NBE-002: A novel anthracycline-based antibody-drug conjugate (ADC) targeting ROR1 for the treatment of advanced solid tumors—A phase 1/2 clinical trial

TPS1108

Background: The receptor tyrosine kinase-like orphan receptor 1 (ROR1) is highly expressed during embryonic development, but is minimally present or absent on post-partum healthy tissues. ROR1 is expressed in a variety of hematological and solid tumors and is associated with aggressive cancer phenotype and poor clinical outcomes. NBE-002 is an ADC targeting ROR1, obtained by site-specific, enzymatic conjugation of the anthracycline-derivative PNU-159682, modified with a non-cleavable linker to a humanized recombinant IgG1 monoclonal antibody, based on a novel anti-human ROR1 monoclonal antibody XBR1-402 (Peng et al. (2017) J. Mol. Biol. 429: 2954-73). Direct anti-tumor activity of NBE-002 was evaluated in immunodeficient, ROR1 expression-low/-intermediate/-high PDX models of several carcinoma and sarcoma subtypes. The most pronounced anti-tumor effect was achieved in triple-negative breast cancer (TNBC), at doses as low as 0.033 mg/kg, suggesting a best-in-class therapeutic index in light of the high tolerability in preclinical toxicology models. Administration in a fully immune competent setting (EMT6/ROR1 orthotopic breast cancer model) led to a strong anti-tumor response and a long-lasting anti-tumor immune protection dependent on CD8 T cells. Methods: NBE-002-01 (NCT04441099) is a first-in-human, open-label, multi-center, phase (Ph) 1/2 study of NBE-002 in adult patients with advanced solid tumors. Ph 1 of the study consists of a Dose Escalation Cohort (DEC), utilizing an accelerated titration design, followed by a traditional 3+3 design, and an optional Safety Expansion Cohort (SEC). Ph 2 will include two parallel Expansion Cohorts (EC), enrolling patients with advanced TNBC (EC1) or other solid tumors (EC2), with Simon’s two-stage design. Key eligibility criteria include Eastern Cooperative Oncology Group performance status of 0-2 (Ph 1) or 0-1 (Ph 2), adequate organ function defined as: hemoglobin ≥9.0 g/dL, neutrophils ≥1500 /µL, platelets ≥100000/µL, aspartate and alanine aminotransferases ≤2.5x the upper limit of normal (ULN), total bilirubin ≤1.5x ULN, creatinine ≤1.5x ULN, left ventricular ejection fraction ≥50%. The primary objectives are to assess safety and tolerability and to establish the recommended dose for further development (Ph 1), and to evaluate anti-tumor activity of (Ph 2). Secondary and exploratory objectives include characterization of immunogenicity as well as pharmacokinetic and pharmacodynamic profiles. NBE-002 is given intravenously once every three weeks until disease progression, unacceptable toxicity, withdrawal of consent, or other protocol-specific criteria are met. Ph 1 dose escalation was initiated on 17 JULY 2020 and is still recruiting in the US. Ph 2 is planned to be initiated in 2022. Clinical trial information: NCT04441099 .

Partially shielded enzymes capable of processing large protein substrates

We report the first method of enzyme protection enabling the production of partially shielded enzymes capable of processing substrates as large as proteins. We show that partially shielded sortase retains its transpeptidase activity and can perform bioconjugation reactions on antibodies. Moreover, a partially shielded trypsin is shown to outperform its soluble counterpart in terms of proteolytic kinetics. Remarkably, partial enzyme shielding results in a drastic increase in temporal stability of the enzyme.

Abstract 1541: Evaluation of ROR1 targeted antibody drug conjugate in ROR1 positive leukemia

Introduction: Receptor tyrosine kinase like orphan receptor 1 (ROR1) is expressed on the surface of multiple blood and solid tumors. There have been increased interest in ROR1 as a potential therapeutic target in ROR1 positive leukemia and lymphomas. In this study, we present a novel first-in-class anti-ROR1 monoclonal antibody drug conjugate (ADC) based on chimeric rabbit/human mAb “XBR1-402” or its humanized version “huXBR1-402”. Conjugated to a highly potent anthracycline derived toxin, PNU-159682 (PNU) via a non-cleavable peptide/amide linker, it shows both in vitro and in vivo efficacy at eliminating ROR1 positive chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), and pre B cell acute lymphocytic leukemia (pre B cell ALL) cells.

Results: We saw a decrease in viability for all ADC treated ROR1 positive leukemic cell lines. We saw a significant decrease in viability of the ROR1 positive pre B cell ALL cell lines 697 and Kasumi-2 when treated with XBR1-402-PNU (EC50 = 77 and 1 ng/mL) when compared to treatment with isotype-matched control Trastuzumab-PNU (Tras-PNU) (EC50 = 2987 and 5330 ng/mL). In in vitro models of MCL, we saw decreased viability for huXBR1-402-PNU treated ROR1 positive Jeko and Mino cell lines but not ROR1 negative Mec-1 cell line when compared to Tras-PNU. While we were unable to observe significant direct cytotoxicity in primary CLL cells, we show that PNU ADCs mediates antibody dependent cellular cytotoxicity and phagocytosis in vitro. We also show that PNU ADCs mediates a G2/M cell cycle arrest in affected cell lines and hypothesize that this is necessary for direct cytotoxicity. As ex vivo primary CLL cells are non-proliferative, we postulate that PNU would target actively dividing cells in proliferation centers (e.g. secondary lymphoid organs). To test this hypothesis, we set up in vivo studies to test with XBR1-402-PNU and huXBR1-402-PNU in disseminated and aggressive ROR1+ ALL and CLL models, respectively. In vivo studies on a murine 697 ALL model suggested XBR1-402-PNU treatment increased overall survival when compared to treatment with Tras-PNU control (Median survival of 32 and 23 days post implantation, p=0.021, n=6). In vivo CLL studies with engrafted murine human-ROR1 expressing TCL1 leukemic cells showed that huXBR1-402-PNU treatment (3 times/week for 1 week) both suppressed leukemia burden and increased overall survival when compared with Tras-PNU control (Median survival of 62 and 41 days post implantation, p=0.0028, n=4). Ongoing proliferation and cell cycling studies will confirm the mechanism of the in vivo cytotoxicity of huXBR1-402-PNU.

Conclusion: Our results suggest that anti-ROR1 ADC huXBR1-402-PNU is an effective and promising targeted cytotoxic therapy for ROR1 positive leukemic cells of CLL, MCL, and pre B ALL and warrants further evaluation for clinical consideration as either a single agent or combination therapy in patients with ROR1 positive leukemia and lymphomas.

