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

Abstract

Objectives Toll-like receptors (TLRs) and complement are two components of the innate immunity. Factor B (cfB) is essential for the alternative pathway (AP) of complement activation. We have recently reported that cfB is significantly up-regulated in the kidney and may contribute to acute tubular injury in an animal model of sepsis. The current study investigates the mechanisms responsible for the cfB up-regulation and its role in sodium transporter expression in tubular cells during sepsis. Design Animal study. Setting Laboratory investigation. Subjects C57BL/6 J wide-type (WT), cfB−/−, and Nfkb1tm1Bal p50−/− mice. Interventions Human proximal tubular cells (HK-2) and mouse tubular epithelial cells (MTECs) were stimulated with TLR agonists. Bay 11-7082 was used to block NF-κB pathway. AP activation was detected by C3 zymosan deposition. Polymicrobial sepsis was created by cecal ligation and puncture (CLP). Sodium transporter gene expression was determined by qRT-PCR. Measurements and Main Results The agonists for TLR4 (LPS) or TLR3 (poly I:C) induced a marked increase in cfB expression in HK-2 and MTECs both at gene and protein levels. The TLR1/2 agonist, Pam3cys, induced cfB production only in HK-2 cells, not MTEC. The TLR9 ligand, CpG, failed to induce cfB production either in HK-2 cells or MTEC. LPS/poly I:C-induced cfB up-regulation was blocked by Bay 11-7082, a potent inhibitor of NF-κB signaling, and in MTECs deficient in p50 subunit of NF-κB. Media from the LPS-treated MTEC cultures contained de novo synthesized cfB and led to functional AP activation. In a CLP model, WT septic mice had down-regulated expression of sodium transporters in the kidney compared with the sham. In comparison, cfB−/− mice or mice treated with anti-cfB displayed preserved levels of Na+/K+ ATPase-α1 following sepsis. Conclusions 1) TLR3/4 activation is sufficient to induce cfB production via NF-κB pathway and to enhance AP activation in the kidney tubular epithelial cells; 2) cfB may contribute to the down-regulation of certain sodium transporter expression during sepsis.

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“…That reduction in CFB also significantly reduced the deposition of C3b on tubular epithelial cells and accumulation of neutrophils in the renal interstitium. Renal tubular cells showed increased CFB production in mice subjected to cecal ligation and puncture model of microbial sepsis and in cultured human proximal tubular cells stimulated with toll-like receptor agonists [ 46 , 47 ]. Thus, increased CFB in the urine of patients undergoing cardiac surgery may identify those patients with underlying tubular cells CFB production that predisposes to complement-mediated tubular cell injury during surgery.…”