Endothelial-specific loss of Krüppel-Like Factor 4 triggers complement-mediated endothelial injury

Thrombotic microangiopathy (TMA) in the kidney represents the most severe manifestation of kidney microvascular endothelial injury. Despite the source of the inciting event, the diverse clinical forms of kidney TMA share dysregulation of endothelial cell transcripts and complement activation. Here, we show that endothelial-specific knockdown of Krüppel-Like Factor 4 (Klf4)^ΔEC, an anti-inflammatory and antithrombotic zinc-finger transcription factor, increases the susceptibility to glomerular endothelial injury and microangiopathy in two genetic murine models that included endothelial nitric oxide synthase knockout mice and aged mice (52 weeks), as well as in a pharmacologic model of TMA using Shiga-toxin 2. In all models, Klf4^ΔEC mice exhibit increased pro-thrombotic and pro-inflammatory transcripts, as well as increased complement factors C3 and C5b-9 deposition and histologic features consistent with subacute TMA. Interestingly, complement activation in Klf4^ΔEC mice was accompanied by reduced expression of a key KLF4 transcriptional target and membrane bound complement regulatory gene, Cd55. To assess a potential mechanism by which KLF4 might regulate CD55 expression, we performed in silico chromatin immunoprecipitation enrichment analysis of the CD55 promotor and found KLF4 binding sites upstream from the CD55 transcription start site. Using patient-derived kidney biopsy specimens, we found glomerular expression of KLF4 and CD55 was reduced in patients with TMA as compared to control biopsies of the unaffected pole of patient kidneys removed due to kidney cancer. Thus, our data support that endothelial Klf4 is necessary for maintenance of a quiescent glomerular endothelial phenotype and its loss increases susceptibility to complement activation and induction of prothrombotic and pro-inflammatory pathways.