Citation Format: Eileen Hu, Priscilla Do, Rajeswaran Mani, Frank Frissora, Rebecca Pearson, Gerard Lozanski, Haiyong Peng, Lorenz Waldmeier, Roger Beerli, Christoph Rader, Ulf Grawunder, John Byrd, Natarajan Muthusamy. Evaluation of ROR1 targeted antibody drug conjugate in ROR1 positive leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1541.

Abstract LB-197: NBE-002, an anthracycline-based immune-stimulatory antibody drug conjugate (iADC) targeting ROR1 for the treatment of triple-negative breast cancer

Introduction: We report on the development and preclinical validation of NBE-002, a next-generation antibody drug conjugate (ADC) for the treatment of triple-negative breast cancer (TNBC). NBE-002 consists of a humanized monoclonal antibody directed against the receptor tyrosine kinase ROR1, expressed on the surface of numerous solid cancers including TNBC, and is site-specifically conjugated to a derivative of the highly potent anthracycline PNU-159682.

Results: Therapeutic efficacy of NBE-002 was evaluated in ROR1-low/-intermediate/-high patient-derived xenograft (PDX) models of TNBC, lung adenocarcinoma, ovarian carcinoma, and a variety of sarcomas. NBE-002 was found to display significant anti-tumor activity in each indication. The most pronounced effect was achieved in TNBC, where complete tumor regression was observed already at the lowest ADC dose tested (0.33 mg/kg), even in models expressing low levels of ROR1 (H-score ≤70). Since these PDX studies were performed in immune-compromised hosts, and anthracyclines are known inducers of immunogenic cell death, anti-tumor activity was also investigated in syngeneic tumor models in immune-competent hosts. Anti-tumor activity of PNU-ADCs involved activation of the immune system, as shown by evaluation of NBE-002 or a Trastuzumab-PNU conjugate (T-PNU) in ROR1- or HER2-positive syngeneic breast cancer models, respectively. Depletion of CD8 T cells severely reduced anti-tumor activity, demonstrating an important role for T cells in driving tumor regression. Furthermore, when tumor free animals were re-challenged with the same tumor, tumor growth was inhibited in the absence of any further ADC administration, indicating the development of an immunological memory. Notably, combination of ADC and checkpoint inhibition, such as α-PD1 or α-CTLA4, significantly enhanced tumor eradication following the treatment.

Conclusion: Our results demonstrate that NBE-002 is a highly effective and promising targeted therapeutic for the treatment of ROR1 positive TNBC and potentially other solid tumor indications that warrants clinical development. Considering the pronounced immune-modulatory functions of the PNU payload, NBE-002 may be particularly well suited for combination therapy with immune checkpoint inhibitors. NBE-002 is currently undergoing GMP manufacturing and initiation of clinical studies is expected in mid-2020.

Citation Format: Roger R. Beerli, Lorenz Waldmeier, Rémy Gébleux, Francesca Pretto, Ulf Grawunder. NBE-002, an anthracycline-based immune-stimulatory antibody drug conjugate (iADC) targeting ROR1 for the treatment of triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-197.

A novel anti-HER2 anthracycline-based antibody-drug conjugate induces adaptive anti-tumor immunity and potentiates PD-1 blockade in breast cancer

Increasing evidence suggests that antibody-drug conjugates (ADCs) can enhance anti-tumor immunity and improve clinical outcome. Here, we elucidate the therapeutic efficacy and immune-mediated mechanisms of a novel HER2-targeting ADC bearing a potent anthracycline derivate as payload (T-PNU) in a human HER2-expressing syngeneic breast cancer model resistant to trastuzumab and ado-trastuzumab emtansine. Mechanistically, the anthracycline component of the novel ADC induced immunogenic cell death leading to exposure and secretion of danger-associated molecular signals. RNA sequencing derived immunogenomic signatures and TCRβ clonotype analysis of tumor-infiltrating lymphocytes revealed a prominent role of the adaptive immune system in the regulation of T-PNU mediated anti-cancer activity. Depletion of CD8 T cells severely reduced T-PNU efficacy, thus confirming the role of cytotoxic T cells as drivers of the T-PNU mediated anti-tumor immune response. Furthermore, T-PNU therapy promoted immunological memory formation in tumor-bearing animals protecting those from tumor rechallenge. Finally, the combination of T-PNU and checkpoint inhibition, such as α-PD1, significantly enhanced tumor eradication following the treatment. In summary, a novel PNU-armed, HER2-targeting ADC elicited long-lasting immune protection in a murine orthotopic breast cancer model resistant to other HER2-directed therapies. Our findings delineate the therapeutic potential of this novel ADC payload and support its clinical development for breast cancer patients and potentially other HER2 expressing malignancies.

Sortase A Enzyme-Mediated Generation of Site-Specifically Conjugated Antibody-Drug Conjugates

Antibody-drug conjugates (ADCs) are highly potent targeted anticancer therapies. They rely on the linking of a selectively targeting antibody moiety with potent cytotoxic payloads to effect antitumoral activity. In recent years, one focus in the ADC field was to create novel methods for site-specifically conjugating payloads to antibodies. The method presented here is based on the S. aureus sortase A-mediated transpeptidation reaction. This method requires antibodies to be engineered in such a way that they possess the sortase recognition pentapeptide motif LPETG on the C-terminus of the immunoglobulin heavy and/or light chains. In addition, the toxin must contain an oligoglycine motif in order to make it a suitable substrate for sortase A. Here we describe a detailed method to conjugate a pentaglycine-modified toxin to the C-termini of LPETG-tagged antibody heavy and light chains using sortase-mediated antibody conjugation (SMAC-Technology™). Highly homogenous, site-specifically conjugated ADCs with controlled drug to antibody ratio and improved overall properties can be obtained with this method.

