CD5-induced apoptosis of B cells in some patients with chronic lymphocytic leukemia

B cell receptor signaling proteins as biomarkers for progression of CLL requiring first-line therapy

The molecular landscape of chronic lymphocytic leukemia (CLL) has been extensively characterized, and various potent prognostic biomarkers were discovered. The genetic composition of the B-cell receptor (BCR) immunoglobulin (IG) was shown to be especially powerful for discerning indolent from aggressive disease at diagnosis. Classification based on the IG heavy chain variable gene (IGHV) somatic hypermutation status is routinely applied. Additionally, BCR IGH stereotypy has been implicated to improve risk stratification, through characterization of subsets with consistent clinical profiles. Despite these advances, it remains challenging to predict when CLL progresses to requiring first-line therapy, thus emphasizing the need for further refinement of prognostic indicators. Signaling pathways downstream of the BCR are essential in CLL pathogenesis, and dysregulated components within these pathways impact disease progression. Considering not only genomics but the entirety of factors shaping BCR signaling activity, this review offers insights in the disease for better prognostic assessment of CLL.

Exploring a peptide nucleic acid-based antisense approach for CD5 targeting in chronic lymphocytic leukemia

We herein report an innovative antisense approach based on Peptide Nucleic Acids (PNAs) to down-modulate CD5 expression levels in chronic lymphocytic leukemia (CLL). Using bioinformatics tools, we selected a 12-mer tract of the CD5 mRNA as the molecular target and synthesized the complementary and control PNA strands bearing a serine phosphate dipeptide tail to enhance their water solubility and bioavailability. The specific recognition of the 12-mer DNA strand, corresponding to the target mRNA sequence by the complementary PNA strand, was confirmed by non-denaturing polyacrylamide gel electrophoresis, thermal difference spectroscopy, circular dichroism (CD), and CD melting studies. Cytofluorimetric assays and real-time PCR analysis demonstrated the downregulation of CD5 expression due to incubation with the anti-CD5 PNA at RNA and protein levels in Jurkat cell line and peripheral blood mononuclear cells from B-CLL patients. Interestingly, we also observed that transfection with the anti-CD5 PNA increases apoptotic response induced by fludarabine in B-CLL cells. The herein reported results suggest that PNAs could represent a potential candidate for the development of antisense therapeutic agents in CLL.

The multiple faces of CD5

Since its discovery, over 30 years ago, CD5 has been used as a marker to identify T cells, B1-a cells, and B cell chronic lymphocytic leukemia cells. Throughout the years, many studies have described the functional relevance of CD5 as a modulator of T and B cell receptor signaling. However, it has not been until recent years that CD5 has emerged as a functional receptor in other areas of the immune system. Here, we review some of the most important aspects of CD5 as a modulator of TCR and BCR signaling, cell survival receptor both in T and B cells during health and disease, as well as the newly discovered roles of this receptor in thymocyte selection, T cell effector differentiation, and immune tolerance. CD5 was found to promote T cell survival by protecting autoreactive T cell from activation-induced cell death, to promote de novo induction of regulatory T cells in the periphery, to modulate Th17 and Th2 differentiation, and to modulate immune responses by modulating dendritic cell functions. CD5 is overexpressed in Tregs and Bregs, which are fundamental to maintain immune homeostasis. The newly established roles of CD5 in modulating different aspects of immune responses identify this receptor as an immune checkpoint modulator, and therefore it could be used as a target for immune intervention in different pathologies such as cancer, autoimmune diseases or infections.

Dynamic variation of CD5 surface expression levels within individual chronic lymphocytic leukemia clones

