K. Derkatz

ABO-incompatible heart transplantation in early childhood: An international multicenter study of clinical experiences and limits

BACKGROUND Intentional blood group (BG)-incompatible (ABOi) heart transplantation in childhood is emerging in many centers. Safety limits remain undetermined. In this multicenter study we have compiled experience on clinical and immunologic boundaries. METHODS Data from six centers in Europe and North America on ABOi transplantation were collected in a standardized survey. RESULTS Fifty-eight ABOi transplants were performed in 57 patients. Median age at transplant was 6.8 months (0.03 to 90 months); post-transplant follow-up was 37.7 months (0.46 to 117 months), accumulating 188 patient-years. Forty-seven percent of the patients received pretransplant mechanical circulatory support. Donors were either blood group A (n = 25), B (n = 18) or AB (n = 15). The median peak antibody titer to the donor BG pretransplant was 1:8 (0 to 1:64) for anti-A and 1:4 (0 to 1:32) for anti-B. Titers against the donor BG were lower post- than pretransplant in B recipients (p = 0.02), whereas third-party antibodies in BG O recipients developed normally post-transplant. Induction immunosuppression included anti-thymocyte globulin (61%), basiliximab (32%) or none (7%). All patients received calcineurin inhibitors, including 62% with mycophenolate mofetil, 10% with azathioprine, 2% with everolimus and 24% with steroids. There were 4 episodes of cellular rejection (Grade≥2R) and 7 antibody-mediated rejections. Five patients underwent antibody removal post-transplant. One patient developed severe graft vasculopathy. Freedom from death or retransplantation was 100%/96%/69% at 1/5/10 years. No graft loss was attributed to BG antibodies. CONCLUSIONS Successful ABOi heart transplantation can be performed at an older age and with higher isohemagglutinin titers than initially assumed and using similar immunosuppressive regimens as for ABO-compatible transplants. Rejection and graft vasculopathy are rare. Persistently low titers of antibodies to the donor BG post-transplant suggest elements of tolerance and/or accommodation.

Absence of Donor-Specific Anti-HLA Antibodies after ABO-Incompatible Heart Transplantation in Infancy – Altered Immunity or Age?

Specific B‐cell tolerance toward donor blood group antigens develops in infants after ABO‐incompatible heart transplantation, whereas their immune response toward protein antigens such as HLA has not been investigated. We assessed de novo HLA‐antibodies in 122 patients after pediatric thoracic transplantation (28 ABO‐incompatible) and 36 controls. Median age at transplantation was 1.7 years (1 day to 17.8 year) and samples were collected at median 3.48 years after transplantation. Antibodies were detected against HLA‐class I in 21 patients (17.2%), class II in 18 (14.8%) and against both classes in 10 (8.2%). Using single‐antigen beads, donor‐specific antibodies (DSAs) were identified in six patients (all class II, one additional class I). Patients with DSAs were significantly older at time of transplantation. In patients who had undergone pretransplant cardiac surgeries, class II antibodies were more frequent, although use of homografts or mechanical heart support had no influence. DSAs were absent in ABO‐incompatible recipients and class II antibodies were significantly less frequent than in children with ABO‐compatible transplants. This difference was present also when comparing only children transplanted below 2 years of age. Therefore, tolerance toward the donor blood group appears to be associated with an altered response to HLA beyond age‐related effects.

Increased Expression of the Inhibitory B-Cell Molecule CD22 on CD27+IgM+ B Cells Containing ABO-Antibody-Secreting Cells: A Role in Susceptibility to B-Cell Tolerance after ABO-Incompatible Infant Heart Transplantation?: 2329

Purpose: Immune immaturity allows ABO-incompatible heart transplantation (ABOi HTx) to be performed safely in infants and generally results in B-cell tolerance to donor ABO antigens. ABO antibodies are thought to arise in a T-independent (TI) manner; TI B-cell activation has been shown to be inhibited by the interaction of the inhibitory B-cell co-receptor CD22 with sialic acids on cells and tissues, leading to B-cell tolerance. It is unknown whether CD22 plays a role in regulating B-cell responses to ABO antigens in the transplant setting. Due to the generally reduced immune response to TI-antigens in early childhood, we hypothesized an enhanced role for CD22 during infancy compared to later in life. In this study we examined CD22 expression on various B-cell subsets. In addition, we performed functional assays to determine CD22 expression after B-cell stimulation and to assess the presence of ABO antigen-specific IgM antibody-secreting cells (ASC). Methods: We analyzed human splenocytes isolated from organ donors (n=41; ages 4 days 74 years). Flow cytometric analysis was performed to quantify the expression levels of CD22, CD27, CD38, IgM and IgG on the surface of CD19+ B cells. CD27+IgM+ B cells and CD27-IgM+ B cells were isolated from human splenocytes (n=5) by automated magnetic activated cell sorting, labelled with Cell Proliferation Dye eFluor450 and stimulated with CpG plus IL-2, IL-10 and IL-15. After one week, CD22 expression was examined by flow cytometry and the frequency of ABO antigen-specific ASC detected by ELISPOT. Results: Significant differences were observed when comparing the median fluorescence intensity (MFI) of CD22 amongst various B-cell subsets (p< 0.0001; Kruskal Wallis test). Post testing using the Dunn‘s multiple comparison test revealed that CD27+IgM+ B cells had higher expression (p< 0.001) than other B-cell subsets. Furthermore, the MFI of CD22 on the CD27+IgM+ B cells was inversely correlated with age (p=0.001), with infant samples having the highest level of CD22, and expression decreasing with increasing age. After culture, downregulated CD22 expression was observed in proliferating CD27+IgM+ B cells compared to non-proliferating cells. ELISPOT analysis showed that the vast majority of ABO antigen-specific ASC were derived from the CD27+IgM+ B-cell population. Conclusion: Based on previous studies showing the inhibitory role of CD22 on B-cell activation, our findings suggest that the increased expression of CD22 on the CD27+IgM+ B-cell subset, which includes precursors of ABO antigen-specific ASC, may cause these infant B cells to be more susceptible to down-regulation of B-cell signaling leading to subsequent inactivation. CD22 may therefore play an inhibitory role in immune responses to ABO antigens in the ABOi HTx setting. Studies are underway to determine the role of CD22 in B-cell signalling and regulation during infancy. 2374