High frequency of normal DJH joints in B cell progenitors in severe combined immunodeficiency mice

Pathological RANK signaling in B cells drives autoimmunity and chronic lymphocytic leukemia

Clinical evidence suggests alterations in receptor activator of NF-κB (RANK) signaling are key contributors to B cell autoimmunity and malignancy, but the pathophysiological consequences of aberrant B cell–intrinsic RANK signaling remain unknown. We generated mice that express a human lymphoma–derived, hyperactive RANK^K240E variant in B lymphocytes in vivo. Forced RANK signaling disrupted B cell tolerance and induced a fully penetrant systemic lupus erythematosus–like disease in addition to the development of chronic lymphocytic leukemia (CLL). Importantly, RANK^K240E transgenic CLL cells as well as CLL cells of independent murine and of human origin depend on microenvironmental RANK ligand (RANKL) for tumor cell survival. Consequently, inhibition of the RANKL–RANK axis with anti-RANKL antibodies killed murine and human CLL cells in vitro and in vivo. These results establish pathological B cell–intrinsic RANK signaling as a potential driver of autoimmunity and B cell malignancy, and they suggest the exploitation of clinically available anti-RANKL compounds for CLL treatment.

Modulation of PKC-α promotes lineage reprogramming of committed B lymphocytes

During hematopoietic lineage development, hematopoietic stem cells sequentially commit toward myeloid or lymphoid lineages in a tightly regulated manner, which under normal circumstances is irreversible. However, studies have established that targeted deletion of the B-lineage specific transcription factor, paired box gene 5 (Pax5), enables B cells to differentiate toward other hematopoietic lineages, in addition to generating progenitor B-cell lymphomas. Our previous studies showed that subversion of protein kinase C (PKC)-α in developing B cells transformed B-lineage cells. Here, we demonstrate that PKC-α modulation in committed CD19(+) B lymphocytes also promoted lineage conversion toward myeloid, NK-, and T-cell lineages upon Notch ligation. This occurred via a reduction in Pax5 expression resulting from a downregulation of E47, a product of the E2A gene. T-cell lineage commitment was indicated by the expression of T-cell associated genes Ptcra, Cd3e, and gene rearrangement at the Tcrb gene locus. Importantly, the lineage-converted T cells carried Igh gene rearrangements reminiscent of their B-cell origin. Our findings suggest that modulation of PKC-α induces hematopoietic-lineage plasticity in committed B-lineage cells by perturbing expression of critical B-lineage transcription factors, and deregulation of PKC-α activity/expression represents a potential mechanism for lineage trans-differentiation during malignancies.

Nanoparticle-based therapy in an in vivo microRNA-155 (miR-155)-dependent mouse model of lymphoma

MicroRNA-155 (miR-155) is an oncogenic microRNA that regulates several pathways involved in cell division and immunoregulation. It is overexpressed in numerous cancers, is often correlated with poor prognosis, and is thus a key target for future therapies. In this work we show that overexpression of miR-155 in lymphoid tissues results in disseminated lymphoma characterized by a clonal, transplantable pre-B-cell population of neoplastic lymphocytes. Withdrawal of miR-155 in mice with established disease results in rapid regression of lymphadenopathy, in part because of apoptosis of the malignant lymphocytes, demonstrating that these tumors are dependent on miR-155 expression. We show that systemic delivery of antisense peptide nucleic acids encapsulated in unique polymer nanoparticles inhibits miR-155 and slows the growth of these "addicted" pre-B-cell tumors in vivo, suggesting a promising therapeutic option for lymphoma/leukemia.

