Immune factors influencing ethnic disparities in kidney transplantation outcomes

Genetic versus self-reported African ancestry of the recipient and neighborhood predictors of kidney transplantation outcomes in 2 multiethnic urban cohorts

African American (AA) kidney recipients have a higher risk of allograft rejection and failure compared to non-AAs, but to what extent these outcomes are due to genetic versus environmental effects is currently unknown. Herein, we tested the effects of recipient self-reported race versus genetic proportion of African ancestry (pAFR), and neighborhood socioeconomic status (SES) on kidney allograft outcomes in multiethnic kidney transplant recipients from Columbia University (N = 1083) and the University of Pennsylvania (N = 738). All participants were genotyped with SNP arrays to estimate genetic admixture proportions. US census tract variables were used to analyze the effect of neighborhood factors. In both cohorts, self-reported recipient AA race and pAFR were individually associated with increased risk of rejection and failure after adjustment for known clinical risk factors and neighborhood SES factors. Joint analysis confirmed that self-reported recipient AA race and pAFR were both associated with a higher risk of allograft rejection (AA: HR 1.61 (1.31-1.96), P = 4.05E-06; pAFR: HR 1.90 (1.46-2.48), P = 2.40E-06) and allograft failure (AA: HR 1.52 (1.18-1.97), P = .001; pAFR: HR 1.70 (1.22-2.35), P = .002). Further research is needed to disentangle the role of genetics versus environmental, social, and structural factors contributing to poor transplantation outcomes in kidney recipients of African ancestry.

Progress made toward equitable transplantation in children and young adults with kidney disease

Racial disparities in pediatric kidney transplantation have been well described over the last two decades and include disparities in preemptive transplantation, waitlisting, time from activation to transplantation, living donation, and graft outcomes. Changes to the organ allocation system including the institution of Share 35 in 2005 and the Kidney Allocation System (KAS) of 2014 have resulted in resolution of some, but not all racial-ethnic disparities. Despite overall improvements in time from waitlist activation to transplant, disparities remain in preemptive transplantation, time to waitlisting, and living donor transplantation. Although improving under the KAS, racial disparities remain in graft survival as well. Racial disparity in kidney transplant access and graft survival is an international problem within pediatric nephrology. Although the racial group affected may differ, various minoritized pediatric groups across the world are affected by transplant disparities. Social determinants of health including financial access, language barriers, and the presence of a healthy living donor play a role in mediating these disparities. Further investigation is needed to better understand and intervene upon modifiable social, biological, and cultural factors driving the remaining disparity in transplant outcomes.

Improving Access to HLA-Matched Kidney Transplants for African American Patients

Introduction

Kidney transplants fail more often in Black than in non-Black (White, non-Black Hispanic, and Asian) recipients. We used the estimated physicochemical immunogenicity for polymorphic amino acids of donor/recipient HLAs to select weakly immunogenic kidney transplants for Black vs. White or non-Black patients.

Methods

OPTN data for 65,040 donor/recipient pairs over a 20-year period were used to calculate the individual physicochemical immunogenicity by hydrophobic, electrostatic and amino acid mismatch scores (HMS, EMS, AMS) and graft-survival outcomes for Black vs. White or vs. non-Black recipients, using Kaplan-Meier survival and Cox regression analyses. Simulations for re-matching recipients with donors were based on race-adjusted HMS thresholds with clinically achievable allocations.

Results

The retrospective median kidney graft survival was 12.0 years in Black vs. 18.6 years in White (6.6-year difference; p>0.001) and 18.4 years in non-Black (6.4-year difference; p>0.01) recipients. Only 0.7% of Blacks received transplants matched at HLA-A/B/DR/DQ (HMS=0) vs. 8.1% in Whites (p<0.001). Among fully matched Blacks (HMS=0), graft survival was 16.1-years and in well-matched Blacks (HMS ≤ 3.0) it was 14.0-years. Whites had 21.6-years survival at HMS ≤ 3.0 and 18.7-years at HMS ≤ 7.0 whereas non-Blacks had 22.0-year at HMS ≤ 3.0 and 18.7-year at HMS ≤ 7.0, confirming that higher HMS thresholds produced excellent survival. Simulation of ABO-compatible donor-recipient pairs using race-adjusted HMS thresholds identified weakly immunogenic matches at HMS=0 for 6.1% Blacks and 18.0% at HMS ≤ 3.0. Despite prioritizing Black patients, non-Black patients could be matched at the same level as in current allocation (47.0% vs 56.5%, at HMS ≤ 7.0).

