Immunophenotypic Characteristics of Leukemic BCells in CLPD Other Than CLL

Among the B-CLPD, CLL is the most common disease, its differential diagnosis from other leukemic B-cell CLPD being a constant challenge. Typically, CLL exhibits a highly characteristic phenotype, different from that found in other B-CLPD (44-48), including weak expression of CD22, CD20/FMC7, CD79b and sIg, together with a relatively high reactivity for the CD5 and CD23 antigens (for more details, see Chapter 7). In the last decade, scoring systems based on the above mentioned markers (46,48) have been proposed for the diagnosis of CLL; these have proved to be extraordinarily helpful in distinguishing typical CLL cases from other leukemic B-CLPD. Accordingly, whereas CLL patients usually (>90% of cases) display either 5/6 or 6/6 of these markers, only a few other B-CLPD (<5%) exhibit them in such high numbers (44,46, 50,51).

Information provided by these and other additional phenotypic markers is also essential for the diagnostic subclassification of B-CLPD other than CLL. Of all antigens tested in the past, a general consensus exists that CD5, CD19, sIg light chains, CD20, CD23, bcl2, and CD10 are essential for the evaluation of B-CLPD (52). Additional markers such as CD22, FMC7, CD11c, CD103, CD38, CD25, CD79b, and sIg heavy chains would be needed for the full characterization of these disease entities (52). In tissue lymphoma, CD30, terminal deoxynucleotidyl transferase (Tdt), CD71, and CD34 might be useful as well, in specific cases (52).

In the following sections the phenotype of each individual diagnostic subgroup of leukemic B-CLPD is described (Table 4).

3.2.1. Prolymphocytic Leukemia

PLL was initially described in 1974 by Galton et al. (53) as a disorder characterized by the presence of massive splenomegaly (in the absence of extensive lymph node involvement) and high white blood cell counts, PB leukocytes corresponding to large lymphocytes with a prominent nucleoli. Despite their apparent morphological immaturity, B-prolymphocytes typically display phenotypic patterns consistent with a more advanced maturation stage than that of the typically small lymphocytes from CLL (8,54) (Table 4). Accordingly, PLL B-prolymphocytes show strong sIg, CD79b, CD22, and CD20/FMC7 expression, they are CD23-, CD24+, and they display a lower reactivity for CD5 than CLL B-cells (8,47,53-55). In addition, prolymphocytes from PLL patients are typically negative for CD103, HC2, and CD10, although they may frequently show weak CD11c expression (8,47,53-56).

It should be noted that intermediate variant forms between CLL and PLL have been described, either presenting at the diagnostic onset of the disease, or as progression from a CLL (1,54,57). In any case, morphologically, these patients have a mixed population of small mature-appearing B-cells and B-prolymphocytes and display intermediate phenotypic features between typical CLL and PLL cases, the relationship to PLL remaining unclear (57,58).

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