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Although purine analogues have significantly improved the outcome of hairy cell leukaemia ( HCL) patients, 30-40% relapse, illustrating the need for minimal residual disease ( MRD) markers that can aid personalized therapeutic management. Diagnostic samples from 34 HCL patients were used to design an 8-colour flow cytometry (8- FC) tube for blood MRD (B/ RD) analysis (188 samples) which was compared to quantitative IGH polymerase chain reaction (Q- PCR) on 83 samples and to qualitative consensus IGH PCR clonality analysis on 165 samples. Despite heterogeneous HCL phenotypes at diagnosis, discrimination from normal B lymphocytes was possible in all cases using a single 8- FC tube, with a robust sensitivity of detection of 10−4, comparable to Q- PCR at this level, but preferable in terms of informativeness, simplicity and cost. B/ RD assessment of 15 patients achieving haematological complete remission after purine analogues was predictive of a clinically significant relapse risk: with a median follow-up of 95 months; only one of the nine patients with reproducible 8- FC B/ RD levels below 10−4 (B/ RDneg) relapsed, compared to 5/6 in the B/ RDpos group ( P = 0·003). These data demonstrate the clinical interest of a robust 8- FC HCL B/ RD strategy that could become a surrogate biomarker for therapeutic stratification and new drug assessment, which should be evaluated prospectively. [ABSTRACT FROM AUTHOR]

The diagnosis of plasmacytoid dendritic cell leukaemia (pDCL) is based on the immunophenotypic profile: CD4+ CD56+ lineageneg CD45RA+/ROneg CD11cneg CD116low CD123+ CD34neg CD36+ HLA-DR+. Several studies have reported pDCL cases that do not express this exact profile or expressing some lineage antigens that could thus be misdiagnosed. This study aimed to validate pDCL-specific markers for diagnosis by flow-cytometry or quantitative reverse transcription polymerase chain reaction on bone marrow samples. Expression of markers previously found in normal pDC was analysed in 16 pDCL, four pDCL presenting an atypical phenotype (apDCL) and 113 non-pDC – lymphoid or myeloid – acute leukaemia. CD123 was expressed at significantly higher levels in pDCL and apDCL. BDCA-2 was expressed on 12/16 pDCL and on 2/4 apDCL, but was never detected in the 113 non-pDC acute leukaemia cases. BDCA-4 expression was found on 13/16 pDCL, but also in 12% of non-pDC acute leukaemia. High levels of LILRA4 and TCL1A transcripts distinguished pDCL and apDCL from all other acute leukaemia (except B-cell acute lymphoblastic leukaemia for TCL1A). We thus propose a diagnosis strategy, scoring first the CD4+ CD56+/− MPOneg cCD3neg cCD79aneg CD11cneg profile and then the CD123high, BDCA-2 and BDCA-4 expression. Atypical pDCL can be also identified this way and non-pDC acute leukaemia excluded: this scoring strategy is useful for diagnosing pDCL and apDCL. [ABSTRACT FROM AUTHOR]

CD4+/CD56+ haematodermic neoplasm or ‘early’ plasmacytoid dendritic cell leukaemia/lymphoma (pDCL) was described as a disease entity in the last World Health Organisation/European Organisation for Research and Treatment of Cancer classification for cutaneous lymphomas. These leukaemia/lymphomas co-express CD4 and CD56 without any other lineage-specific markers and have been identified as arising from plasmacytoid dendritic cells. Despite a fairly homogeneous pattern of markers expressed by most pDCL, numerous distinctive features (e.g. cytological aspects and aberrant marker expression) have been reported. This may be related to the ‘lineage-independent developmental’ programme of dendritic cells, which may be able to develop from either immature or already committed haematopoietic progenitors. This highlights the need for specific validated markers to diagnose such aggressive leukaemia. Here, we propose –among others (e.g. T-cell leukaemia 1) – blood dendritic cell antigen-2 and high levels of CD123 expression as potential markers. In addition, we propose a multidisciplinary approach including several fields of haematology to improve pDCL diagnosis. [ABSTRACT FROM AUTHOR]