Conclusions

Race-adjusted HMS (EMS, AMS)-based allocation increased the number of weakly immunogenic donors for Black patients, while still providing excellent options for non-Black recipients.

Feature Importance of Acute Rejection among Black Kidney Transplant Recipients by Utilizing Random Forest Analysis: An Analysis of the UNOS Database

Background: Black kidney transplant recipients have worse allograft outcomes compared to White recipients. The feature importance and feature interaction network analysis framework of machine learning random forest (RF) analysis may provide an understanding of RF structures to design strategies to prevent acute rejection among Black recipients. Methods: We conducted tree-based RF feature importance of Black kidney transplant recipients in United States from 2015 to 2019 in the UNOS database using the number of nodes, accuracy decrease, gini decrease, times_a_root, p value, and mean minimal depth. Feature interaction analysis was also performed to evaluate the most frequent occurrences in the RF classification run between correlated and uncorrelated pairs. Results: A total of 22,687 Black kidney transplant recipients were eligible for analysis. Of these, 1330 (6%) had acute rejection within 1 year after kidney transplant. Important variables in the RF models for acute rejection among Black kidney transplant recipients included recipient age, ESKD etiology, PRA, cold ischemia time, donor age, HLA DR mismatch, BMI, serum albumin, degree of HLA mismatch, education level, and dialysis duration. The three most frequent interactions consisted of two numerical variables, including recipient age:donor age, recipient age:serum albumin, and recipient age:BMI, respectively. Conclusions: The application of tree-based RF feature importance and feature interaction network analysis framework identified recipient age, ESKD etiology, PRA, cold ischemia time, donor age, HLA DR mismatch, BMI, serum albumin, degree of HLA mismatch, education level, and dialysis duration as important variables in the RF models for acute rejection among Black kidney transplant recipients in the United States.

Change in Mycophenolate and Tacrolimus Exposure by Transplant Vintage and Race

OBJECTIVES

Although both tacrolimus and mycophenolate have improved outcomes after kidney transplant, studies regarding effects of exposure on outcomes, specifically related to racial disparities, are sparse.

MATERIALS AND METHODS

In this 8-year longitudinal cohort study of adult kidney transplant recipients, mycophenolate and tacrolimus levels were compared across transplant vintage stratified by non-African Americans versus African Americans. Data were analyzed with standard univariate tests and multivariable regression models.

RESULTS

Our study included 1217 patients (transplanted from 2005-2013) who had tacrolimus and myco-phenolate exposure data, with follow-up through 2015 (53.7% were African Americans). Mean mycophenolate dose was 1672 ± 463 mg/day during the first 3 years posttransplant. Although transplant vintage did not appreciably impact mycophenolate dosing in non-African Americans (0.7 mg/day/y; P = .903), doses significantly decreased in African Americans across transplant vintage (-20.5 mg/day/y; P < .001). Rate of mycophenolate being held or discontinued based on transplant vintage significantly increased in African Americans but did not change in non-African Americans. At the beginning of the study, mean tacrolimus levels were lower in African Americans; however, levels then slightly decreased in non-African Americans (-0.03 ng/mL/y; P = .279) and slightly increased in African Americans (+0.03 ng/mL/y; P = .247), with similar levels by 2013. Higher tacrolimus levels were protective against rejection in African Americans only but were protective against death-censored graft loss in both race/ethnicity groups. Mycophenolate dosing had no appreciable impact on outcomes in African Americans, but higher mycophenolate dosing was a significant risk factor for death-censored graft loss in non-African Americans.

CONCLUSIONS

Tacrolimus and mycophenolate exposure levels have significantly changed over time and differed by race/ethnicity. In non-African Americans, those transplanted more recently tended to have lower tacrolimus but similar mycophenolate exposure. Although mycophenolate exposure in African Americans has recently decreased, tacrolimus has increased. Differences in outcomes likely reflect improved understanding of immunosuppressant tolerability by recipient race/ethnicity.