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of clonally derived mature CD5high B cells; however, the cellular origin of CLL is still unknown. Patients with CLL also harbor variable numbers of CD5low B cells, but the clonal relationship of these cells to the bulk disease is unknown and can have important implications for monitoring, treating, and understanding the biology of CLL. Here, we use B-cell receptors (BCRs) as molecular barcodes to first show by single-cell BCR sequencing that the great majority of CD5low B cells in the blood of CLL patients are clonally related to CD5high CLL B cells. We investigate whether CD5 state switching was likely to occur continuously as a common event or as a rare event in CLL by tracking somatic BCR mutations in bulk CLL B cells and using them to reconstruct the phylogenetic relationships and evolutionary history of the CLL in four patients. Using statistical methods, we show that there is no parsimonious route from a single or low number of CD5low switch events to the CD5high population, but rather, large-scale and/or dynamic switching between these CD5 states is the most likely explanation. The overlapping BCR repertoires between CD5high and CD5low cells from CLL patient peripheral blood reveal that CLL exists in a continuum of CD5 expression. The major proportion of CD5low B cells in patients are leukemic, thus identifying CD5low B cells as an important component of CLL, with implications for CLL pathogenesis, clinical monitoring, and the development of anti-CD5-directed therapies.

Antitumour effects of single or combined monoclonal antibodies directed against membrane antigens expressed by human B cells leukaemia

Background

The increasing availability of different monoclonal antibodies (mAbs) opens the way to more specific biologic therapy of cancer patients. However, despite the significant success of therapy in breast and ovarian carcinomas with anti-HER2 mAbs as well as in non-Hodkin B cell lymphomas with anti-CD20 mAbs, certain B cell malignancies such as B chronic lymphocytic leukaemia (B-CLL) respond poorly to anti-CD20 mAb, due to the low surface expression of this molecule. Thus, new mAbs adapted to each types of tumour will help to develop personalised mAb treatment. To this aim, we analyse the biological and therapeutic properties of three mAbs directed against the CD5, CD71 or HLA-DR molecules highly expressed on B-CLL cells.

Results

The three mAbs, after purification and radiolabelling demonstrated high and specific binding capacity to various human leukaemia target cells. Further in vitro analysis showed that mAb anti-CD5 induced neither growth inhibition nor apoptosis, mAb anti-CD71 induced proliferation inhibition with no early sign of cell death and mAb anti-HLA-DR induced specific cell aggregation, but without evidence of apoptosis. All three mAbs induced various degrees of ADCC by NK cells, as well as phagocytosis by macrophages. Only the anti-HLA-DR mAb induced complement mediated lysis. Coincubation of different pairs of mAbs did not significantly modify the in vitro results. In contrast with these discrete and heterogeneous in vitro effects, in vivo the three mAbs demonstrated marked anti-tumour efficacy and prolongation of mice survival in two models of SCID mice, grafted either intraperitoneally or intravenously with the CD5 transfected JOK1-5.3 cells. This cell line was derived from a human hairy cell leukaemia, a type of malignancy known to have very similar biological properties as the B-CLL, whose cells constitutively express CD5. Interestingly, the combined injection of anti-CD5 with anti-HLA-DR or with anti-CD71 led to longer mouse survival, as compared to single mAb injection, up to complete inhibition of tumour growth in 100% mice treated with both anti-HLA-DR and anti-CD5.

Conclusions

Altogether these data suggest that the combined use of two mAbs, such as anti-HLA-DR and anti-CD5, may significantly enhance their therapeutic potential.

CD5 promotes IL-10 production in chronic lymphocytic leukemia B cells through STAT3 and NFAT2 activation

B lymphocytes from chronic lymphocytic leukemia (CLL) display some CD5 transcripts for CD5 containing the known exon 1 (E1A) and other CD5 transcripts containing the new exon 1 (E1B). These malignant B cells, as well as B cell lines transfected with cDNA for E1A-cd5 or with cDNA for E1B-cd5 produce IL-10, raising the possibility that CD5 participates in the secretion of IL-10. We identified transcription factors involved in this production in CD5(+) B lymphocytes from CLL patients and in E1A-cd5-transfected or E1B-cd5-transfected Jok cells. STAT3 is activated via phosphorylation of serine 727 but also NFAT2 through its translocation into the nucleus. Chromatin immunoprecipitation experiments confirmed the role of STAT3 and allowed the discovery of a role for NFAT2 in IL-10 production. Both transcription factors bind not only to the enhancer of the Il-10 gene but also to the promoter of the Il-5 and Il-13 genes. Furthermore, transfection of B cell lines with E1A-cd5 or E1B-cd5 established that activation of STAT3 and NFAT2 is regulated by CD5. The same holds true for the production of IL-10, IL-5, and IL-13 and the expression of the receptors for these cytokines. This interpretation was confirmed by two experiments. In the first, downregulation of CD5 by small interfering RNAs lowered the production of IL-10. In the second experiment, transfection of the GFP-NFAT2 gene into B lymphocytes induced nuclear translocation of NFAT2 in CD5(+) but not in CD5(-) B cells. Thus, CD5 expression is associated with NFAT2 activity (and mildly STAT3 activity), indicating that CD5 controls IL-10 secretion.

