Cell type- and stage-specific expression of the CD20/B1 antigen correlates with the activity of a diverged octamer DNA motif present in its promoter

Advancements in cancer immunotherapies targeting CD20: from pioneering monoclonal antibodies to chimeric antigen receptor-modified T cells

CD20 located predominantly on the B cells plays a crucial role in their development, differentiation, and activation, and serves as a key therapeutic target for the treatment of B-cell malignancies. The breakthrough of monoclonal antibodies directed against CD20, notably exemplified by rituximab, revolutionized the prognosis of B-cell malignancies. Rituximab, approved across various hematological malignancies, marked a paradigm shift in cancer treatment. In the current landscape, immunotherapies targeting CD20 continue to evolve rapidly. Beyond traditional mAbs, advancements include antibody-drug conjugates (ADCs), bispecific antibodies (BsAbs), and chimeric antigen receptor-modified (CAR) T cells. ADCs combine the precision of antibodies with the cytotoxic potential of drugs, presenting a promising avenue for enhanced therapeutic efficacy. BsAbs, particularly CD20xCD3 constructs, redirect cytotoxic T cells to eliminate cancer cells, thereby enhancing both precision and potency in their therapeutic action. CAR-T cells stand as a promising strategy for combatting hematological malignancies, representing one of the truly personalized therapeutic interventions. Many new therapies are currently being evaluated in clinical trials. This review serves as a comprehensive summary of CD20-targeted therapies, highlighting the progress and challenges that persist. Despite significant advancements, adverse events associated with these therapies and the development of resistance remain critical issues. Understanding and mitigating these challenges is paramount for the continued success of CD20-targeted immunotherapies.

The Role of the Transcriptional Coactivator BOB.1/OBF.1 in Adaptive Immunity

BOB.1/OBF.1 is a transcriptional coactivator involved in octamer-dependent transcription. Thereby, BOB.1/OBF.1 is involved in the transcriptional regulation of genes important for lymphocyte physiology. BOB.1/OBF.1-deficient mice reveal multiple B- and T-cell developmental defects. The most prominent defect of these mice is the complete absence of germinal centers (GCs) resulting in severely impaired T-cell-dependent immune responses. In humans, BOB.1/OBF.1 is associated with several autoimmune and inflammatory diseases but also linked to liquid and solid tumors. Although its role for B-cell development is relatively well understood, its exact role for the GC reaction and T-cell biology has long been unclear. Here, the contribution of BOB.1/OBF.1 for B-cell maturation is summarized, and recent findings regarding its function in GC B- as well as in various T-cell populations are discussed. Finally, a detailed perspective on how BOB.1/OBF.1 contributes to different pathologies is provided.

Reverse effect of Semaphorin-3F on rituximab resistance in diffuse large B-cell lymphoma via the Hippo pathway

BACKGROUND

Exploring the underlying mechanism of rituximab resistance is critical to improve the outcomes of patients with diffuse large B-cell lymphoma (DLBCL). Here, we tried to identify the effects of the axon guidance factor semaphorin-3F (SEMA3F) on rituximab resistance as well as its therapeutic value in DLBCL.

METHODS

The effects of SEMA3F on the treatment response to rituximab were investigated by gain- or loss-of-function experiments. The role of the Hippo pathway in SEMA3F-mediated activity was explored. A xenograft mouse model generated by SEMA3F knockdown in cells was used to evaluate rituximab sensitivity and combined therapeutic effects. The prognostic value of SEMA3F and TAZ (WW domain-containing transcription regulator protein 1) was examined in the Gene Expression Omnibus (GEO) database and human DLBCL specimens.

RESULTS

We found that loss of SEMA3F was related to a poor prognosis in patients who received rituximab-based immunochemotherapy instead of chemotherapy regimen. Knockdown of SEMA3F significantly repressed the expression of CD20 and reduced the proapoptotic activity and complement-dependent cytotoxicity (CDC) activity induced by rituximab. We further demonstrated that the Hippo pathway was involved in the SEMA3F-mediated regulation of CD20. Knockdown of SEMA3F expression induced the nuclear accumulation of TAZ and inhibited CD20 transcriptional levels via direct binding of the transcription factor TEAD2 and the CD20 promoter. Moreover, in patients with DLBCL, SEMA3F expression was negatively correlated with TAZ, and patients with SEMA3F low TAZ high had a limited benefit from a rituximab-based strategy. Specifically, treatment of DLBCL cells with rituximab and a YAP/TAZ inhibitor showed promising therapeutic effects in vitro and in vivo .

CONCLUSION

Our study thus defined a previously unknown mechanism of SEMA3F-mediated rituximab resistance through TAZ activation in DLBCL and identified potential therapeutic targets in patients.

Hodgkin Lymphoma: A Special Microenvironment

Classical Hodgkin's lymphoma (cHL) is one of the most particular lymphomas for the few tumor cells surrounded by an inflammatory microenvironment. Reed-Sternberg (RS) and Hodgkin (H) cells reprogram and evade antitumor mechanisms of the normal cells present in the microenvironment. The cells of microenvironment are essential for growth and survival of the RS/H cells and are recruited through the effect of cytokines/chemokines. We summarize recent advances in gene expression profiling (GEP) analysis applied to study microenvironment component in cHL. We also describe the main therapies that target not only the neoplastic cells but also the cellular components of the background.

L-Type Cav 1.2 Calcium Channel-α-1C Regulates Response to Rituximab in Diffuse Large B-Cell Lymphoma

PURPOSE

One third of patients with diffuse large B-cell lymphoma (DLBCL) succumb to the disease partly due to rituximab resistance. Rituximab-induced calcium flux is an important inducer of apoptotic cell death, and we investigated the potential role of calcium channels in rituximab resistance.

