Abstract Purpose Enhanced B-cell presentation of donor alloantigen relative to presentation of HLA-mismatched reference alloantigen is associated with acute cellular rejection (ACR), when expressed as a ratio called the antigen presenting index (API) in an exploratory cohort of liver and intestine transplant (LT, IT) recipients. Methods To test clinical performance, we measured the API using the previously described 6-hour assay in 84 LT and 54 IT with median age 3.3 years (0.05-23.96). Recipients experiencing ACR within 60 days after testing were termed rejectors. Results We first confirmed that B-cell uptake and presentation of alloantigen induced and thus reflected the alloresponse of T-helper cells, which were incubated without and with cytochalasin and primaquine to inhibit antigen uptake and presentation, respectively. Transplant recipients included 76 males and 62 females. Rejectors were tested at median 3.6 days before diagnosis. The API was higher among rejectors compared with non-rejectors (2.2 ± 0.2 vs 0.6 ± 0.04, p-value=1.7E-09). In logistic regression and ROC analysis, API ≥ 1.1 achieved sensitivity, specificity, positive and negative predictive values for predicting ACR in 99 training set samples. Corresponding metrics ranged from 80-88% in 32 independent post-transplant samples, and 73-100% in 20 independent pre-transplant samples. In time-to-event analysis, API ≥ 1,1 predicted higher incidence of late DSA after API measurements in LT (p=0.011) and graft loss in IT recipients (p=0.008), compared with recipients with API&lt;1.1, respectively. Conclusion Enhanced donor antigen presentation by circulating B-cells predicts rejection after liver or intestine transplantation as well as higher incidence of DSA and graft loss late after transplantation

Enhanced donor antigen presentation by B-cells predicts acute cellular rejection and late outcomes after transplantation