Circulating B Cells With Memory and Antibody-Secreting Phenotypes Are Detectable in Pediatric Kidney Transplant Recipients Before the Development of Antibody-Mediated Rejection

Background. Development of anti-human leukocyte antigen donor-specific antibodies (DSAs) is associated with antibody-mediated rejection (AMR) and reduced allograft survival in kidney transplant recipients. Whether changes in circulating lymphocytes anticipate DSA or AMR development is unclear. Methods. We used time-of-flight mass cytometry to analyze prospectively collected peripheral blood mononuclear cells (PBMC) from pediatric kidney transplant recipients who developed DSA (DSA-positive recipients [DSA(POS)], n = 10). PBMC were obtained at 2 months posttransplant, 3 months before DSA development, and at DSA detection. PBMC collected at the same time points posttransplant from recipients who did not develop DSA (DSA-negative recipients [DSA(NEG)], n = 11) were used as controls. Results. DSA(POS) and DSA(NEG) recipients had similar baseline characteristics and comparable frequencies of total B and T cells. Within DSA(POS) recipients, there was no difference in DSA levels (mean fluorescence intensity [MFI]: 13 687 +/- 4159 vs 11 375 +/- 1894 in DSA(POS)AMR-positive recipients (AMR(POS)) vs DSA(POS)AMR-negative recipients (AMR(NEG)), respectively; P = 0.630), C1q binding (5 DSA(POS)AMR(POS) [100%] vs 4 DSA(POS)AMR(NEG) [80%]; P = 1.000), or C3d binding (3 DSA(POS)AMR(POS) [60%] vs 1 DSA(POS)AMR(NEG) [20%]; P = 0.520) between patients who developed AMR and those who did not. However, DSA(POS) patients who developed AMR (n = 5; 18.0 +/- 3.6 mo post-DSA detection) had increased B cells with antibody-secreting (IgD(-)CD27(+)CD38(+); P = 0.002) and memory (IgD(-)CD27(+)CD38(-); P = 0.003) phenotypes compared with DSA(NEG) and DSA(POS)AMR(NEG) recipients at DSA detection. Conclusions. Despite the small sample size, our comprehensive phenotypic analyses show that circulating B cells with memory and antibody-secreting phenotypes are present at DSA onset, >1 year before biopsy-proven AMR in pediatric kidney transplant recipients.