Purification and analysis of kidney-infiltrating leukocytes in a mouse model of lupus nephritis.

Renal injury often occurs as a complication in autoimmune diseases such as systemic lupus erythematosus (SLE). It is estimated that a minimum of 20% SLE patients develop lupus nephritis, a condition that can be fatal when the pathology progresses to end-stage renal disease. Studies in animal models showed that incidence of immune cell infiltrates in the kidney was linked to pathological injury and correlated with severe lupus nephritis. Thus, preventing immune cell infiltration into the kidney is a potential approach to impede the progression to an end-stage disease. A requirement to investigate the role of kidney-infiltrating leukocytes is the development of reproducible and efficient protocols for purification and characterization of immune cells in kidney samples. This chapter describes a detailed methodology that discriminates tissue-resident leukocytes from blood-circulating cells that are found in kidney. Our protocol was designed to maximize cell viability and to reduce variability among samples, with a combination of intravascular staining and magnetic bead separation for leukocyte enrichment. Experiments included as example were performed with FcγRIIb[KO] mice, a well-characterized murine model of SLE. We identified T cells and macrophages as the primary leukocyte subsets infiltrating into the kidney during severe nephritis, and we extensively characterized them phenotypically by flow cytometry.

Dysfunction of CD169+ macrophages and blockage of erythrocyte maturation as a mechanism of anemia in Plasmodium yoelii infection.

Plasmodium parasites cause malaria with disease outcomes ranging from mild illness to deadly complications such as severe malarial anemia (SMA), pulmonary edema, acute renal failure, and cerebral malaria. In young children, SMA often requires blood transfusion and is a major cause of hospitalization. Malaria parasite infection leads to the destruction of infected and noninfected erythrocytes as well as dyserythropoiesis; however, the mechanism of dyserythropoiesis accompanied by splenomegaly is not completely understood. Using Plasmodium yoelii yoelii 17XNL as a model, we show that both a defect in erythroblastic island (EBI) macrophages in supporting red blood cell (RBC) maturation and the destruction of reticulocytes/RBCs by the parasites contribute to SMA and splenomegaly. After malaria parasite infection, the destruction of both infected and noninfected RBCs stimulates extramedullary erythropoiesis in mice. The continuous decline of RBCs stimulates active erythropoiesis and drives the expansion of EBIs in the spleen, contributing to splenomegaly. Phagocytosis of malaria parasites by macrophages in the bone marrow and spleen may alter their functional properties and abilities to support erythropoiesis, including reduced expression of the adherence molecule CD169 and inability to support erythroblast differentiation, particularly RBC maturation in vitro and in vivo. Therefore, macrophage dysfunction is a key mechanism contributing to SMA. Mitigating and/or alleviating the inhibition of RBC maturation may provide a treatment strategy for SMA.

MAVS Positively Regulates Mitochondrial Integrity and Metabolic Fitness in B Cells.

Activated B cells experience metabolic changes that require mitochondrial remodeling, in a process incompletely defined. In this study, we report that mitochondrial antiviral signaling protein (MAVS) is involved in BCR-initiated cellular proliferation and prolonged survival. MAVS is well known as a mitochondrial-tethered signaling adaptor with a central role in viral RNA-sensing pathways that induce type I IFN. The role of MAVS downstream of BCR stimulation was recognized in absence of IFN, indicative of a path for MAVS activation that is independent of viral infection. Mitochondria of BCR-activated MAVS-deficient mouse B cells exhibited a damaged phenotype including disrupted mitochondrial morphology, excess mitophagy, and the temporal progressive blunting of mitochondrial oxidative capacity with mitochondrial hyperpolarization and cell death. Costimulation of MAVS-deficient B cells with anti-CD40, in addition to BCR stimulation, partially corrected the mitochondrial structural defects and functionality. Our data reveal a (to our knowledge) previously unrecognized role of MAVS in controlling the metabolic fitness of B cells, most noticeable in the absence of costimulatory help.

Plasmodium curtails autoimmune nephritis via lasting bone marrow alterations, independent of hemozoin accumulation.