EXPERIMENTAL DESIGN

The distinctive expression of calcium channel members was compared between patients sensitive and resistant to rituximab, cyclophosphamide, vincristine, doxorubicin, prednisone (RCHOP) regimen. The observation was further validated through mechanistic in vitro and in vivo studies using cell lines and patient-derived xenograft mouse models.

RESULTS

A significant inverse correlation was observed between CACNA1C expression and RCHOP resistance in two independent DLBCL cohorts, and CACNA1C expression was an independent prognostic factor for RCHOP resistance after adjusting for International Prognostic Index, cell-of-origin classification, and MYC/BCL2 double expression. Loss of CACNA1C expression reduced rituximab-induced apoptosis and tumor shrinkage. We further demonstrated direct interaction of CACNA1C with CD20 and its role in CD20 stabilization. Functional modulators of L-type calcium channel showed expected alteration in rituximab-induced apoptosis and tumor suppression. Furthermore, we demonstrated that CACNA1C expression was directly regulated by miR-363 whose high expression is associated with worse prognosis in DLBCL.

CONCLUSIONS

We identified the role of CACNA1C in rituximab resistance, and modulating its expression or activity may alter rituximab sensitivity in DLBCL.

A molecular perspective on rituximab: A monoclonal antibody for B cell non Hodgkin lymphoma and other affections

Rituximab (a chimeric anti-CD20 monoclonal antibody) is the first Food and Drug Administration approved anti-tumor antibody. Immunotherapy by rituximab, especially in combination-therapy, is a mainstay for a vast variety of B-cell malignancies therapy. Its therapeutic value is unquestionable, yet the mechanisms of action responsible for anti-tumor activity of rituximab and rituximab resistance mechanisms are not completely understood. Investigation of the mechanisms of action that contribute to the rituximab activity have eventually directed to a suite of novel combinations and novel treatment schedules, and also have resulted new generations of antibodies with more desired effects. Although, further investigations are needed to define the mechanisms of rituximab resistance and prominent effector activity of the altered next generation anti-CD20 to improve their efficacies and develop new anti-CD20 monoclonal antibodies in NHL treatment. This article focuses on the properties of CD20 which led scientists to select it as an effective therapeutic target and the molecular details of mechanisms of rituximab action and resistance. We also discuss about the impact of rituximab in monotherapy and in combination with chemotherapy regimens. Finally, we comparatively summarize the next generations of anti CD20 monoclonal antibodies to highlight their advantages relative to their ancestor: Rituximab.

Inhibitors of SRC kinases impair antitumor activity of anti-CD20 monoclonal antibodies

Clinical trials with SRC family kinases (SFKs) inhibitors used alone or in a combination with anti-CD20 monoclonal antibodies (mAbs) are currently underway in the treatment of B-cell tumors. However, molecular interactions between these therapeutics have not been studied so far. A transcriptional profiling of tumor cells incubated with SFKs inhibitors revealed strong downregulation of MS4A1 gene encoding CD20 antigen. In a panel of primary and established B-cell tumors we observed that SFKs inhibitors strongly affect CD20 expression at the transcriptional level, leading to inhibition of anti-CD20 mAbs binding and increased resistance of tumor cells to complement-dependent cytotoxicity. Activation of the AKT signaling pathway significantly protected cells from dasatinib-triggered CD20 downregulation. Additionally, SFKs inhibitors suppressed antibody-dependent cell-mediated cytotoxicity by direct inhibition of natural killer cells. Abrogation of antitumor activity of rituximab was also observed in vivo in a mouse model. Noteworthy, the effects of SFKs inhibitors on NK cell function are largely reversible. The results of our studies indicate that development of optimal combinations of novel treatment modalities with anti-CD20 mAbs should be preceded by detailed preclinical evaluation of their effects on target cells.

Rituximab therapy in nephrotic syndrome: implications for patients' management

Rituximab offers an alternative to current immunosuppressive therapies for difficult-to-treat nephrotic syndrome. The best outcomes are seen in patients with steroid-dependent nephrotic syndrome who have failed to respond to multiple therapies. By contrast, the benefits of rituximab therapy are limited in patients with steroid-resistant nephrotic syndrome, particularly those with focal segmental glomerulosclerosis (FSGS). Therapy with plasma exchange and one or two doses of rituximab has shown success in patients with recurrent FSGS. Young patients and those with normal serum albumin at recurrence of nephrotic syndrome are most likely to respond to rituximab therapy. A substantial proportion of rituximab-treated patients with idiopathic membranous nephropathy show complete or partial remission of proteinuria, and reduced levels of phospholipase A(2) receptor autoantibodies, which are implicated in the pathogenesis of this disorder. Successful rituximab therapy induces prolonged remission and enables discontinuation of other medications without substantially increasing the risk of infections and other serious adverse events. However, the available evidence of efficacy of rituximab therapy is derived chiefly from small case series and requires confirmation in prospective, randomized, controlled studies that define the indications for use and predictors of response to this therapy.

Regulation of CD20 in rituximab-resistant cell lines and B-cell non-Hodgkin lymphoma

PURPOSE

The aim of this research was to further investigate the contribution of CD20 antigen expression to rituximab activity and define the mechanisms responsible for CD20 downregulation in rituximab-resistant cell lines (RRCL).

EXPERIMENTAL DESIGN

Rituximab-sensitive cell lines, RRCL, and primary neoplastic B cells were evaluated by chromium-51 release assays, ImageStream image analysis, immunohistochemical staining, flow cytometric analysis, CD20 knockdown, promoter activity, chromatin immunoprecipitation (ChIP) analysis of CD20 promoter, and CD20 plasmid transfection experiments to identify mechanisms associated with CD20 regulation in RRCL.