Selection of the alternative exon 1 from the cd5 gene down-regulates membrane level of the protein in B lymphocytes

The human cd5 gene has two alternative exons 1: exon 1A (E1A) which encodes the full-length (FL) CD5 protein and exon 1B (E1B) which encodes a truncated (TR) isoform. The FL variant of CD5 protein is translocated to the plasma membrane, while its TR variant is retained in the cytoplasm. Because there is an inverse relationship between the levels of FL-CD5 and TR-CD5 in B cells, we have addressed the issue of how the selection of exon 1 is determined. In leukemic B cells, DNA methyltransferase (DNMT)1-induced methylation of E1B prevents its transcription. Furthermore, the level of mRNA for DNMT1 correlates inversely with that of mRNA for CD5-E1B. However, suppression of E1B transcription is incomplete, and some molecules of TR-CD5 continue to be synthesized. Bortezomid-induced inhibition of the proteasome establishes that these TR-CD5 molecules are cleared through the ubiquitin-proteasome pathway. Transfection of CD5 mutants into COS-1 cells locates the ubiquitin-binding site at the second destruction box of the extracellular region of CD5. Activation of the B cells by anti-IgM, Staphylococcus aureus Cowan I (SAC), or PMA up-regulates DNMT1, and thereby CD5-E1A mRNA at the expense of CD5-E1B mRNA. Aberrant synthesis of TR-CD5 is thus offset by balanced degradation of excessive protein. Dysregulation of these mechanisms reduces the expression level of membrane CD5, and thereby diminishes the threshold of the response by cells expressing CD5.

Natural phosphorylation of CD5 in chronic lymphocytic leukemia B cells and analysis of CD5-regulated genes in a B cell line suggest a role for CD5 in malignant phenotype

Chronic lymphocytic leukemia (CLL) results in the accumulation of B cells, presumably reflecting the selection of malignant cell precursors with Ag combined with complex alterations in protein activity. Repeated BCR stimulation of normal B cells leads to anergy and CD5 expression, both of which are features of CLL. Because CD5 is phosphorylated on tyrosine following BCR engagement and negatively regulates BCR signaling in normal B cells, we investigated its phosphorylation status and found it to be naturally phosphorylated on tyrosine but not on serine residues in CLL samples. To analyze the role of CD5, we established a B cell line in which CD5 is phosphorylated. Gene profiling of vector vs CD5-transfected B cells pointed out gene groups whose expression was enhanced: Apoptosis inhibitors (BCL2), NF-kappaB (RELB, BCL3), Wnt, TGFbeta, VEGF, MAPKs, Stats, cytokines, chemokines (IL-10, IL-10R, IL-2R, CCL-3, CCL-4, and CCR7), TLR-9, and the surface Ags CD52, CD54, CD70, and CD72. Most of these gene groups are strongly expressed in CLL B cells as compared with normal B cells. Unexpectedly, metabolic pathways, namely cholesterol synthesis and adipogenesis, are also enhanced by CD5. Conversely, CD5 inhibited genes involved in RNA splicing and processing, ribosome biogenesis, proteasome, and CD80 and CD86 Ags, whose expression is low in CLL. Comparison of CD5- vs tailless CD5-transfected cells further demonstrated the role of CD5 phosphorylation in the regulation of selected genes. These results support a model where CLL cells are chronically stimulated, leading to CD5 activation and cell survival. In addition to CD5 itself, we point to several CD5-induced genes as potential therapeutic targets.