Novel Antibody Drug Conjugates Targeting Tumor-Associated Receptor Tyrosine Kinase ROR2 by Functional Screening of Fully Human Antibody Libraries Using Transpo-mAb Display on Progenitor B Cells

Receptor tyrosine kinase-like orphan receptor 2 (ROR2) has been identified as a highly relevant tumor-associated antigen in a variety of cancer indications of high unmet medical need, including renal cell carcinoma and osteosarcoma, making it an attractive target for targeted cancer therapy. Here, we describe the de novo discovery of fully human ROR2-specific antibodies and potent antibody drug conjugates (ADCs) derived thereof by combining antibody discovery from immune libraries of human immunoglobulin transgenic animals using the Transpo-mAb mammalian cell-based IgG display platform with functional screening for internalizing antibodies using a secondary ADC assay. The discovery strategy entailed immunization of transgenic mice with the cancer antigen ROR2, harboring transgenic IgH and IgL chain gene loci with limited number of fully human V, D, and J gene segments. This was followed by recovering antibody repertoires from the immunized animals, expressing and screening them as full-length human IgG libraries by transposon-mediated display in progenitor B lymphocytes ("Transpo-mAb Display") for ROR2 binding. Individual cellular "Transpo-mAb" clones isolated by single cell sorting and capable of expressing membrane-bound as well as secreted human IgG were directly screened during antibody discovery, not only for high affinity binding to human ROR2, but also functionally as ADCs using a cytotoxicity assay with a secondary anti-human IgG-toxin-conjugate. Using this strategy, we identified and validated 12 fully human, monoclonal anti-human ROR2 antibodies with nanomolar affinities that are highly potent as ADCs and could be promising candidates for the therapy of human cancer. The screening for functional and internalizing antibodies during the early phase of antibody discovery demonstrates the utility of the mammalian cell-based Transpo-mAb Display platform to select for functional binders and as a powerful tool to improve the efficiency for the development of therapeutically relevant ADCs.

Mining Naïve Rabbit Antibody Repertoires by Phage Display for Monoclonal Antibodies of Therapeutic Utility

Owing to their high affinities and specificities, rabbit monoclonal antibodies (mAbs) have demonstrated value and potential primarily as basic research and diagnostic reagents, but, in some cases, also as therapeutics. To accelerate access to rabbit mAbs bypassing immunization, we generated a large naïve rabbit antibody repertoire represented by a phage display library encompassing >10 billion independent antibodies in chimeric rabbit/human Fab format and validated it by next-generation sequencing. Panels of rabbit mAbs selected from this library against two emerging cancer targets, ROR1 and ROR2, revealed high diversity, affinity, and specificity. Moreover, ROR1- and ROR2-targeting rabbit mAbs demonstrated therapeutic utility as components of chimeric antigen receptor-engineered T cells, further corroborating the value of the naïve rabbit antibody library as a rich and virtually unlimited source of rabbit mAbs.

Generation of an artificial human B cell line test system using Transpo-mAbTM technology to evaluate the therapeutic efficacy of novel antigen-specific fusion proteins

The antigen-specific targeting of autoreactive B cells via their unique B cell receptors (BCRs) is a novel and promising alternative to the systemic suppression of humoral immunity. We generated and characterized cytolytic fusion proteins based on an existing immunotoxin comprising tetanus toxoid fragment C (TTC) as the targeting component and the modified Pseudomonas aeruginosa exotoxin A (ETA') as the cytotoxic component. The immunotoxin was reconfigured to replace ETA' with either the granzyme B mutant R201K or MAPTau as human effector domains. The novel cytolytic fusion proteins were characterized with a recombinant human lymphocytic cell line developed using Transpo-mAb™ technology. Genes encoding a chimeric TTC-reactive immunoglobulin G were successfully integrated into the genome of the precursor B cell line REH so that the cells could present TTC-reactive BCRs on their surface. These cells were used to investigate the specific cytotoxicity of GrB(R201K)-TTC and TTC-MAPTau, revealing that the serpin proteinase inhibitor 9-resistant granzyme B R201K mutant induced apoptosis specifically in the lymphocytic cell line. Our data confirm that antigen-based fusion proteins containing granzyme B (R201K) are suitable candidates for the depletion of autoreactive B cells.

Transpo-mAb display: Transposition-mediated B cell display and functional screening of full-length IgG antibody libraries

In vitro antibody display and screening technologies geared toward the discovery and engineering of clinically applicable antibodies have evolved from screening artificial antibody formats, powered by microbial display technologies, to screening of natural, full-IgG molecules expressed in mammalian cells to readily yield lead antibodies with favorable properties in production and clinical applications. Here, we report the development and characterization of a novel, next-generation mammalian cell-based antibody display and screening platform called Transpo-mAb Display, offering straightforward and efficient generation of cellular libraries by using non-viral transposition technology to obtain stable antibody expression. Because Transpo-mAb Display uses DNA-transposable vectors with substantial cargo capacity, genomic antibody heavy chain expression constructs can be utilized that undergo the natural switch from membrane bound to secreted antibody expression in B cells by way of alternative splicing of Ig-heavy chain transcripts from the same genomic expression cassette. We demonstrate that stably transposed cells co-express transmembrane and secreted antibodies at levels comparable to those provided by dedicated constructs for secreted and membrane-associated IgGs. This unique feature expedites the screening and antibody characterization process by obviating the need for intermediate sequencing and re-cloning of individual antibody clones into separate expression vectors for functional screening purposes. In a series of proof-of-concept experiments, we demonstrate the seamless integration of antibody discovery with functional screening for various antibody properties, including binding affinity and suitability for preparation of antibody-drug conjugates.

Sortase Enzyme-Mediated Generation of Site-Specifically Conjugated Antibody Drug Conjugates with High In Vitro and In Vivo Potency

Antibody drug conjugates (ADCs) have recently been proven to be highly potent anti-tumor drugs, typically exceeding the efficacy of conventional monoclonal antibodies (mAbs). ADCs are currently produced by chemical conjugation of a small-molecule toxin to the mAb through lysine or cysteine side chains. This leads to heterogeneous mixtures of ADCs in which variable numbers of drugs are conjugated to individual antibodies and in which the site of conjugation cannot be defined. Consequently, there is currently significant interest in further development of drug conjugation technologies, with a particular focus on site-specific payload conjugation. Here, we present an enzymatic conjugation platform based on the S. aureus sortase A-mediated transpeptidation reaction, allowing the efficient generation of ADCs with toxins conjugated to pre-defined sites at pre-defined drug-to-antibody ratios. For this, two modifications were introduced: first, immunoglobulin heavy (IgH) and light (IgL) chains were modified at their C-termini by addition of the sortase A recognition motif LPETG, and second, the small molecule tubulin polymerization inhibitors monomethylauristatin E (MMAE) and maytansine were modified by addition of a pentaglycine peptide, thus making them suitable substrates for sortase A-mediated transpeptidation. We demonstrate efficient generation and characterization of the anti-CD30 ADC Ac10-vcPAB-MMAE, an enzymatically conjugated counterpart of brentuximab vedotin (Adcetris), as well as several anti-HER-2 ADCs including trastuzumab-maytansine, the counterpart of trastuzumab emtansine (Kadcyla). ADCs generated in this manner were found to display in vitro cell killing activities indistinguishable from the classic conjugates. Further, when tested in vivo in a HER-2-overexpressing ovarian cancer xenograft mouse model, enzymatically generated trastuzumab-maytansine was found to lead to complete regression of established tumors, similar to Kadcyla.

