B-cell apoptosis induced by antigen receptor crosslinking is blocked by a T-cell signal through CD40

Developments in kidney xenotransplantation

The search for kidney xenografts that are appropriate for patients with end-stage renal disease has been ongoing since the beginning of the last century. The major cause of xenograft loss is hyperacute and acute rejection, and this has almost been overcome via scientific progress. The success of two pre-clinical trials of α1,3-galactosyltransferase gene-knockout porcine kidneys in brain-dead patients in 2021 triggered research enthusiasm for kidney xenotransplantation. This minireview summarizes key issues from an immunological perspective: the discovery of key xenoantigens, investigations into key co-stimulatory signal inhibition, gene-editing technology, and immune tolerance induction. Further developments in immunology, particularly immunometabolism, might help promote the long-term outcomes of kidney xenografts.

NF-kB's contribution to B cell fate decisions

NF-κB signaling is essential to an effective innate and adaptive immune response. Many immune-specific functional and developmental outcomes depend in large on NF-κB. The formidable task of sorting out the mechanisms behind the regulation and outcome of NF-κB signaling remains an important area of immunology research. Here we briefly discuss the role of NF-κB in regulating cell fate decisions at various times in the path of B cell development, activation, and the generation of long-term humoral immunity.

FcγRs and Their Relevance for the Activity of Anti-CD40 Antibodies

Inhibitory targeting of the CD40L-CD40 system is a promising therapeutic option in the field of organ transplantation and is also attractive in the treatment of autoimmune diseases. After early complex results with neutralizing CD40L antibodies, it turned out that lack of Fcγ receptor (FcγR)-binding is the crucial factor for the development of safe inhibitory antibodies targeting CD40L or CD40. Indeed, in recent years, blocking CD40 antibodies not interacting with FcγRs, has proven to be well tolerated in clinical studies and has shown initial clinical efficacy. Stimulation of CD40 is also of considerable therapeutic interest, especially in cancer immunotherapy. CD40 can be robustly activated by genetically engineered variants of soluble CD40L but also by anti-CD40 antibodies. However, the development of CD40L-based agonists is biotechnologically and pharmacokinetically challenging, and anti-CD40 antibodies typically display only strong agonism in complex with FcγRs or upon secondary crosslinking. The latter, however, typically results in poorly developable mixtures of molecule species of varying stoichiometry and FcγR-binding by anti-CD40 antibodies can elicit unwanted side effects such as antibody-dependent cellular cytotoxicity (ADCC) or antibody-dependent cellular phagocytosis (ADCP) of CD40 expressing immune cells. Here, we summarize and compare strategies to overcome the unwanted target cell-destroying activity of anti-CD40-FcγR complexes, especially the use of FcγR type-specific mutants and the FcγR-independent cell surface anchoring of bispecific anti-CD40 fusion proteins. Especially, we discuss the therapeutic potential of these strategies in view of the emerging evidence for the dose-limiting activities of systemic CD40 engagement.

Deconvolution of B cell receptor repertoire in multiple sclerosis patients revealed a delay in tBreg maturation

Background

B lymphocytes play a pivotal regulatory role in the development of the immune response. It was previously shown that deficiency in B regulatory cells (Bregs) or a decrease in their anti-inflammatory activity can lead to immunological dysfunctions. However, the exact mechanisms of Bregs development and functioning are only partially resolved. For instance, only a little is known about the structure of their B cell receptor (BCR) repertoires in autoimmune disorders, including multiple sclerosis (MS), a severe neuroinflammatory disease with a yet unknown etiology. Here, we elucidate specific properties of B regulatory cells in MS.

Methods

We performed a prospective study of the transitional Breg (tBreg) subpopulations with the CD19+CD24highCD38high phenotype from MS patients and healthy donors by (i) measuring their content during two diverging courses of relapsing-remitting MS: benign multiple sclerosis (BMS) and highly active multiple sclerosis (HAMS); (ii) analyzing BCR repertoires of circulating B cells by high-throughput sequencing; and (iii) measuring the percentage of CD27+ cells in tBregs.

Results

The tBregs from HAMS patients carry the heavy chain with a lower amount of hypermutations than tBregs from healthy donors. The percentage of transitional CD24highCD38high B cells is elevated, whereas the frequency of differentiated CD27+ cells in this transitional B cell subset was decreased in the MS patients as compared with healthy donors.

Conclusions

Impaired maturation of regulatory B cells is associated with MS progression.

