Flow Cytometric Immunophenotyping in Myelodysplasia: Discovery and Diagnosis
Authors: Van de Loosdrecht Arjan A , Westers Theresia M. 
Published: October 2019

Abstract
Diagnosis of myelodysplastic syndromes (MDS) is based on the integration of results from diagnostic tools. Initial laboratory assessments in patients with suspected MDS comprise analysis of peripheral blood and bone marrow with the gold standard of cytomorphology, conventional cytogenetics and interphase fluorescence-in-situ hybridization (FISH). Single nucleotide polymorphism (SNP) array and next-generation sequencing (NGS) are emerging techniques. The pathological hallmark of MDS is dysplasia. Flow cytometry (FC) can identify aberrancies in antigen expression and differentiation patterns that are indicative of dysplasia. FC is regarded instrumental and now even recommended in the diagnostic work-up of (suspected) MDS, especially when dysplasia by cytomorphology is minimal and cytogenetics shows no (typical) abnormalities. No single FC marker has been identified that is specific for MDS. The WHO-2008 classification recommends the presence of three or more FC aberrancies as highly suggestive for MDS. Minimal requirements to analyze dysplasia by FC have been proposed by the European LeukemiaNet (ELNet) working group (ELNet iMDS-Flow. ELNet recommendations should enable a categorization of FC results in cytopenic patients as "normal", "suggestive of", or “high probability of” MDS. FC as a single technique is not sufficient for the diagnosis of MDS and should be part of an integrated diagnostic work-up. Within the ELNet-iMDS-Flow group, a score based on four cardinal parameters was validated in MDS patients with <5% blasts and non-clonal cytopenic controls. This diagnostic score comprises: a) SSC of granulocytes (reflecting granularity) as ratio to lymphocytes; b) percentage of CD34+ myeloid progenitors of nucleated cells; c) percentage of B cell progenitors within the total CD34+ compartment; and d) expression of CD45 on CD34+ myeloid progenitors as ratio to lymphocytes. An abnormal value for every single parameter is assigned 1 point; MDS is highly suggestive when a score of 2 or more is obtained. Sensitivity of this diagnostic score was 70% and specificity 92%. It was demonstrated that this score also separates distinct subgroups with respect to prognosis within IPSS-R risk groups. Most flow scores mainly incorporate markers that cover the myelomonocytic lineage, e.g. the flow cytometric scoring system (FCSS). The latter separates patients with no and mild-to-moderate dysplasia from those with severe dysplasia. The FCSS has recently been shown to add significantly in separating patients into low or high risk disease in the revised IPSS subgroups. Finally, FCSS originally designed as a prognostic score, but can also be applied as a diagnostic score as combined with the 4-parameter FC diagnostic score to further improve sensitivity and specificity. Flow profiles based on the myelomonocytic lineage may fail to recognize MDS patients that exclusively show erythroid and/or megakaryocytic dysplasia. Analysis of megakaryocytes is hampered by their paucity. Recent advances shows that when adding erythroid markers such as CD71, CD36 and CD117/CD105 may add significantly to the diagnostic score of MDS by FC. Interestingly, aberrancies in the myelomonocytic lineage have been shown in patients with solely erythroid dysplasia with impact on prognosis. In addition, aberrant FC profile of myeloid progenitors has been associated with high transfusion requirements and disease progression as well as with a short duration of response or even lack of response to growth factor and azacitidine treatment. In conclusion, FC is a useful tool in the diagnostic work-up of MDS. Further studies on the value of FC in diagnosis, prognosis and predicting response to treatment are ongoing, as well as correlation of FC results with data obtained by SNP and NGS within prospective multicenter clinical trials in low and high risk MDS.

Disclosures
Van de Loosdrecht: celgene: Honoraria, Research Funding; alexion: Research Funding.