Homeostatic and functional analysis of mature B cells in λ5-deficient mice

The peripheral B-cell pool is in dynamic equilibrium and is controlled by a variety of factors. The rate of generation of B cells can influence both the composition and size of the peripheral B-cell compartment. Mice deficient for lambda5 gene expression have a block in early B-cell development leading to a markedly reduced number of peripheral B cells. To address the question of how this early developmental block influences the composition of the B-cell pool, we have analyzed mature B-cell subpopulations in lambda5-deficient mice. In analogy with other situations of B lymphopenia, the proportion was increased but not the absolute number of marginal-zone B cells, whereas those of follicular B cells were decreased. Immunohistology revealed that B-cell follicles were smaller in overall size and contained a prominent B-cell replete marginal zone. BrdU labelling kinetics showed slower turnover of follicular as well as of marginal-zone B cells. Functionally, lambda5(-/-) mice were able to mount not only primary but also secondary thymus-dependent as well as thymus-independent responses, albeit mostly at reduced levels.

How to make ends meet in V(D)J recombination

In most vertebrate species analyzed so far, the diversity of soluble or membrane-bound antigen-receptors expressed by B and T lymphocytes is generated by V(D)J recombination. During this process, the coding regions for the variable domains of antigen-receptors are created by the joining of subexons that are randomly selected from arrays of tandemly repeated V, D (sometimes) and J gene segments. This involves the site-specific cleavage of chromosomal DNA by the lymphocyte-specific recombination-activating gene (RAG)-1/2 proteins, which appear to have originated from an ancient transposable element. The DNA double-strand breaks created by RAG proteins are subsequently processed and rejoined by components of the nonhomologous DNA end-joining pathway, which is conserved in all eukaryotic organisms - from unicellular yeast up to highly complex mammalian species.

Normal V(D)J recombination in cells from patients with Nijmegen breakage syndrome

The majority of antigen receptor diversity in mammals is generated by V(D)J recombination. During this process DNA double strand breaks are introduced at recombination signals by lymphoid specific RAG1/2 proteins generating blunt ended signal ends and hairpinned coding ends. Rejoining of all DNA ends requires ubiquitously expressed DNA repair proteins, such as Ku70/86 and DNA ligase IV/XRCC4. In addition, the formation of coding joints depends on the function of the scid gene encoding the catalytic subunit of DNA-dependent protein kinase, DNA-PK(CS), that is somehow required for processing of coding end hairpins. Recently, it was shown that purified RAG1/2 proteins can cleave DNA hairpins in vitro, but the same activity was also described for a protein complex of the DNA repair proteins Nbs1/Mre11/Rad50. This leaves the possibility that either protein complex might be involved in coding end processing in V(D)J recombination. We have therefore analyzed V(D)J recombination in cells from patients with Nijmegen breakage syndrome, carrying a mutation in the nbs1 gene. We find that V(D)J recombination frequencies and the quality of signal and coding joining are comparable to wild-type controls, as analyzed by a cellular V(D)J recombination assay. In addition, we did not detect significant differences in CDR3 sequences of endogenous Ig lambdaL and kappaL chain gene loci cloned from peripheral blood lymphocytes of an NBS patient and of healthy individuals. These findings suggest that the Nbs1/Mre11/Rad50 complex is not involved in coding end processing of V(D)J recombination.

The nonhomologous DNA end joining pathway is important for chromosome stability in primary fibroblasts

There are two types of chromosome instability, structural and numerical, and these are important in cancer. Many structural abnormalities are likely to involve double-strand DNA (dsDNA) breaks. Nonhomologous DNA end joining (NHEJ) and homologous recombination are the major pathways for repairing dsDNA breaks. NHEJ is the primary pathway for repairing dsDNA breaks throughout the G0, G1 and early S phases of the cell cycle [1]. Ku86 and DNA ligase IV are two major proteins in the NHEJ pathway. We examined primary dermal fibroblasts from mice (wild type, Ku86(+/-), Ku86(-/-), and DNA ligase IV(+/-)) for chromosome breaks. Fibroblasts from Ku86(+/-) or DNA ligase IV(+/-) mice have elevated frequencies of chromosome breaks compared with those from wild-type mice. Fibroblasts from Ku86(-/-) mice have even higher levels of chromosome breaks. Primary pre-B cells from the same animals did not show significant accumulation of chromosome breaks. Rather the pre-B cells showed increased cell death. These studies demonstrate that chromosome breaks arise frequently and that NHEJ is required to repair this constant spontaneous damage.

DNA ligase IV is essential for V(D)J recombination and DNA double-strand break repair in human precursor lymphocytes

Nonhomologous DNA end joining (NHEJ) is the major pathway for repairing double-strand DNA breaks. V(D)J recombination is a double-strand DNA breakage and rejoining process that relies on NHEJ for the joining steps. Here we show that the targeted disruption of both DNA ligase IV alleles in a human pre-B cell line renders the cells sensitive to ionizing radiation and ablates V(D)J recombination. This phenotype can only be reversed by complementation with DNA ligase IV but not by expression of either of the remaining two ligases, DNA ligase I or III. Hence, DNA ligase IV is the activity responsible for the ligation step in NHEJ and in V(D)J recombination.

Requirement for an interaction of XRCC4 with DNA ligase IV for wild-type V(D)J recombination and DNA double-strand break repair in vivo

The XRCC4 gene is required for the repair of DNA double-strand breaks in mammalian cells. Without XRCC4, cells are hypersensitive to ionizing radiation and deficient for V(D)J recombination. It has been demonstrated that XRCC4 binds and stimulates DNA ligase IV, which has led to the hypothesis that DNA ligase IV is essential for both of these processes. In this study deletion mutants of XRCC4 were tested for their ability to associate with DNA ligase IV in vitro and for their ability to reconstitute XRCC4-deficient cells in vivo. We find that a central region of XRCC4 from amino acids 100-250 is necessary for DNA ligase IV binding and that deletions within this region functionally inactivates XRCC4. Deletions within the C-terminal 84 amino acids neither affect DNA ligase IV binding nor the in vivo function of XRCC4. The correlation between the ability or inability of XRCC4 to bind DNA ligase IV and its ability or failure to reconstitute wild-type DNA repair in vivo, respectively, demonstrates for the first time that the physical interaction with DNA ligase IV is crucial for the in vivo function of XRCC4. Deletions within the N-terminal 100 amino acids inactivate XRCC4 in vivo but leave DNA ligase IV binding unaffected. This indicates further DNA ligase IV-independent functions of XRCC4.