The host response against infection with Plasmodium commonly raises self-reactivity as a side effect, and antibody deposition in kidney has been cited as a possible cause of kidney injury during severe malaria. In contrast, animal models show that infection with the parasite confers long-term protection from lethal lupus nephritis initiated by autoantibody deposition in kidney. We have limited knowledge of the factors that make parasite infection more likely to induce kidney damage in humans, or the mechanisms underlying protection from autoimmune nephritis in animal models. Our experiments with the autoimmune-prone FcγR2B[KO] mice have shown that a prior infection with P. yoelii 17XNL protects from end-stage nephritis for a year, even when overall autoreactivity and systemic inflammation are maintained at high levels. In this report we evaluate post-infection alterations, such as hemozoin accumulation and compensatory changes in immune cells, and their potential role in the kidney-specific protective effect by Plasmodium. We ruled out the role of pigment accumulation with the use of a hemozoin-restricted P. berghei ANKA parasite, which induced a self-resolved infection that protected from autoimmune nephritis with the same mechanism as parasitic infections that accumulated normal levels of hemozoin. In contrast, adoptive transfer experiments revealed that bone marrow cells were altered by the infection and could transmit the kidney protective effect to a new host. While changes in the frequency of bone marrow cell populations after infection were variable and unique to a particular parasite strain, we detected a sustained bias in cytokine/chemokine expression that suggested lower fibrotic potential and higher Th1 bias likely affecting multiple cell populations. Sustained changes in bone marrow cell activation profile could have repercussions in immune responses long after the infection was cleared.

Dynamic chromatin accessibility licenses STAT5- and STAT6-dependent innate-like function of TH9 cells to promote allergic inflammation.

Allergic diseases are a major global health issue. Interleukin (IL)-9-producing helper T (TH9) cells promote allergic inflammation, yet TH9 cell effector functions are incompletely understood because their lineage instability makes them challenging to study. Here we found that resting TH9 cells produced IL-9 independently of T cell receptor (TCR) restimulation, due to STAT5- and STAT6-dependent bystander activation. This mechanism was seen in circulating cells from allergic patients and was restricted to recently activated cells. STAT5-dependent Il9/IL9 regulatory elements underwent remodeling over time, inactivating the locus. A broader 'allergic TH9' transcriptomic and epigenomic program was also unstable. In vivo, TH9 cells induced airway inflammation via TCR-independent, STAT-dependent mechanisms. In allergic patients, TH9 cell expansion was associated with responsiveness to JAK inhibitors. These findings suggest that TH9 cell instability is a negative checkpoint on bystander activation that breaks down in allergy and that JAK inhibitors should be considered for allergic patients with TH9 cell expansion.

Cerebral Malaria Is Regulated by Host-Mediated Changes in Plasmodium Gene Expression.

Cerebral malaria (CM), the deadliest complication of Plasmodium infection, is a complex and unpredictable disease. However, our understanding of the host and parasite factors that cause CM is limited. Using a mouse model of CM, experimental CM (ECM), we performed a three-way comparison between ECM-susceptible C57BL/6 mice infected with ECM-causing Plasmodium ANKA parasites [ANKA(C57BL/6)], ECM-resistant BALB/c mice infected with Plasmodium ANKA [ANKA(BALB/c)], and C57BL/6 mice infected with Plasmodium NK65 that does not cause ECM [NK65(C57BL/6)]. All ANKA(C57BL/6) mice developed CM. In contrast, in ANKA(BALB/c) and NK65(C57BL/6), infections do not result in CM and proceed similarly in terms of parasite growth, disease course, and host immune response. However, parasite gene expression in ANKA(BALB/c) was remarkably different than that in ANKA(C57BL/6) but similar to the gene expression in NK65(C57BL/6). Thus, Plasmodium ANKA has an ECM-specific gene expression profile that is activated only in susceptible hosts, providing evidence that the host has a critical influence on the outcome of infection. IMPORTANCE Hundreds of thousands of lives are lost each year due to the brain damage caused by malaria disease. The overwhelming majority of these deaths occur in young children living in sub-Saharan Africa. Thus far, there are no vaccines against this deadly disease, and we still do not know why fatal brain damage occurs in some children while others have milder, self-limiting disease progression. Our research provides an important clue to this problem. Here, we showed that the genetic background of the host has an important role in determining the course and the outcome of the disease. Our research also identified parasite molecules that can potentially be targeted in vaccination and therapy approaches.

CCL17-producing cDC2s are essential in end-stage lupus nephritis and averted by a parasitic infection.

