Structures, interactions and interferences in the immediate and long-term protection by innate immune macromolecular complexes. – STR-ASS-DEF-COL

The mammalian immune system has evolved innate and adaptive components that cooperate to protect the host against microbial infections. The innate system involves constitutive recognition molecules able to sense danger signals and to trigger effector mechanisms aimed at limiting early infection and eliminating apoptotic cells, while preserving host integrity. The soluble defence collagens under study (C1q, MBL, and ficolins) are essential mediators of innate immunity, due to their capacity to identify microbes and elements of altered self and to interact with complement activating proteases or specific receptors such as calreticulin (CRT) and CR1. However, inappropriate activation of complement by endogenous ligands may be at the origin of inflammatory and/or autoimmune pathologies, whereas some pathogens have developed infection strategies by interfering with the host innate immune defense mechanisms CRT is well known as a molecular chaperone in the endoplasmic reticulum, but it has also been identified at the surface of mammalian cells and appears to act as a marker of apoptotic cells, thereby facilitating their phagocytosis. CRT homologues have been identified in various human endoparasites and recently proposed to play a general role in parasites' adaptation to their environment by facilitating their evasion from the host immune defence. According to this hypothesis, studies on T. cruzi , the parasite responsible for Chagas' disease, suggest that TcCRT interacts with C1q, which might divert the complement activating ability of C1q and participate in invasion of host cells by facilitating parasite internalization. The detailed understanding of the molecular interactions of the defence collagens involved in complement activation, in enhancement of phagocytosis, and in the mechanisms of parasite evasion appears thus crucial for the development of novel therapeutic strategies. The defence collagens are unique oligomeric molecules composed of collagen stalks and globular recognition domains. The association of their collagen-like regions to modular dimeric proteases (MASPs) or tetrameric (C1r2-C1s2) or to the CR1 receptor gives raise to large (> 600 kDa), very flexible complexes. Deciphering the structural bases of these assemblies requires to associate classical methods of crystallography, applied to the different functional domains of the partner proteins, to more global techniques of electron microscopy. CRTs from human and T. cruzi comprise a flexible central arm (P-domain) likely inserted into a globular domain homologous to that of calnexin, and a very acidic C-terminal extremity. The C1q binding sites have been located in a fragment encompassing most of the P-domain and part of the N-domain, which seems unlikely to represent a structural sub-domain. Thus, a new structural dissection strategy should be applied to decipher the molecular determinants involved in the interactions between CRT and defence collagens. The project brings together two French partners with complementary expertise in the field of the structure-function relationships of defence collagens and their associated proteases. The partner in structural biology brings together specialists in different disciplines with a view to release the scientific bottlenecks linked to the particular properties of the complexes under study. External collaborations guarantee the functional relevance of the studies on calreticulins: human CRT with G. Houen (Copenhagen), and T. cruzi CRT with A. Feirrera (Chile) which will eventually allow to connect the T. cruzi CRT-host proteins interactions to be studied to their functional consequences for in vivo parasite infectivity.