DNA ligase IV binds to XRCC4 via a motif located between rather than within its BRCT domains

The covalent rejoining of DNA ends at single-stranded or double-stranded DNA breaks is catalyzed by DNA ligases. Four DNA ligase activities (I-IV) have been identified in mammalian cells [1]. It has recently been demonstrated that DNA ligase IV interacts with and is catalytically stimulated by the XRCC4 protein [2,3], which is essential for DNA double-strand break repair and the genomic rearrangement process of V(D)J recombination [4]. Together with the finding that the yeast DNA ligase IV homologue is essential for nonhomologous DNA end joining [5-7], this has led to the hypothesis that mammalian DNA ligase IV catalyzes ligation steps in both of these processes [8]. DNA ligase IV is characterized by a unique carboxy-terminal tail comprising two BRCT (BRCA1 carboxyl terminus) domains. BRCT domains were initially identified in the breast cancer susceptibility protein BRCA1 [9], but are also found in other DNA repair proteins [10]. It has been suggested that DNA ligase IV associates with XRCC4 via its tandem BRCT domains and that this may be a general model for protein-protein interactions between DNA repair proteins [3]. We have performed a detailed deletional analysis of DNA ligase IV to define its XRCC4-binding domain and to characterize regions essential for its catalytic activity. We find that a region in the carboxy-terminal tail of DNA ligase IV located between rather than within BRCT domains is necessary and sufficient to confer binding to XRCC4. The catalytic activity of DNA ligase IV is affected by mutations within the first two-thirds of the protein including a 67 amino-acid amino-terminal region that was previously thought not to be present in human DNA ligase IV [11].

Antigen receptor gene rearrangement

Two specialized forms of site-directed double-strand (ds) DNA breakage and rejoining are part of the physiologic program of lymphocytes. One is recombination of the V, D and J gene sequences, termed V(D)J recombination, occurring during early B- and T-cell development, and the other is class-switch recombination occurring exclusively in mature B cells. For V(D)J recombination significant progress has been made recently elucidating the biochemistry of the reaction. In particular our understanding of how DNA ds breaks are both generated and rejoined has increased. For class-switch recombination no definitive information is known about the nucleases required for making the ds breaks, but recent evidence suggests that the joining phase shares activities also required for V(D)J recombination and general DNA ds break repair.

Activity of DNA ligase IV stimulated by complex formation with XRCC4 protein in mammalian cells

Mutation of the XRCC4 gene in mammalian cells prevents the formation of the signal and coding joints in the V(D)J recombination reaction, which is necessary for production of a functional immunoglobulin gene, and renders the cells highly sensitive to ionizing radiation. However, XRCC4 shares no sequence homology with other proteins, nor does it have a biochemical activity to indicate what its function might be. Here we show that DNA ligase IV co-immunoprecipitates with XRCC4 and that these two proteins specifically interact with one another in a yeast two-hybrid system. Ligation of DNA double-strand breaks in a cell-free system by DNA ligase IV is increased fivefold by purified XRCC4 and seven- to eightfold when XRCC4 is co-expressed with DNA ligase IV. We conclude that the biological consequences of mutating XRCC4 are primarily due to the loss of its stimulatory effect on DNA ligase IV: the function of the XRCC4-DNA ligase IV complex may be to carry out the final steps of V(D)J recombination and joining of DNA ends.

Yeast DNA ligase IV mediates non-homologous DNA end joining

The discovery of homologues from the yeast Saccharomyces cerevisiae of the human Ku DNA-end-binding proteins (HDF1 and KU80) has established that this organism is capable of non-homologous double-strand end joining (NHEJ), a form of DNA double-strand break repair (DSBR) active in mammalian V(D)J recombination. Identification of the DNA ligase that mediates NHEJ in yeast will help elucidate the function of the four mammalian DNA ligases in DSBR, V(D)J recombination and other reactions. Here we show that S. cerevisiae has two typical DNA ligases, the known DNA ligase I homologue CDC9 and the previously unknown DNA ligase IV homologue DNL4. dnl4 mutants are deficient in precise and end-processed NHEJ. DNL4 and HDF1 are epistatic in this regard, with the mutation of each having equivalent effects. dnl4 mutants are complemented by overexpression of Dnl4 but not of Cdc9, and deficiency of Dnl4 alone does not impair either cell growth or the Cdc9-mediated responses to ionizing and ultraviolet radiation. Thus, S. cerevisiae has two distinct and separate ligation pathways.

A complex of RAG-1 and RAG-2 proteins persists on DNA after single-strand cleavage at V(D)J recombination signal sequences

The recombination activating gene (RAG) 1 and 2 proteins are required for initiation of V(D)J recombination in vivo and have been shown to be sufficient to introduce DNA double-strand breaks at recombination signal sequences (RSSs) in a cell-free assay in vitro. RSSs consist of a highly conserved palindromic heptamer that is separated from a slightly less conserved A/T-rich nonamer by either a 12 or 23 bp spacer of random sequence. Despite the high sequence specificity of RAG-mediated cleavage at RSSs, direct binding of the RAG proteins to these sequences has been difficult to demonstrate by standard methods. Even when this can be demonstrated, questions about the order of events for an individual RAG-RSS complex will require methods that monitor aspects of the complex during transitions from one step of the reaction to the next. Here we have used template-independent DNA polymerase terminal deoxynucleotidyl transferase (TdT) in order to assess occupancy of the reaction intermediates by the RAG complex during the reaction. In addition, this approach allows analysis of the accessibility of end products of a RAG-catalyzed cleavage reaction for N nucleotide addition. The results indicate that RAG proteins form a long-lived complex with the RSS once the initial nick is generated, because the 3'-OH group at the nick remains obstructed for TdT-catalyzed N nucleotide addition. In contrast, the 3'-OH group generated at the signal end after completion of the cleavage reaction can be efficiently tailed by TdT, suggesting that the RAG proteins disassemble from the signal end after DNA double-strand cleavage has been completed. Therefore, a single RAG complex maintains occupancy from the first step (nick formation) to the second step (cleavage). In addition, the results suggest that N region diversity at V(D)J junctions within rearranged immunoglobulin and T cell receptor gene loci can only be introduced after the generation of RAG-catalyzed DNA double-strand breaks, i.e. during the DNA end joining phase of the V(D)J recombination reaction.