The impact of time-varying clinical surrogates on disparities in African-American kidney transplant recipients - a retrospective longitudinal cohort study

An improved understanding of the impact of clinical surrogates on disparities in African-American (AA) kidney transplantation (KTX) is needed. We conducted a 10-year retrospective longitudinal cohort study of electronically abstracted clinical data assessing the impact of surrogates on disparities in KTX. Clinical surrogates were assessed by posttransplant year (1, 2, 3 or 4) and defined as acute rejection (Banff ≥1A), mean SBP >140 mmHg, tacrolimus variability (CV) >40%, mean glucose >160 mg/dl and mean hemoglobin <10 g/dl. We utilized landmark methodology to minimize immortal time bias and logistic and survival regression to assess outcomes; 1610 KTX were assessed (54.2% AAs), with 1000, 468, 368 and 303 included in the year 1, 2, 3 and 4 complete case analyses, respectively. AAs had significantly higher odds of developing a clinical surrogate, which increased in posttransplant years three and four [OR year 1 1.99 (1.38-2.88), year 2 1.77 (1.20-2.62), year 3 2.35 (1.49-3.71), year 4 2.85 (1.72-4.70)]. Adjusting for the five clinical surrogates in survival models explained a significant portion of the higher risks of graft loss in AAs in post-transplant years three and four. Results suggest focusing efforts on improving late clinical surrogate management within AAs may help mitigate racial disparities in KTX.

Transplant Center Variability in Disparities for African-American Kidney Transplant Recipients

Background

Disparities research has traditionally focused on patient-level variables to ascertain predominant risk factors driving differences in outcomes for African-American (AA) kidney transplant recipients. Our objectives were to determine the magnitude and impact of transplant center variability for graft outcome disparities.

Material/Methods

This was a retrospective cohort study analyzing 25 years of U.S. national transplant registry data at both the patient and center levels using univariate descriptive statistics and multivariable modeling.

Results

A total of 257,024 recipients from 191 centers were analyzed; AAs represented 31.1% of recipients. After adjusting for baseline characteristics, AAs had 42% higher risk of graft loss (aHR 1.42, 95% CI 1.39 to 1.45; p<0.001). Center variability for graft outcome disparities in AAs was significant (race*center interaction term p<0.05), with the aHRs ranging from 0.5 to 4.9; 46% of centers demonstrated a non-statistically significant disparity (aHR p>0.05) and 25% of centers had a large AA disparity (aHR >1.75). In a more recent transplant time period (2000–14), overall racial disparities decreased but center-level disparities increased in variability. Center-level factors significantly associated with increasing disparity included higher acute rejection rates, fewer transplants per year, longer length of stay, lower use of calcineurin inhibitors (CNI), and lower living donor rates.

Conclusions

There is evidence of significant center-level variability in graft outcome disparities for AA kidney recipients. Further, there appears to be a number of center-level factors associated with this variability, including acute rejection rates, CNI use, number of transplants per year, and, in recent years, low living donor rates.

Tacrolimus Trough Concentration Variability and Disparities in African American Kidney Transplantation

BACKGROUND

Low tacrolimus concentrations have been associated with higher risk of acute rejection, particularly within African American (AA) kidney transplant recipients; little is known about intrapatient tacrolimus variabilities impact on racial disparities.

METHODS

Ten year, single-center, longitudinal cohort study of kidney recipients. Intrapatient tacrolimus variability was assessed using the coefficient of variation (CV) measured between 1 month posttransplant and the clinical event, with a comparable period assessed in those without events. Pediatrics, nontacrolimus/mycophenolate regimens, and nonrenal transplants were excluded. Multivariable Cox regression models were used to analyze data.

RESULTS

One thousand four hundred eleven recipients were included (54.4% AA) with 39 521 concentrations used to assess intrapatient tacrolimus CV. Overall, intrapatient tacrolimus CV was higher in AAs versus non-AAs (39.9 ± 19.8 % vs 34.8 ± 15.8% P < 0.001). Tacrolimus variability was a significant risk factor for deleterious clinical outcomes. A 10% increase in tacrolimus CV augmented the risk of acute rejection by 20% (adjusted hazard ratio, 1.20, 1.13-1.28; P < 0.001) and the risk of graft loss by 30% (adjusted hazard ratio, 1.30, 1.23-1.37; P < 0.001), with significant effect modification by race for acute rejection, but not graft loss. High tacrolimus variability (CV >40%) was a significant explanatory variable for disparities in AAs; the crude relative risk of acute rejection in AAs was reduced by 46% when including tacrolimus variability in modeling and reduced by 40% for graft loss.

