Phorbol ester activates CD5+ leukaemic B cells via a T cell-independent mechanism

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

TLR9 Ligand (CpG Oligodeoxynucleotide) Induces CLL B-Cells to Differentiate into CD20(+) Antibody-Secreting Cells

B-cell chronic lymphocytic leukemia (CLL) is the most frequent adult leukemia in the Western world. It is a heterogeneous disease characterized by clonal proliferation and the accumulation of CD5⁺ mature B lymphocytes. However, the normal counterpart from which the latter cells arise has not yet been identified. CD27 expression and gene expression profiling data suggest that CLL cells are related to memory B-cells. In vitro, memory B-cells differentiate into plasma cells when stimulated with CpG oligodeoxynucleotide (CpG). The objective of the present study was therefore to investigate the ability of CpG, in the context of CD40 ligation, to induce the differentiation of CLL B-cells into antibody-secreting cells (ASCs). CD20⁺CD38⁻ CLL B-cells were stimulated with a combination of CpG, CD40 ligand and cytokines (CpG/CD40L/c) in a two-step, 7-day culture system. We found that the CpG/CD40L/c culture system prompted CLL B-cells to differentiate into CD19⁺CD20⁺CD27⁺CD38⁻ASCs. These cells secreted large amounts of IgM and had the same shape as plasma cells. However, only IgMs secreted by ASCs that had differentiated from unmutated CLL B-cells were poly/autoreactive. Class-switch recombination (CSR) to IgG and IgA was detected in cells expressing the activation-induced cytidine deaminase gene (AICDA). Although these ASCs expressed high levels of the transcription factors PRDM1 (BLIMP1), IRF4, and XBP1s, they did not downregulate expression of PAX5. Our results suggest that CLL B-cells can differentiate into ASCs, undergo CSR and produce poly/autoreactive antibodies. Furthermore, our findings may be relevant for (i) identifying the normal counterpart of CLL B-cells and (ii) developing novel treatment strategies in CLL.

Phorbol myristate acetate, but not CD40L, induces the differentiation of CLL B cells into Ab-secreting cells

In this study, we investigated the capacity of chronic lymphocytic leukemia (CLL) B cells to undergo terminal differentiation into Ig-secreting plasma cells in T cell-independent and T cell-dependent responses. We used a two-step model involving stimulation with phorbol myristate acetate (PMA) and CD40L, together with cytokines (PMA/c and CD40L/c), for 7 days. We describe immunophenotypic modifications, changes in the levels of mRNA and protein for transcription factors and morphological and functional events occurring during the differentiation of CLL B cells into antibody-secreting cells (ASCs). The induction of differentiation differed significantly between the CD40L/c and PMA/c culture systems. The PMA/c culture system allowed CLL B cells to differentiate into IgM-secreting cells with an immunophenotype and molecular profile resembling those of preplasmablasts. By contrast, CD40L/c-stimulated cells had a phenotype and morphology similar to those of activated B cells and resembling those of the CLL B cells residing in the lymph node and bone marrow. These data suggest that the CLL B cells are not frozen permanently at a stage of differentiation and are able to differentiate into ASCs as appropriate stimulation are provided. The data presented here raise questions about the molecular processes and stimulation required for CLL B-cell differentiation and about the inability of CD40 ligand to induce differentiation of the CLL B cells.

IgM expressed by leukemic CD5(+) B cells binds mouse immunoglobulin light chain

Mouse immunoglobulin (Ig) molecules have previously been shown to bind to the surface of CD5(+) B cells from patients with B-cell chronic lymphocytic leukemia (B-CLL). The results indicated that surface IgM was involved in the interaction and suggested the phenomenon was an example of the polyreactive binding capacity of the surface Ig (sIg) expressed by these malignant cells. This article describes the further characterization of the interaction between human IgM and mouse Ig molecules and subunits. Mouse Ig molecules of both kappa and lambda light chain classes bound to the B-CLL cell surface. The dissociation constant for the interaction of mouse IgG1 (K121) with the B-CLL cell surface was 3.6 x 10(-7) M. To confirm the involvement of the human IgM expressed by the B-CLL cells in the interaction, the malignant cells were stimulated in vitro to induce secretion of human IgM. Enzyme immunoassay was used to show that secreted human IgM bound to intact mouse Ig, as occurred with the cell surface analysis. The mouse Ig epitope recognized by the purified secreted human IgM was shown by Western blot analysis to be located on the light chain of the mouse Ig molecule and to be conformationally dependent. K121 light chain was cloned and expressed in E. coli and the recombinant light chain bound to the surface of CLL B cells. The results confirm that human IgM is the reactive ligand in the interaction with mouse Ig and indicate that the interaction of polyreactive IgM with mouse IgG occurs via the light chain component of IgG.