OncomiR addiction in an in vivo model of microRNA-21-induced pre-B-cell lymphoma

MicroRNAs (miRNAs) belong to a recently discovered class of small RNA molecules that regulate gene expression at the post-transcriptional level. miRNAs have crucial functions in the development and establishment of cell identity, and aberrant metabolism or expression of miRNAs has been linked to human diseases, including cancer. Components of the miRNA machinery and miRNAs themselves are involved in many cellular processes that are altered in cancer, such as differentiation, proliferation and apoptosis. Some miRNAs, referred to as oncomiRs, show differential expression levels in cancer and are able to affect cellular transformation, carcinogenesis and metastasis, acting either as oncogenes or tumour suppressors. The phenomenon of 'oncogene addiction' reveals that despite the multistep nature of tumorigenesis, targeting of certain single oncogenes can have therapeutic value, and the possibility of oncomiR addiction has been proposed but never demonstrated. MicroRNA-21 (miR-21) is a unique miRNA in that it is overexpressed in most tumour types analysed so far. Despite great interest in miR-21, most of the data implicating it in cancer have been obtained through miRNA profiling and limited in vitro functional assays. To explore the role of miR-21 in cancer in vivo, we used Cre and Tet-off technologies to generate mice conditionally expressing miR-21. Here we show that overexpression of miR-21 leads to a pre-B malignant lymphoid-like phenotype, demonstrating that mir-21 is a genuine oncogene. When miR-21 was inactivated, the tumours regressed completely in a few days, partly as a result of apoptosis. These results demonstrate that tumours can become addicted to oncomiRs and support efforts to treat human cancers through pharmacological inactivation of miRNAs such as miR-21.

V(D)J recombination in zebrafish: Normal joining products with accumulation of unresolved coding ends and deleted signal ends

V(D)J recombination proceeds from a site-specific cleavage to an imprecise end joining, via generation and resolution of recombination ends. Although rearranged antigen receptor genes isolated from zebrafish (Danio rerio) resemble those made in mammals, differences may arise during evolution from lower to higher vertebrates, in regard to efficiency, fidelity and regulation of this recombination. To elucidate the V(D)J recombination reaction in zebrafish, we characterized recombination ends transiently produced by zebrafish lymphocytes, as well as joining products. Similar to their mammalian counterpart, zebrafish lymphocytes make perfect signal joints and normal coding joints, indicating their competent end resolution machinery. However, recombination ends recovered from the same zebrafish lymphoid tissues exhibit some features that are not readily seen in normal mammalian counterpart: deleted signal ends and accumulation of opened coding ends. These results indicate that the recombination reaction in zebrafish lymphocytes is inefficient and less stringently regulated, which may result from unstable post-cleavage complexes, and/or slow transition from cleavage to resolution. Our data suggests that the V(D)J recombination machinery may have undergone evolution selection to become more efficient in higher jawed vertebrates.

Subversion of protein kinase C alpha signaling in hematopoietic progenitor cells results in the generation of a B-cell chronic lymphocytic leukemia-like population in vivo

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of long-lived mature B cells with the distinctive phenotype CD19(hi) CD5+ CD23+ IgM(lo), which are refractory to apoptosis. An increased level of apoptosis has been observed on treatment of human B-CLL cells with protein kinase C (PKC) inhibitors, suggesting that this family of protein kinases mediate survival signals within B-CLL cells. Therefore, to investigate the ability of individual PKC isoforms to transform developing B cells, we stably expressed plasmids encoding PKC mutants in fetal liver-derived hematopoietic progenitor cells (HPC) from wild-type mice and then cultured them in B-cell generation systems in vitro and in vivo. Surprisingly, we noted that expression of a plasmid-encoding dominant-negative PKC alpha (PKC alpha-KR) in HPCs and subsequent culture both in vitro and in vivo resulted in the generation of a population of cells that displayed an enhanced proliferative capacity over untransfected cells and phenotypically resemble human B-CLL cells. In the absence of growth factors and stroma, these B-CLL-like cells undergo cell cycle arrest and, consistent with their ability to escape growth factor withdrawal-induced apoptosis, exhibited elevated levels of Bcl-2 expression. These studies therefore identify a unique oncogenic trigger for the development of a B-CLL-like disease resulting from the subversion of PKC alpha signaling. Our findings uncover novel avenues not only for the study of the induction of leukemic B cells but also for the development of therapeutic drugs to combat PKC alpha-regulated transformation events.

Lack of MSH2 involvement differentiates V(D)J recombination from other non-homologous end joining events

V(D)J recombination and class switch recombination are the two DNA rearrangement events used to diversify the mouse and human antibody repertoires. While their double strand breaks (DSBs) are initiated by different mechanisms, both processes use non-homologous end joining (NHEJ) in the repair phase. DNA mismatch repair elements (MSH2/MSH6) have been implicated in the repair of class switch junctions as well as other DNA DSBs that proceed through NHEJ. MSH2 has also been implicated in the regulation of factors such as ATM and the MRN (Mre11, Rad50, Nbs1) complex, which are involved in V(D)J recombination. These findings led us to examine the role of MSH2 in V(D)J repair. Using MSH2^−/− and MSH2^+/+ mice and cell lines, we show here that all pathways involving MSH2 are dispensable for the generation of an intact pre-immune repertoire by V(D)J recombination. In contrast to switch junctions and other DSBs, the usage of terminal homology in V(D)J junctions is not influenced by MSH2. Thus, whether the repair complex for V(D)J recombination is of a canonical NHEJ type or a separate microhomology-mediated-end joining (MMEJ) type, it does not involve MSH2. This highlights a distinction between the repair of V(D)J recombination and other NHEJ reactions.

