Table 1.

Remaining questions for the development of intestinal organoid transplantation therapy for IBD patients.

Questions	Suggested solution(s)
At the cell culture level
Can we expand patient derived organoids in vitro at a proliferation efficiency comparable to those derived from healthy donors?	• May be possible depending on culture conditions and cell source (REF 44).
Can we expand patient derived organoids in a completely xeno-free culture condition? Or otherwise in a fully defined culture condition?	• Adaptation or modification of the collagen-based method may be suitable (REF 40).
What kind of tests should we apply for the quality assurance and quality control of donor organoids?	• Evaluation of stem cell function and/or content may be required.
How could we exclude the tumorigenicity of the donor organoids?	• Data of in vivo transplantation may be highly supportive (REF 43).
Can we expand patient derived organoids without enhancing the risk of infectious pathogen-related adverse events?	• Standard sterility tests, endotoxin tests should be confirmed. In addition, viruses or mycoplasmas should be examined following the standard methods for clinical grade products.
At the cell transplantation level
Is it better to deliver ISCs as an organoid, or otherwise as a cell sheet?	• Engraftment ability confirmed only for organoids.
What kind of device is suitable to efficiently deliver organoids through an endoscopic procedure?	• Readily available endoscopic tools, or custom-made devices should be tested.
Is an additional technique/procedure required to promote the engraftment of the donor organoids?	• Additional procedure may be required to let the organoids stay at the desired region until they finish the initial engraftment process.
Is there any host mucosal condition that is beneficial or inversely unfavorable for the engraftment of the donor organoids?	• Needs to be evaluated in pre-clinical studies using colitis mice models, and further evaluated in initial-phase clinical trials.
At the clinical level
What will be the best index to evaluate the clinical effect of organoid transplantation?	• Needs to be evaluated in early-phase clinical trials.
What kind of patients is the best candidate of organoid transplantation? UC or CD?	• Needs to be evaluated in mid ~ late-phase clinical trials.
Can we identify the donor cell derived crypts within the recipient mucosa?	• Identification of a reliable donor-cell marker, or clinically available method for donor-cell labeling should be developed.
Is it better to perform an allogenic organoid transplantation from a healthy donor instead of an autologous transplantation?	• Pre-clinical evidence should be established using transplantation to the colitis model.