RESULTS

RRCL exhibited a gradual loss of CD20 surface expression with repeated exposure to rituximab. We identified a CD20 antigen surface threshold level required for effective rituximab-associated complement-mediated cytotoxicity (CMC). However, a direct correlation between CD20 surface expression and rituximab-CMC was observed only in rituximab-sensitive cell lines. CD20 promoter activity was decreased in RRCL. Detailed analysis of various CD20 promoter fragments suggested a lack of positive regulatory factors in RRCL. ChIP analysis showed reduced binding of several key positive regulatory proteins on CD20 promoter in RRCL. Interleukin-4 (IL-4) induced higher CD20 promoter activity and CD20 expression but modestly improved rituximab activity in RRCL and in primary B-cell lymphoma cells. Forced CD20 expression restored cytoplasmic but not surface CD20, suggesting the existence of a defect in CD20 protein transport in RRCL.

CONCLUSIONS

We identified several mechanisms that alter CD20 expression in RRCL and showed that, whereas CD20 expression is important for rituximab activity, additional factors likely contribute to rituximab sensitivity in B-cell lymphoma.

Oct1 is a switchable, bipotential stabilizer of repressed and inducible transcriptional states

Little is known regarding how the Oct1 transcription factor regulates target gene expression. Using murine fibroblasts and two target genes, Polr2a and Ahcy, we show that Oct1 recruits the Jmjd1a/KDM3A lysine demethylase to catalyze the removal of the inhibitory histone H3K9 dimethyl mark and block repression. Using purified murine T cells and the Il2 target locus, and a colon cancer cell line and the Cdx2 target locus, we show that Oct1 recruits the NuRD chromatin-remodeling complex to promote a repressed state, but in a regulated manner can switch to a different capacity and mediate Jmjd1a recruitment to block repression. These findings indicate that Oct1 maintains repression through a mechanism involving NuRD and maintains poised gene expression states through an antirepression mechanism involving Jmjd1a. We propose that, rather than acting as a primary trigger of gene activation or repression, Oct1 is a switchable stabilizer of repressed and inducible states.

HDAC inhibitors augment cytotoxic activity of rituximab by upregulating CD20 expression on lymphoma cells

Anti-CD20 antibody rituximab is now essential for the treatment of CD20-positive B-cell lymphomas. Decreased expression of CD20 is one of the major mechanisms underlying both innate and acquired resistance to rituximab. In this study, we show that histone deacetylase (HDAC) inhibitors augment the cytotoxic activity of rituximab by enhancing the surface expression of CD20 antigen on lymphoma cells. HDAC inhibitors, valproic acid (VPA) and romidepsin, increased CD20 expression at protein and mRNA levels in B-cell lymphoma cell lines with relatively low CD20 expression levels. The VPA-mediated increase in CD20 expression occurred at 1 m, which is clinically achievable and safe, but insufficient for inducing cell death. Chromatin immunoprecipitation assays revealed that HDAC inhibitors transactivated the CD20 gene through promoter hyperacetylation and Sp1 recruitment. HDAC inhibitors potentiated the activity of rituximab in complement-dependent cytotoxic assays. In mouse lymphoma models, HDAC inhibitors enhanced CD20 expression along with histone hyperacetylation in transplanted cells, and acted synergistically with rituximab to retard their growth. The combination with HDAC inhibitors may serve as an effective strategy to overcome rituximab resistance in B-cell lymphomas.

Identification of a suppressor element in the amelogenin promoter

Amelogenin expression is regulated in a cell-type- specific manner. Investigators have previously identified an enhancer element by using the 5' flanking sequence of the amelogenin promoter. However, the cell-type-specific regulation of the amelogenin gene remains poorly understood. In some genes, the first intron regulates tissue-specific expression. We hypothesized that intron 1 is important for the cell-type-specific regulation of amelogenin expression. We identified a suppressor element between -74 and -464. We also found enhancer activity in intron 1. Additionally, we found that the suppressor element in the promoter region suppressed intron 1 enhancer activity. The suppressor and the enhancers acted in an ameloblast-like cell line, but not in HeLa cells. Mutation of the Oct-1 binding sites reversed the suppressor activity, suggesting that Oct-1 sites are essential for suppression. These results suggest that Oct-1 and intron 1 may contribute to cell-type-specific amelogenin expression.

New transcription factors in diagnostic hematopathology

The transcription factors (TFs) that controls the intricate machinery of multistep differentiation and activation programs of the lymphoid system, represent a complex array of proteins, whose identification and function has only in part been completed. TFs are usually expressed during specific differentiation or activation cellular programs, making them interesting tools in diagnostic immunohistochemistry. In fact, the specificity of some of these TFs for lineage or activation/differentiation passages or their abnormal expression in specific disease entity, represents a feature that has been exploited in diagnostic/prognostic immunohistochemistry. Bcl-6 was the prototype of this class of markers. Currently, the expanding knowledge of the TFs involved in the differentiation programs and in the activation processes of T-lymphocyte and B-lymphocyte in normal and neoplastic conditions and the availability of antibodies able to efficiently recognize these TFs in histologic material, represent a powerful tool in diagnostic hematopathology. In this review we will consider the basic biologic aspects and the applications in hematopathology of some of the lymphocyte-related TFs, including Pax5/BSAB, MUM1/IRF4, BOB1, Oct-2, T-bet, and FOXP3. This field is rapidly evolving, as witnessed by the ongoing growing number of novel TFs with possible diagnostic applications appearing in the literature.

