Circulating Erythroblasts Are Associated with Specific Characteristics in Adult Patients with Sickle Cell Disease: The Drepablast Study

Introduction Circulating erythroblasts (cEB) in the peripheral blood of patients with sickle cell disease (SCD) may be detected in acute situations such as vaso-occlusive events or at steady-state, and the significance of their presence in this context is unknown. Our objective was to compare clinical and biological characteristics of adult patients with SCD depending on the presence or not of cEB at steady state. Methods We conducted a monocentric, retrospective, observational study in a French Referral Center, involving patients ≥ 18 years with SCD, who had at least two visits (with a minimum interval of 3 months) at steady state (consultation or day hospital), with a biological evaluation comprising a complete blood count, between 01/01/2017 and 07/31/2023. Two groups of patients were defined: presence (cEB+) or absence (cEB-) of cEB (threshold: 1/100 leukocytes) at inclusion. Visits occurring after a transfusion within 3 months or after a bone marrow transplantation were excluded. Data were extracted from the hospital's clinical data warehouse and the diagnosis of hospitalized vaso-occlusive crisis (HVOC) was determined based on the presence of the International Classification of Diseases code D57.0. Continuous variables were compared using Kruskal-Wallis rank sum test, categorical variables with Pearson's Chi-squared test, and ordinal variables with trend test. P-values of baseline covariates were adjusted for multiple comparisons following the Benjamini and Hochberg procedure of false discovery rate. Time to new HVOC was analyzed using Cox proportional Hazard model. Results We included 384 patients with a median age of 26.0 [19.5;34.1] years: 201 (52.3%) women, 278 (76.8%) had HbSS, 60 (16.6%) HbSC, 9 (2.5%) HbS/β0-thalassemia, and 15 (4.1%) HbS/β+-thalassemia genotype, respectively. Median age, sex ratio or rate of chronic complications showed no statistically significant difference between the 2 groups. Patients with cEB+ (n = 212, 55.2%) more often had a history of acute chest syndrome (158/212 (74.5%) vs 100/172 (58.1%), p <0.001) and HbSS or HbS/β0-thalassemia genotype (184/212 (86.7%) vs 103/172 (66.9%), p <0.001). These patients had a lower hemoglobin (Hb) level (8.9 [7.9;10.1] vs 10 [9.1;11.4] g/dL, p <0.001), a higher mean corpuscular volume (MCV) (88.5 [79;98] vs 79 [70;89] fL, p <0.001), a higher reticulocyte count (224 [149;307.5] vs 159 [112;241.5] G/L, p <0.001) and a higher ferritin level (136 [55;287] vs 58 [32;168], p <0.001). Lactate dehydrogenase (LDH) and total bilirubin levels were also higher in cEB+ patients: 378 [292;504] vs 320 [250;425] UI/L, p <0.001; 41 [25;67] vs 29 [19.8;46.2] µmol/L, p <0.001, respectively. In the HbSS and HbS/β0-thalassemia population (n=287), cEB+ patients (n=184, 64.1%) had no specific clinical features but a significantly lower Hb level (8.7 [7.7;9.8] vs 9.4 [8.4;10] g/dL, p = 0.033), a higher MCV (90 [81;99] vs 85 [78;92] fL, p = 0.011) and a higher ferritin level (150 [62;305.8] vs 73 [36;254] µmol/L, p = 0.048). Moreover, higher cEB (in logarithmic scale) correlated linearly with lower Hb (g/dL) level and higher MCV (fL) values. No significant difference were observed between cEB+ and cEB- patients concerning reticulocyte count (233 [154.5;319.5] vs 211 [146;263.5] G/L, p = 0.102), LDH (409 [319;521.8] vs 364 [293;467] UI/L, p = 0.115), total bilirubin (44 [28;70] vs 37 [26;56] µmol/L, p = 0.277), hydroxyurea use, β-globin haplotype distribution or presence of alpha-thalassemia. In multivariate analysis, during follow-up, every increase of 10% of cEB was independently associated with a shorter time to a new HVOC (HR 1.015 (IC 95 [1.003-1.028]), p = 0.02). We characterized cEB of 5 HbSS patients by flow cytometry and May-Grünwald Giemsa staining. cEB represented all stages of terminal erythroid differentiation, with similar proportions to those of the bone marrow. In vitro erythroid differentiation of circulating CD34+ cells from cEB- and cEB+ patients showed accelerated differentiation in cEB+ patients. Conclusion The presence of cEB in adult patients with SCD appears as a frequent feature (more than half of patients) and is associated with lower Hb level with inadequate reticulocyte response, higher MCV, and shorter time to a new HVOC, but not with chronic complications. Other studies are needed to better characterize these cEB, and to better explore dyserythropoiesis in this population as suggested by our preliminary results.