CONCLUSIONS

These data demonstrate that intrapatient tacrolimus variability is strongly associated with acute rejection in AAs and graft loss in all patients. Tacrolimus variability is a significant explanatory variable for disparities in AA recipients.

Cytolytic Induction Therapy Improves Clinical Outcomes in African-American Kidney Transplant Recipients

OBJECTIVE

Determine the impact of cytolytic versus IL-2 receptor antibody (IL-2RA) induction on acute rejection, graft loss and death in African-American (AA) kidney transplant (KTX) recipients.

BACKGROUND

AAs are underrepresented in clinical trials in transplantation; thus, there is controversy regarding the optimal choice of perioperative antibody induction in KTX to improve outcomes.

METHODS

National cohort study using US transplant registry data from January 1, 2000 to December 31, 2009 in adult solitary AA KTX recipients, with at least 5 years of follow-up. Multivariable logistic and Cox regression were utilized to assess the outcomes of acute rejection, graft loss, and mortality, with interaction terms to assess effect modification.

RESULTS

Twenty-five thousand eighty-four adult AAs receiving solitary KTX were included, 16,927 (67.5%) received cytolytic induction and 8157 (32.5%) received IL-2RA induction. After adjustment for recipient sociodemographics, donor, and transplant characteristics, the use of cytolytic induction therapy reduced the risk of acute rejection by 32% (OR 0.68, 0.62-0.75), graft loss by 9% (HR 0.91, 0.86-0.97), and death by 12% (HR 0.88, 0.83-0.94). There were a number of significant effect modifiers, including public insurance, panel reactive antibody, delayed graft function, and steroid withdrawal; in these groups, cytolytic induction substantially improved clinical outcomes.

CONCLUSIONS

These data demonstrate that cytolytic induction therapy, as compared with IL-2RA, reduces the risk of rejection, graft loss, and death in adult AA KTX recipients, particularly in those who are sensitized, receive public insurance, develop delayed graft function, or undergo steroid withdrawal.

A Comparative Effectiveness Analysis of Early Steroid Withdrawal in Black Kidney Transplant Recipients

BACKGROUND AND OBJECTIVES

There is continued debate whether early steroid withdrawal is safe to use in high-immunologic risk patients, such as blacks. The goal of this study was to use comparative effectiveness methodology to elucidate the safety of early steroid withdrawal in blacks with kidney transplants.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Our cohort study used United Network of Organ Sharing data including all adult black kidney transplant recipients from 2000 to 2009 followed through 2014. Propensity score matching was used to equalize baseline risk between continued steroid and early steroid withdrawal groups. Interaction terms were used to assess if the effect of early steroid withdrawal on outcomes varied by baseline and post-transplant factors. Of 26,582 eligible black patients with kidney transplants (5825 [21.9%] with early steroid withdrawal), 5565 patients with early steroid withdrawal were matched to 5565 blacks on continued steroid use.

RESULTS

Black patients with early steroid withdrawal had similar risk of graft loss (hazard ratio, 0.98; 95% confidence interval, 0.92 to 1.04; P=0.42) and lower risk of death (hazard ratio, 0.91; 95% confidence interval, 0.84 to 0.99; P=0.02), primarily driven by a late mortality advantage (>4 years post-transplant). Delayed graft function, cytolytic induction, tacrolimus, and mycophenolate significantly modified the effect of early steroid withdrawal on outcomes (P<0.05). Acute rejection rates were slightly higher in the continued steroid group (13.0% versus 11.3%, respectively; P<0.01), but this was not associated with graft or patient survival.

CONCLUSIONS

Overall, early steroid withdrawal in black kidney transplant recipients was not associated with graft loss but seemed to be associated with better long-term patient survival. Early steroid withdrawal in blacks not receiving cytolytic induction, tacrolimus, and mycophenolate or those with delayed graft function was associated with higher risk of graft loss and death.

Outcome disparities between African Americans and Caucasians in contemporary kidney transplant recipients

BACKGROUND

Racial disparities in African-American (AA) kidney transplant have persisted for nearly 40 years, with limited data available on the scope of this issue in the contemporary era of transplantation.

METHODS

Descriptive retrospective cohort study of US registry data including adult solitary kidney transplants between Jan 1, 2005 to Dec 31, 2009.

