Post‐haematopoietic cell transplantation outcomes: why ST2 became a ‘golden nugget’ biomarker

Summary Immunotherapies have emerged as highly promising approaches to treat cancer patients. Allogeneic haematopoietic cell transplantation (HCT) is the most validated tumour immunotherapy available to date but its clinical efficacy is limited by toxicities, such as graft‐versus‐host disease (GVHD) and treatment resistance leading to relapse. The problems with new cellular therapies and checkpoint inhibitors are similar. However, development of biomarkers post‐HCT, particularly for toxicities, has taken off in the last decade and has expanded greatly. Thanks to the advances in genomics, transcriptomics, proteomics and cytomics technologies, blood biomarkers have been identified and validated in promising diagnostic tests, prognostic tests stratifying for future occurrence of GVHD, and predictive tests for responsiveness to GVHD therapy and non‐relapse mortality. These biomarkers may facilitate timely and selective therapeutic intervention. This review outlines a path from biomarker discovery to first clinical correlation, focusing on soluble STimulation‐2 (sST2) ‒ the interleukin (IL)‐33‐decoy receptor ‒ which is the most validated biomarker.