Coordinated Circulating T Follicular Helper and Activated B Cell Responses Underlie the Onset of Antibody-Mediated Rejection in Kidney Transplantation
Authors: Louis Kevin , Macedo Camila , Bailly Elodie , Lau Louis , Ramaswami Bala , Marrari Marilyn , Landsittel Douglas , Chang Alexander , Chandran Uma , Fadakar Paul , Yamada Masaki , Chalasani Geetha , Randhawa Parmjeet , Zeevi Adriana , Singh Harinder , Lefaucheur Carmen , Metes Diana 
Published: July 2020

Significance Statement
Most studies investigating the mechanisms of antibody-mediated rejection, a major cause of kidney allograft failure, have focused on characterizing the role of donor-specific antibodies (DSAs), whereas the alloreactive cellular component has been less studied. On the basis of a multidimensional and concomitant profiling of circulating T follicular helper (TFH) cells and B cells, the authors identified highly coordinated responses of circulating TFH cells and activated B cells at phenotypic, functional, and transcriptional levels in patients with antibody-mediated rejection. The levels of circulating TFH cell and B cell activation were predictive of DSA pathogenicity, histologic severity, and allograft loss. This study provides novel mechanistic insights into the cellular and molecular processes underlying antibody-mediated rejection and a rationale for monitoring and therapeutic targeting of circulating TFH cell–B cell interaction during antibody-mediated rejection.

Background
Although antibody-mediated rejection (ABMR) has been long recognized as a leading cause of allograft failure after kidney transplantation, the cellular and molecular processes underlying the induction of deleterious donor-specific antibody (DSA) responses remain poorly understood.

Methods
Using high-dimensional flow cytometry, in vitro assays, and RNA sequencing, we concomitantly investigated the role of T follicular helper (TFH) cells and B cells during ABMR in 105 kidney transplant recipients.

Results
There were 54 patients without DSAs; of those with DSAs, ABMR emerged in 20 patients, but not in 31 patients. We identified proliferating populations of circulating TFH cells and activated B cells emerging in blood of patients undergoing ABMR. Although these circulating TFH cells comprised heterogeneous phenotypes, they were dominated by activated (ICOS+PD-1+) and early memory precursor (CCR7+CD127+) subsets, and were enriched for the transcription factors IRF4 and c-Maf. These circulating TFH cells produced large amounts of IL-21 upon stimulation with donor antigen and induced B cells to differentiate into antibody-secreting cells that produced DSAs. Combined analysis of the matched circulating TFH cell and activated B cell RNA-sequencing profiles identified highly coordinated transcriptional programs in circulating TFH cells and B cells among patients with ABMR, which markedly differed from those of patients who did not develop DSAs or ABMR. The timing of expansion of the distinctive circulating TFH cells and activated B cells paralleled emergence of DSAs in blood, and their magnitude was predictive of IgG3 DSA generation, more severe allograft injury, and higher rate of allograft loss.

Conclusions
Patients undergoing ABMR may benefit from monitoring and therapeutic targeting of TFH cell–B cell interactions.