Tying loose ends: roles of Ku and DNA-dependent protein kinase in the repair of double-strand breaks

A convergence of information from biochemistry, yeast and mammalian genetics, immunology, and radiation biology has permitted identification of some of the protein participants - Ku, DNA-PK, XRCC4 - and the reaction intermediates in DNA end joining, suggesting how broken chromosomal ends may be recognized and repaired in eukaryotic cells. Some components may be defective in inherited disorders.

Expression of DNA-dependent protein kinase holoenzyme upon induction of lymphocyte differentiation and V(D)J recombination

Murine preB lymphocytes grow in tissue culture in the presence of stromal cells and interleukin 7 (IL-7), and can be induced to differentiate to surface-immunoglobulin-positive B cells in vitro by withdrawal of IL-7. Upon differentiation, proliferation ceases, and upregulation of Rag-1 and Rag-2 expression, and induction of V(D)J immunoglobulin-gene rearrangements occur. DNA-dependent protein kinase (DNA-PK) is required for effective V(D)J recombination and repair of DNA double-strand breaks. The holoenzyme comprises a catalytic subunit (DNA-PKcs) and the Ku heterodimer (Ku70/Ku80). We have analyzed expression of Ku70, Ku80 and DNA-PKcs upon induction of differentiation in preB cells derived from wild-type, severe combined immunodeficiency (SCID) and Rag-2-/- mice. Protein levels of Ku80 and Ku70 moderately decrease after induction in all three cell types. A distinct polypeptide that crossreacts with anti-Ku Ig appears in the cytoplasm of wild-type and Rag-2-/- cells, but not of SCID cells. In mouse preB cells, Ku70 and Ku80 are present in the nuclei and cytoplasm before and after onset of differentiation. In vivo, Ku70 is predominantly expressed in V(D)J-recombination-active, early-preB and CD4-/CD8- thymocyte cell populations. Upon differentiation, protein levels of DNA-PKcs are unaltered. DNA-PK activity, which is not detectable in SCID cells, increases in wild-type and Rag-2-/- cells more than twofold shortly after induction of differentiation, then falls back to about 50% of starting levels.

The half-life of RAG-1 protein in precursor B cells is increased in the absence of RAG-2 expression

Site-specific recombination of immunoglobulin and T cell receptor gene segments in B and T lymphocytes is dependent on the expression of two recombinant activation genes, Rag-1 and Rag-2. Here, we show that RAG-1 protein turnover in pre-B cells depends on the expression of RAG-2. The apparent half-life of RAG-1 protein is increased when RAG-2 is not expressed in differentiating pre-B cells.

Phenotypic and functional analysis of B lymphopoiesis in interleukin-7-transgenic mice: expansion of pro/pre-B cell number and persistence of B lymphocyte development in lymph nodes and spleen

Transgenic mice in which mouse interleukin (IL)-7 cDNA is expressed under the control of the mouse major histocompatibility complex (MHC) class II (E alpha) promoter develop a lymphoproliferative disease characterized by the early polyclonal expansion of T cells followed in many cases by the development of lymphomas of immature B cells. Here, we have analyzed B cell development in these transgenic mice. Phenotypic analysis using monoclonal antibodies to B220, IgM, IgD, c-kit, IL-7 receptor, MHC class II, AA4.1, CD19, CD23, CD25, CD40 and CD43 shows that B lymphopoiesis in the bone marrow is dramatically altered and the number of pro/pre-B and immature B cells is significantly increased. Interestingly, pro/pre-B and immature B cells persist in the spleens of adult transgenic mice and are also present in lymph nodes and blood. Cell cycle analysis of lymph node cells shows that subpopulations of developing B cells retain the cell cycle profiles of their bone marrow counterparts. Limiting dilution analysis shows that the number of clonable pre-B cells is significantly increased and that at limiting dilution, growth of transgenic pre-B cells is still dependent on exogenous IL-7. Using semiquantitative polymerase chain reaction (PCR) and in situ hybridization, the level of IL-7 transcripts in the spleen was found to decrease between 2 and 4 weeks in control mice with levels in transgenics mice being approximately 50 times greater. These transgenic mice represent an interesting model with which to study the effects of IL-7 overexpression in the bone marrow and raise interesting questions regarding the regulation of B lymphopoiesis in normal mice.

A novel molecular complex expressed on immature B cells: a possible role in T cell-independent B cell development

To identify surface molecules that may play a role in regulating ileal Peyer's patch (PP) B cell growth, we generated monoclonal antibodies (mAbs) and then selected them for a unique reactivity with ileal PP B cells. Flow cytometric analysis identified a mAb (SIC4.8R) that labeled 97% of ileal and 50-60% of jejunal PP sIgM+B cells. SIC4.8R also labeled a subpopulation of cortical thymocytes buy few B or T cells in other lymphoid tissues, including bone marrow. Immunohistochemistry revealed intense SIC4.8R staining of B cells in the cortex of ileal PP follicles. SIC4.8R also labeled bovine PP B cells, a murine pro-B cell line, and pre-B cells in human bone marrow. Protein chemistry revealed that a structurally similar molecular complex was expressed on sheep ileal PP B cells and thymocytes and murine pro-B cells. Addition of soluble SIC4.8R to cultured ileal PP B cells reduced apoptotic cell death, elevated proliferative responses, partially inhibited anti-Ig-induced cell death, and induced IL-4 responsiveness. In contrast, soluble SIC4.8R had an antiproliferative effect on a mouse pro-B cell line. Finally, SIC4.8R labeling declined following the stimulation of ileal PP B cells with CD40 ligand. In conclusion, the present investigation determined that SIC4.8R identified a novel molecular complex that is expressed at several stages of T cell-independent B cell development in a variety of mammalian species. This observation confirmed that PP B cells are developmentally distinct from other B cell populations in sheep and suggested that the bone marrow may not be a site of B lymphopoiesis in young lambs.

Induction of sterile transcription from the kappa L chain gene locus in V(D)J recombinase-deficient progenitor B cells

B cell development in RAG-2-deficient (RAG-2T) mice is impeded at an early stage, due to the inability of these animals to rearrange their endogenous ig gene loci. Expression of an E mu-bcl-2 transgene in these mice did not change this phenotype. However, stromal cell/IL-7-reactive B cell progenitors (pro-B cells) were found in fetal live and bone marrow of RAG-2T and RAG-2T/E mu-bcl-2 transgenic mice in numbers comparable to normal mice. Like cells from normal mice they are c-kit+, surrogate L chain+ and CD25-, and can proliferate in vitro for long periods of time. Upon IL-7 deprivation, they can be induced to differentiate into c-kit-, surrogate L chain- and CD25+ cells that are no longer clonable on stromal cells and IL-7. Furthermore, sterile transcription from the kappa L chain gene loci is induced. The latter was also observed with pro-B cells directly isolated ex vivo from the bone marrow of RAG-2-deficient animals. The results suggest that progenitor B cell differentiation can occur in cells from V(D)J recombinase-deficient mice to the stage where kL chain gene rearrangements would normally be initiated. It further indicates that some molecular programs of early B cell differentiation can take place in the absence of Ig gene rearrangements.

