Characteristics and Distribution of Erythroid Progenitors and Precursors Correlate with Erythropoiesis Stimulating Agents (ESAs) Response in Lower Risk Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are an heterogenous group of clonal myeloid disorders characterized by dysplastic ineffective hematopoiesis, peripheral blood cytopenias and a variable risk of progression to acute myeloid leukemia (AML) evaluated by IPSS-R score. Early stage erythropoiesis is dependent on growth factors like EPO, whereas late erythroid differentiation is iron-dependent. The entire process requires a fine regulation between apoptosis and cell survival. Anemia is the most common cytopenia in IPSS-R lower risk MDS (LR-MDS) and the mechanism underlying dyserythropoiesis is multifactorial, leading to apoptosis of erythroid progenitors and precursors. LR-MDS anemia is mainly treated with ESAs (e.g., Erythropoietin or EPO), but response is not optimal and eventually patients will suffer from chronic anemia and transfusion dependence. Moreover, the only strong predictor of ESAs treatment is the endogenous concentration of serum EPO (sEPO).

The primary aim of this study was to detect quantitative and qualitative flow cytometry (FC) alterations of erythroid populations in the bone marrow (BM) of LR-MDS patients associated with resistance or response to ESAs and compared them with age-matched non cytopenic BM controls (CTRLs, 6). Secondly, we wanted to investigate if different WHO diagnostic MDS subgroups displayed specific pattern of erythroid maturation and/or FC abnormalities.

We selected 97 LR-MDS anemic patients treated with ESAs at MDS Unit, Hematology dpt. AOU Careggi (Florence) and divided them according to type of ESAs response (e.g., primary refractory, PR; long responders, LR, with DOR longer than 24 months and secondary refractory, SR, with DOR shorter than 24 months). Baseline BM FC analysis was performed using an erythroid panel containing antibodies for CD33, CD34, CD36, HLA-DR, CD45, CD71, CD105, CD117. We also measured the coefficient of variance (CV) of CD71 and CD36 to assess qualitative FC abnormalities within the erythroid populations. We then identified all stages of erythroid maturation from A) CD34+ committed erythroid cells (CD34+, CD45dim, CD117+, CD71+, CD33+, HLA-DR +), or erythroid progenitors (COM-E), expressed both as fraction of all CD34+ cells and all 117+ progenitor cells, to B) CD34- erythroid precursors (from proerythroblasts, PROE, to orthochromatic erythroblasts).

CV CD71 was increased in PR patients compared to CTRLs (p=0,009) and to responders (p=0,008). COM-E/tot.CD34+ cells were increased in all response categories, but there was a significant trend of higher values in LR vs. PR (p=0,055). Finally, (COM-E + PROE)/tot.117+ progenitors were only enriched in LR compared to CTRLs (p=0,009). Regarding WHO subgroups, MDS 5q- displayed a decrease in COM-E/CD34+ cells and in (COM-E + PROE)/tot.117+ cells compared to MDS-RS (p=0,00094 and p=0,009 respectively) and to CTRLs for the latter population (p=0,03). Moreover, total erythroid precursors were significantly reduced compared to MDS-SLD, -MLD and -RS. MDS-RS showed a marked erythroid hyperplasia and elevation of CV CD71 compared to CTRLs (p=0,0079 and p=0,024 respectively).

Taken together, these results show that in LR-MDS, the increase of CV CD71 in erythroid cells is associated with resistance to ESAs, whereas response to ESAs is present in cases with enriched EPO sensitive COM-E and PROE populations compared to CTRLs, especially for LR vs PR (p=0,055). Finally, MDS-5q- and MDS-RS were the WHO subgroups with the highest rate of PR in our cohort (46% and 59% respectively), irrespective of sEPO. These 2 subgroups of MDS exhibited different repartition of erythroid subpopulations with characteristics associated with ESAs resistance. In 5q- COM-E and PROE were depauperate and, consequently, erythroid precursors reduced, while MDS-RS displayed an expansion of erythroid precursors and higher CV of CD71 compared to CTRLs.

Finally, these FC parameters, easily and rapidly assessed at baseline BM evaluation in MDS patients, correlate with response to ESAs and with different WHO subgroups. FC erythroid abnormalities could partly explain the complex pathobiology underlying dyserythropoiesis and its treatment. The different repartition of erythroid progenitors observed could lead to molecular and transcriptome analyses focused on these altered subpopulations in order to investigate the exact mechanism of anemia in LR-MDS(s).

Sanna:Abbvie: Honoraria, Other: travel, Speakers Bureau; Astra Zeneca: Honoraria, Speakers Bureau; Janssen: Consultancy, Honoraria, Other: travel, Speakers Bureau. Santini:Gilead: Membership on an entity's Board of Directors or advisory committees; Menarini: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Geron: Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees; Otsuka: Membership on an entity's Board of Directors or advisory committees; Servier: Membership on an entity's Board of Directors or advisory committees; Syros: Membership on an entity's Board of Directors or advisory committees.