Early neutrophil incompetence following allogeneic stem cell transplant.

6565 Background: Neutrophils are key immune effector cells against fungal pathogens such as Candida albicans . After allogeneic stem cell transplant (allo-SCT), absolute neutrophil count (ANC) is used to measure myeloid engraftment and estimate infection risk, but post-transplant neutrophil function remains poorly understood. Our prior work showed impaired neutrophil function after both myeloablative (MAC) and reduced-intensity conditioning (RIC) regimens. Here, we assess broader neutrophil functions and hypothesize post-transplant neutrophil impairment persists for at least one month despite numeric recovery. Methods: Peripheral blood was collected from patients undergoing RIC or MAC allo-SCT and healthy donors at four time points: immediately post-engraftment (P1), and at one (P2), three (P3), and seven (P4) months. Neutrophil function was analyzed by multiparametric flow cytometry to assess phagocytosis, degranulation (CD66b), reactive oxygen species (ROS) production (dihydrorhodamine-123), and ectodomain shedding (CD62L) after incubation with fluorescent C. albicans . At P1, neutrophil deformability was quantified to generate a sepsis risk index: Band 1 (low risk, 0.1-4.9), Band 2 (intermediate risk, 5-6.2) and Band 3 (high risk, 6.3-10). Neutrophil antifungal activity was assessed in vitro using microscale platforms to quantify and image swarming (cumulative neutrophil response) during restriction of C. albicans growth over 12 hours. Results: Nineteen subjects were included. The most common underlying malignancy was acute myeloid leukemia (12/19, 63%). Conditioning regimens included fludarabine/melphalan (14/19, 74%), busulfan/fludarabine (4/19, 21%), or fludarabine/total body irradiation (1/19, 5%), with post-transplant cyclophosphamide and tacrolimus. The majority received grafts from matched unrelated donors (14/19, 74%). Median ANC (K/uL) was 3.59 at P1 (15/19 subjects, 79%), 4.55 at P2 (14/19, 74%), 4.03 at P3 (5/19, 26%) and 2.94 at P4 (7/19, 37%). Functional analyses revealed significant impairment in allo-SCT recipients compared with controls at P1, with trends of persistent impairment at P2, and evidence of recovery by P3-P4. At P1, eight recipients demonstrated altered neutrophil deformability, yielding a higher sepsis risk index [median (IQR) 5.93 (3.88–6.80)] versus controls [median (IQR) 0.95 (0.33–2.33)]. Swarming analysis revealed impaired collective behavior in fungal containment compared with controls. Conclusions: Despite normalization of ANC after allo-SCT, early post-engraftment neutrophils exhibit significant functional and biophysical impairments, indicating transient immune incompetence. This early window of vulnerability may persist for at least one month post-transplant. Ongoing follow-up and expanded analyses will further characterize the underlying mechanisms and define the trajectory of delayed functional neutrophil recovery.