Epigenetic processes play a major role in B-cell-specific gene silencing in classical Hodgkin lymphoma

Many B-lineage-specific genes are down-regulated in Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL). We investigated the involvement of epigenetic modifications in gene silencing in cHL cell lines and in microdissected primary HRS cells. We assessed the expression and methylation status of CD19, CD20, CD79B, SYK, PU.1, BOB.1/OBF.1, BCMA, and LCK, all of which are typically down-regulated in cHL. We could reactivate gene expression in cHL cell lines with the DNA demethylating agent 5-aza-deoxycytidine (5-aza-dC). Using methylation-specific polymerase chain reaction (MSP), bisulfite genomic sequencing, and digestion with methylation-sensitive endonuclease followed by polymerase chain reaction (PCR), we determined the methylation status of promoter regions of PU.1, BOB.1/OBF.1, CD19, SYK, and CD79B. Down-regulation of transcription typically correlated with hypermethylation. Using bisulfite genomic sequencing we found that in microdissected HRS cells of primary cHL SYK, BOB.1/OBF.1, and CD79B promoters were also hypermethylated. Ectopic expression of both Oct2 and PU.1 in a cHL cell line potentiated endogenous PU.1 and SYK expression after 5-aza-dC treatment. These observations indicate that silencing of the B-cell-specific genes in cHL may be the consequence of a compromised regulatory network where down-regulation of a few master transcription factors results in silencing of numerous genes.

Enhanced Expression of CD20 in Human Tumor B Cells Is Controlled through ERK-Dependent Mechanisms

Rituximab, a chimeric Ab directed against CD20, induces apoptosis in targeted cells. Although the majority of B cell malignancies express the CD20 Ag, only approximately 50% of patients will respond to single-agent rituximab. The available data suggest that a decreased CD20 expression could account for the lack of response observed in some patients treated with rituximab. Despite the potential critical role of CD20 in the biology of B cell malignancies, the mechanisms controlling its expression are poorly understood. We evaluated the effect of the immune modulator agent bryostatin-1 on the expression of CD20 in non-Hodgkin's lymphoma cells. Using the B cell lines, DB and RAMOS, as well as tumor cells derived from a chronic lymphocytic leukemia patient, we demonstrated that bryostatin-1 enhanced the expression of both CD20 mRNA and protein. The enhanced expression of CD20 was associated with increased transcriptional activity of the CD20 gene, whereas the stability of CD20 mRNA was not affected. The effect of bryostatin-1 on CD20 expression in non-Hodgkin's lymphoma cells was mediated through the MAPK kinase/ERK signal transduction pathway and involved protein kinase C, but was independent of p38 MAPK and was insensitive to dexamethasone. Cells pretreated with bryostatin-1 were more susceptible to the proapoptotic effect of anti-CD20 Ab. Overall, these data demonstrate for the first time that ERK phosphorylation is required for the up-regulated expression of CD20 on B cell malignancies. The findings also suggest that bryostatin-1 and rituximab could be a valuable combined therapy for B cell malignancies.

Cellular and molecular signal transduction pathways modulated by rituximab (rituxan, anti-CD20 mAb) in non-Hodgkin's lymphoma: implications in chemosensitization and therapeutic intervention

The clinical application of rituximab (chimeric mouse anti-human CD20 mAb, Rituxan, IDEC-C2B8), alone and/or combined with chemotherapy, has significantly ameliorated the treatment outcome of patients with relapsed and refractory low-grade or follicular non-Hodgkin's lymphoma (NHL). The exact in vivo mechanisms of action of rituximab are not fully understood, although antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and apoptosis have been suggested. We have proposed that modifications of the cellular signaling pathways by rituximab may be crucial for its clinical response. The B-cell restricted cell surface phosphoprotein CD20 is involved in many cellular signaling events including proliferation, activation, differentiation, and apoptosis upon crosslinking. Monomeric rituximab chemosensitizes drug-resistant NHL cells via selective downregulation of antiapoptotic factors through the type II mitochondrial apoptotic pathway. Several signaling pathways are affected by rituximab which are implicated in the underlying molecular mechanisms of chemosensitization. ARL (acquired immunodeficiency syndrome (AIDS)-related lymphoma) and non-ARL cell lines have been examined as in vitro model systems. In ARL, rituximab diminishes the activity of the p38MAPK signaling pathway resulting in inhibition of the interleukin (IL)-10/IL-10R autocrine/paracrine cytokine autoregulatory loop leading to the inhibition of constitutive STAT-3 activity and subsequent downregulation of Bcl-2 expression leading to chemosensitization. Rituximab upregulates Raf-1 kinase inhibitor protein (RKIP) expression in non-ARL cells. Through physical association with Raf-1 and nuclear factor kappaB (NF-kappa B)-inducing kinase (NIK), RKIP negatively regulates two major survival pathways, namely, the extracellular signal-regulated kinase1/2 (ERK1/2) and the NF-kappa B pathways, respectively. Downmodulation of the ERK1/2 and NF-kappa B pathways inhibits the transcriptional activity of AP-1 and NF-kappa B transcription factors, respectively, both of which lead to the downregulation of Bcl-(xL) (Bcl-2 related gene (long alternatively spliced variant of Bcl-x gene)) transcription and expression and sensitization to drug-induced apoptosis. Bcl-(xL)-overexpressing cells corroborated the pivotal role of Bcl-(xL) in chemosensitization. The specificity of rituximab-mediated signaling and functional effects were corroborated by the use of specific pharmacological inhibitors. Many patients do not respond and/or relapse and the mechanisms of unresponsiveness are unknown. Rituximab-resistant B-NHL clones were generated to investigate the acquired resistance to rituximab-mediated signaling, and chemosensitization. Resistant clones display different phenotypic, genetic and functional properties compared to wild-type cells. This review summarizes the data highlighting a novel role of rituximab as a signal-inducing antibody and as a chemosensitizing agent through negative regulation of major survival pathways. Studies presented herein also reveal several intracellular targets modified by rituximab, which can be exploited for therapeutic and prognostic purposes in the treatment of patients with rituximab- and drug-refractory NHL.

