(A) Left panel: CSF-1 mediates HK2 TEC proliferation. CSF-1 stimulates proliferation of human HK2 TECs. Right panel: anti–CSF-1R Abs suppress CSF-1–mediated HK2 TEC proliferation. MTT assay. (B) Left panel: TEC injury increases TEC secretion of CSF-1. Following exposure of cultured HK2 TECs to increasing concentrations of actinomycin D (act-D) for 72 hours, we analyzed CSF-1 in the supernatant by ELISA. Right panel: anti–CSF-1R Abs inhibit TEC survival/proliferation in response to TEC injury. HK2 TECs were incubated with the indicated concentrations of actinomycin D in the presence and absence of anti–CSF-1R Abs for 72 hours prior to determination of cell mass by MTT assay. (C) CSF-1 expression, CSF-1R expression, and proliferation of TECs correlate in human transplants with tubular injury. Using serial sections of kidney biopsy specimens from patients with tubular injury and impaired renal function, we probed for CSF-1, CSF-1R, and proliferation in tubules. CSF-1R and CSF-1 are coexpressed on TECs by immunostaining. Note proliferating Ki67⁺ TECs in tubules coexpressing CSF-1 and CSF-1R. Right panels: CSF-1R and CSF-1 Ab specificity was verified using peptide preabsorption. Representative photomicrographs and correlation graphs. Original magnification, ×20. Data show means ± SEM. n = 8 per group. (D) CSF-1R is tyrosine phosphorylated in human transplants with tubular injury. Using serial sections of kidney biopsy specimens, we probed for CSF-1R and CSF-1R phosphorylation at Y723. CSF-1R and tyrosine-phosphorylated CSF-1R are coexpressed on TECs by immunostaining. CSF-1R and phospho-Y723 CSF-1R Ab specificity was verified using peptide preabsorption and rabbit IgG, respectively. Original magnification, ×40.