The normal counterpart to the chronic lymphocytic leukemia B cell

Chronic lymphocytic leukemia (CLL) is characterized by the monoclonal expansion of small mature-looking B cells that accumulate in the blood, marrow, and lymphoid organs, and have a remarkable phenotypic homogeneity. By definition, CLL cells co-express CD5 and CD23 with faint to undetectable amounts of monoclonal surface immunoglobulins (sIg). The concept of phenotypic homogeneity has been reinforced by gene expression profiling data, which suggest that the pathogenesis of CLL has to be associated with a fairly common mechanism of transformation. In recent years the biology of CLL has been enriched by an unprecedented flurry of new observations that are leading to a better understanding of the natural history of the disease. Still CLL cells have so far defied any attempt to satisfactorily answer the simple time-honored question of what their cell of origin is. It is the purpose of this review to discuss the features a cell must possess to be considered with reasonable approximation the normal counterpart of a CLL B cell.

CD5 provides viability signals to B cells from a subset of B-CLL patients by a mechanism that involves PKC

B-chronic lymphocytic leukaemia (B-CLL) is a heterogeneous disease characterized by an accumulation of B lymphocytes expressing CD5. To date, the biological significance of this molecule in B-CLL B cells remains to be elucidated. In this study, we have analysed the functional consequences of the binding of an anti-CD5 antibody on B-CLL B cells. To this purpose, we have measured the percentage of viability of B-CLL B cells in the presence or in the absence of anti-CD5 antibodies and also examined some of the biochemical events downstream the CD5-signalling. We demonstrate that anti-CD5 induces phosphorylation of protein tyrosine kinases and protein kinase C (PKC), while no activation of Akt/PKB and MAPKs is detected. This signalling cascade results in viability in a group of patients in which we observe an increase of Mcl-1 levels, whereas the levels of bcl-2, bcl-x(L) and XIAP do not change. We also report that this pathway leads to IL-10 production, an immunoregulatory cytokine that might act as an autocrine growth factor for leukaemic B cells. Inhibition of PKC prevents the induction of Mcl-1 and IL-10, suggesting that the activation of PKC plays an important role in the CD5-mediated survival signals in B cells from a subset of B-CLL patients.

High Mda-7 expression promotes malignant cell survival and p38 MAP kinase activation in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL)-B-cells are quiescent differentiated cells that produce interleukin (IL)-10 and accumulate due to resistance to apoptosis. The mechanisms underlying such resistance are poorly understood. Herein we show that all CLL B-cells tested (30/30) display high mRNA and protein expression of the tumor suppressor Mda-7/IL-24, an IL-10 family member, in comparison to normal B cells. A downstream Mda-7 signaling target, p38 mitogen-activated protein kinase (MAPK) was highly phosphorylated in all CLL cells but not in normal B-cells. Mda-7 expression and p38 MAPK phosphorylation diminished in culture and the latter could be reinduced by recombinant (r)-IL-24 or LPS and Mda-7 transfection. Mda-7/IL-24 siRNA specifically inhibited p38 MAPK phosphorylation in CLL without affecting p38 MAPK, bcl2, or Lyn expression, further demonstrating the direct role of Mda-7/IL-24 in p38 MAPK activation. Both pharmacological inhibition of p38 MAPK and Mda-7 silencing augmented spontaneous apoptosis by three-fold in CLL cells cultured in autologous serum, which was reversed by LPS and r-IL-24. We established the role of p38 MAPK in CLL cell survival and demonstrated a paradoxical effect, whereby Mda-7 and IL-24, inducers of apoptosis in diverse cancer cells, promote the survival of CLL B-cells through p38 MAPK activation.

Differential expression of CD180 and IgM by B-cell chronic lymphocytic leukaemia cells using mutated and unmutated immunoglobulin VH genes

We have studied the surface expression of the Toll-like receptor family member CD 180 on cells from 78 patients with B-chronic lymphocytic leukaemia (B-CLL). B-CLL cells had variable levels of CD 180 expression, but this was always less than that expressed by normal blood B cells and was stable for 24 months. Significantly higher levels of CD 180 were expressed by B-CLL cells with mutated IGVH genes compared with those using unmutated IGVH genes. This was in contrast to the higher levels of expression of surface immunoglobulin M by B-CLL cells using unmutated, rather than mutated IGVH genes. CD 180 was functional on B-CLL cells from some of the patients, as shown by the increased expression of CD 86 following incubation in vitro with anti-CD 180. The differential expression of CD 180 amongst B-CLL patients is one more marker that may define more precisely the different biological properties of this heterogeneous disease.