Phospholipase Cgamma2 provides survival signals via Bcl2 and A1 in different subpopulations of B cells

PLCgamma2 plays a critical role in B cell receptor (BCR) signaling and its targeted deletion results in defective B cell development and function. Here, we show that PLCgamma2 deficiency specifically blocks B cell maturation at the transitional type 2 (T2) to follicular (FO) B cell transition and the PLCgamma2 pathway regulates survival of B cells. BCR-induced apoptosis is dramatically enhanced in all subsets of splenic PLCgamma2-deficient B cells, especially in T2 and FO B cell subpopulations. We also find that all splenic PLCgamma2-deficient B cell subpopulations express abnormally low levels of Bcl-2 protein. In addition, PLCgamma2 deficiency disrupts BCR-mediated induction of A1 expression. Enforced expression of Bcl-2 prevents BCR-induced apoptosis in all splenic PLCgamma2-deficient B cell subpopulations and partially restores the numbers of PLCgamma2-deficient FO B cells. In contrast to Bcl-2, enforced expression of A1 preferentially prevents BCR-induced apoptosis in PLCgamma2-deficient FO B cells and partially restores the numbers of these B cells. Therefore, the PLCgamma2 pathway provides a survival signal via regulation of Bcl-2 in all splenic B cell subpopulations and via additional induction of A1 in mature FO B cells.

Development and function of diabetogenic T-cells in B-cell-deficient nonobese diabetic mice

Insulin-dependent diabetes (type 1 diabetes) in the NOD mouse is a T-cell-mediated autoimmune disease. However, B-cells may also play a critical role in disease pathogenesis, as genetically B-cell-deficient NOD mice (NOD.microMT) have been shown to be protected from type 1 diabetes and to display reduced responses to certain islet autoantigens. To examine the requirements for B-cells in the development of type 1 diabetes, we generated a B-cell-naive T-cell repertoire by transplantation of NOD fetal thymuses (FTs) into NOD.scid recipients. Surprisingly, these FT-derived NOD T-cells were diabetogenic in 36% of NOD.scid recipients, despite the absence of B-cells. In addition, T-cells isolated from NOD.microMT mice were diabetogenic in 22% of NOD.scid recipients. Together, these results indicate that B-cells are not an absolute requirement for the generation or effector function of an islet-reactive T-cell repertoire in NOD mice. We suggest that conditions favoring rapid lymphocyte expansion can reveal autoreactive T-cell activity and precipitate disease in genetically susceptible individuals.

Separation-of-function mutants reveal critical roles for RAG2 in both the cleavage and joining steps of V(D)J recombination

The only established physiological function of the V(D)J recombinase, comprising RAG1 and RAG2, is to perform DNA cleavage. The molecular roles of RAG2 in cleavage, the mechanisms used to join the broken DNA ends, and the identity of nuclease(s) that open the hairpin coding ends have been unknown. Site-directed mutagenesis targeting each conserved basic amino acid in RAG2 revealed several separation-of-function mutants that address these questions. Analysis of these mutants reveals that RAG2 helps recognize or cleave distorted DNA intermediates and plays an essential role in the joining step of V(D)J recombination. Moreover, the discovery that some mutants block RAG-mediated hairpin opening in vitro provides a critical link between this biochemical activity and coding joint formation in vivo.

Joining-deficient RAG1 mutants block V(D)J recombination in vivo and hairpin opening in vitro

The RAG proteins cleave at V(D)J recombination signal sequences then form a postcleavage complex with the broken ends. The role of this complex in end processing and joining, if any, is undefined. We have identified two RAG1 mutants proficient for DNA cleavage but severely defective for coding and signal joint formation, providing direct evidence that RAG1 is critical for joining in vivo and strongly suggesting that the postcleavage complex is important in end joining. We have also identified a RAG1 mutant that is severely defective for both hairpin opening in vitro and coding joint formation in vivo. These data suggest that the hairpin opening activity of the RAG proteins plays an important physiological role in V(D)J recombination.