Lupus nephritis is a severe organ manifestation in systemic lupus erythematosus leading to kidney failure in a subset of patients. In lupus-prone mice, controlled infection with Plasmodium parasites protects against the progression of autoimmune pathology including lethal glomerulonephritis. Here, we demonstrate that parasite-induced protection was not due to a systemic effect of infection on autoimmunity as previously assumed, but rather to specific alterations in immune cell infiltrates into kidneys and renal draining lymph nodes. Infection of lupus-prone mice with a Plasmodium parasite did not reduce the levels or specificities of autoreactive antibodies, vasculitis, immune complex-induced innate activation, or hypoxia. Instead, infection uniquely reduced kidney-infiltrating CCL17-producing bone marrow-derived type 2 inflammatory dendritic cells (iDC2s). Bone marrow reconstitution experiments revealed that infection with Plasmodium caused alterations in bone marrow cells that hindered the ability of DC2s to infiltrate the kidneys. The essential role for CCL17 in lupus nephritis was confirmed by in vivo depletion with a blocking antibody, which reduced kidney pathology and immune infiltrates, while bypassing the need for parasitic infection. Therefore, infiltration into the kidneys of iDC2s, with the potential to prime local adaptive responses, is an essential regulated event in the transition from manageable glomerulonephritis to lethal tubular injury.

RTP4 inhibits IFN-I response and enhances experimental cerebral malaria and neuropathology.

Infection by malaria parasites triggers dynamic immune responses leading to diverse symptoms and pathologies; however, the molecular mechanisms responsible for these reactions are largely unknown. We performed Trans-species Expression Quantitative Trait Locus analysis to identify a large number of host genes that respond to malaria parasite infections. Here we functionally characterize one of the host genes called receptor transporter protein 4 (RTP4) in responses to malaria parasite and virus infections. RTP4 is induced by type I IFN (IFN-I) and binds to the TANK-binding kinase (TBK1) complex where it negatively regulates TBK1 signaling by interfering with expression and phosphorylation of both TBK1 and IFN regulatory factor 3. Rtp4-/- mice were generated and infected with malaria parasite Plasmodiun berghei ANKA. Significantly higher levels of IFN-I response in microglia, lower parasitemia, fewer neurologic symptoms, and better survival rates were observed in Rtp4-/- than in wild-type mice. Similarly, RTP4 deficiency significantly reduced West Nile virus titers in the brain, but not in the heart and the spleen, of infected mice, suggesting a specific role for RTP4 in brain infection and pathology. This study reveals functions of RTP4 in IFN-I response and a potential target for therapy in diseases with neuropathology.

The E3 ubiquitin ligase MARCH1 regulates antimalaria immunity through interferon signaling and T cell activation.

Malaria infection induces complex and diverse immune responses. To elucidate the mechanisms underlying host-parasite interaction, we performed a genetic screen during early (24 h) Plasmodium yoelii infection in mice and identified a large number of interacting host and parasite genes/loci after transspecies expression quantitative trait locus (Ts-eQTL) analysis. We next investigated a host E3 ubiquitin ligase gene (March1) that was clustered with interferon (IFN)-stimulated genes (ISGs) based on the similarity of the genome-wide pattern of logarithm of the odds (LOD) scores (GPLS). March1 inhibits MAVS/STING/TRIF-induced type I IFN (IFN-I) signaling in vitro and in vivo. However, in malaria-infected hosts, deficiency of March1 reduces IFN-I production by activating inhibitors such as SOCS1, USP18, and TRIM24 and by altering immune cell populations. March1 deficiency increases CD86+DC (dendritic cell) populations and levels of IFN-γ and interleukin 10 (IL-10) at day 4 post infection, leading to improved host survival. T cell depletion reduces IFN-γ level and reverse the protective effects of March1 deficiency, which can also be achieved by antibody neutralization of IFN-γ. This study reveals functions of MARCH1 (membrane-associated ring-CH-type finger 1) in innate immune responses and provides potential avenues for activating antimalaria immunity and enhancing vaccine efficacy.

A single-nucleotide polymorphism in a Plasmodium berghei ApiAP2 transcription factor alters the development of host immunity.

The acquisition of malaria immunity is both remarkably slow and unpredictable. At present, we know little about the malaria parasite genes that influence the host's ability to mount a protective immune response. Here, we show that a single-nucleotide polymorphism (SNP) resulting in a single amino acid change (S to F) in an ApiAP2 transcription factor in the rodent malaria parasite Plasmodium berghei (Pb) NK65 allowed infected mice to mount a T helper cell 1 (TH1)-type immune response that controlled subsequent infections. As compared to PbNK65S, PbNK65F parasites differentially expressed 46 genes, most of which are predicted to play roles in immune evasion. PbNK65F infections resulted in an early interferon-γ response and a later expansion of germinal centers, resulting in high levels of infected red blood cell-specific TH1-type immunoglobulin G2b (IgG2b) and IgG2c antibodies. Thus, the Pb ApiAP2 transcription factor functions as a critical parasite virulence factor in malaria infections.

