Incidence Of Atypical Chronic Lymphocytic Leukemia In 1819 Patients With B Chronic Lymphoproliferative Disorder

Flow cytometry is the diagnostic tool of choice to study abnormal lymphoid population detected in peripheral blood by morphological analysis. The main diagnosed chronic lymphoproliferative disorder (CLPD) is chronic lymphocytic leukemia (CLL). In a significant number of cases, a B-CLPD non-CLL can be diagnosed. Further molecular and histological examinations are then compulsory to characterize such hematologic malignancies. The objective of this study was to determine the incidence of atypical CLL among all B-CLPD diagnosed by flow cytometry.

We retrospectively studied the B-CLPD consecutively diagnosed at the hospital of Caen (Normandy, France) between 2000 and 2013. The diagnosis of B-CLPD was based on the detection by flow cytometry of circulating lymphoid abnormal B cells. Multiparametric flow cytometry included markers CD19, CD20, CD22, CD79b, CD5, CD10, CD23, CD43, FMC7, CD38 and light chains (kappa and lambda) of surface immunoglobulin. The diagnosis of CLL was based on the criteria defined by Hallek et al (Blood 2008). The non-CLL B-CLPD were then explored by molecular analyses driven by the phenotype of B-cells (overexpression of cyclin D1 in case of CD5+/CD10-/CD23- B-CLPD and BCL2-JH rearrangement in case of CD10+ B-CLPD). In addition, histological evidence was necessary to classify the B-CLPD non-CLL.

1819 B-CLPD were detected by flow cytometry. The distribution of B-CLPD was as follows: 1156 cases (64%) of CLL or immunophenotypic equivalent (leukemic phase of small lymphocytic lymphoma (SLL) and monoclonal B-cell lymphocytosis (MBL)), 297 cases (16%) of marginal zone lymphoma (MZL), 84 cases (5%) of mantle cell lymphoma (MCL), 39 cases (2%) of follicular lymphoma (FL), 26 cases (1%) of hairy cell leukemia (HCL), 13 cases (<1%) of diffuse large B-cell lymphoma (DLBCL), 9 cases (<1%) of Waldenstrom's macroglobulinemia (WM) and 3 cases (<1%) of B-cells prolymphocytic leukemia (B-PLL). 65 cases (4%) remained unclassified due to lack of histological and molecular data. 127 cases (7%) did not meet the diagnostic criteria of CLL established by Hallek et al but were classified as atypical CLL because of the detection of a clonal B-cell proliferation expressing CD5+ / CD23+ / CD43+ / CD10- / FMC7+ / CD79b+ with moderate or high intensity and light chain kappa or lambda with moderate or strong intensity (absence of molecular or histological argument of MZL or MCL was required). CD20 marker was highly expressed in 113 cases (89%) of atypical CLL.

We particularly studied the 1532 cases (84%) of B-CLPD expressing CD5 (table). CD5+ CD23+ B-CLPD cases accounted for 1293 (84%) with 1153 cases of CLL, 13 cases of MCL and 127 cases of atypical CLL. CD5+ CD23- B-CLPD accounted for 239 cases (16%) with 72 cases of MCL, 158 cases of MZL, 4 cases of FL, 2 cases of WM, 2 cases of CD23- CLL and one case of B-PLL.

WHO classification of hematologic malignancies do not include atypical CLL as defined by a clonal proliferation of B-cells expressing CD5+, CD23+, cyclin D1- with no histological evidence of MZL or MCL, and which do not meet all the diagnostic criteria of CLL (Hallek et al, Blood 2008). This concept of atypical CLL, first described by Criel et al (BJH 1997), is particularly interesting, because such B-CLPD seems to have a different outcome as compared with CLL (Oscier et al, BJH 1997) and to have a different biological presentation with atypical morphology of CLL cells (Criel et al, BJH 1997), more frequent trisomy 12 (Matutes et al, BJH 1996) and a stronger intensity of CD20 (Ugo et al, Leuk Lymphoma 2006).

Diagnosis of B-CLPD relies on a multidisciplinary approach combining morphological, immunophenotypic, molecular and histological analyses. Despite detailed information of these analyses, there are B-CLPD which remain unclassifiable according WHO classification, especially CD5 positive B-CLPD. The concept of atypical CLL seems to take all its meaning to help define such unclassifiable entities.