Signaling capacity of FcγRII isoforms in B-CLL cells

Two main isoforms of Fcgamma receptor II (CD 32) have been described in humans: activatory FcgammaRIIA and inhibitory FcgammaRIIB. We have previously reported that B cells from a subset of chronic lymphocytic leukemia (B-CLL) patients express not only FcgammaRIIB, as normal B lymphocytes, but also the myeloid FcgammaRIIA. The aim of this study was to evaluate the signaling capacity of both FcgammaRII isoforms in B-CLL cells. We found that FcgammaRIIA expressed by leukemic cells failed to induce Ca(2+) mobilization or protein tyrosine phosphorylation, suggesting that the receptor is not functional. By contrast, FcgammaRIIB effectively diminished BCR-triggered ERK 1 phosphorylation, which indicates that it is able to transduce inhibitory signals in B-CLL cells. Moreover, we found that FcgammaRIIB homoaggregation in either B-CLL or non-malignant tonsillar B cells did not result in apoptosis as was reported for murine B splenocytes. Together, these results show that FcgammaRIIB, but not FcgammaRIIA is biologically active in B-CLL cells and might influence leukemic cell physiology in vivo.

An alternative exon 1 of the CD5 gene regulates CD5 expression in human B lymphocytes

T lymphocytes and a subpopulation of B lymphocytes express the CD5 coreceptor. Its functional importance is evident from the multiple levels and developmental stages of the regulation of its expression. We here report the discovery of a novel regulatory exon upstream of the noncoding region of the CD5 gene in humans. This alternate exon 1 is designated E1B (with the conventional exon 1 renamed E1A) and was shown to regulate the expression of CD5. E1B-containing transcripts existed exclusively in B lymphocytes and encoded a protein that was truncated and retained intracellularly. As a consequence, the amount of E1A-containing transcripts was down-regulated and the membrane CD5 expression was diminished in the presence of E1B-containing transcripts. High levels of E1A transcripts were found in chronic lymphocytic leukemia, and there were no E1A transcripts in 697 pre-B cells, which have no membrane CD5. Introduction of E1B into Jurkat cells reduced their membrane expression of CD5, and sequence analysis revealed that the E1B motif is a defective human endogenous retrovirus. A balance between the 2 alternative exons 1 might be central to the regulation of membrane CD5 in human B cells, and, through CD5-associated SH2-containing phosphatase 1, to the modulation of B-cell antigen receptor-transduced signals.

B cell response to surface IgM cross-linking identifies different prognostic groups of B-chronic lymphocytic leukemia patients

On the basis of responses to surface IgM (sIgM) cross-linking, B cells from 41 patients with B-chronic lymphocytic leukemia were categorized as 15 nonresponders (group I) and 26 responders (group II). The latter cases were subclassified as those seven where proliferation was induced (subgroup IIa) and the remaining 19 in whom apoptosis occurred (subgroup IIa). Signal disruption in group I was confirmed by the absence of Ca2+ mobilization. Activation of PI3K was constitutive in subgroup IIa, but not in subgroup IIb, and that of Akt induced by anti-mu in subgroup IIa, but not in subgroup IIb. Among the MAPK, ERK was more highly activated relative to p38 in subgroup IIa, whereas activation of p38 predominated over that of ERK in subgroup IIb. For subgroup IIb cells, based on tyrosine phosphorylation and translocation into lipid rafts, sIgM signaling was shown to be enhanced by Zap70. The different consequences of signaling through sIgM were associated with biological prognosis indicators. These included high levels of CD38, lack of mutations in the IgVH chain genes, preferential usage of full-length CD79b, and severe clinical stage. Thus, modification of sIgM-induced signaling could be a therapeutic approach.