Irradiation promotes V(D)J joining and RAG-dependent neoplastic transformation in SCID T-cell precursors

Defects in the nonhomologous end-joining (NHEJ) pathway of double-stranded DNA break repair severely impair V(D)J joining and selectively predispose mice to the development of lymphoid neoplasia. This connection was first noted in mice with the severe combined immune deficient (SCID) mutation in the DNA-dependent protein kinase (DNA-PK). SCID mice spontaneously develop thymic lymphoma with low incidence and long latency. However, we and others showed that low-dose irradiation of SCID mice dramatically increases the frequency and decreases the latency of thymic lymphomagenesis, but irradiation does not promote the development of other tumors. We have used this model to explore the mechanistic basis by which defects in NHEJ confer selective and profound susceptibility to lymphoid oncogenesis. Here, we show that radiation quantitatively and qualitatively improves V(D)J joining in SCID cells, in the absence of T-cell receptor-mediated cellular selection. Furthermore, we show that the lymphocyte-specific endonuclease encoded by the recombinase-activating genes (RAG-1 and RAG-2) is required for radiation-induced thymic lymphomagenesis in SCID mice. Collectively, these data suggest that irradiation induces a DNA-PK-independent NHEJ pathway that facilitates V(D)J joining, but also promotes oncogenic misjoining of RAG-1/2-induced breaks in SCID T-cell precursors.

Unexpected rearrangement and expression of the immunoglobulin lambda1 locus in scid mice

In severe combined immunodeficient (scid) mice, V(D)J recombination is severely impaired due to a recessive mutation (scid). Thus, we were surprised to find in this study that Vlambda1-Jlambda1 rearrangement is routinely detectable in scid fetal liver, adult bone marrow, and spleen in the apparent absence of completed VH-DJH and Vkappa-Jkappa rearrangements. Particularly surprising, we found the level of Vlambda1-Jlambda1 rearrangement in scid fetal liver to be comparable to that in fetal liver of wild-type mice. The majority of scid Vlambda1-Jlambda1 rearrangements contained abnormal deletions at the VJ junction, consistent with the known effect of scid. However, approximately 15% of Vlambda1-Jlambda1 rearrangements lacked abnormal deletions. Productive lambda1 transcripts resulting from in-frame rearrangements were readily detectable in scid adult bone marrow and spleen, consistent with our ability to detect lambda1-expressing cells by flow cytometry in the spleens of bcl-2-transgenic scid mice. Strikingly, lambda1 transcripts from individual scid mice often showed VJ junctional sequences with the same recurring palindromic (P) additions of three, four, or five nucleotides. To account for these findings, we suggest that (a) nonhomologous end joining of Vlambda1 and Jlambda1 coding ends in fetal B lineage cells may not be (severely) impaired by scid; (b) recurring P additions in scid lambda1 transcripts may reflect certain molecular constraints imposed by scid on the resolution of Vlambda1 and Jlambda1 hairpin coding ends; and (c), scid lymphocytes with productively rearranged Vlambda1 and Jlambda1 elements may differentiate into recombinase-inactive cells and emigrate from bone marrow to spleen.

Gamma-Irradiation Directly Affects the Formation of Coding Joints in SCID Cell Lines

SCID mice have a defect in the catalytic subunit of the DNA-dependent protein kinase, causing increased sensitivity to ionizing radiation in all tissues and severely limiting the development of B and T cell lineages. SCID T and B cell precursors are unable to undergo normal V(D)J recombination: coding joint and signal joint products are less frequently formed and often will exhibit abnormal structural features. Paradoxically, irradiation of newborn SCID mice effects a limited rescue of T cell development. It is not known whether irradiation has a direct impact on the process of V(D)J joining, or whether irradiation of the thymus allows the outgrowth of rare recombinants. To investigate this issue, we sought to demonstrate an irradiation effect ex vivo. Here we have been able to reproducibly detect low-frequency coding joint products with V(D)J recombination reporter plasmids introduced into SCID cell lines. Exposure of B and T lineage cells to 100 cGy of gamma irradiation made no significant difference with respect to the number of coding joint and signal joint recombination products. However, in the absence of irradiation, the coding joints produced in SCID cells had high levels of P nucleotide insertion. With irradiation, markedly fewer P insertions were seen. The effect on coding joint structure is evident in a transient assay, in cultured cells, establishing that irradiation has an immediate impact on the process of V(D)J recombination. A specific proposal for how the DNA-dependent protein kinase catalytic subunit influences the opening of hairpin DNA intermediates during coding joint formation in V(D)J recombination is presented.