RESULTS

60,695 recipients were included; 41,426 Caucasians (68%) and 19,269 AAs (32%). At baseline, AAs were younger, had lower college graduation rates, were more likely to be receiving public health insurance and have diabetes. At one-year post-transplant, AAs had 62% higher risk of graft loss (RR 1.62, 95% CI 1.50-1.75) which increased to 93% at five years (RR 1.93, 95% CI 1.85-2.01). Adjusted risk of graft loss, accounting for baseline characteristics, was 60% higher in AAs (HR 1.61 [1.52-1.69]). AAs had significantly higher risk of acute rejection and delayed graft function.

CONCLUSION

AAs continue to experience disproportionately high rates of graft loss within the contemporary era of transplant, which are related to a convergence of an array of socioeconomic and biologic risk factors.

Twenty years of evolving trends in racial disparities for adult kidney transplant recipients

Disparities in outcomes for African American (AA) kidney transplant recipients have persisted for 40 years without a comprehensive analysis of evolving trends in the risks associated with this disparity. Here we analyzed U.S. transplant registry data, which included adult Caucasian or AA solitary kidney recipients undergoing transplantation between 1990 and 2009 comprising 202,085 transplantations. During this 20-year period, the estimated rate of 5-year graft loss decreased from 27.6% to 12.8%. Notable trends in baseline characteristics that significantly differed by race over time included the following: increased prevalence of diabetes from 2001 to 2009 in AAs (5-year slope difference: 3.4%), longer time on the waiting list (76.5 more days per 5 years in AAs), fewer living donors in AAs from 2003 to 2009 (5-year slope difference: -3.36%), more circulatory death donors in AAs from 2000-09 (5-year slope difference: 1.78%), and a slower decline in delayed graft function in AAs (5-year slope difference: 0.85%). The absolute risk difference between AAs and Caucasians for 5-year graft loss significantly declined over time (-0.92% decrease per 5 years), whereas the relative risk difference actually significantly increased (3.4% increase per 5 years). These results provide a mixed picture of both promising and concerning trends in disparities for AA kidney transplant recipients. Thus, although the disparity for graft loss has significantly improved, equity is still far off, and other disparities, including living donation rates and delayed graft function rates, have widened during this time.

Impact of Cardiovascular Risk Factors on Graft Outcome Disparities in Black Kidney Transplant Recipients

Although outcome inequalities for non-Hispanic black (NHB) kidney transplant recipients are well documented, there is paucity in data assessing the impact of cardiovascular disease (CVD) risk factors on this disparity in kidney transplantation. This was a longitudinal study of a national cohort of veteran kidney recipients transplanted between January 2001 and December 2007. Data included baseline characteristics acquired through the United States Renal Data System linked to detailed clinical follow-up information acquired through the Veterans Affairs electronic health records. Analyses were conducted using sequential multivariable modeling (Cox regression), incorporating blocks of variables into iterative nested models; 3139 patients were included (2095 non-Hispanic whites [66.7%] and 1044 NHBs [33.3%]). NHBs had a higher prevalence of hypertension (100% versus 99%; P<0.01) and post-transplant diabetes mellitus (59% versus 53%; P<0.01) with reduced control of hypertension (blood pressure <140/90 60% versus 69%; P<0.01), diabetes mellitus (A1c <7%, 35% versus 47%; P<0.01), and low-density lipoprotein (<100 mg/dL, 55% versus 61%; P<0.01). Adherence to medications used to manage CVD risk was significantly lower in NHBs. In the fully adjusted models, the independent risk of graft loss in NHBs was substantially reduced (unadjusted hazard ratio, 2.00 versus adjusted hazard ratio, 1.49). CVD risk factors and control reduced the influence of NHB race by 9% to 18%. Similar trends were noted for mortality, and estimates were robust across in sensitivity analyses. These results demonstrate that NHB kidney transplant recipients have significantly higher rates of CVD risk factors and reduced CVD risk control. These issues are likely partly related to medication nonadherence and meaningfully contribute to racial disparities for graft outcomes.

Impact of CD4+ lymphocytes and HIV infection on Anti-Müllerian Hormone levels in a large cohort of HIV-infected and HIV-uninfected women

PROBLEM

Effects of HIV infection on ovarian function and aging are unclear.

