are independent of the activation of the classical pathway. This research topic is aimed at providing a state-of-the-art overview of the classical pathway, including, but not restricted to emerging functions of C1q and of the C1 complex, as well as pathological consequences of C1 activation or of the presence of anti-C1q autoantibodies . Contributions are included in the areas such as structural basis of C1q ligand recognition, C1q family proteins, inhibitors of the classical pathway identified in pathogens and improved derived inhibitors, structural determinants of the substrate specificities of C1r and C1s, elucidation of the architecture of C1, structural and functional homology of C1 with the initiating complexes of the lectin complement pathway, and novel involvement of C1q in processes such as ageing, cancer, synaptic pruning, and pregnancy