Analysis of transcription factor OCT.1, OCT.2 and BOB.1 expression using tissue arrays in classical Hodgkin's lymphoma

Hodgkin's lymphoma can be considered in most cases a B-cell lymphoma due to the presence of potentially functional immunoglobulin (Ig) gene rearrangements in the neoplastic cells. In contrast to lymphocyte-predominant Hodgkin's lymphoma, Hodgkin/Reed-Sternberg (HRS) cells from classical Hodgkin's lymphoma have low frequency of B-cell marker expression and lack Ig light and Ig heavy messenger RNA. Recent studies have shown transcription machinery deficiency in Hodgkin's lymphoma caused by an absence of the transcription factors OCT.1, OCT.2 and/or BOB.1. By using the tissue microarray technique, we have performed an immunohistochemical study of OCT.1, OCT.2 and BOB.1 in 325 classical Hodgkin's lymphoma cases. The results have been correlated with the expression of the B-cell markers CD20, CD79a, B-cell-specific activator protein (BSAP) and MUM.1, the presence of Epstein-Barr virus and the histological subtype. The percentage of CD20 and CD79a positivity was low (18 and 18%, respectively), whereas MUM.1 and BSAP were positive in the majority of cases. Considering the positive cases with independence of the intensity of staining, 62% of them expressed OCT.2, 59% OCT.1 and 37% BOB.1. Nevertheless, when we considered only the strongly positive cases, the results were similar to those previously described by others. No statistical association was found between the transcription factor expression, histological subtype and Epstein-Barr virus presence. To our knowledge, this is the largest series of classical Hodgkin's lymphoma cases in which the expression of transcription factors has been studied. We have found a notorious percentage of cases displaying weak positivity for OCT.2 and BOB.1 factors in HRS cells. We propose that other mechanisms different from the absence of transcription factors OCT.2 and BOB.1 might be involved in the control of Ig transcription and B lineage in classical Hodgkin's lymphoma.

From the bench to the bedside: ways to improve rituximab efficacy

Rituximab (MabThera, Rituxan) is a chimeric IgG1 monoclonal antibody that specifically targets the CD20 surface antigen expressed on normal and neoplastic B-lymphoid cells. Rituximab is currently used in the treatment of both follicular and aggressive B-cell non-Hodgkin lymphomas. Despite its demonstrated clinical effectiveness, its in vivo mechanisms of action remain unknown and could differ by subtype of lymphoma. Rituximab has been shown to induce apoptosis, complement-mediated lysis, and antibody-dependent cellular cytotoxicity in vitro, and some evidence points toward an involvement of these mechanisms in vivo. Rituximab also has a delayed therapeutic effect as well as a potential “vaccinal” effect. Here, we review the current understanding of the mechanism of action of rituximab and discuss approaches that could increase its clinical activity. A better understanding of how rituximab acts in vivo should make it possible to develop new and more effective therapeutic strategies. (Blood. 2004;104:2635-2642)

Embryonic lethality, decreased erythropoiesis, and defective octamer-dependent promoter activation in Oct-1-deficient mice

Oct-1 is a sequence-specific DNA binding transcription factor that is believed to regulate a large group of tissue-specific and ubiquitous genes. Both Oct-1 and the related but tissue-restricted Oct-2 protein bind to a DNA sequence termed the octamer motif (5'-ATGCAAAT-3') with equal affinity in vitro. To address the role of Oct-1 in vivo, an Oct-1-deficient mouse strain was generated by gene targeting. Oct-1-deficient embryos died during gestation, frequently appeared anemic, and suffered from a lack of Ter-119-positive erythroid precursor cells. This defect was cell intrinsic. Fibroblasts derived from these embryos displayed a dramatic decrease in Oct-1 DNA binding activity and a lack of octamer-dependent promoter activity in transient transfection assays. Interestingly, several endogenous genes thought to be regulated by Oct-1 showed no change in expression. When crossed to Oct-2(+/-) animals, transheterozygotes were recovered at a very low frequency. These findings suggest a critical role for Oct-1 during development and a stringent gene dosage effect with Oct-2 in mediating postnatal survival.

B cell development and immunoglobulin transcription in Oct-1-deficient mice

The POU domain transcription factors Oct-1 and Oct-2 interact with the octamer element, a motif conserved within Ig promoters and enhancers, and mediate transcription from the Ig loci. Inactivation of Oct-2 by gene targeting results in normal B cell development and Ig transcription. To study the role of Oct-1 in these processes, the lymphoid compartment of RAG-1(-/-) animals was reconstituted with Oct-1-deficient fetal liver hematopoietic cells. Recipient mice develop B cells with levels of surface Ig expression comparable with wild type, although at slightly reduced numbers. These B cells transcribe Ig normally, respond to antigenic stimulation, undergo class switching, and use a normal repertoire of light chain variable segments. However, recipient mice show slight reductions in serum IgM and IgA. Thus, the Oct-1 protein is dispensable for B cell development and Ig transcription.

Immunotherapy with anti-CD20 compounds

The B-cell surface antigen CD20 is currently the prime target for near-selective treatment of mature B-cell malignancies and a range of reactive B-cell associated disorders (including virus-associated lymphoproliferation or autoimmune conditions). CD20 is strongly and homogeneously expressed on the majority of mature B-cell neoplasms except chronic lymphocytic leukaemia cells, and on all mature reactive B-cells. This review will summarise the modes of action of various reagents targeting CD20. Treatment results following their use in single and combination therapy for B-cell disorders are reviewed.