Extended duration of DH-JH rearrangement in immunoglobulin heavy chain transgenic mice: implications for regulation of allelic exclusion

Here we show that suppression of VH-DJH rearrangement in mice bearing a mu heavy (H) chain transgene (mu-tg mice) is associated with an extended period of DH-JH rearrangement, the first step of Immunoglobulin H chain gene rearrangement. Whereas DH-JH rearrangement is normally initiated and completed at the pro-B cell stage, in mu-tg mice it continues beyond this stage and occurs most frequently at the small (late) pre-B stage. Despite ongoing DH-JH rearrangement in late pre-B cells of mu-tg mice, VH-DJH rearrangement is not detectable in these cells. We infer that the lack of VH-DJH rearrangement primarily reflects tg-induced acceleration of B cell differentiation past the stage at which rearrangement of VH elements is permissible. In support of this inference, we find that the normal representation of early B lineage subsets is markedly altered in mu-tg mice. We suggest that the effect of a productive VH-DJH rearrangement at an endogenous H chain allele may be similar to that of a mu-tg; i.e., cells that make a productive VH-DJH rearrangement on the first attempt rapidly progress to a developmental stage that precludes VH-DJH rearrangement at the other allele (allelic exclusion).

V(D)J recombination in Ku86-deficient mice: distinct effects on coding, signal, and hybrid joint formation

Ku, a heterodimer of 70 and 86 kDa subunits, plays a critical but poorly understood role in V(D)J recombination. Although Ku86-deficient mice are defective in coding and signal joint formation, rare recombination products have been detected by PCR. Here, we report nucleotide sequences of 99 junctions from Ku86-deficient mice. Over 90% of the coding joints, but not signal or hybrid joints, exhibit short sequence homologies, indicating that homology is required to join coding ends in the absence of Ku86. Our results suggest that Ku86 may normally have distinct functions in the formation of these different types of junctions. Furthermore, Ku86(-/-) joints are unexpectedly devoid of N-region diversity, suggesting a novel role for Ku in the addition of N nucleotides by terminal deoxynucleotidyl transferase.

Biochemical and genetic defects in the DNA-dependent protein kinase in murine scid lymphocytes

The scid gene product has been identified as the 460-kDa catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs p460), a member of the phosphatidylinositol 3-kinase family. DNA-PK activity is undetectable in scid cells, but the molecular basis for this defect has not been identified. Here we report that expression of p460 in scid lymphocyte precursors is detectable but is reduced at least 10-fold relative to that in wild-type lymphocytes. In addition, we show that the scid mutation disturbs p460 nuclear association, presumably affecting its role in DNA repair pathways. To examine the molecular basis for our observations, we used a degenerate PCR strategy to clone the C-terminal p460 kinase domain from wild-type and scid thymocytes. Northern (RNA) analysis with these probes revealed normal steady-state p460 mRNA levels in scid cells, suggesting that the reduced abundance of p460 protein is due to a posttranscriptional defect. Sequence comparisons identified a single-base-pair alteration in the scid C-terminal p460 kinase domain, resulting in a premature stop codon. This mutation is predicted to truncate p460 by approximately 8 kDa, but it preserves the conserved motifs required for kinase activity in members of the phosphoinositidyl 3-kinase family. Despite a computed molecular weight alteration of less than 2%, we were able to visualize this difference by Western blot (immunoblot) analysis of wild-type and scid p460. These data demonstrate that the scid DNA-PKes mutation is not a null allele and suggest a molecular rationale for the well-described leakiness of the scid phenotype.