Down-regulation of RAG1 and RAG2 gene expression in preB cells after functional immunoglobulin heavy chain rearrangement

Two waves of immunoglobulin gene rearrangements, first of the heavy, then of the light chain chain gene loci form functional immunoglobulin genes during B cell development. In mouse bone marrow the differential surface expression of B220 (CD45R), c-kit, CD25, and surrogate light chain as well as the cell cycle status allows FACS separation of the cells in which these two waves of rearrangements occur. The gene products of two recombination activating genes, RAG1 and RAG2 are crucial for this rearrangement process. Here, we show that the expression of the RAG genes is twice up- and down-regulated, at the transcriptional level for RAG1 and RAG2, and at the postranscriptional level for RAG2 protein. Expression levels are high in D-->JH and VH-->DJH rearranging proB and preB-I cells, low in preB cells expressing the preB cell receptor on the cell surface, and high again in VL-->JL rearranging small preB-II cells. In immature B cells expressing on the cell surface RAG1 and RAG2 mRNA is down-regulated, whereas RAG2 protein levels are maintained. Down-regulation of RAG1 and RAG2 gene expression after productive rearrangement at one heavy chain allele might be part of the mechanisms that prevent further rearrangements at the other allele.

In-vitro analyses of mechanisms of B-cell development

B-cell lymphopoiesis in vivo is very complex due to the influences of cooperating cells, cytokines and other receptor-ligand interactions which appear to occur developmentally at different cellular stages. Therefore in-vitro models will help to unravel this complex situation. Here, we review our and others' work on in-vitro models of B-cell development. The role of stromal cells, cytokines, surrogate light chain and products of rearranged Ig-loci in the developmentally different cellular stages will be discussed.

Positive and negative selection events during B lymphopoiesis

Early in B-cell development, large numbers of cells have to be generated, each of which expresses only one type of B-cell receptor (i.e. Ig) on its surface. This is achieved by the surface expression of a pre-B cell receptor containing a mu heavy chain/surrogate light chain which differentially provides signals for two responses of precursor B cells at this stage of development. On the one hand, it signals inhibition of further rearrangements of variable heavy chain to diverse-joining heavy chain loci to achieve allelic exclusion at the heavy-chain locus. On the other hand, it signals proliferative expansion by factors between 20 and 100. Later in B-cell development, tolerance to autoantigens must be established and maintained. Tolerance is achieved by developmental arrest and induction of secondary light-chain gene rearrangements in those IgM+ immature B cells that are reactive to autoantigens presented in the primary B-cell generating organs. Even later in development, when mature surface (s)IgM+/sIgD+ B cells encounter autoantigens presented to them in the periphery, either deletion or anergy of the autoantigen-reactive cells occurs. Anergic cells have a sIg-dependent, sIg-proximal defect in signaling and are short-lived. Anergy can be broken in vitro by polyclonal activation via ligation of CD40 in the presence of IL-4. A small part of the remaining immature B cells not reactive to autoantigens are selected to become mature, antigen-reactive sIgM+/sIgD+ B cells. Molecules which might guide such positive selection of B cells still remain to be identified.

IL-2 receptor alpha chain (CD25, TAC) expression defines a crucial stage in pre-B cell development

The analysis of the expression of the alpha chain of the IL-2 receptor (CD25, TAC) on the surface of B lineage cells in mouse bone marrow reveals that it is a useful marker to distinguish pre-B-I from pre-B-II cells. CD25 is not expressed on CD45R(B220)+ c-kit+ CD43+ TdT+ lambda 5+ c mu- sIg-IgH chain locus DJH-rearranged pre-B-I cells of mouse bone marrow. It is expressed on large cycling CD45R(B220)+ c-kit- CD43+ TdT- lambda 5+ c mu+ sIg- and on small resting CD45R(B220)+ c-kit- CD43- TdT- lambda 5- c mu- sIg- IgH chain locus VHDJH-rearranged pre-B-II cells. Therefore, the transition from pre-B-I to large pre-B-II cells is marked by the downregulation of c-kit and terminal deoxynucleotidyl transferase (TdT), and by the upregulation of CD25. SCID, RAG-2T, microMT and lambda 5T mutant mice do have normal, if not elevated numbers of pre-B-I cells but lack all CD25+ pre-B-II cells in their bone marrow. The expression of a transgenic H chain under control of the microH chain enhancer in RAG-2T bone marrow B lineage precursors allows the development of large and small CD25+ pre-B-II cells. The results suggest that the differentiation of pre-B-I to pre-B-II cells in mouse bone marrow requires the expression of microH chains and surrogate L chains in membranes, probably on the surface of precursor B cells.

Roles of IgH and L chains and of surrogate H and L chains in the development of cells of the B lymphocyte lineage

Proteins expressed from productively rearranged H and L chain gene loci have been implied in the regulation of Ig gene rearrangements during B lymphopoiesis. However, recent findings suggest that early B cell development can occur without expression of surrogate L chain, without deposition of microH chains into membranes, without productive H chain gene rearrangements, and even without any rearrangements of Ig gene loci. In bone marrow, 2-5% of all B220-, sIgM-, c-kit+ cells are pro B cells that undergo differentiation from B220- via B220+, c-kit+, CD43+, clonable long-term proliferating pre B-I cells to B220+, c-kit-, CD43-, IL-2 receptor+ pre B-II cells and immature B cells, only to die by apoptosis in situ within less than 4 days. A membrane-bound complex of surrogate H chain (gp130/gp35-65) and surrogate L chain expressed on pro B and pre B-I cells has apparently no influence on this early development. Pre B-I cells carrying DHJH-rearrangements in reading frame (rf) II are counter-selected, probably because they can express an Ig-like complex of truncated DHJHC mu-protein and surrogate L chain, while pre B-I cells DHJH-rearranged in rf I or III are not suppressed. Immature sIg+ B cells, also from bcl-2 transgenic mice, can continue to rearrange L chain gene loci. Thus, mere membrane deposition of Ig, even with concomitant expression of bcl-2, terminates neither expression of RAG-1 and 2, nor secondary L chain gene rearrangements, nor does it allow the development of mature B cells. Membrane-bound expression of an Ig-like complex of microH chains and surrogate L chains appears to be needed to generate the 50-70 million pre B-II cells in bone marrow. However, the membrane-bound expression of Ig is mandatory for negative and positive selection of immature B cells. Autoantigens delete or anergize self-reactive B cells. We speculate that all mature, resting, primary antigen-reactive B cells in the periphery have been selected from immature sIg+ B cells by unknown antigens and have, thereby, changed their lifestyle from rapid death by apoptosis to longevity.

