RhD mismatch does not affect haematopoietic recovery, graft‐versus‐host disease and survival in allogeneic haematopoietic cell transplantation: A Japanese registry‐based study

ABO blood group mismatch between the donor and the recipient can affect the success of the transplant as well as problems with the red blood cells during allogeneic haematopoietic cell transplantation (HCT). However, the impact of the Rhesus (Rh) D mismatch on transplant outcomes in allogeneic HCT has been poorly elucidated.

The Rhesus (Rh) D polypeptide is a highly immunogenic antigen, and anti-RhD antibodies can cause clinically immunogenic haemolytic reactions, such as haemolytic disease of the foetus and newborn and acute haemolytic transfusion reactions [14].In allogeneic HCT, there have been reports of a few cases of immunogenic haemolytic reactions in RhD mismatch settings [4,6,7,13].The distribution of negative RhD blood groups, the most problematic of the Rh types, varies between populations and races.That is most common in individuals of European and North American descent (15%-17%), is relatively decreased in the regions of Africa and India (3%-8%) and is rare in Asia (0.1%-0.3%) [15].In Japan, the frequency of negative RhD individuals is very low, at 1 in 200 (0.5%).Even in allogeneic HCT, in which donors are selected mainly according to human leukocyte antigen (HLA), HCTs using RhD-negative donors or HCTs for RhDnegative recipients are sometimes performed.Still, the impact of the RhD mismatch on transplant outcomes in allogeneic HCT has been poorly elucidated [9,11].In this study, we evaluated the clinical impact of the RhD mismatch on clinical outcomes in allogeneic HCT using Japanese registry data.

Statistical analysis
To assess differences in baseline characteristics between groups, a chi-squared or Fisher exact test was performed for categorical data, T A B L E 1 Baseline characteristics of patients and transplantations.and the Kruskal-Wallis test was used for continuous variables.Unadjusted probabilities of haematopoietic recovery, GVHD, NRM and relapse were calculated using cumulative incidence estimates, while competing hazards were taken into consideration.Death prior to each event was a competing event for both GVHD and haematopoietic recovery.Relapse and NRM were opposing phenomena.The Kaplan-Meier method was used to calculate the unadjusted probability of OS.Grey's test for haematological recovery, GVHD, NRM and relapse, as well as a log-rank test for OS, were used in univariate analyses.A Cox proportional hazard model with adjustments for all endpoints was used for multivariate analysis, and the results were presented as a hazard ratio (HR) and 95% confidence interval (CI).The following variables for multivariate analysis were considered: recipient age (0-29 vs. 30-59 vs. ≥60 years), performance status (PS) (0-1 vs. 2-4), disease risk of HCT (standard risk vs. high risk), HCT period (2000-2010 vs. 2011-2021), graft source (bone marrow vs. peripheral blood stem cell [PBSC] vs. cord blood), donor type (related vs. unrelated), HLA disparities based on antigen level HLA-A, HLA-B and HLA-DR in the GVHD direction (match vs. mismatch), previous history of HCT (no vs. yes), sex compatibility between donor and recipient (match vs. male to female vs. female to male), ABO compatibility between donor and recipient (match vs. minor mismatch vs. major mismatch vs. bidirectional mismatch), conditioning regimen (myeloablative conditioning vs. reduced-intensity conditioning), GVHD prophylaxis

RESULTS
The characteristics of patients and transplants are shown in Table 1.
RhD mismatch was not associated with grades II-IV acute GVHD and chronic GVHD, overall mortality, NRM or relapse in univariate and multivariate analyses (Figure 1d-h, Table 2; Tables S2 and S3).

DISCUSSION
Unlike ABO alloantibodies, RhD alloantibodies do not occur naturally and require an immune response through exposure to RhD-positive RBCs through pregnancy, transfusion and transplantation.Therefore, RhD alloimmunization has occurred after allogeneic HCT, which means that the majority of RhD-mismatched HCT represents de novo RhD alloimmunization [17].However, the incidence of de novo RhD alloimmunization events has been infrequent following RhDmismatched HCT, ranging from 0% to 43% [3][4][5][8][9][10][11][12][13].Moreover, recipients with RhD alloimmunization did not always experience haemolysis during and after allogeneic HCT [17].However, in our study, the incidence of de novo RhD alloimmunization and clinical and laboratory haemolysis following RhD-mismatched HCT could not be evaluated because of incomplete data.Interestingly, although the RhD (À/À) group contained higher proportions of use of PBSC, related donors and HLA match, our data showed that the last transfusion days of RBC and platelets were slightly later in RhD mismatch groups compared with RhD match groups, which suggests that the number of RBC and platelet transfusions could be higher in RhD mismatch groups compared with RhD match groups.This is consistent with a previous study showing that an RhD mismatch led to a significant increase in RBC transfusions in the second month after HCT [9].
Therefore, the frequency and severity of de novo RhD immunization and haemolysis after RhD-mismatched HCT warrant further study with a larger cohort.
Apart from haemolysis after HCT, previous studies demonstrated that ABO incompatibility affected neutrophil and platelet recovery, acute GVHD, NRM or OS after allogeneic HCT [1,2].By contrast, there were only two reports evaluating the impact of RhD mismatch on transplant outcomes [9,11].Erker et al. [9] in Germany reported that the OS of RhD-mismatched groups was significantly lower than that of RhD-matched groups in 143 allogeneic PBSC transplants.By contrast, Wirk et al. [11]  In summary, our registry-based study demonstrated that RhD mismatch between recipient and donor did not significantly impact haematopoietic recovery, GVHD, NRM, relapse or OS after allogeneic HCT.Since the frequency of negative RhD individuals is very low in Japan (about 0.5%), it is anticipated that the challenges associated with obtaining allogeneic donors and blood transfusion supplies would raise the risk of allogeneic HCT.However, RhD mismatches may not need to be avoided for recipient and donor combinations in allogeneic HCT.
Clinical data were provided by the Transplant Registry Unified Management Program of the Japanese Data Center for Haematopoietic Cell Transplantation and the Japanese Society for Transplantation and Cellular Therapy.Patients who underwent allogeneic HCT between 2000 and 2021 in Japan were included in this study.We excluded patients who lacked data on RhD between recipients and donors and survival.Finally, 64,923 patients were eligible for this study.Details for the methods are provided in the Supplementary Methods.
in the United States reported that the RhD mismatch was not associated with OS, NRM or acute and chronic GVHD in 258 allogeneic HCTs, which is consistent with our results.Multivariate analysis of RhD compatibility for the outcomes after allogeneic HCT.
and the total number of transfusions.Second, the distribution of negative RhD blood groups varies between populations and races.Therefore, our results should be interpreted cautiously when extended to other racial cohorts.Despite some limitations, this study included the largest patient population, which could show no impact of the RhD mismatch on post-transplant outcomes after allogeneic HCT.T A B L E 2Abbreviations: CI, confidence interval; GVHD, graft-versus-host disease; HCT, haematopoietic cell transplantation; HR, hazard ratio; NRM, nonrelapse mortality; OS, overall survival; RhD, rhesus D.