Complement activation and toll-like receptor-2 signaling contribute to cytokine production after renal ischemia/reperfusion

Abstract

The innate immune system causes tissue inflammation and injury after renal ischemia/reperfusion (I/R). The complement system is activated on ischemic tubular epithelial cells (TECs) and induces the cells to produce pro-inflammatory chemokines. TECs also express toll-like receptors (TLRs)-2 and -4. Signaling through the TLRs induces TECs to produce a variety of chemokines, some of which can also be induced by complement activation fragments. We sought to determine whether the effects of complement activation and TLR signaling in TECs are redundant, or whether additive protection can be achieved by blocking both of these innate immune systems. To confirm that the complement system, TLR-2 signaling, and TLR-4 signaling induce production of a similar repertoire of inflammatory chemokines, we stimulated TECs with complement sufficient serum or with TLR-2 and TLR-4 ligands in vitro. We found that all three of these stimuli induce TECs to produce KC, MIP-2, IL-6, and TNF-α, and that there was a trend toward greater production of KC in cells exposed to two stimuli. Based upon these results, we hypothesized that mice deficient in both complement activation and TLR-2 signaling would demonstrate greater protection from I/R than mice deficient only in the complement system. To test this hypothesis we induced ischemic acute kidney injury (AKI) in wild-type mice, mice with targeted deletion of complement factor B (fB(-/-) mice), or mice with targeted deletion of factor B and TLR-2 (fB(-/-)TLR2(-/-) mice). Surprisingly, we found that fB(-/-)TLR2(-/-) mice developed more severe injury than those with single deficiency of factor B. Our results indicate that blockade of the complement system may be more protective than simultaneous blockade of both the complement system and TLR-2 in ischemic AKI.