V(D)J recombination activates a p53-dependent DNA damage checkpoint in scid lymphocyte precursors

Double-stranded DNA breaks (DSBs) trigger p53-mediated cell cycle arrest or apoptosis pathways that limit the oncogenic consequences of exposure to genotoxic agents, but p53-mediated responses to DSB generated by normal physiologic events have not been documented. "Broken" V(D)J coding ends accumulate in scid lymphocyte precursors as a consequence of a mutation in DNA-dependent protein kinase (DNA-PK). The ensuing failure to rearrange efficiently antigen receptors arrests lymphoid development. Here we show that scid thymocytes express high levels of p53 protein, attributable to recombinase activating gene (RAG)-dependent generation of DSB adjacent to V, D, and J gene segments. To examine the functional importance of p53 expression in vivo, we bred p53-/- scid mice. The absence of p53 facilitated production of in-frame V(D)Jbeta coding joints and developmental progression of scid thymocytes, in addition to a dramatic accumulation of pro-B cells. All mice developed disseminated pro-B or immature T cell lymphoma/leukemia by 7-12 weeks of age. We present evidence that p53 deficiency prolongs the survival of scid lymphocyte precursors harboring broken V(D)J coding ends, allowing the accumulation of aneuploid cells. These results demonstrate that a p53-mediated DNA damage checkpoint contributes to the immune deficiency characteristic of the scid mutation and limits the oncogenic potential of DSBs generated during V(D)J recombination.

Ku86-deficient mice exhibit severe combined immunodeficiency and defective processing of V(D)J recombination intermediates

Ku is a heterodimeric DNA end binding complex composed of 70 and 86 kDa subunits. Here, we show that Ku86 is essential for normal V(D)J recombination in vivo, as Ku86-deficient mice are severely defective for formation of coding joints. Unlike severe combined immunodeficient (scid) mice, Ku86-deficient mice are also defective for signal joint formation. Both hairpin coding ends and blunt full-length signal ends accumulate. Contrary to expectation, Ku86 is evidently not required for protection of either type of V(D)J recombination intermediate. Instead, V(D)J recombination appears to be arrested after the cleavage step in Ku86-deficient mice. We suggest that Ku86 may be required to remodel or disassemble DNA-protein complexes containing broken ends, making them available for further processing and joining.

Variable region usage in B cell deficient mice in a model of experimental herpes simplex virus retinitis

By using PCR, we have found previously that differences in T cell receptor alpha/beta chain variable region (TCR alpha/beta) gene expression in lymph nodes (LN) existed between susceptible and resistant mice following anterior chamber (AC) inoculation with herpes simplex virus (HSV). We now report and compare the immunoglobulin gene variable region (Ig VH) and the TCR V beta mRNA expression in normal congenic mice (BALB/c and C.B-17) and also in B cell modulated C.B-17 mice (B-). RNA prepared from spleen and LN of these mice before and after HSV-AC inoculation was analysed by PCR using oligoprimers specific for 11 VH gene families and 19 V beta gene families. Densitometry analysis revealed that VH family mRNA expression levels in spleen and lymph nodes did not correlate with HSV susceptibility or resistance patterns in B-, BALB/c and C.B-17 mice. Analysis of TCR V beta chain genes showed that spleen of resistant C.B-17 and B-Ab transferred mice showed a preference for V beta 11 gene expression not seen in susceptible mice. In contrast, LN of BALB/c and B-, both susceptible mice, made TCR V beta transcripts that were indistinguishable from those generated by resistant C.B-17 mice, following HSV-AC inoculation. Finally, TCR V beta gene family repertoire appears to be severely diminished in uninfected B- mice (50% in LN and 45% in spleen) by anti-mu treatment.

Efficient nonhomologous and homologous recombination in scid cells

The severe combined immunodeficiency (scid) mutation affects both coding joint formation during immunoglobulin and T-cell receptor V(D)J recombination and double-strand break repair. We analyzed scid cells for their ability to undergo other types of DNA end joining: nonhomologous and homologous recombination. Using plasmid constructs carrying antibiotic resistance genes, we observed that the efficiency of nonhomologous integration in scid cells was equal to that in wildtype cell lines. In addition, there was no obvious difference in the fidelity of the integration and in the expression of the resistance genes. Moreover, scid cells were able to carry out homologous recombination of extrachromosomal substrates just as well as wildtype cells. These results suggest a mechanistic difference between nonhomologous integration and homologous recombination on the one hand and V(D)J recombination and double-strand break repair on the other.

Development of B cells in scid mice with immunoglobulin transgenes: implications for the control of V(D)J recombination

The inability of scid pro-B cells to progress to the pre-B and B cell stages is believed to be caused by a defective recombinase activity that fails to resolve chromosomal breaks resulting from attempted V(D)J recombination. In support of this model, we report that certain immunoglobulin transgenes, specifically those which strongly inhibit endogenous VH-to-DJH and V kappa-to-J kappa rearrangement in wild-type mice, allow scid pro-B cells to progress to the pre-B and B cell stages. This rescue of scid B cell differentiation is associated with a dramatic reduction in expression of the recombination activation genes, RAG1 and RAG2, and with reduced transcription of the kappa locus.