Analysis of a miR-148a Targetome in B Cell Central Tolerance

A microRNA (miRNA) often regulates the expression of hundreds of target genes. A fundamental question in the field of miRNA research is whether a miRNA exerts its biological function through regulating a small number of key targets or through small changes in the expression of hundreds of target genes. We addressed this issue by performing functional analysis of target genes regulated by miR-148a. We previously identified miR-148a as a critical regulator of B cell central tolerance and found 119 target genes that may mediate its function. We selected 4 of them for validation and demonstrated a regulatory role for Bim, Pten, and Gadd45a in this process. In this study, we performed functional analysis of the other miR-148a target genes in in vitro and in vivo models of B cell central tolerance. Our results show that those additional target genes play a minimal role, if any, in miR-148a-mediated control of B cell central tolerance, suggesting that the function of miRNAs is mediated by a few key target genes. These findings have advanced our understanding of molecular mechanisms underlying miRNA regulation of gene expression and B cell central tolerance.

Role of inhibitory B cell co-receptors in B cell self-tolerance to non-protein antigens

Antibodies to non-protein antigens such as nucleic acids, polysaccharides, and glycolipids play important roles in both host defense against microbes and development of autoimmune diseases. Although non-protein antigens are not recognized by T cells, antibody production to non-protein antigens involve T cell-independent mechanisms such as signaling through TLR7 and TLR9 in antibody production to nucleic acids. Although self-reactive B cells are tolerized by various mechanisms including deletion, anergy, and receptor editing, T cell tolerance is also crucial in self-tolerance of B cells to protein self-antigen because self-reactive T cells induce autoantibody production to these self-antigens. However, presence of T cell-independent mechanism suggests that T cell tolerance is not able to maintain B cell tolerance to non-protein self-antigens. Lines of evidence suggest that B cell response to non-protein self-antigens such as nucleic acids and gangliosides, sialic acid-containing glycolipids, are suppressed by inhibitory B cell co-receptors CD72 and Siglec-G, respectively. These inhibitory co-receptors recognize non-protein self-antigens and suppress BCR signaling induced by these antigens, thereby inhibiting B cell response to these self-antigens. Inhibitory B cell co-receptors appear to be involved in B cell self-tolerance to non-protein self-antigens that can activate B cells by T cell-independent mechanisms.

CD24 and IgM Stimulation of B Cells Triggers Transfer of Functional B Cell Receptor to B Cell Recipients Via Extracellular Vesicles

Extracellular vesicles (EVs) are membrane-encapsulated nanoparticles that carry bioactive cargo, including proteins, lipids, and nucleic acids. Once taken up by target cells, EVs can modify the physiology of the recipient cells. In past studies, we reported that engagement of the glycophosphatidylinositol-anchored receptor CD24 on B lymphocytes (B cells) causes the release of EVs. However, a potential function for these EVs was not clear. Thus, we investigated whether EVs derived from CD24 or IgM-stimulated donor WEHI-231 murine B cells can transfer functional cargo to recipient cells. We employed a model system where donor cells expressing palmitoylated GFP (WEHI-231-GFP) were cocultured, after stimulation, with recipient cells lacking either IgM (WEHI-303 murine B cells) or CD24 (CD24 knockout mouse bone marrow B cells). Uptake of lipid-associated GFP, IgM, or CD24 by labeled recipient cells was analyzed by flow cytometry. We found that stimulation of either CD24 or IgM on the donor cells caused the transfer of lipids, CD24, and IgM to recipient cells. Importantly, we found that the transferred receptors are functional in recipient cells, thus endowing recipient cells with a second BCR or sensitivity to anti-CD24-induced apoptosis. In the case of the BCR, we found that EVs were conclusively involved in this transfer, whereas in the case in the CD24 the involvement of EVs is suggested. Overall, these data show that extracellular signals received by one cell can change the sensitivity of neighboring cells to the same or different stimuli, which may impact B cell development or activation.

Intrinsic and extrinsic regulation of IgE B cell responses

Stringent regulation of IgE antibody production is critical for constraining allergic responses. This review discusses recent advances in understanding cell-intrinsic and extrinsic mechanisms that regulate the genesis and fate of IgE B cells. B cell-intrinsic regulation of IgE is orchestrated by the IgE B Cell Receptor (BCR). Through its antigen-independent signaling and low surface expression, the IgE BCR drives IgE B cells to differentiate into short-lived plasma cells and/or undergo apoptosis, restricting IgE-expressing cells from entering long-lived compartments. The pivotal extrinsic regulators of IgE responses are T follicular helper cells (T_FH). T_FH produce IL-4 and IL-21, which, respectively, are the major activating and inhibitory cytokines for IgE class-switching. Other newly identified T follicular subsets also contribute to IgE regulation. Although IgE responses are normally constrained, recent studies suggest that specific conditions can induce the formation of IgE responses with enhanced affinity or longevity, effectively 'breaking the rules' of IgE regulation.