Role of B-cell antigen receptor-associated molecules and lipid rafts in CD5-induced apoptosis of B CLL cells

A total of 40 patients with B-CLL were investigated for CD5-triggered apoptosis and categorized as 20 resistant (group I) and 20 sensitive patients (group II). The densities of surface IgM (sIgM) and CD5 were lower in group I than group II, as were the percentages of CD79b+, CD38+, and Zap70-expressing B cells. CD5 signaling was mediated through the BCR in group II B cells, as established by coimmunoprecipitation of CD5 and CD79a and tyrosine phosphorylation of CD79a. Following colocalization of CD5 and sIgM in membrane lipid rafts (LRs), Syk became associated with these molecules, whereas SHP-1 was uncoupled from CD5. Nonresponsiveness to CD5 cross-linking in group I was ascribed to three possible abnormalities, and defined as three subgroups of patients. In subgroups Ia and Ib, CD5 and sIgM colocalized within the LRs. SHP-1 remained attached to the BCR in subgroup Ia, but not in subgroup Ib, where signal transduction was associated with an excess of truncated CD79b. In subgroup Ic, CD5 and sIgM segregated into different LRs, resulting in no signaling of apoptosis.

Engagement of the B-cell antigen receptor (BCR) allows efficient transduction of ZAP-70-positive primary B-CLL cells by recombinant adeno-associated virus (rAAV) vectors

Engagement of the B-cell antigen receptor (BCR) by crosslinking of the surface immunoglobulin (sIg) homodimer was studied for recombinant adeno-associated virus (rAAV)-mediated gene transfer into B-cell chronic lymphocytic leukaemia (B-CLL) cells. Leukemic cells obtained from 20 patients were stimulated with anti-sIg-directed antibodies and transduced with rAAV vectors coding for enhanced green fluorescent protein (EGFP) (AAV/EGFP) or CD40L (AAV/CD40L). Transduction of B-CLL cells was enhanced after BCR engagement compared to unstimulated controls (P=0.0356). BCR crosslinking induced a significant, dose- and time-dependent upregulation of heparan sulfate proteoglycan (HSPG), the primary receptor for AAV, on B-CLL cells (mean: 38.2 versus 1.7%; P=0.0006). A correlation of HSPG expression after BCR crosslinking with transduction efficiency by AAV/EGFP (P=0.0153) and AAV/CD40L (P=0.0347) was observed. High expression of zeta-associated protein 70 (ZAP-70) in B-CLL cells correlated with a better transduction efficiency by AAV/EGFP (P<0.0001) and AAV/CD40L (P=0.002), respectively: 48 h after transduction of ZAP-70-positive samples, transgene expression was seen in a mean of 33.8% (s.e.m. 3.7%) and 28.9% (s.e.m. 6.7%) of cells, respectively, and could be specifically blocked by heparin, a soluble competitor of HSPG (P<0.0001). In summary, engagement of the BCR on ZAP-70 positive B-CLL cells allows efficient rAAV-mediated gene delivery.

Differential signaling via surface IgM is associated with VH gene mutational status and CD38 expression in chronic lymphocytic leukemia

The mutational status of tumor immunoglobulin V(H) genes is providing a powerful prognostic marker for chronic lymphocytic leukemia (CLL), with patients having tumors expressing unmutated V(H) genes being in a less favorable subset. However, the biologic differences correlating with V(H) gene status that could determine the clinical course of the disease are unknown. Here we show that differing responses to IgM ligation are closely associated with V(H) gene status. Specifically, 80% of cases with unmutated V(H) genes showed increased global tyrosine phosphorylation following IgM ligation, whereas only 20% of samples with mutated V(H) genes responded (P =.0002). There was also an association between response to IgM ligation and expression of CD38 (P =.015). The Syk kinase, critical for transducing B-cell receptor (BCR)- derived signals, was constitutively present in all CLL samples, and there was a perfect association between global phosphorylation and induction of phosphorylation/activation of Syk. Nonresponsiveness to anti-IgM could be circumvented by ligation of IgD (10 of 15 samples tested) or the BCR-associated molecule CD79alpha (12 of 15 samples tested). These results suggest that multiple mechanisms underlie nonresponsiveness to anti-IgM in CLL and that retained responsiveness to anti-IgM contributes to the poor prognosis associated with the unmutated subset of CLL. The prognostic power of the in vitro response to IgM ligation remains to be determined in a large series, but the simple technology involved may present an alternative or additional test for predicting clinical course.