Interleukin-10 inhibits the in vitro proliferation of human activated leukemic CD5+ B-cells

B-cell chronic lymphocytic leukemia (B-CLL) is characterised by the proliferation and accumulation of sIgM+/CD5+ B-cells that fail to progress to the final stages of B-cell development. Despite their developmental arrest, leukemic CD5+ B-cells can undergo proliferation in vitro in the presence of different activators including phorbol esters, antibodies to cell surface antigens and human cytokines. Interleukin-10 (IL-10) has recently been found to inhibit CLL B-cell function in vitro by inducing apoptosis and down-regulating expression of bcl-2. Here, we examined the effect of IL-10 on proliferation, RNA synthesis, immunoglobulin (IgM) secretion and viability of leukemic CD5+ B-cells induced by activation with the phorbol ester PMA, alone or in combination with anti-Ig. IL-10 reduced PMA and PMA/anti-Ig induced proliferation and RNA synthesis by 50-80% and 15-40% respectively. Although proliferation and RNA synthesis induced by PMA/anti-Ig could be enhanced by the addition of IL-2, IL-4, IL-13, IFN-gamma or TNF-alpha, the presence of these cytokines failed to abrogate the IL-10-mediated inhibition of leukemic CD5+ B-cell activation. In contrast to the effects on proliferation and RNA synthesis, IL-10 did not inhibit IgM secretion, and had only a minimal effect on the viability of activated cells. Our results indicate that IL-10 inhibits proliferation of leukemic CD5+ B-cells by a mechanism distinct from induction of apoptosis and support the proposal for the utilisation of IL-10 in the therapy of B-CLL.

Cytokines and cross-linking of sIgM augment PMA-induced activation of human leukaemic CD5+ B cells

Purified leukaemic CD5+ B cells obtained from patients with B cell chronic lymphocytic leukaemia (B-CLL) undergo activation and differentiation following in vitro stimulation with optimal concentrations of the phorbol ester PMA. This paper examines the ability of exogenous cytokines, anti-Ig antibodies, or combinations of these, to enhance or replace the activation signal provided by PMA to different populations of leukaemic B cells. Proliferation induced by PMA was enhanced 2-20-fold when the cells were co-cultured with either anti-Ig, IL-2, IL-4, IL-13, IFN-gamma or TNF-alpha. Moreover, the combination of anti-Ig, PMA and any one of the above cytokines further enhanced proliferation. Anti-Ig and exogenous cytokines were also capable of inducing proliferation in leukaemic B cells cultured with a non-mitogenic concentration of PMA. When taken together with the finding that IL-2, IL-4, IL-13, IFN-gamma and TNF-alpha prevent in vitro apoptosis of leukaemic CD5+ B cells, the results presented here suggest that these cytokines, in conjunction with signals delivered via sIg, may play a role in the proliferation of leukaemic B cells in vivo and, consequently, the pathogenesis of B-CLL.

Human cytokines suppress apoptosis of leukaemic CD5+ B cells and preserve expression of bcl-2

Leukaemic CD5+ B cells obtained from B cell chronic lymphocytic leukaemia (B-CLL) patients rapidly undergo apoptosis during in vitro culture. This is associated with down-regulation in expression of bcl-2. Spontaneous apoptosis of these cells contrasts their enhanced longevity in vivo and suggests that apoptosis-inhibitory factors may be responsible for the accumulation of leukaemic cells in B-CLL. The effect of different cytokines on apoptosis and bcl-2 expression was examined in six populations of leukaemic CD5+ B cells. Consistent with previous data, IL-4 and IFN-gamma suppressed apoptosis in 6/6 and 5/6 cell populations, respectively. Interestingly, the ability to suppress apoptosis in leukaemic CD5+ B cells was also found to be a property of IL-2, IL-6, IL-13 and TNF-alpha. In the presence of these cytokines, 10-40% more viable cells were detected, compared with unstimulated cultures. Enhancement of cell viability and suppression of apoptosis were associated with a delay in down-regulation of bcl-2. These results suggest a role for autocrine and paracrine growth factors in the pathogenesis of B-CLL, and indicate that cytokines which prevent apoptosis in vitro may be targets for treating this malignancy.