The natural killer-cellular humoral activation test as a functional biomarker of antibody-mediated rejection after kidney transplantation.

Antibody-mediated rejection (ABMR) is a major cause of graft failure after kidney transplantation, yet current diagnostics do not directly assess effector immune activity. Natural killer (NK) cells contribute to ABMR through antibody-dependent cell-mediated cytotoxicity, but clinically applicable functional assays are lacking. We evaluated the Natural Killer-Cellular Humoral Activation Test (NK-CHAT), a functional assay quantifying antibody-dependent NK cell activation, as a non-invasive biomarker of ABMR. Fourteen kidney transplant recipients within six months post-transplantation were studied, including biopsy-confirmed ABMR (n = 7) and no major abnormalities (n = 7). NK cell activation was assessed longitudinally by CD107a degranulation and interferon-γ (IFN-γ) production after exposure to donor cells coated with recipient serum. CD107a-based NK-CHAT activity was significantly higher in ABMR recipients and detectable early after transplantation, whereas IFN-γ production did not differ between groups. These findings suggest that NK-CHAT captures functional NK cell activation associated with ABMR and may serve as a proof-of-concept functional assay with potential relevance to ABMR-associated immune activation.