Activation of tubular epithelial cells by mesangial-derived TNF-alpha: glomerulotubular communication in IgA nephropathy.
BACKGROUND
IgA nephropathy (IgAN), characterized by mesangial IgA deposition, runs a variable clinical course with tubulointerstitial damage and renal failure in no less than 30% of patients. Histologically, IgA is rarely detected in renal tubules. The direct toxicity by IgA on renal tubules remains uncertain. We hypothesize that mediators released from human mesangial cells (HMC) triggered by IgA deposition may lead to activation of proximal tubular epithelial cells (PTEC).


METHODS
The binding of IgA to PTEC or HMC was assessed by flow cytometry. IgA-HMC medium was prepared by collecting the spent medium in which growth arrested HMC were incubated with IgA isolated from patients with IgAN, healthy control subjects, or other nephritic control patients. PTEC was cultured with the IgA-HMC medium in the presence or absence of neutralizing antibodies to TNF-alpha, IL-1beta, TGF-beta, or PDGF. Gene expression and protein synthesis of TNF-alpha, MIF, or ICAM-1 by PTEC were determined by RT-PCR and ELISA, respectively.


RESULTS
The binding of IgA isolated from patients with IgAN to PTEC was increased when compared to binding of IgA from healthy control subjects (P < 0.005). However, the binding to PTEC was less than one tenth that of HMC in IgAN. The binding to PTEC was not mediated through known IgA receptors, as shown by competitive binding assays and gene expression of the receptors. Despite the in vitro binding, PTEC cultured with isolated IgA exhibited no increased cell proliferation or enhanced synthesis of TNF-alpha, MIF, or sICAM-1. However, when PTEC were cultured with IgA-HMC medium prepared from IgAN patients, there was enhanced proliferation of PTEC (P < 0.001) and increased synthesis of TNF-alpha, MIF, and sICAM-1 when compared with PTEC cultured with IgA-HMC medium from control subjects (P < 0.001). The synthesis of MIF and sICAM-1 by PTEC cultured with IgA-HMC medium was reduced by neutralizing antibodies to TNF-alpha (P < 0.001) but not by neutralizing antibodies to IL-1beta, TGF-beta, or PDGF.


CONCLUSIONS
Our finding implicates that TNF-alpha released from the mesangium after IgA deposition activates renal tubular cells. The glomerulotubular communication could play an important role in the pathogenesis of tubulointerstitial damage in IgAN.
Chan
Loretta Y Y
LY
Department of Medicine, Queen Mary Hospital, The University of Hong Kong, Hong Kong.
Leung
Joseph C K
JC
Tsang
Anita W L
AW
Tang
Sydney C W
SC
Lai
Kar Neng
KN
D049109:Cell Proliferation; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000473:pathology; D005260:Female; D005920:Glomerular Mesangium / Q000502:physiology*; D005922:Glomerulonephritis, IGA / Q000473:pathology*; D006801:Humans; D007070:Immunoglobulin A / Q000378:metabolism; D018799:Intercellular Adhesion Molecule-1 / Q000502:physiology; D007687:Kidney Tubules, Proximal / Q000473:pathology*; D008263:Macrophage Migration-Inhibitory Factors / Q000502:physiology; D008297:Male; D011961:Receptors, Fc / Q000032:analysis; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*
C045281:IgA receptor; D007070:Immunoglobulin A; D008263:Macrophage Migration-Inhibitory Factors; D011961:Receptors, Fc; D014409:Tumor Necrosis Factor-alpha; D018799:Intercellular Adhesion Molecule-1