Human Oct-1L isoform has tissue-specific expression pattern similar to Oct-2

POU homeodomain proteins are important regulators of ubiquitous as well as tissue-specific transcription. Ubiquitously expressed Oct-1 and tissue-specific Oct-2 proteins are members of the POU family and contain very similar DNA-binding POU domains. While Oct-1 is ubiquitous, Oct-2 is predominantly expressed in B cells, in activated T cells and in nervous system. Oct-1 is involved in regulation of some houskeeping genes-histone H2B, snRNAs as well as in tissue-specific regulation of immunoglobuline gene transcription and of some other genes. Here we report that novel alternatively spliced product of the human Oct-1 gene encode Oct-1L isoform with tissue-specific expression pattern, similar to Oct-2. Oct-1L differ from ubiquitously expressed Oct-1A in 5'-terminal exon (exon 1L). Analysis of nucleotide sequences from Human Genome Data Bank has located exon 1L about 108 kbp downstream ubiquitously expressed exon 1U. Amino terminus of Oct-1L show extensive similarity to amino terminus of Oct-2. We suppose, that Oct-1L may has a specific role in gene expression in lymphoid tissues and brain.

Chicken HDAC2 down-regulates IgM light chain gene promoter activity

In a chicken B cell line, DT40, the disruption of HDAC2 (chHDAC2) gene causes an alteration of several gene expressions including chicken IgM light chain (chIgM-L) gene by 2D-PAGE analysis. To investigate the transcriptional function of chHDAC2, we employed the chIgM-L promoter reporter plasmid. We found that chHDAC2 represses activated chIgM-L promoter activity. In transient expression experiments in NIH 3T3 cell, the specific histone deacetylase inhibitor tricostatin A (TSA) increased transactivation of chIgM-L promoter activity mediated by chicken Oct-1 and OBF-1 proteins. In transient coexpression of the three class I chicken histone deacetylases (chHDAC1-3) tested, only chHDAC2 repressed the activated chIgM-L promoter activity. These findings suggest that chHDAC2 might be recruited to the chIgM-L promoter and specifically repress chIgM-L transcription.

Cloning and characterization of the chick Oct binding factor OBF-1

We have cloned the chicken homolog of OBF-1, chOBF-1, which comprises 256 amino acids, and exhibits only 65% overall identity to the human and mouse OBF-1 proteins. Amino acid sequence alignment revealed the putative Oct-binding sequence, RPYQGVRVKEPVKELL(K/R)RKRG, which is conserved among chicken, mouse and human. chOBF-1 protein was demonstrated to bind chicken Oct-1 protein by the in vitro immunoprecipitation experiment, and chOBF-1 was shown to functionally activate the chicken immunoglobulin (Ig) light chain promoter in the NIH 3T3 cell. Taken together, these data indicate that the Ig gene transcription machinery, including Oct-1 and OBF-1, has been highly conserved in vertebrate evolution.

Non-DNA-binding Ikaros isoform gene expressed in adult B-precursor acute lymphoblastic leukemia

Ikaros, a zinc finger transcription factor, is essential for lymphoid development. Mutant mice expressing dominant-negative Ikaros gene (Ikaros) isoforms develop an aggressive form of lymphoid malignancies. We examined the expression of Ikaros isoforms in 11 leukemic cell lines and adult acute lymphoblastic leukemia cells from 36 patients with B-precursor acute lymphoblastic leukemia (pre-B ALL) and nine with T-precursor acute lymphoblastic leukemia (pre-T ALL), using reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. In one pre-B ALL cell line, INC cells, and primary leukemic cells from 16 patients with pre-B ALL, we found the predominant expression of a non-DNA-binding Ikaros isoform, Ik-6. However, Ik-6 was not detected in pre-T ALL cells. All of pre-B ALL cells expressing Ik-6 were CD10(+), whereas CD10(-) pre-B ALL cells did not express Ik-6. The expression of Ik-6 was not related to karyotype abnormalities such as t(9;22) and t(4;11). Proteins from the cells that expressed Ik-6 alone failed to bind to the Ikaros protein-specific binding sequence in DNA. Ikaros proteins lacking the DNA binding sequences were detected in the cytoplasm but not in the nucleus of the cells. When INC and primary pre-B ALL cells that express Ik-6 alone were irradiated and cultured in the absence of serum, these cells produced functional Ikaros isoforms, Ik-1 and Ik-2. Purified CD19(+) CD10(-) and CD19(+) CD10(+) cells from normal human bone marrow did not express Ik-6. The predominant expression of Ik-6, which is the result of post-transcription dysregulation, is characteristic of adult pre-B ALL, especially CD10(+) pre-B ALL.

Analysis of octamer-binding transcription factors Oct2 and Oct1 and their coactivator BOB.1/OBF.1 in lymphomas

Oct1 and Oct2 are transcription factors of the POU homeo-domain family that bind to the Ig gene octamer sites, regulating B-cell-specific genes. The function of these transcription factors is dependent on the activity of B-cell-restricted coactivators such as BOB.1/OBF.1. Independent studies of the expression of these proteins in non-Hodgkin's lymphoma have been restricted to single markers, and most lack data concerning immunohistochemical expression. Thus, we have investigated the expression of Oct1, Oct2, and BOB.1/OBF.1 in human reactive lymphoid tissue and in a series of 140 Hodgkin and non-Hodgkin's lymphomas. None of these proteins was found to be restricted to B cells, although only B cells expressed high levels of all three markers. Additionally, germinal center B cells showed stronger Oct2 and BOB.1/OBF.1 staining. Consequently, most B-cell lymphomas showed reactivity for all three antibodies. Oct2 expression was significantly higher in germinal center-derived lymphomas, although other B-cell lymphomas also displayed a high level of Oct2 expression. Although T-cell lymphomas and Hodgkin's lymphomas expressed some of these proteins, they commonly exhibited less reactivity than B-cell lymphomas. Despite not being entirely cell-specific, the strong nuclear expression of Oct2 and BOB.1/OBF.1 by germinal center- derived lymphomas makes these antibodies a potentially useful tool in lymphoma diagnosis.

