*Purpose: Antibody-mediated rejection (AMR) accounts for >50% of premature kidney graft loss. AMR is caused by donor specific antibodies (DSA) against human leukocyte antigens (HLA) on the graft endothelium. DSA can directly injure the endothelium via extracellular signal-regulated kinase (ERK) and mTOR signaling. Since HLA does not have a signalling domain, direct injury by DSA remains poorly understood. Using unbiased proteome analysis, we compared laser-captured glomeruli of 7 clinical biopsies with AMR to 23 matched ‘non-AMR’ biopsies. Among the proteins significantly increased in AMR glomeruli, we identified galectin-1 (LGALS1), an extracellular matrix-associated protein involved in regulatory immune functions. We found that LGALS1 is induced in human glomerular microvascular endothelial cells (GMECs) upon stimulation with anti-HLA class I antibody. We hypothesize that LGALS1 is expressed in GMECs, and that anti-HLA antibody signals via ERK to induce LGALS1 expression.

*Methods: To characterize the cell origin of LGALS1 in the kidney, we examined a feature plot from our recent single-cell RNA-sequencing study of 19 living donor biopsies. We optimized LGALS1 stain in a wedge biopsy from a control kidney. GMECs were stained with anti-LGALS1 antibody and counter-stained with phalloidin and DAPI.

*Results: Single-cell RNA-sequencing data of living donor kidney biopsies demonstrated LGALS1 expression in endothelial and immune cells (Fig.1A). LGALS1 protein was localized to the glomeruli (Fig.1B) and expressed in the cytoplasm and membranes of GMECs (Fig.1C). Anti-HLA class I antibody resulted in ERK phosphorylation which peaked at 30 minutes post-stimulation.

*Conclusions: GMECs express LGALS1, which increases in conjunction with ERK phosphorylation upon stimulation with anti-HLA class I antibody. Our next step is to decipher the signalling cascade through which anti-HLA class I increases LGALS1 expression in GMECs.

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