Toward an Integrated View of Operational Tolerance in Human Renal Transplantation: A Systems Biology Perspective

Operational tolerance (OT) is the phenomenon occurring in human renal and liver transplantation in which the body does not reject the organ after discontinuing immunosuppression for at least a year. We revisited the data generated by The Brazilian Multicenter Study on Operational Tolerance involving different conceptual fields - antigen-specific cytokine response, immune cell numbers and repertoire, signaling pathways, and epigenetics. We integrated our data to pave the way to systems biology thinking and harness debate on potential mechanisms in OT. We present original data on systems biology in OT, connecting potential mechanistic players. Using bioinformatics, we identified three dominant features that discriminate OT from its opposing clinical outcome, chronic rejection (CR). The OT-CR discriminative molecules were FOXP3, GATA3 and STAT6, each corresponding to a differential profile: (1) In FOXP3, OT presents preserved regulatory T cell (Treg) numbers but decreased numbers in CR; (2) in GATA3, increased expression is seen in OT; and (3) in STAT6, decreased monocyte activation is seen in OT. With these variables, we built molecular networks to identify interactions related to OT versus CR. Our first systems biology endeavor gave rise to novel potentially relevant interconnected players in OT mechanisms: FOXP3 connecting to interleukin-9 (IL-9) and IL-35 signaling, suggesting their immunoregulatory involvement in OT. Likewise, GATA3/FOXP3 interactions incrementing/stabilizing FOXP3 transcription suggest participation in keeping healthy FOXP3+ Tregs in OT. We envision that systems biology thinking will greatly contribute to advancing knowledge in human transplantation tolerance in an interactive perspective.