Demonstration of fibrinogen-FcRn binding at acidic pH by means of Fluorescence Correlation Spectroscopy

The neonatal Fc receptor (FcRn) interacts with IgG and albumin at acidic pH within endosomes, thus protecting these plasma proteins from degradation. Recently, we proposed fibrinogen as a new binding partner of FcRn. This work was aimed at providing a direct demonstration of FcRn-fibrinogen binding at acidic pH by Fluorescence Correlation Spectroscopy. The increase in diffusion time between free and fibrinogen-bound FITC-labelled FcRn was assumed as the binding indicator. We observed that, at acidic pH (pH = 5.3), FcRn diffusion time shifted from ≈730 μs (FITC-labelled FcRn alone) to >1200 μs (FITC-labelled FcRn added with fibrinogen). A similar trend was exhibited by albumin, a known FcRn interactor, while no significant variations in diffusion time were observed upon incubation with catalase as negative control. Our results demonstrate a binding interaction between fibrinogen, one of the most abundant plasma proteins, and FcRn, a receptor involved in the regulation of the levels of IgG and albumin. This interaction is likely responsible for fibrinogen protection from intracellular degradation and recycling in plasma. Fibrinogen is crucial not only in haemostasis but also in acute inflammatory response and in some pathological conditions. The interaction with FcRn can influence not only the levels of fibrinogen in plasma and other tissues, but also the levels of other FcRn binding partners, among which are some plasma proteins of clinical relevance.