Diagnostic Utility of Flow Cytometric Immunophenotyping in Chronic Myelomonocytic Leukemia

Abstract

INTRODUCTIONThe diagnosis of chronic myelomonocytic leukemia (CMML) according to WHO 2017 requires the presence of ≥1x109/L and ≥10% of monocytes in peripheral blood (PB). Establish an accurate diagnostic is difficult since many clinical situations present persistent monocytosis. The presence of dysplasia, mainly dysgranulopoiesis, is frequent but not always present in CMML. Cytogenetic aberrations are infrequent in this disease (20-25% of cases). Although 85-90% of CMML patients present at least one mutation in TET2, SRSF2 or ASXL1 genes, the use of NGS panels is not widespread. Furthermore, mutations in these genes are among the most frequently observed in age-related clonal hematopoiesis. Therefore, complementary techniques are required to support the diagnosis of this entity. The study of the peripheral monocyte subsets by flow cytometry (FC) has gained special interest due to a high sensitivity and specificity for the diagnosis of CMML (S = 90.6%, E = 95.1%, Selimoglu-Buet et al., Blood, 2015). An increase in the fraction of classical monocytes (Mo1) to >94% of total monocytes is an event frequently observed in CMML. There are no specific bone marrow (BM) FC panels for the diagnosis of CMML and very few have been validated for the diagnosis of MDS. “Ogata score”, the only multicenter validated score in MDS, has not been applied in CMML. The aim of our study was to evaluate the usefulness of FC in PB and BM for the diagnosis of CMML.METHODSTwenty-two CMML were prospectively studied from 02/2016 to 04/2018. Patients' characteristics are summarized in Table 1. Diagnostic procedure consisted of morphological, cytochemical (Perls, myeloperoxidase, nonspecific esterase), cytogenetic and FC studies in BM, and morphological and FC studies in PB. “Ogata Score” was applied in BM samples (Table 2). Aberrant coexpression of CD2, CD7 and CD56 in BM monocytes was assessed. Immunophenotypic maturation profile of the monocytic elements in BM distinguishes: promonocytes (CD34-/CD117-/CD64++/CD14- or dim/CD45+/HLA-DR+++), mature monocytes (CD34-/CD117-/CD64++/CD14++/CD45++/HLA-DR++) and mature monocytes in terminal stage (CD300e+). In PB, the monocytes FC subsets (Mo1, Mo2 and Mo3) were studied, as well as the aberrant coexpression of CD2, CD7 and CD56 (Table 3).RESULTSThe presence of ≥2 aberrations in Ogata Score predicted properly the diagnosis of CMML in all patients analyzed (100% sensitivity). Due to the study design, we could not obtain results about specificity.An increase in Mo1 (classical monocytes) > 94% was detected in 18/20 patients (Table 3). This method predicted the diagnosis of CMML with a sensitivity of 91%, a result almost identical to the original study (Selimoglu-Buet et al., Blood, 2015).A good positive correlation was established between the percentage of BM promonocytes detected by morphology and by FC (Rho Spearman 0.61, P = 0.003).A negative correlation was found between the percentage of promonocytes by FC in MO and the expression of CD56 (Rho Spearman -0.612, P = 0.002). Similarly, CD56+ CMML presented a percentage of promonocytes by FC significantly lower than the CD56- CMML group (median: 24.5% (14-40) vs. 41% (23-71), P = 0.005). The expression of CD56 seems to be related to a more mature immunophenotypic profile of the monocytic population. On the other hand, the correlation between the percentage of CD56+ monocytes in BM and PB was almost perfect (Rho Spearman 0.928, P <0.001).CONCLUSIONOur findings support the usefulness of flow cytometry in bone marrow and peripheral blood for the diagnosis of CMML. DisclosuresNo relevant conflicts of interest to declare.