Major complications after hematopoietic stem cell transplantation (HSCT) include severe sepsis, kidney injury, and graft-versus-host disease (GVHD) and are the main causes of mortality and low survival rates. Despite overall outcomes after HSCT improving significantly in recent decades, transplant-related complications are still the most important causes of late morbidity and mortality. Along with a greater understanding of the gut microbiota (GM), increasing evidence has suggested that there is a close link between GM and transplant-related complications.

GM has a symbiotic relationship with the human host and plays a key role in maintaining human wellness. Dysbiosis, defined as a disruption of the microbiome natural balance, seems to be a contributor to the development and maintenance of several diseases. Patients undergoing HSCT can experience significant shifts in GM due to their underlying malignancies and exposure to extensive chemotherapy and systemic antibiotics, which may lead to biological disorders. The imbalance of GM may impair the intestinal mucosal barrier and induce systemic inflammation, and its metabolites can also exert direct or indirect effects on transplant-related complications after HSCT. Among the many effects and impacts of GM, its role in the production and regulation of OS is increasingly recognized as one of the most significant. OS, an imbalance between oxidants and antioxidants in living organisms, can lead to cellular damage caused by reactive oxygen species (ROS). ROS, a kind of signaling molecule, may contribute to cell viability and tissue oxygen metabolism or inhibit the expression of related genes. Aberrant production of nitric oxide (NO) generates ROS associated with cellular damage, neuroinflammation, neurodegenerative disorders, and axonal degeneration. The dysbiosis of GM may result in excessive bioavailability of ROS and contribute to an increase in OS. Thus, the interaction between OS and GM is expected to become a new potential therapeutic target in complications after HSCT.

In this Special Issue, we aim to provide new insights into the involvement of OS and GM in transplant-related complications after HSCT. Knowledge of the close link between OS and GM, and their role in major complications after HSCT will help develop strategies to reduce morbidity and mortality. We invite researchers to contribute original research as well as review articles.

Potential topics include but are not limited to the following:

• Specific changes in GM and its metabolites during different engraftment periods
• The production and regulation of OS in transplant-related complications after HSCT
• The relationship and regulatory mechanisms between OS and GM in transplant-related complications after HSCT
• Bacterial and intestinal flora metabolites as potential biomarkers or therapeutic targets for preventing major complications after HSCT
• Timely and early maintenance of the balance of GM and OS as strategies to prevent major complications after HSCT