METHOD OF STUDY

Anti-Müllerian Hormone (AMH) levels were analyzed in 2621 HIV-infected and 941 uninfected participants using left-censored longitudinal models.

RESULTS

Age-adjusted AMH levels were 16% lower in women with undetectable viraemia and 26% lower in detectable viraemia, relative to uninfected women. Current CD4 count associated with higher AMH in both HIV-infected and HIV-uninfected women. After controlling for current and nadir CD4, AMH was ~15% higher in HIV-infected relative to uninfected women, regardless of HIV viraemia. Gravidity, amenorrhea, and nadir total lymphocyte counts associated with higher AMH; hormonal contraceptive use and past weight loss associated with lower AMH.

CONCLUSIONS

CD4 +  lymphocyte counts were associated with AMH in both HIV-infected and uninfected women. After adjustment for CD4 counts and age, HIV infection was associated with higher AMH. CD4 T cells and cellular activation may influence ovarian granulosa cell function.

Long-term outcomes in African American kidney transplant recipients under contemporary immunosuppression: a four-yr analysis of the Mycophenolic acid Observational REnal transplant (MORE) study

Mycophenolic acid Observational REnal transplant (MORE) was a prospective, observational study of de novo kidney transplant patients receiving mycophenolic acid (MPA). Four-yr data on 904 patients receiving tacrolimus and enteric-coated mycophenolate sodium (EC-MPS) or mycophenolate mofetil (MMF) were analyzed to evaluate immunosuppression and graft outcomes in African American (AA, n = 218) vs. non-AA (n = 686) patients. Mean tacrolimus dose was higher in AA vs. non-AA patients but mean tacrolimus trough concentration was similar. Use of the recommended MPA dose in AA patients decreased from 78.9% at baseline to 33.1% at year 3. More AA patients received the recommended MPA dose with EC-MPS than MMF at month 6 (56.2% vs. 35.7%, p = 0.016) and month 36 (46.6% vs. 16.7%, p = 0.029), with no safety penalty. Significantly, more AA patients received corticosteroids than non-AA patients. Biopsy-proven acute rejection was higher in AA vs. non-AA patients (18.9% vs. 10.7%, p = 0.003), as was graft loss (10.9% vs. 4.4%, p = 0.003); differences were confirmed by Cox regression analysis. Patient survival was similar. Estimated GFR was comparable in AA vs. non-AA patients. Kidney allograft survival remains lower for AA vs. non-AA recipients even under the current standard of care.

Hemolytic disease of the fetus and newborn with late-onset anemia due to anti-M: a case report and review of the Japanese literature

Hemolytic disease of the fetus and newborn (HDFN) attributed to M/N-incompatibility varies from asymptomatic to lethally hydropic. Case reports are rare, and the clinical significance of anti-M is not completely understood. A challenging case of HDFN due to anti-M prompted an investigation of the Japanese literature, in order to characterize the clinical spectrum of M/N-incompatibility pregnancies in Japan and report results to English-language readers. Japanese reports of HDFN attributed to M/N incompatibility were compiled. Abstracted data include maternal antibody titers at delivery, fetal direct antiglobulin test, hemoglobin, total bilirubin, reticulocyte count at birth, and therapeutic interventions. We investigated characteristics of HDFN due to M/N-incompatible pregnancies in Japan after encountering a case of severe HDFN along with late-onset anemia in an infant born to a woman carrying IgG anti-M with a titer of 1. In total, thirty-three babies with HDFN due to anti-M and one due to anti-N have been reported in Japan since 1975. The median maternal antibody titer was 64 at delivery and was 16 or less in 10 of 34 women (29%). Five of 34 babies (15%) were stillborn or died as neonates. Twenty-one of 29 survivors (72%) had severe hemolytic anemia and/or hydrops fetalis. The reticulocyte count of neonates with anemia stayed below the reference interval. Sixteen (55%) developed late-onset anemia and 14 (48%) were transfused with M-negative RBCs. Significant positive correlation (P < .05) between the hemoglobin value and the reticulocyte count within 4 days of birth was obtained in 16 babies with anti-M HDFN. In the Japanese population, 21 of 34 cases of M/N-incompatible HDFN (72%) have manifested as severe hemolytic anemia and/or hydrops fetalis. Low reticulocyte count in neonates with late-onset anemia is consistent with suppressed erythropoiesis due to anti-M.