Effect of fecal microbiota transplantation on gut microbiota functional profile in recipients of allogeneic hematopoietic cell transplantation.

Intestinal dysbiosis has been associated with both the effectiveness and toxicity of immunotherapy in cancer patients, inspiring multiple trials investigating fecal microbiota transplantation (FMT) in these patients. FMT restores microbial community structures damaged by antibiotics and enriches the microbiota with beneficial bacteria. However, the precise mechanism through which FMT exerts its effects and provides clinical benefits remains incompletely understood. Efforts to date have primarily focused on characterizing taxonomic changes following FMT. We hypothesized that FMT may also modify the functional pathways and metabolic capabilities of the gut microbiota, with possible clinical impact. To investigate this, we conducted a study involving 17 patients with blood disorders who received prophylactic FMT from one of the three healthy donors shortly after hematopoietic cell transplantation (HCT). By analyzing shotgun metagenomic profiles of the baseline, pre-FMT, and post-FMT gut microbiota, we demonstrate that FMT effectively restored pathways that had been depleted following HCT. However, it did not significantly reduce pathways that had expanded, indicating that FMT operates primarily through a restorative mechanism, reestablishing lost functional capabilities in the microbiota rather than suppressing overactive pathways. These findings highlight the potential for optimizing FMT protocols and identifying patient populations where FMT may be particularly beneficial.