Human genetic deficiencies reveal the roles of complement in the inflammatory network: Lessons from nature

TLDR

Complement component C5 is essential for inflammatory tissue damage in animal models, yet its role in human inflammation remains incompletely understood, and naturally occurring complement deficiencies provide unique tools to probe protein function. The study examined gram‑negative bacterial responses in individuals with naturally occurring complement deficiencies—including a novel C5 defect—using a whole‑blood lepirudin‑based model that preserves complement activity and permits cross‑talk among inflammatory pathways. Expression of tissue factor, adhesion molecules, and oxidative burst was driven by C5a, whereas granulocyte enzyme release relied mainly on C3; cytokine profiles varied, with IL‑1β and IL‑8 being complement‑dependent, IFN‑γ and IL‑6 CD14‑dependent, and IL‑1ra/IP‑10 fully CD14‑dependent and up‑regulated by complement inhibition, showing that granulocyte responses are largely complement‑dependent, monocyte responses more CD14‑dependent, and that dual complement‑CD14 blockade abolishes all responses, highlighting complement’s comprehensive role in gram‑negative bacterial inflammation.

Abstract

Complement component C5 is crucial for experimental animal inflammatory tissue damage; however, its involvement in human inflammation is incompletely understood. The responses to gram-negative bacteria were here studied taking advantage of human genetic complement-deficiencies--nature's own knockouts--including a previously undescribed C5 defect. Such deficiencies provide a unique tool for investigating the biological role of proteins. The experimental conditions allowed cross-talk between the different inflammatory pathways using a whole blood model based on the anticoagulant lepirudin, which does not interfere with the complement system. Expression of tissue factor, cell adhesion molecules, and oxidative burst depended highly on C5, mediated through the activation product C5a, whereas granulocyte enzyme release relied mainly on C3 and was C5a-independent. Release of cytokines and chemokines was mediated to varying degrees by complement and CD14; for example, interleukin (IL)-1beta and IL-8 were more dependent on complement than IFN-gamma and IL-6, which were highly dependent on CD14. IL-1 receptor antagonist (IL-1ra) and IFN-gamma inducible protein 10 (IP-10) were fully dependent on CD14 and inversely regulated by complement, that is, complement deficiency and complement inhibition enhanced their release. Granulocyte responses were mainly complement-dependent, whereas monocyte responses were more dependent on CD14. Notably, all responses were abolished by combined neutralization of complement and CD14. The present study provides important insight into the comprehensive role of complement in human inflammatory responses to gram-negative bacteria.