Inversions produced during V(D)J rearrangement at IgH, the immunoglobulin heavy-chain locus

Diversity in immunoglobulin antigen receptors is generated in part by V(D)J recombination. In this process, different combinations of gene elements are joined in various configurations. Products of V(D)J recombination are coding joints, signal joints, and hybrid junctions, which are generated by deletion or inversion. To determine their role in the generation of diversity, we have examined two sorts of recombination products, coding joints and hybrid junctions, that have formed by inversion at the mouse immunoglobulin heavy-chain locus. We developed a PCR assay for quantification and characterization of inverted rearrangements of DH and JH gene elements. In primary cells from adult mice, inverted DJH rearrangements are detectable but they are rare. There were approximately 1,100 to 2,200 inverted DJH coding joints and inverted DJH hybrid junctions in the marrow of one adult mouse femur. On day 16 of gestation, inverted DJH rearrangements are more abundant. There are approximately 20,000 inverted DJH coding joints and inverted DJH hybrid junctions per day 16 fetal liver. In fetal liver cells, the number of inverted DJH rearrangements remains relatively constant from day 14 to day 16 of gestation. Inverted DJH rearrangements to JH4, the most 3' JH element, are more frequently detected than inverted DJH rearrangements to other JH elements. We compare the frequencies of inverted DJH rearrangements to previously determined frequencies of uninverted DJH rearrangements (DJH rearrangements formed by deletion). We suggest that inverted DJH rearrangements are influenced by V(D)J recombination mechanistic constraints and cellular selection.

Murine B cell development: commitment and progression from multipotential progenitors to mature B lymphocytes

B lymphocytes, the cellular source of antibody, are critical components of the immune response. They develop from multipotential stem cells, progressively acquiring the traits that allow them to function as mature B lymphocytes. This developmental program is dependent on appropriate interactions with the surrounding environment. These interactions, mediated by cell-cell and cell-matrix interactions, provide the growth and differentiation signals that promote progression along the developmental pathway. This chapter addresses the properties of developing B lineage cells and the nature of the environmental signals that support B lineage progression.

Rescue of T cell-specific V(D)J recombination in SCID mice by DNA-damaging agents

Assembly of antigen receptor V (variable), D (diversity), and J (joining) gene segments requires lymphocyte-specific genes and ubiquitous DNA repair activities. Severe combined immunodeficient (SCID) mice are defective in general double-strand (ds) DNA break repair and V(D)J coding joint formation, resulting in arrested lymphocyte development. A single treatment of newborn SCID mice with DNA-damaging agents restored functional, diverse, T cell receptor beta chain coding joints, as well as development and expansion of thymocytes expressing both CD4 and CD8 coreceptors, but did not promote B cell development. Thymic lymphoma developed in all mice treated with DNA-damaging agents, suggesting an interrelation between V(D)J recombination, dsDNA break repair, and lymphomagenesis.

Bcl-2 expression promotes B- but not T-lymphoid development in scid mice

Expression of antigen receptors is vital for the development of B and T lymphocytes. In mice with the scid mutation, which are unable to make productive rearrangements of their immunoglobulin and T-cell receptor (TCR) genes, lymphopoiesis aborts at an early stage. The death of the immature lymphocytes by apoptosis is postulated to result from a failure to receive a survival signal induced by receptor engagement. Consistent with this hypothesis, introduction of immunoglobulin or TCR transgenes into scid mice promoted an increase in B- or T-lymphoid cells, respectively. As the protein encoded by the bcl-2 gene can inhibit cell death, we tested whether lymphopoiesis could be rescued in scid mice by crossing in a bcl-2 transgene. Strikingly, the bcl-2/scid mice accumulated almost normal numbers of B-lymphoid cells which lacked surface immunoglobulin but expressed markers of maturity. T-cell development remained blocked. Introducing a TCR transgene enabled bcl-2/scid mice to develop normal numbers of CD4+8+ thymocytes even in the absence of immunological selection, suggesting that T cells become competent to respond to bcl-2 protein only after the TCR complex is displayed at the cell surface.