Age impacts risk of mixed chimerism following ric hct for non-scid inborn errors of immunity

Alemtuzumab, fludarabine and melphalan containing reduced intensity conditioning (RIC) is commonly used in patients undergoing allogeneic hematopoietic cell transplantation (HCT) for definitive treatment of high-risk inborn errors of immunity (IEI). Although survival is favorable, there is increased risk of mixed chimerism leading to secondary graft failure. Evaluate factors associated with risk of developing mixed chimerism, particularly the influence of age in patients undergoing allogeneic HCT for non-SCID IEI who received a uniform RIC regimen that included intermediate schedule alemtuzumab, fludarabine and melphalan. We hypothesized that age would impact incidence of mixed chimerism. We retrospectively reviewed records of patients who underwent HCT for non-SCID IEIs with a uniform RIC regimen that included intermediate schedule alemtuzumab (1 mg/kg divided over days -14 to - 10), fludarabine (150 mg/m2 or 5 mg/kg if weight <10 kg divided over days -9 to -4), and melphalan (140 mg/m2 or 4.7 mg/kg if weight <10 kg on day-3) between 2010 and 2020 at our institution. Mixed chimerism was defined as <95% donor on 2 or more consecutive occasions on whole blood. Ninety-three patients who underwent RIC-HCT for non-SCID IEI using intermediate schedule alemtuzumab, fludarabine, and melphalan were evaluated and categorized into 3 groups: <1, 1-5, and > 5 years of age. Forty-nine patients (52.7%) developed mixed chimerism at a median of 34 days post-HCT (range, 10-1396 days). Mixed chimerism developed in 88.9 % (n=16/18) for <1 year, 57.1% (n=20/35) for 1-5 years, and 35% (n=14/40) for those >5 years. Patients <5 years of age were significantly more likely to develop mixed chimerism (X2 (3, N = 93) = 14.8, p =0.001). We observed a significantly increased cumulative incidence of developing mixed chimerism if < 1 year of age (p=0.0002). Competing risk regression analysis demonstrated an increase in odds of development of mixed chimerism for age <1 (OR 3.72, p = 0.006, 95% CI 1.46-19.46) compared to age >5 years and decrease in odds of mixed chimerism in patients who developed acute GVHD prior to any intervention (OR 0.24, p=0.005, 95% CI 0.09-0.65) There was no significant association between mixed chimerism and graft source, graft type, CD34+ or CD3+ cell dose, HLA match or underlying disease (HLH vs non-HLH). Additionally, the need for secondary intervention was evaluated; 27 patients (29.0%) required one or more secondary intervention(s) (DLI, CD34 Boost, or second HCT). Patients <1 year of age with mixed chimerism were significantly more likely to require secondary intervention for mixed chimerism than patients > 5 years (p =0.004). Our study demonstrates that young age <5 years, especially age <1 year is associated with an increased risk of developing mixed chimerism in patients undergoing RIC-HCT for non-SCID IEI using intermediate schedule alemtuzumab, fludarabine, and melphalan. Our data suggest tailoring regimen intensity based on age to reduce the incidence of mixed chimerism. Children <5 years, particularly those <1 year of age, require a higher intensity regimen. Possible strategies include adding thiotepa or using a busulfan-based reduced toxicity regimen.