CD20-directed serotherapy in patients with multiple myeloma: biologic considerations and therapeutic applications

Clonotypic B cells circulating in patients with multiple myeloma (MM) express CD20, and it has been suggested that these cells may be clonogenic. Furthermore, 20% of patients with MM express CD20 on their bone marrow plasma cells (BMPCs). Therefore, the authors began a phase II clinical study to determine the activity of the anti-CD20 monoclonal antibody rituximab in MM patients. Nineteen previously treated MM patients received 375 mg/m2 rituximab per week for 4 weeks. Three months after initiation of treatment, patients were assessed for response and received a second course of therapy if their disease was stable (SD) or they achieved a partial response (PR). Six of 19 (32%) patients had either a PR (n = 1) or SD (n = 5), with a median time to treatment failure of 5.5 months (mean, 10.3 months; range, 3-27+ months). All six patients who had a PR or SD had CD20+ BMPC. Overall, rituximab therapy was well tolerated. Because most patients with MM poorly express CD20 on their BMPCs, the authors evaluated agents for their ability to induce CD20 expression and thereby facilitate rituximab binding on MM cells. These studies show that interferon-gamma (IFN-y) induced CD20 expression on MM BMPCs, MM B cells, and healthy donor BMPCs. In contrast, CD20 expression on chronic lymphocytic leukemia, follicular non-Hodgkin's lymphoma, healthy donor B cells, and progenitor cells was unaffected by IFN-y. Rituximab binding to the BMPCs of MM patients was also increased after culture with pharmacologically attainable levels of IFN-gamma (1-100 U/mL). In conclusion, these studies suggest that MM patients with CD20+ BMPCs may benefit from rituximab therapy. Furthermore, IFN-gamma induces CD20 expression on MM BMPCs and B cells and facilitates rituximab binding to MM BMPCs, providing the rationale for clinical trials to examine its use with CD20-directed serotherapies in MM.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

B cell development and immunoglobulin gene transcription in the absence of Oct-2 and OBF-1

Oct-2 and OBF-1 (also called OCA-B or Bob-1) are B cell-specific transcription factors that bind to the conserved octamer site of immunoglobulin promoters, yet their role in immunoglobulin transcription has remained unclear. We generated mice in which the lymphoid compartment was reconstituted with cells that lack both Oct-2 and OBF-1. Even in the absence of these two transcription factors, B cells develop normally to the membrane immunoglobulin M-positive (IgM+) stage and immunoglobulin gene transcription is essentially unaffected. These observations imply that the ubiquitous factor Oct-1 plays a previously unrecognized role in the control of immunoglobulin gene transcription and suggest the existence of another, as yet unidentified, cofactor. In addition, both factors are essential for germinal center formation, although OBF-1 is more important than Oct-2 for IgG production after immunization.

Transcriptional regulation during B cell development

Information is increasingly available concerning the molecular events that occur during primary and antigen-dependent stages of B cell development. In this review the roles of transcription factors and coactivators are discussed with respect to changes in expression patterns of various genes during B cell development. Transcriptional regulation is also discussed in the context of developmentally regulated immunoglobulin gene V(D)J recombination, somatic hypermutation, and isotype switch recombination.

An Intronic Silencer Regulates B Lymphocyte Cell- and Stage-Specific Expression of the Human Complement Receptor Type 2 (CR2, CD21) Gene

Human CR2 (CD21) is a B lymphocyte protein whose surface expression is restricted primarily to the mature cell stage during development. To study the transcriptional mechanisms that govern cell- and stage-restricted CR2 expression, we first performed transient transfection analysis using constructs extending from −5 kb to +75 bp (−5 kb/+75) in the CR2 promoter. The promoter was found to be broadly active, with no evidence of cell- or stage-specific reporter gene expression. However, the addition of a 2.5-kb intronic gene segment (containing a DNase I hypersensitive site) to the (−5-kb/+75) construct resulted in appropriate reporter gene expression, defined as the silencing of the (−5-kb/+75) promoter activity only in non-CR2-expressing cells. Interestingly, appropriate reporter gene expression required stable transfection of the constructs in cell lines, suggesting nuclear matrix or chromatin interactions may be important for appropriate CR2 gene expression. Importantly, transgenic mice also required the intronic silencer to generate lymphoid tissue-specific reporter gene expression. Some transgenic founder lines did not demonstrate reporter gene expression, however, indicating that additional transcriptional regulatory elements are present in other regions of the CR2 gene. In summary, these data support the hypothesis that human CR2 expression is regulated primarily by an intronic silencer with lineage- and B cell stage-specific activity.

PU.1/Pip and Basic Helix Loop Helix Zipper Transcription Factors Interact With Binding Sites in the CD20 Promoter to Help Confer Lineage- and Stage-Specific Expression of CD20 in B Lymphocytes

CD20 is a B-lineage–specific gene expressed at the pre–B-cell stage of B-cell development that disappears on differentiation to plasma cells. As such, it serves as an excellent paradigm for the study of lineage and developmental stage-specific gene expression. Using in vivo footprinting we identified two sites in the promoter at −45 and −160 that were occupied only in CD20+ B cells. The −45 site is an E box that binds basic helix-loop-helix-zipper proteins whereas the −160 site is a composite PU.1 and Pip binding site. Transfection studies with reporter constructs and various expression vectors verified the importance of these sites. The composite PU.1 and Pip site likely accounts for both lineage and stage-specific expression of CD20 whereas the CD20 E box binding proteins enhance overall promoter activity and may link the promoter to a distant enhancer.