Plasmodium yoelii Erythrocyte-Binding-like Protein Modulates Host Cell Membrane Structure, Immunity, and Disease Severity.

Erythrocyte-binding-like (EBL) proteins are known to play an important role in malaria parasite invasion of red blood cells (RBCs); however, any roles of EBL proteins in regulating host immune responses remain unknown. Here, we show that Plasmodium yoelii EBL (PyEBL) can shape disease severity by modulating the surface structure of infected RBCs (iRBCs) and host immune responses. We identified an amino acid substitution (a change of C to Y at position 741 [C741Y]) in the protein trafficking domain of PyEBL between isogenic P. yoelliinigeriensis strain N67 and N67C parasites that produce different disease phenotypes in C57BL/6 mice. Exchanges of the C741Y alleles altered parasite growth and host survival accordingly. The C741Y substitution also changed protein processing and trafficking in merozoites and in the cytoplasm of iRBCs, reduced PyEBL binding to band 3, increased phosphatidylserine (PS) surface exposure, and elevated the osmotic fragility of iRBCs, but it did not affect invasion of RBCs in vitro The modified iRBC surface triggered PS-CD36-mediated phagocytosis of iRBCs, host type I interferon (IFN-I) signaling, and T cell differentiation, leading to improved host survival. This study reveals a previously unknown role of PyEBL in regulating host-pathogen interaction and innate immune responses, which may be explored for developing disease control strategies.IMPORTANCE Malaria is a deadly parasitic disease that continues to afflict hundreds of millions of people every year. Infections with malaria parasites can be asymptomatic, with mild symptoms, or fatal, depending on a delicate balance of host immune responses. Malaria parasites enter host red blood cells (RBCs) through interactions between parasite ligands and host receptors, such as erythrocyte-binding-like (EBL) proteins and host Duffy antigen receptor for chemokines (DARC). Plasmodium yoelii EBL (PyEBL) is known to play a role in parasite invasion of RBCs. Here, we show that PyEBL also affects disease severity through modulation of host immune responses, particularly type I interferon (IFN-I) signaling. This discovery assigns a new function to PyEBL and provides a mechanism for developing disease control strategies.

Soft-Tissue Chondroma in the Right Hallux: A Case Report.

Soft-tissue chondroma is a rare, benign tumor. It is predominantly found in the hands and feet, but rarely in the toes. In this article, we report a digital soft-tissue chondroma that presented as a painful nodule of 5 years' duration in a 67-year-old man. Physical examination revealed a round, solid, movable nodule measuring 7 mm in diameter. Radiographs showed faint linear calcifications in the nodule under the right hallux proximal phalanx neck. The mass was completely excised, and pathologic observation revealed a mass composed of mature chondrocytes in a cartilaginous matrix, consistent with a chondroma. Even though this is a benign tumor, it needs to be differentiated from other tumors, including schwannoma, leiomyoma, chondrosarcoma, and others. Surgical excision is the preferred treatment.

Antiviral Adaptor MAVS Promotes Murine Lupus With a B Cell Autonomous Role.

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by increased production of autoantibodies, which commonly target nuclear antigens, and concomitant deposition of immune complexes that cause inflammation in tissues. SLE is often associated with increased systemic expression of type I interferons, in some cases due to dysregulation in nucleic acid-sensing innate pathways. There is strong genetic evidence for a link between cytoplasmic RNA sensing pathways (RIG-I/MDA5) and SLE, both in human patients and murine models, however questions still remain regarding pathway initiation, cell types involved and downstream effects. Here we show that MAVS, an essential adaptor for RIG-I/MDA5 signaling, is necessary for all symptoms of autoimmune disease that develop spontaneously in the lupus model FcγRIIB-/- mice. This effect was independent of type I interferon signaling, TLR7 expression or STING, all three factors that have been connected to autoimmunity. Mixed bone marrow reconstitution experiments showed reduced occurrence in autoimmune germinal centers and diminished autoantibody production by MAVS-deficient B cells. Thus, MAVS plays a B cell intrinsic role in autoreactive B cell activation that is independent of its anti-viral functions and independent of elevated type I interferon expression.

Schwannoma of Foot and Ankle: Seven Case Reports and Literature Review.

