Stable Blood Group Chimerism in Recipients of ABO Incompatible Allogeneic Stem Cell Transplantation

Background: The ability to express the ABH antigens in oral mucosa and saliva is present in about 80% of the general population (hence called secretors) and is regulated by the Sec gene on chromosome 19q13.3, which encodes for the enzyme fut2 (α(1,2)Fucosyltransferase), resulting in the expression of blood types in body fluids, including saliva. The gene encoding for the ABH antigens that determines ABO blood type, is located on chromosome 9q34, and transcribes to the enzyme fut1. While fut1 is mainly expressed in erythroid tissue, fut2is expressed in secretory cells. The aim of the present study was to evaluate the ABO type in saliva and red blood cells of patients undergoing allogeneic SCT from ABO mismatched donors

Methods: Secretion status and ABO type in saliva and blood were analyzed in patients with different hematological malignancies undergoing alloSCT from ABO incompatible donors and from healthy donors serving as controls. All study participants signed informed consent. For the determination of ABO type in saliva, following mouth rinse, 5 cc of saliva were collected from each participant into fresh tube and immediately frozen at -80⁰C. Saliva ABH antigens were extracted and enzymes were inactivated. Secretor status and ABO type in patients' saliva were determined by inhibition test. Agglutination of diluted A, B or O typed donor red cells was tested macroscopically in the presence or absent of the extracted ABH antigens pre-incubated with anti-A, anti-B or anti-H, respectively. ABO blood type was routinely determined in patients at least every 2 weeks and time to ABO type conversion was recorded as along with all transplant-related clinical data

Results: The study cohort included 30 patients (16 males and 14 females; median age 54.2, 18.8-68.5), who underwent alloSCT between Dec 2009 and Feb 2014 from an ABO incompatible donor and were available for routine follow-up. Median follow-up time from transplant to last ABO determination in saliva was 613 days (153-2789).Donors were matched related in 11, matched unrelated in 16 and mismatched unrelated in 3 cases. All grafts were from mobilized peripheral blood. Transplant from major, minor and bidirectional ABO incompatible donors was present in 9, 16 and 5 recipients, respectively. All patients engrafted. Chimerism analysis at day 30 and 100 post transplant by PCR for STR was 100% in 24/25 and 23/25 of tested patients, respectively. Median days to ABO type conversion were 64 (21-290). Of 30 patients, 26 were found to be secretors (87%).In the secretor group, 29/30(96.6%) retained original blood group in the saliva, while one patient originally typed as AB and transplanted from an A type donor, did not retain his original AB blood type in the saliva. It is not clear whether the lack of B-antigen is attributed to the acute mucosal GvHD or to a true conversion of the ABH antigen expression in the saliva

Conclusion: To the best of our knowledge, this is the first report of stable chimerism of the ABO blood groups post ABO-incompatible allogeneic transplantation, such that the majority of recipients (96.6%) retained the recipient ABO group in the saliva, while expressing the donor ABO group in the blood. The significance of these findings and correlation with long-term outcome need to be further studied in larger patient cohort