Complement Factor B Production in Renal Tubular Cells and Its Role in Sodium Transporter Expression During Polymicrobial Sepsis

Objectives: Toll-like receptors and complement are two components of the innate immunity. Complement factor B is essential for the alternative pathway of complement activation. We have recently reported that complement factor B is significantly up-regulated in the kidney and may contribute to acute tubular injury in an animal model of sepsis. This study investigates the mechanisms responsible for the complement factor B up-regulation and its role in sodium transporter expression in tubular cells during sepsis. Design: Animal study. Setting: Laboratory investigation. Subjects: C57BL/6 J wild-type, complement factor B-/-, and NNfkb1(tm1Bal) p5(0-/-) mice. Interventions: Human proximal tubular cells and mouse tubular epithelial cells were stimulated with Toll-like receptor agonists. Bay 11-7082 was used to block nuclear factor-kappa B pathway. Alternative pathway activation was detected by C3 zymosan deposition. Polymicrobial sepsis was created by cecal ligation and puncture. Sodium transporter gene expression was determined by quantitative reverse transcriptase-polymerase chain reaction. Measurements and Main Results: The agonists for Toll-like receptor 4 (lipopolysaccharide) or Toll-like receptor 3 (polyinosinic-polycytidylic acid) induced a marked increase in complement factor B expression in human proximal tubular cells and mouse tubular epithelial cells both at gene and protein levels. The Toll-like receptor 1/2 agonist, Pam3cys, induced complement factor B production only in human proximal tubular cells, not in mouse tubular epithelial cells. The Toll-like receptor 9 ligand, CpG oligodeoxynucleotides failed to induce complement factor B production either in human proximal tubular cells or in mouse tubular epithelial cells. Lipopolysaccharide/polyinosinic-polycytidylic acid-induced complement factor B upregulation was blocked by Bay 11-7082, a potent inhibitor of nuclear factor-.B signaling, and in mouse tubular epithelial cells deficient in p50 subunit of nuclear factor-kappa B. Media from the lipopolysaccharide- treated mouse tubular epithelial cell cultures contained de novo synthesized complement factor B and led to functional alternative pathway activation. In a cecal ligation and puncture model, wild-type septic mice had down-regulated expression of sodium transporters in the kidney compared with the sham. In comparison, complement factor B-/- mice or mice treated with anti-complement factor B displayed preserved levels of Na+/K+ ATPase-alpha 1 following sepsis. Conclusions: 1) Toll-like receptor 3/4 activation is sufficient to induce complement factor B production via nuclear factor-kappa B pathway and to enhance alternative pathway activation in the kidney tubular epithelial cells. 2) Complement factor B may contribute to the down-regulation of certain sodium transporter expression during sepsis. (Crit Care Med 2016; 44: e289- e299)