*Purpose: Chronic-active antibody mediated rejection (c-aABMR) contributes significantly to late renal allograft failure. Non-invasive biomarkers of c-aABMR are currently not available but could be valuable for early detection. In this study the activation profiles of relevant immune cell populations in peripheral blood of patients with c-aABMR were evaluated as potential biomarker.

*Methods: The peripheral blood mononuclear cells of the kidney transplant recipients included were used for flow cytometric analysis. A panel of monoclonal antibodies was used designed to characterize both the specific cell type (T and B cells, γδ T cells , NK cells and monocytes ) and their activation status. Cases with biopsy proven c-aABMR (c-aABMRpos, N=25) were compared to matched controls (c-aABMRneg, N=25).

*Results: No significant differences were found in the total percentage and distribution of NK cells, B cells and T cells. There was however a higher percentage of monocytes present in c-aABMRpos cases (19.5% vs. 14.4%, p<0.05). Additionally, differences were found in activation status of circulating monocytes, NK cells and γδ T cells. The c-aABMRpos cases had a significantly higher percentage of monocytes expressing the activation marker CD38 (p=0.04) as well as higher expression of CD38 on NK cells (p=0.02). CD16 (Fcγ III receptor) expression on NK cells was significantly higher in c-aABMRpos cases (MFI 56965 vs. 34345, p<0.01) but significantly lower in γδ T cells (MFI 837 vs. 1277, p=0.02). Although statistically significant, these differences were not sufficient to readily identify patients with c-aABMR.

*Conclusions: Cases with c-aABMR express a different CD16 and CD38 expression profile on circulating NK cells, γδ T cells and monocytes. Increased CD16 expression on circulating NK cells suggest that an interaction with antibodies on renal endothelial cells has taken place.

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