BACKGROUND: Schwannoma is most often grown on the trunk, upper and lower extremities, and head and neck, but rarely on the foot. This study aimed to reveal clinical presentations, histopathology and treatment options for schwannoma of the foot. MATERIALS AND METHODS: Seven schwannomas out of 174 soft-tissue tumors on the foot and ankle were retrieved from our Institute in a 3-year period, and 42 schwannomas on the foot and ankle in the literature in a 30-year period were reviewed. RESULTS: The incidence of schwannoma of foot was found to be 4.0%. The patient age ranged from 8 to 84 years, with a mean of 47.4 years. More than 80% of tumors were located on the ankle, heel and plantar aspect. Overall, 77.6% of patients complained about a painful mass. Magnetic resonance imaging revealed a well-circumscribed, round or ovoid mass with iso-intensity signal compared with surrounding neuromuscular tissues on T1-weighted images and hyper-intensity signal on T2. Forty-eight out of 49 patients were treated with surgical excision or enucleation without recurrence in follow-up from 2 months to 4 years. Histologically, schwannoma was composed of hypercellular Antoni A zone with palisaded spindle cells with strong immunostaining for S-100 and hypocellular Antoni B zone with vascularization in myxoid stroma. CONCLUSION: Schwannoma of the foot and ankle is a rare, painful, indurated tumor. Magnetic resonance imaging reveals the location, size, texture and relationships with surrounding neuromuscular structures. Surgical excision is the primary treatment option with excellent outcome.

Interleukin-1ß-induced IRAK1 ubiquitination is required for TH-GM-CSF cell differentiation in T cell-mediated inflammation.

Accumulating evidence suggests granulocyte macrophage-colony stimulating factor (GM-CSF) can function as an inflammatory mediator, but whether GM-CSF-producing CD4+ T cells (TH-GM-CSF) are a distinct T helper cell subset is lacking. Herein we demonstrate that interleukin (IL)-1ß exclusively drives differentiation of naïve CD4+ T cells into TH-GM-CSF cells via inducing ubiquitination of IL-1 receptor-associated kinase 1 (IRAK1) and subsequent activation of the transcription factor NF-kappaB (NF-κB), independent of RAR-related orphan receptor gamma (RORγt) required for TH17 differentiation. In vivo, TH-GM-CSF cells are present in murine Citrobacter Rodentium infections and mediate colitis following adoptive transfer of CD4+ T cells into Rag1-/- mice via GM-CSF-induced macrophage activation. The TH-GM-CSF cell phenotype is stable and distinct from the TH17 genetic program, but IL-1ß can convert pre-formed TH17 cells into TH-GM-CSF cells, thereby accounting for previously reported associations between IL-17 and GM-CSF. Together, our results newly identify IL-1ß/NF-κB-dependent TH-GM-CSF cells as a unique T helper cell subset and highlight the importance of CD4+ T cell-derived GM-CSF induced macrophage activation as a previously undescribed T cell effector mechanism.

Transcription factors IRF8 and PU.1 are required for follicular B cell development and BCL6-driven germinal center responses.

The IRF and Ets families of transcription factors regulate the expression of a range of genes involved in immune cell development and function. However, the understanding of the molecular mechanisms of each family member has been limited due to their redundancy and broad effects on multiple lineages of cells. Here, we report that double deletion of floxed Irf8 and Spi1 (encoding PU.1) by Mb1-Cre (designated DKO mice) in the B cell lineage resulted in severe defects in the development of follicular and germinal center (GC) B cells. Class-switch recombination and antibody affinity maturation were also compromised in DKO mice. RNA-seq (sequencing) and ChIP-seq analyses revealed distinct IRF8 and PU.1 target genes in follicular and activated B cells. DKO B cells had diminished expression of target genes vital for maintaining follicular B cell identity and GC development. Moreover, our findings reveal that expression of B-cell lymphoma protein 6 (BCL6), which is critical for development of germinal center B cells, is dependent on IRF8 and PU.1 in vivo, providing a mechanism for the critical role for IRF8 and PU.1 in the development of GC B cells.

T follicular helper cells restricted by IRF8 contribute to T cell-mediated inflammation.

