The origin of the classical complement pathway remains open during chordate evolution. A C1q-like member, BjC1q, was identified in the basal chordate amphioxus. It is predominantly expressed in the hepatic caecum, hindgut, and notochord, and is significantly upregulated following challenge with bacteria or lipoteichoic acid and LPS. Recombinant BjC1q and its globular head domain specifically interact with lipoteichoic acid and LPS, but BjC1q displays little lectin activity. Moreover, rBjC1q can assemble to form the high molecular weight oligomers necessary for binding to proteases C1r/C1s and for complement activation, and binds human C1r/C1s/mannan-binding lectin-associated serine protease-2 as well as amphioxus serine proteases involved in the cleavage of C4/C2, and C3 activation. Importantly, rBjC1q binds with human IgG as well as an amphioxus Ig domain containing protein, resulting in the activation of the classical complement pathway. This is the first report showing that a C1q-like protein in invertebrates is able to initiate classical pathway, raising the possibility that amphioxus possesses a C1q-mediated complement system. It also suggests a new scenario for the emergence of the classical complement pathway, in contrast to the proposal that the lectin pathway evolved into the classical pathway.Keywords: Amphioxus r C1q r Classical complement pathway r IgG r IgSF Additional supporting information may be found in the online version of this article at the publisher's web-site
IntroductionThe complement system, which helps Abs and phagocytic cells to clear pathogens from an organism, is a central component of innate immunity and a link between innate and adaptive immunity. The complement system is composed of more than 30 distinct plasma proteins and membrane-associated proteins that include Correspondence: Prof. Shicui Zhang e-mail: sczhang@ouc.edu.cn both effectors for foreign cell clearance and regulators for hostcell protection [1,2]. There are three pathways by which the complement system can be activated: the classical pathway, the lectin pathway, and the alternative pathway. Among them, the classical pathway is initiated by the binding of C1q, the first component of complement (C1), to immunoglobulins (IgG or IgM) within immune complexes, and its associated two serine proteases C1r and C1s, which leads to the subsequent cleavage of C4 and C2, followed by C3 activation [3]. From an evolutionary perspective, the classical complement pathway appears to have emerged only in the jaw vertebrates, when the adaptive immune system C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2014. 44: 3680-3695 Innate immunity 3681 was established in the cartilaginous lineage [4,5]. In the jawless fish lamprey, which lacks the conventional classical pathway, C1q is found to act as a lectin to bind N-acetyl-D-glucosamine (GlcNAc) and to initiate the complement system through the lectin pathway [6]. This has fueled the possibility that the lectin pathway evolved into the classical pat...