Chronic allograft nephropathy (CAN), characterized by interstitial fibrosis, vascular intimal thickening, and glomerulosclerosis, has remained as leading cause of allograft failure. The role of innate immunity as a first responder and its implication in CAN has remained unclear. We have previously demonstrated the interaction between human monocytes and vascular endothelial cells (ECs) instigates upregulation of monocyte-derived costimulatory molecules. We hypothesized that the initial interaction between monocytes and ECs, the first barrier between host immunity and allograft, may play a critical role in initiating the transformation of cells skewed toward a pro-fibrotic mesenchymal cell phenotype. Allogeneic monocyte-ECs co-cultures at 24, 48, and 72 hours were evaluated by real time PCR (RT-PCR) for pro-fibrosis gene transcript signatures indicating endothelial-to-mesenchymal transition, myofibroblast formation, collagen biosynthesis, and related factors. The co-culture supernatants were collected and analyzed by multiplex cytokine array. The initial interaction of monocytes with ECs revealed uptake of allogeneic EC membranes by CD14+ monocytes as detected by flow cytometry and fluorescent microscopic analysis. The release of cytokines IL-6, IL-1α, TNF-α, and MIP-1α along with chemokines I-TAC and IP-10 were detected in culture supernatants as early as 24 hours after coculture. Additionally, neutralizing antibody infliximab specific for TNF-α inhibited chemokine (IP-10, I-TAC) but not cytokine (IL-6, IL-1α, MIP-1α) production during monocyte-EC interaction. The upregulation of gene transcripts for vimentin, aSMA, and gremlin-1 was detected at 24-hour co-incubation, and continually increased thereafter. The interaction between monocytes and endothelial cells resulted in upregulation of transcripts for heat shock protein 47, collagen type 1-α, and collagen type 4-α of 5. Additionally, transcripts for insulin-like growth factor 1, MMP-2, and MMP-9 but not CTGF were also up-regulated as early as 24-hours after interaction. This study demonstrates initial responses of allogeneic monocyte-EC interaction result in a dramatic release of pro-inflammatory cytokines and potent chemotactic agent IP-10 and I-TAC. The interaction between allogeneic monocyte and ECs also results in a pro-fibrosis transcriptome. Our results suggest that the initial monocyte-EC interaction is a significant contributor in the initiation of allograft fibrosis.

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