The follicular helper T cell (TFH) are established regulators of germinal center (GC) B cells, whether TFH have pathogenic potential independent of B cells is unknown. Based on in vitro TFH cell differentiation, in vivo T cell transfer animal colitis model, and intestinal tissues of inflammatory bowel disease (IBD) patients, TFH and its functions in colitis development were analyzed by FACS, ChIP, ChIP-sequencing, WB, ELISA and PCR. Herein we demonstrate that intestinal tissues of patients and colon tissues obtained from Rag1-/- recipients of naïve CD4+ T cells with colitis, each over-express TFH-associated gene products. Adoptive transfer of naïve Bcl6-/- CD4+ T cells into Rag1-/- recipient mice abrogated development of colitis and limited TFH differentiation in vivo, demonstrating a mechanistic link. In contrast, T cell deficiency of interferon regulatory factor 8 (IRF8) resulted in augmentation of TFH induction in vitro and in vivo. Functional studies showed that adoptive transfer of IRF8 deficient CD4+ T cells into Rag1-/- recipients exacerbated colitis development associated with increased gut TFH-related gene expression, while Irf8-/-/Bcl6-/- CD4+ T cells abrogated colitis, together indicating that IRF8-regulated TFH can directly cause colon inflammation. Molecular analyses revealed that IRF8 suppresses TFH differentiation by inhibiting transcription and transactivation of the TF IRF4, which is also known to be essential for TFH induction. Our documentation showed that IRF8-regulated TFH can function as B-cell-independent, pathogenic, mediators of colitis suggests that targeting TFH could be effective for treatment of IBD.

Duox1 Regulates Primary B Cell Function under the Influence of IL-4 through BCR-Mediated Generation of Hydrogen Peroxide.

Engagement of the BCR with Ags triggers signaling pathways for commitment of B lymphocyte responses that can be regulated, in part, by reactive oxygen species. To investigate the functional relevance of reactive oxygen species produced in primary B cells, we focused on the role of the hydrogen peroxide generator Duox1 in stimulated splenic B cells under the influence of the TH2 cytokine IL-4. We found that H2O2 production in wild type (WT) and Nox2-deficient CD19+ B cells was boosted concomitantly with enhanced expression of Duox1 following costimulation with BCR agonists together with IL-4, whereas stimulated Duox1-/- cells showed attenuated H2O2 release. We examined whether Duox1-derived H2O2 contributes to proliferative activity and Ig isotype production in CD19+ cells upon BCR stimulation. Duox1-/- CD19+ B cells showed normal responses of Ig production but a higher rate of proliferation than WT or Nox2-deficient cells. Furthermore, we demonstrated that the H2O2 scavenger catalase mimics the effect of Duox1 deficiency by enhancing proliferation of WT CD19+ B cells in vitro. Results from immunized mice reflected the in vitro observations: T cell-independent Ag induced increased B cell expansion in germinal centers from Duox1-/- mice relative to WT and Nox2-/- mice, whereas immunization with T cell-dependent or -independent Ag elicited normal Ig isotype secretion in the Duox1 mutant mice. These observations, obtained both by in vitro and in vivo approaches, strongly suggest that Duox1-derived hydrogen peroxide negatively regulates proliferative activity but not Ig isotype production in primary splenic CD19+ B cells.

3'Igh enhancers hs3b/hs4 are dispensable for Myc deregulation in mouse plasmacytomas with T(12;15) translocations.

Myc-deregulating T(12;15) chromosomal translocations are the hallmark cytogenetic abnormalities of murine plasmacytomas (PCTs). In most PCTs, the immunoglobulin heavy chain (Igh) locus is broken between the Eµ enhancer and the 3' regulatory region (3'RR), making the latter the major candidate for orchestrating Myc deregulation. To elucidate the role of the Igh3'RR in tumorigenesis, we induced PCTs in Bcl-xL-transgenic mice deficient for the major Igh3'RR enhancer elements, hs3b and hs4 (hs3b-4-/-). Contrary to previous observations using a mouse lymphoma model, which showed no tumors with peripheral B-cell phenotype in hs3b-4-/- mice, these animals developed T(12;15)-positive PCTs, although with a lower incidence than hs3b-4+/+ (wild-type, WT) controls. In heterozygous hs3b-4+/- mice there was no allelic bias in targeting Igh for T(12;15). Molecular analyses of Igh/Myc junctions revealed dominance of Sµ region breakpoints versus the prevalence of Sγ or Sα in WT controls. Myc expression and Ig secretion in hs3b-4-/- PCTs did not differ from WT controls. We also evaluated the effect of a complete Igh3'RR deletion on Myc expression in the context of an established Igh/Myc translocation in ARS/Igh11-transgenic PCT cell lines. Cre-mediated deletion of the Igh3'RR resulted in gradual reduction of Myc expression, loss of proliferative activity and increased cell death, confirming the necessity of the Igh3'RR for Myc deregulation by T(12;15).