Soluble fms-Like Tyrosine Kinase 1 Localization in Renal Biopsies of CKD

Abstract

Introduction: Soluble fms-like tyrosine kinase 1 (sFLT1) is a splice variant of the vascular endothelial growth factor (VEGF) receptor lacking the transmembrane and cytoplasmic domains and acts as a powerful antagonist of VEGF signaling. Plasma sFLT1 levels are higher in patients with chronic kidney disease (CKD) and correlate with renal dysfunction. The source of plasma sFLT1 in CKD is unclear.

Methods: Fifty-two renal biopsies were studied for sFLT1 expression using immunohistochemistry and evaluated on a 0-4 grading scale of positive cells within inflammatory infiltrates. These included drug-induced interstitial nephritis (6); allografts (12), with polyomavirus nephritis (3); diabetes mellitus (10); lupus glomerulonephritis (6); pauci-immune vasculitis (7); IgA nephropathy (6); and miscellaneous CKD (5).

Results: Forty-seven biopsies had inflammatory infiltrates of which 37 had sFLT1-positive cells: of these biopsies, 3 were grade 4, i.e., had cells that constituted more than 50% of the inflammatory infiltrate, 9 were grade 3 (25%-50%), 5 were grade 2 (10%-25%), 3 were grade 1 (10%), and 17 were grade 0.5 (<10%). There was a robust correlation (r² = 0.89) between degree of inflammation and sFLT1-positive cells. CD68/sFLT1 co-immunostaining studies indicated that sFLT1-positive cells were histiocytes. The surrounding capillary network was reduced.

Conclusion: sFLT1-positive histiocytes are generally part of the inflammatory infiltrates noted in CKD and are particularly abundant in forms of interstitial nephritis. Their presence promotes an anti-angiogenic state locally in the tubulointerstitium that could inhibit capillary repair, contribute to peritubular capillary loss, and enhance fibrosis in CKD.

Keywords: CKD; histiocytes/macrophages; sFLT1.

Figure 1

Grading scale for immunohistochemical staining of soluble fms-like tyrosine kinase 1 (sFLT1) in kidney biopsies: sFLT1-positive cells that constituted more than 50% of the inflammatory infiltrate (grade 4), 25%–50% (grade 3), 10%–25% (grade 2), 10% (grade 1), and <10% (grade 0.5). Bars = 20 μm.

Figure 2

The expression of soluble fms-like tyrosine kinase 1 (sFLT1) in kidney biopsies is positively correlated with kidney inflammation (n = 52 biopsies). sFLT1 protein expression localized in kidney biopsies by immunohistochemistry. A 4-tier grading system was used to semiquantitatively measure sFLT1 and inflammation in the kidney biopsies.

Figure 3

Representative images of immunohistochemical assay for soluble fms-like tyrosine kinase 1 (sFLT1), CD68 (macrophage marker), and CD34 (endothelial marker) immunoreactivity (brown precipitate) in renal biopsies. Bars = 20 μm. G, glomerulus.

Figure 4

Upper panels (interstitial nephritis case): representative images of immunofluorescence assay for soluble fms-like tyrosine kinase 1 (sFLT1; red) and CD34 (endothelial marker; green) in a renal biopsy. The areas to the right of the asterisks show reduced or absent CD34 staining. Bars = 100 μm. Lower panels (anti-neutrophilic cytoplasmic antibody case): representative images of immunofluorescence assay for sFLT1 (red), and CD68 (macrophage marker; green) in a renal biopsy. Yellow immunofluorescence demonstrates colocalization of the 2 antigens, which demonstrates that macrophages express sFLT1 in kidney interstitium. Bars = 20 μm.