Presentations

Abstract

Introduction: In sickle cell disease (SCD), neutrophils are abnormally activated and contribute to vascular occlusion, leading to complications such as end-organ damage, acute chest syndrome, and stroke.1,2 Red cell exchange (RCE) is a therapy provided to SCD patients to remove sickled RBCs and replace them with healthy donor RBCs.3 Prior work from our lab has shown that RCE resulted in decreased plasma levels of matrix metallopeptidase 9 (MMP9 ), indicating a reduction in neutrophil tertiary granule release.4 Further investigation on the modulation of RCE on neutrophil primary and secondary granule release is necessary to fully understand the effects of RCE on neutrophil degranulation and hyperreactivity in SCD patients. We hypothesize that RCE will decrease the plasma levels of myeloperoxidase (MPO), which is representative of neutrophil primary granules.

Methods: We aim to establish the effects of RCE on neutrophil primary granule release, by comparing neutrophil degranulation responses and release of MPO in SCD patients before and after receiving RCE therapy. Whole blood samples from 22 SCD patients before and after receiving RCE therapy were collected in ethylenediaminetetraacetic acid (EDTA) collection tubes. Following published methods , whole blood before (pre-RCE) and after RCE (post-RCE) were incubated with the following agonists at 37°C for 30 minutes: buffer, phorbol myristate acetate (PMA, 500 nM), N-Formyl-Met-Leu-Phe (fMLF, 1 M), or lipopolysaccharide (LPS, 100 ng/ml). The buffer condition served to measure plasma levels of circulating MPO, and also represented spontaneous neutrophil degranulation. We assessed neutrophil degranulation responses by quantifying plasma levels of myeloperoxidase using commercially available enzyme-linked immunosorbent assay (ELISA) kits (R&D Systems, Minneapolis, MN).

Results: Our findings are obtained from the same study cohort as was included in our recent work.4 In total, 22 patients with sickle hemoglobin (HbSS) receiving steady-state RCE from an outpatient clinic were recruited (Table 1). 11 patients were male and 11 were female. The mean age was 35.4 years (range 20-60 years) and the median age was 33.5 years. From our study population, one patient received a transfusion > 15 months before the study and received a pre-operative RCE before undergoing surgery. Patients received RCE therapy for various indications. At the time of the study, the average number of RCE procedures each patient received was 66.6 ± 74.4, over an average duration of 7.2 ± 8.3 years. The mean HbS was 33 ± 14%, ranging from 15.9 to 64.6%. Our findings demonstrate a significant decrease in the mean levels of released MPO among 22 patients with SCD following treatment with RCE (Figure 1). Neutrophils, when incubated with various agonists, showed the following trends in MPO release: incubation with buffer resulted in 1208.97 pg/mL before RCE and 677.2 pg/mL after RCE; incubation with PMA resulted in 3184.28 pg/mL before RCE and 1892.71 pg/mL after RCE; incubation with fMLF resulted in 4870.24 pg/mL before RCE and 2734.23 pg/mL after RCE; and incubation with LPS resulted in 3261.48 pg/mL before RCE and 1256.68 pg/mL after RCE. Overall, RCE reduced neutrophil degranulation of MPO by 44% in buffer incubation, 40.45% in PMA incubation, 43.86% in fMLF incubation, and 61.47% in LPS incubation.

Conclusion: Experimental results support our hypothesis that RCE therapy not only decreases tertiary granule release but also inhibits neutrophil degranulation of primary granules. This is demonstrated by the overall decrease in plasma MPO in patients after receiving RCE, as compared to before receiving RCE. Building upon our previous research showing that RCE also reduces the release of MMP9 from tertiary granules, we uncover the role of RCE therapy in modulating neutrophil degranulation.4 In SCD, abnormally activated neutrophils exhibit enhanced adhesion to the endothelium, platelets, and RBCs, consequently leading to complications including end-organ damage, acute chest syndrome, and stroke.1,2 Our study underscores the role that RCE plays in modulating neutrophil hyperreactivity in SCD patients. Future studies will focus on analyzing neutrophil degranulation responses and the release of lactoferrin, which is representative of secondary granules.