*Purpose: Development of de novo donor-specific antibodies (DSAs) is associated with antibody-mediated rejection and allograft failure. More particularly, HLA-II DSAs, especially those against DQ are associated with a worst outcome. However, some patients with circulating DSAs escape graft dysfunction or rejection by an unknown mechanism. We hypothesized that DSA-mediated injury is modulated by the variability in expression of HLA-II antigen subclasses (DR, DP and DQ) on endothelial cells and that this variability could be the result of different signaling pathways for each subclass.

*Methods: We measured in vitro HLA-I and II (DR, DP and DQ subclasses) antigen expression on 2 glomerular endothelial cell lines and on endothelial-colony forming cells (ECFC) from patients’ collected PBMCs (n=24). Endothelial cells were treated following a time-course of either IFN-γ alone or combination of IFN-γ and TNF-α for 2 to 10 days. HLA antigen expression was assessed by flow cytometry over time. Unstimulated cells were used as controls.

*Results: Following treatment with IFN-γ alone, we observed a hierarchy of expression where DR > DP > DQ in all patients (96±3 vs. 94±9 vs. 27±26% for maximal percentage of HLA-DR vs. -DP, vs -DQ respectively, p<0.001). Surprisingly, treatment with IFN-γ and TNF-α led to different effects. The combination of cytokines slightly decreased HLA-DR, substantially decreased HLA-DP and increased HLA-DQ expression (92±6 vs. 40±27 vs. 36±24% for HLA -DR vs. -DP, vs -DQ respectively, p<0.001). Results were similar at different time-points (2 to 4 days) and with different concentrations of cytokines (IFN-γ 100 or 500 U/ml with TNF-α 100 or 200 U/ml). MFI followed a similar pattern to the percentage of positive cells above.

*Conclusions: For a given individual, the glomerular endothelial expression of HLA II varies by subclass. In addition, while IFN-γ stimulation leads to increased expression for all subclass, the response of each subclass following stimulation with a combination of IFN-γ and TNF-α varies in opposite directions. These data suggest that their endothelial expression is regulated by different intracellular signaling pathways. This could allow targeting a single pathway based on the patient’s DSA profile to prevent antibody-mediated rejection without jeopardizing the entire expression of HLAs on the endothelial cells.

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