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Abstract: PB1677

Type: Publication Only

Session title: Myeloma and other monoclonal gammopathies - Clinical

Background

Plasma cell leukemia (PCL) is rare, aggressive plasma cell (PC) disorder, characterized by presence of circulating PC (cPC) in peripheral blood (PB). PCL can be divided in 2 groups - primary PCL (pPCL) which originates de novo and secondary PCL (sPCL), a leukemic transformation of MM. Detection of cPC by flow cytometry (FC) is important for early diagnostics and for distinguishing PCL from reactive plasmacytosis as well, but less about PCs phenotype profile is known.

Aims
Analyses of differences in number and phenotypic profile PCs between pPCL and sPCL aiming at their association with clinical outcomes and evaluation of the prognostic value of PCs immunophenotype.

Methods

Investigation of PB and/or bone marrow (BM) of 18 pPCL and 15 sPCL patients was done by FC. Expression of surface antigens (CD19, CD20, CD27, CD28, CD44, CD56, CD81, CD117 and CD200) with intracellular nestin was studied. Medians of relative expression and positivity (cut-off 20%) were used.

Results

There were found no significant differences in clinical characteristics of pPCL and sPCL patients except increased level of platelets in pPCL compared to sPCL (median 129.5 vs 49.1x10⁹L, p=0.004, respectively). Median OS was 17.6 months in pPCL, 0.9 months in sPCL; (p<0.001). Significantly higher number of cPCs was identified by FC when compared to morphology (26.7 vs 13.5%, p=0.02). The number of cPCs was lower in pPCL than in sPCL (25.1% vs. 30.2%), the number of PCs in BM was significantly lower in pPCL than in sPCL (39.2% vs. 64.9%; p=0.02).

Phenotypic profile of pPCL was very close to sPCL in PB and/or BM, although there were some alterations. Unlike pPCL, no CD19 and CD20 positive patients were detected in sPCL. Interestingly, co-expression of CD28 and CD56 with no expression of CD117 was observed in 75% (3/4) CD20⁺ pPCL cases. Low level of expression and similar positivity of CD27, CD28, CD81, and CD117 were found in PB and/or BM of both PCLs. On the other hand, missing CD56 was not a dominating event in both PCLs, although its decreased expression was found in PB when compared to BM. Lower CD200 expression was found in sPCL when compared to pPCL in PB (51.5% vs 99.7%) and similarly in BM. Positivity of nestin was similar for both PCL, but lower expression was found in PB of pPCL (9.3% vs. 32.0%). Total lack of CD20 and higher difference in number of CD81⁺ PC in BM compared to PB was associated with higher risk of death. Patients with platelets below 70x10⁹/l had 3.9 times higher risk of death, patients with ECOG 2–3 had three times higher risk of death compared to patients with ECOG 0–1.

Conclusion

Even when limited number of PCL patients was analyzed, it was proved that polychromatic FC analysis of cPCs is more accurate than morphology assessment and should be used diagnostics of all patients suspected with PCL. Phenotype differences were mostly not significant in both PCLs, but lack of CD20 and decreased CD81 in PB seems to be risk factors of shorter survival.

Supported by grant AZV NV18-03-00203.

Keyword(s): Flow cytometry, Immunophenotype, Monoclonal gammopathy, Plasma cells