*Purpose: The effects of solid organ (SOT) and allogeneic stem cell transplantation (SCT) on the neutrophil function is not well described. In this study, we measure human neutrophil anti-fungal and anti-bacterial activity in SOT and SCT patients.

*Methods: SOT and SCT patients were identified and consented from 09/2018 until 12/2018. Healthy control patients (HC) were identified at primary care clinics. A 10 mL sample of EDTA peripheral blood were obtained from healthy and transplant patients’ days 60 – 100 post-transplant. Neutrophils were isolated by negative selection. C. albicans or S. aureus were incubated for 2 hours with and without human neutrophils at MOI of 10, 5 and 1 (C. albicans) or dilutions of 1:176; 1:352 and 1:704 (S. aureus). Following mammalian cell lysis, percent remaining live pathogen was measured using a viability dye. In addition, growth inhibition of C. albicans by neutrophil swarming to C. albicans spotted onto glass slides was also assessed by live cell imaging.

*Results: 12 SOT (9 kidney, 4 liver), 7 SCT and 6 HC were enrolled. No differences in baseline characteristics were found between groups. All patients had absolute neutrophil counts above 500 cells/uL. Neutrophils from SCT and SOT recipients had lower C. albicans killing percentages compared to controls (HC 75%, SCT: 64% SOT: 61% [SCT vs HC p=0.05 Mann-Whitney test]). Neutrophil from SCT and SOT exhibited diminished killing of S. aureus as compared to HC (HC: 44%, SCT: 24%, SOT: 16%, [SCT vs HC p=0.05; SOT vs HC p=0.04 Mann-Whitney test]). Similar results were found at a dilution of 1:704 (HC: 99%, SCT: 66%, SOT: 22% [SOT vs HC p=0.02 Mann-Whitney test]). Furthermore, neutrophil swarming to a spot of C. albicans was significantly less inhibited by neutrophils from SCT when compared to SOT and controls.

*Conclusions: Our data indicates that the neutrophil function and the capacity to control pathogen is profoundly impaired in SOT and SCT recipients at 60-100 days post-transplant. These results provide insight to the continued risk of infections in these susceptible populations despite normal circulating neutrophil counts. Functional neutrophil profiling to predict infectious risk could be a useful diagnostic in susceptible populations.

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