In this retrospective, single-center study we originally tested the hypothesis that pre-transplant administration of rituximab in patients with major or bidirectional ABO incompatibility improves RBC engraftment. Adding rituximab to conditioning resulted in a statistically significant reduction of the time to achieve transfusion independence after transplant compared to a historical cohort of patients who did not receive any preventive strategy for the reduction of complications related to ABO major incompatibility. Rituximab is currently used in ABO-incompatible solid organ transplantation as part of desensitization methods (17, 18). To our knowledge, in the setting of ABO-incompatibility HSCT, this is the first experience with the use of rituximab during conditioning with the specific aim of reducing the immunological complications of major blood group incompatibility.
Current guidelines for the prevention of delayed red blood cell engraftment and of PRCA include the following measures: to select, when possible, a donor without ABO major incompatibility, to reduce graft RBC to ensure an RBC volume < 20 mL and to remove host isohemagglutinins by therapeutic plasma exchange before transplant and pre-transplant immunoabsorption.(13). The use of plasma exchange alone for the reduction of isohemagglutinins seems to have only a temporarily limited effect since the rebound of anti-donor isohemagglutinins titer is described(13). Pre-transplant immunoadsorption may be performed with an infusion of donor-type fresh frozen plasma positive in A or B antigens to neutralize respective isohemagglutinins(19)(20) or with donor-type RBC transfusion (21)(22). The study of Stussi et al(14) reported the successful use of pretransplant reduction of host anti-donor isoagglutinins with different methods. In this study, 153 patients undergoing transplant with major ABO-incompatible donors were treated with plasmapheresis alone (n = 6), in vivo absorption (n = 70), a combination of both (n = 22) or only with the removal of RBC from the graft (n = 55). Twelve patients (8%) developed PRCA: 3/98 (3%) with, and 9/55 (16%) without prior isoagglutinin reduction. Patients who underwent anti-donor isoagglutinin reduction had faster RBC engraftment. Although effective, in vivo immunoabsorption requires high expertise in transfusion medicine, and the transfusion reactions reported after this procedure (hemolysis, fever, rigors, hematuria, and lumbar pain) may limit the feasibility of this method.
In our center, plasma exchange or in vivo immunoadsorption do not represent standard procedures to prevent PRCA or other RBC engraftment complications in ABO major incompatibility transplants. As a consequence, only 3 over 55 patients were treated with plasma exchange in the rituximab group due to very high anti-donor isohemagglutinins titer, while none of the patients in the control group were treated with this procedure. This prevents us from speculating on a comparison between rituximab and plasma exchange in reducing transfusion burden after transplant or the time to red blood cell engraftment.
We did not find a significant difference in the incidence of immunological complications such as PRCA (2% of patients in the rituximab group, 6% of patients in the control group) or post-transplant immune hemolytic anemia (none of the patients in both groups). This observation may be due to the limited number of patients enrolled in this study and requires further studies.
The use of rituximab in pre-transplant conditioning has already been explored in other settings since different conditioning regimens for lymphoproliferative disorders include this drug to improve disease control after transplant(23–25). These data support the feasibility of adding rituximab to conditioning. In our experience, rituximab added to conditioning was well tolerated with no immediate adverse reactions, and neutrophils and platelet engraftment were not impaired by rituximab. Moreover, TRM was not significantly different for the Rituximab or control groups.
This study has several limitations that deserve to be analyzed. The first one is the retrospective and monocentric nature of the study. Moreover, the decision to administer rituximab in patients with major or bidirectional ABO incompatibility was started in 2016 and since then 15 out of 66 patients did not receive rituximab because of a low anti-donor isohemagglutinins titer. Despite this possible negative selection bias, the rituximab group had a better outcome concerning time to red blood cell engraftment compared with the control group, so we may speculate that a comparison between more homogeneous groups should at least confirm the results of this study. A matched analysis for anti-donor isohemagglutinins titer would resolve this issue but it was not statistically feasible due to the small number of patients in the two cohorts.
Despite the better time to red blood cell engraftment, we could not demonstrate a significant reduction in the median number of red blood cell units transfused after transplant for patients treated with rituximab. We are tempting to speculate that this may be related to the high heterogeneity of the patient populations and to the broad variability of the number of red blood cell units required by the two groups.
The use of rituximab in the peri-transplant setting can potentially increase the post-transplant infection risk. In our study, patients who received rituximab experienced a significant impairment in the post-transplant CD19+ B cells reconstitution up to 180 days after transplant. This laboratory finding was associated with an increased rate of CMV reactivations for patients treated with rituximab compared to the control group. With the use of pre-emptive therapy, none of the patients in the rituximab group experienced CMV-disease. The problem of CMV reactivation could probably be mitigated by the introduction of prophylaxis with Letermovir, which was not available in the years investigated in this study. Moreover, it must be considered that rituximab and Letermovir prophylaxis significantly increase the economic burden of the transplant procedure.
Previous reports suggested that rituximab administration before transplant may have a role in reducing the risk of both acute(26, 27) or chronic GVHD(28, 29). In our study, we did not find a significant difference in the incidence of acute or chronic GVHD between patients in the rituximab group and in the control group. This could be explained by the broad use of anti-thymocyte globulin for the prophylaxis of GVHD in the two study groups, which may have limited the potential effect of rituximab on reducing the GVHD risk.
The multivariable analysis supported that rituximab use was associated with better RBC engraftment. Other factors associated with transfusion independence were the disease status at transplant and the total number of CD34 + cells infused. Since this last finding, it is advisable to warrant a good infusion of CD34 + cells in a major ABO incompatibility transplant setting.
In conclusion, the result of this study confirms that rituximab added to conditioning regimens is feasible, safe, and significantly improves time to red blood cell engraftment after a major ABO-incompatible transplant. Additional studies are needed to understand the role of rituximab on infection risk, GVHD risk, and long-term effects on engraftment and transplant outcome.