Inhibition of G-protein-mediated MAP kinase activation by a new mammalian gene family

A general property of signal transduction pathways is that prolonged stimulation decreases responsiveness, a phenomenon termed desensitization. Yeast cells stimulated with mating pheromone activate a heterotrimeric G-protein-linked, MAP-kinase-dependent signalling pathway that induces G1-phase cell-cycle arrest and morphological differentiation (reviewed in refs 1, 2). Eventually the cells desensitize to pheromone and resume growth. Genetic studies have demonstrated the relative importance of a desensitization mechanism that uses the SST2 gene product, Sst2p. Here we identify a mammalian gene family termed RGS (for regulator of G-protein signalling) that encodes structural and functional homologues of Sst2p. Introduction of RGS family members into yeast blunts signal transduction through the pheromone-response pathway. Like SST2 (refs 8-10), they negatively regulate this pathway at a point upstream or at the level of the G protein. The RGS family members also markedly impair MAP kinase activation by mammalian G-protein-linked receptors, indicating the existence and importance of an SST2-like desensitization mechanism in mammalian cells.

Coordinate suppression of myeloma-specific genes and expression of fibroblast-specific genes in myeloma X fibroblast somatic cell hybrids

In most instances, fusion of differentiated cell types with fibroblasts has resulted in the extinction of the differentiation-specific traits of the non-fibroblast parental cell. To explore the genetic basis of this phenomenon, we have studied a series of somatic cell hybrids between mouse myeloma and fibroblasts. All the hybrids were adherent having a fibroblast-like phenotype. Molecular analysis revealed that plasma cell specific genes like the productively rearranged Ig genes, the J chain gene and genes for the cell surface markers CD20 and PC1, were extinguished in the hybrids. In contrast, fibroblast specific genes like fibronectin, alpha 2(I) and III collagens, as well as the receptor for fibroblast growth factor (flg), were expressed. Extinction was not due to chromosomal loss or lack of the relevant genes. To learn about the mechanism(s) of this phenomenon we have looked for the presence of positive and negative transcription factors in our hybrids. Expression of the PU.1 transcription factor, a member of the Ets transcription factor family normally expressed in B cells and macrophages, was lost in the cell hybrids. Interestingly, we found that the B-cell-specific Oct-2 transcription factor was still expressed at somewhat variable levels in several of the hybrid cell lines. In contrast, expression of the recently identified octamer coactivator BOB.1/OBF.1 was extinguished in all cell hybrids. This supports a critical role of this transcriptional coactivator for B-cell-specific gene expression. In addition, the Id and HLH462 genes coding for proteins known to repress bHLH transcription factors by formation of heterodimers, were found to be expressed at increased levels in fibroblasts and in the hybrids, indicating that their increased levels might also contribute to the suppression of myeloma-specific genes. Our results show that in myeloma x fibroblast hybrids, the phenotype of the fibroblast is dominant. It is suggested that fibroblasts contain regulatory "master" genes that are responsible for activation of the fibroblast differentiation pathway and suppress differentiation programs of other cell types.

Functional characterization of the murine homolog of the B cell-specific coactivator BOB.1/OBF.1

B cell-specific transcriptional promoter activity mediated by the octamer motif requires the Oct1 or Oct2 protein and additional B cell-restricted cofactors. One such cofactor, BOB.1/OBF.1, was recently isolated from human B cells. Here, we describe the isolation and detailed characterization of the murine homolog. Full-length cDNAs and genomic clones were isolated, and the gene structure was determined. Comparison of the deduced amino acids shows 88% sequence identity between mouse and human BOB.1/OBF.1. The NH2-terminal 126 amino acids of BOB.1/OBF.1 are both essential and sufficient for interaction with the POU domains of either Oct1 or Oct2. This protein-protein interaction does not require the simultaneous binding of Oct proteins to DNA, and high resolution footprinting of the Oct-DNA interaction reveals that binding of BOB.1/OBF.1 to Oct1 or Oct2 does not alter the interaction with DNA. BOB.1/OBF.1 can efficiently activate octamer-dependent promoters in fibroblasts; however, it fails to stimulate octamer-dependent enhancer activity. Fusion of subdomains of BOB.1/OBF.1 with the GAL4 DNA binding domain reveals that both NH2- and COOH-terminal domains of BOB.1/OBF.1 contribute to full transactivation function, the COOH-terminal domain is more efficient in this transactivation assay. Consistent with the failure of full-length BOB.1/OBF.1 to stimulate octamer-dependent enhancer elements in non B cells, the GAL4 fusions likewise only stimulate from a promoter-proximal position.

Oct-2 is required early in T cell-independent B cell activation for G1 progression and for proliferation

Oct-2, a POU homeodomain protein expressed primarily in B cells, is a powerful transcriptional activator that binds to DNA at sites appropriately placed for major effects on immunoglobulin gene expression. Our examination of B cell development and function in Oct-2 null mice did not support an essential role for Oct-2 early in B cell development. Rather, Oct-2 was required later, when B cells were induced to differentiate to antibody-secreting cells. We show here that Oct-2 is not required for normal immunoglobulin production by mature B lymphocytes. Instead, it is essential for a normal proliferative response to polyclonal mitogens. Responses to signals from activated T cells are unaffected. The requirement for Oct-2 maps to an early activation step in G1, during which B cells make the commitment to progress through the cell cycle and to divide.

Molecular mechanisms regulating CD19, CD20 and CD22 gene expression

The CD19, CD20 and CD22 genes encode transmembrane proteins that are of vital importance to B-cell function. Similar to the immunoglobulin (Ig) genes, they are expressed in a lineage-specific and developmentally regulated manner. Here, John Kehrl and colleagues describe how an understanding of the transcriptional regulation of the CD19, CD20 and CD22 genes is leading to valuable insights into some of the important molecular events that occur in B-cell development and differentiation.