Rearrangement and expression of kappa light chain genes can occur without mu heavy chain expression during differentiation of pre-B cells

The kinetics of kappa light (kappa L) chain gene rearrangement and expression on mRNA and protein level has been studied with four stromal cell/IL-7 reactive, long-term in vitro proliferating pre-B cell lines and clones, two from fetal liver of normal mice and two from fetal liver of E microH-bcl-2 transgenic (bcl-2-tg) mice. These pre-B cell lines and clones are DJH-rearranged on both H chain alleles. Two of the clones harbor H chain rearrangements which do not allow the expression of VHDJH rearranged H chain genes as microH chain proteins. Upon removal of IL-7 from the pre-B cell cultures all four cell lines rearrange VH-DJH and VL-JL gene segments, loose the surface expression of c-kit, CD43, and surrogate light chain, as well as the capacity to be clonable on stromal cells in the presence of IL-7. Pre-B cells from normal mice die by apoptosis during differentiation, while those from bcl-2-tg mice do not. All four lines and clones express comparable levels of mRNA for microH and kappa L chains with the same time kinetics during 3 days of differentiation. However, only two of the four pre-B cell lines and clones express microH chain protein, whereas all four pre-B cell lines and clones express kappa L chain protein at comparable levels between 2 x 10(5) and 1.4 x 10(6) kappa L chain molecules per cell. These results suggest that microH chain expression is not mandatory for rearrangement and normal expression of kappa L chain genes when pre-B cells differentiate to B cells.

Immature surface Ig+ B cells can continue to rearrange kappa and lambda L chain gene loci

Pro and pre B cells possess the long-term capacity to proliferate in vitro on stromal cells and interleukin 7 (IL-7) and can differentiate to surface immunoglobulin (sIg+) cells upon removal of IL-7 from the cultures. A key event in this differentiation is the extensive cell loss due to apoptosis. Because the proto-oncogene bcl-2 can promote cell survival, we established pre-B cell lines from E mu-bcl-2 transgenic mice. These pre-B cells have the same properties as those derived from non-bcl-2 transgenic mice except that they do not die by apoptosis. This allowed us to study the fate of newly formed B cells in vitro for a longer period of time. Here we show that early during the differentiation of pre-B cells, upregulation of RAG-1 and RAG-2 expression go hand in hand with rearrangements of the Ig gene loci. Moreover, the newly formed sIg+ B cells continue to express RAG-1 and RAG-2 and continue to rearrange L chain gene loci, even in the absence of proliferation, in an orderly fashion, so that kappa L+ sIg+ cells can become lambda L+ sIg+ or sIg- cells, whereas lambda L+ sIg+ cells can become sIg-, but not kappa L+ sIg+ cells. Thus, deposition of a complete Ig molecule on the surface of a B cell does not automatically stop the Ig-rearrangement machinery.

B cell development in mice with a defective lambda 5 gene

The surrogate light chain encoded by the two pre-B cell-specific genes VpreB and lambda 5 plays a critical role in B cell development of the mouse. It has been shown that targeted disruption of the lambda 5 gene results in a depletion of B220+ CD43- IgM-pre-B cells in bone marrow, and in a delayed appearance both of CD5+ as well as CD5- surface immunoglobulin (sIg)+ B cells in the periphery. In this report we show that DHJH-rearranged B220- and B220+, CD43+, c-kit+, sIgM- pro- and pre-B-I cells with long-term capacity to proliferate in vitro on stromal cells in the presence of interleukin-7 are present in normal numbers in the bone marrow of lambda 5 T/lambda 5 T mice at various ages. They express normal levels of VpreB mRNA but, in contrast to normal pre-B-I cells, do not express surrogate light chain on their surface. Pre-B-I cells from fetal liver and bone marrow of lambda 5 T/lambda 5 T mice differentiate with normal kinetics and in normal numbers to sIg+, mitogen-reactive B cells. These results suggest that the delayed generation of sIg+ B cells in the peripheral, mature compartments of CD5+ and CD5- cells could be accounted for by the daily production of approximately 5 x 10(5) sIg+ B cells from the pre-B-I cell pool in the absence of a normal pool of pre-B-II cells.

Interferon-gamma arrests proliferation and causes apoptosis in stromal cell/interleukin-7-dependent normal murine pre-B cell lines and clones in vitro, but does not induce differentiation to surface immunoglobulin-positive B cells

Normal pre-B cells from fetal liver or bone marrow of the mouse proliferate for long periods of time in tissue culture on stromal cells in the presence of interleukin-7 (IL-7). Their IgH loci are partly in germ-line, partly in DHJH-rearranged configuration, while their light chain loci are in germ-line configuration. They express the pre-B cell-specific genes VpreB and lambda 5. Proliferation of these pre-B cells is inhibited by interferon (IFN)-gamma, with half-maximal inhibition at concentrations between 0.1 and 1 unit/ml. Normal pre-B cells exposed to IFN-gamma die by apoptosis, as is evidenced by the disintegration of pre-B cell DNA into oligonucleosomal multimers of 180-200 bp. While the proliferation of pre-B cells from E mu-bcl-2 transgenic (tg) mice is inhibited by IFN-gamma, these cells do not die by apoptosis. IFN-gamma does not induce differentiation to more mature B lineage cells. In the absence of IL-7 normal pre-B cells differentiate to VHDHJH/VLJL-rearranged, surface immunoglobulin-positive B cells expressing the alpha chain of the IL-2 receptor. They also down-regulate the expression of VpreB and lambda 5, and lose the capacity to proliferate on stromal cells in the presence of IL-7. In contrast, both normal and E mu-bcl-2 tg pre-B cells exposed to IFN-gamma in the presence of stromal cells and IL-7 fail to differentiate, i.e. do not express surface immunoglobulin, retain expression of VpreB and lambda 5, do not express the alpha chain of the IL-2 receptor, and retain the capacity to proliferate on stromal cells in the presence of IL-7, once IFN-gamma is removed. The potential usefulness of a treatment of acute lymphocytic leukemia of the B cell lineage (pre B-ALL) with IFN-gamma is discussed.