C1q-mediated efferocytosis: from molecular and cellular mechanisms to self-tolerance or autoimmunity – C1qEffero

The C1q complement protein, first identified as a microbial sensor involved in innate immune defence, is now considered as a multifunctional protein with a prominent role in the safe clearance of altered self-elements. The C1qEffero project focuses on the particular role of C1q in the phagocytosis of dying cells, a process called efferocytosis, which is essential in maintenance of tissue homeostasis, resolution of inflammation and immune self-tolerance. Actually, defective efferocytosis may lead to a breach in tolerance and trigger an immune response at the origin of autoimmune and inflammatory pathologies. Clinical studies have thus revealed a very strong association of C1q deficiencies with the development of severe systemic autoimmune diseases such as lupus.
Efferocytosis is a complex cellular process involving numerous molecules such as “eat-me” signals of target cells, phagocytic receptors, signalling molecules and bridging molecules (opsonins) among which C1q was identified as a main actor. We have recently characterised the structural bases of the recognition of apoptotic cells by C1q and shown that cell surface calreticulin (CALR) enhances the phagocytosis of these cells. The objective of C1qEffero is now to decipher the complex molecular interplay of C1q at the apoptotic cells-phagocytes interface (efferocytic synapse) and to investigate how given interactions influence the phagocyte response, in relation with immune tolerance.
Several C1q-dependent efferocytosis pathways have been described, each involving distinct phagocyte receptors, but the relative contribution of the different receptors, the possible formation of multimolecular complexes, and the molecular determinants of the C1q-receptor interactions remain to be investigated. We will target the endocytic C1q receptors SREC-I, LRP1 and RAGE, including their possible association with CALR, and the inhibitory receptor LAIR-1, involved in the maintenance of immune tolerance.
Our research programme involves a multidisciplinary and integrated approach combining:
- protein engineering, biochemistry and structural biology (cryo-electron microscopy, X-ray crystallography) to dissect the C1q-receptors interactions, get structural information on soluble complexes and engineer variants with modified functionalities
- imaging of the C1q-receptor complexes at the efferocytic synapse by super-resolution fluorescence microscopy, an approach which will give access to the localisation and the organisation of complexes involving C1q at the nanoscale
- cell biology to (i) analyse the interactions between apoptotic cells opsonised by C1q or its variants and the receptors of different phagocyte types, and (ii) investigate the immune consequences of C1q-opsonised dying cell capture on the maintenance of immune tolerance, both in vitro and ex vivo (using C1q-deficient mice)
This collaborative project relies on the experience of two teams with recognised expertise in the field of C1q structure-function and recognition of apoptotic cells by C1q (IBS, Grenoble), and in the biology of phagocytes in pathologic contexts, including autoimmunity (CRCNA, Angers). An essential and unique tool for this project resides in full-length recombinant C1q, recently produced by partner 1 (international patent application). C1q engineering is now possible and will allow to map the interaction sites of C1q with its partners and to generate C1q variants endowed with specific binding properties to analyse their role in a cellular context.
This programme aiming at deciphering the role of C1q in efferocytosis and immune tolerance will allow to better understand how C1q contributes to the triggering of autoimmune diseases. It might help in identifying new therapeutic targets for these pathologies of rising prevalence, which represent a major health issue in modern societies.