Role of ribosomal protein S19 oligomer-C5a receptor system in acute inflammation resolution

Abstract

Ribosomal protein S19 (RP S19) is a component of the small ribosome subunit and essential for ribosome biogenesis. RP S19 is also present in blood plasma, forming a complex with prothrombin. Cellular RP S19 is inter-molecularly cross-linked by an intracellular transglutaminase during apoptosis, and plasma RP S19 is similarly cross-linked by activated coagulation factor XIII during blood coagulation, forming an isopeptide bond between Lys122 and Gln137 in both cases. The cross-linked RP S19 oligomers thus formed gain a ligand capacity to the C5a receptor and express various kinds of extra-ribosomal functions.
The cells undergoing apoptosis de novo synthesize the C5a receptor. The RP S19 oligomers liberated by the apoptotic cells hasten the apoptosis execution on one hand and recruit phagocytic macrophages on the other, completing the prompt clearance of the apoptotic cells. Isolated neutrophils spontaneously undergo apoptosis and generate the RP S19 oligomers. The RP S19 oligomers do not elicit chemotactic response of neutrophils but rather speed up the apoptotic process of the cells, while these molecules induce chemotactic migration of monocytes/macrophages. We currently made a hypothesis that this would be a crucial mechanism in resolution of acute inflammation. This hypothesis has been experimentally supported. For instance, when the RP S19 oligomers were immunologically neutralized in a carrageenan-induced mouse pleurisy model, neutrophil number in the pleural exudate greatly increased and the inflammation spread to lung parenchyma. Similar phenomena were observed in the carrageenan pleurisy induced in Gln137Glu-RP S19 knock-in mice without the neutralization.
Regarding the discrimination by the RP S19 oligomers but not by complement C5a between neutrophils and monocytes/macrophages, we made a hypothesis that a molecule(s) that disconnects the RP S19 oligomer/C5a receptor complex but not the C5a/C5a receptor complex from the intracellular signal transduction pathway is present near C5a receptor in neutrophils but not in monocytes/macrophages. To examine the hypothesis and identify the disconnecter molecule(s), we prepared a recombinant C5a/RP S19 chimeric protein which reproduces the functions of RP S19 oligomers as a monomeric protein. Delta-lactoferrin (δ-Lf) was co-separated with C5a receptor when ligated by C5a/RP S19 but not by C5a in neutrophils. δ-Lf is an intracellular protein, and it is not synthesized by monocytes/macrophages. When δ-Lf mRNA translation was blocked, HL-60-derived neutrophil-like phenotypes changed to chemotactically respond to C5a/RPS19. δ-Lf seems to be the disconnecter molecule.