Soluble CD40 ligand inhibits the growth of non-Hodgkin's lymphoma cells through the JNK signaling pathway

The incidence of non-Hodgkin's lymphoma (NHL) has been increasing annually and has become a serious threat to human health. However, the pathogenesis of NHL remains unclear. The present study aimed to investigate the effect of soluble CD40 ligand (sCD40L) on NHL cells and its underlying mechanism. Cell Counting kit-8 assay and flow cytometry apoptosis experiments were conducted to investigate the effects of sCD40L on cell proliferation and apoptosis. Western blotting was performed to detect the protein expression levels of BAX, Bcl-2, ERK, p-ERK, JNK, p-JNK, p38, p-p38 and c-JUN. The results of the present study demonstrated that exogenous sCD40L significantly inhibited the proliferation and promoted the apoptosis of Raji and CA46 cells. Additionally, exogenous sCD40L promoted the apoptosis of lymphoma cells by activating the JNK signaling pathway.

Determination of membrane capacitance and cytoplasm conductivity by simultaneous electrorotation

Membrane capacitances and cytoplasm conductivities of hematopoietic cells were investigated by simultaneous electrorotation (ROT) systems of multiple cells. Simultaneous ROT was achieved by the rotation of electric fields in grid arrays formed with three-dimensional interdigitated array (3D-IDA) electrodes that can be easily fabricated using two substrates with IDA electrodes. When AC signals were applied to four microband electrodes with a 90° phase difference to each electrode, cells dispersed randomly in the 3D-IDA device started to rotate and moved to the center of each grid. Multiple cells were simultaneously rotated at the center of grids without friction from contact with other cells and substrates. The averages and variance of ROT rates of cells at each frequency can be measured during a single operation of the device within 5 min, resulting in the acquisition of ROT spectra. Membrane capacitances and cytoplasm conductivities of hematopoietic cells (K562 cells, Jurkat cells, and THP-1 cells) were determined by fitting ROT spectra obtained experimentally to the curves calculated theoretically. The values determined by using the simultaneous ROT systems well coincided with the values reported previously. The membrane capacitances and cytoplasm conductivities of WEHI-231 cells were firstly determined to be 8.89 ± 0.25 mF m^-2 and 0.28 ± 0.03 S m^-1, respectively. Furthermore, the difference of the ROT rates based on the difference of the electric properties of cells was applied to discriminate the types of cells. The acquisition of rotation rates of multiple cells within a single operation makes the statistical analysis extremely profitable for determining the electrical properties of cells.

The Use of Anti-CD40 mAb in Cancer

Immunomodulatory monoclonal antibody (mAb) therapy is at the forefront of developing cancer therapeutics with numerous targeted agents proving highly effective in selective patients at stimulating protective host immunity, capable of eradicating established tumours and leading to long-term disease-free states. The cell surface marker CD40 is expressed on a range of immune cells and transformed cells in malignant states whose signalling plays a critical role in modulating adaptive immune responses. Anti-CD40 mAb therapy acts via multiple mechanisms to stimulate anti-tumour immunity across a broad range of lymphoid and solid malignancies. A wealth of preclinical research in this field has led to the successful development of multiple anti-CD40 mAb agents that have shown promise in early-phase clinical trials. Significant progress has been made to enhance the engagement of antibodies with immune effectors through their interactions with Fcγ receptors (FcγRs) by the process of Fc engineering. As more is understood about how to best optimise these agents, principally through the fine-tuning of mAb structure and choice of synergistic partnerships, our ability to generate robust, clinically beneficial anti-tumour activity will form the foundation for the next generation of cancer therapeutics.

The B cell novel protein 1 (BCNP1) regulates BCR signaling and B cell apoptosis

The BCR plays a central role in B cell development, survival, activation, and differentiation. We have identified the B cell novel protein 1 (BCNP1) as a new regulator of BCR signaling. BCNP1 contains a pleckstrin homology domain, three proline-rich motifs, and a potential SH2 binding site, and is predominantly expressed by B cells. We found that BCNP1 overexpression in WEHI231 immature B cells potentiated α-IgM-induced apoptosis. Conversely, BCNP1-deficient WEHI231 cells, generated by CRISPR-Cas9-mediated genome editing, exhibited reduced apoptosis after BCR crosslinking. Biochemical analyses revealed that BCNP1 physically interacted with the B cell linker protein (BLNK), Grb2, and PLCγ2. Moreover, absence of BCNP1 resulted in accelerated dephosphorylation of BLNK, reduced phosphorylation of SYK and PLCγ2, and decreased Ca²⁺ influx after BCR crosslinking. These results demonstrate that BCNP1 promotes BCR signaling by modulating the phosphorylation of BLNK, SYK, and PLCγ2.

Ectonucleotidase Expression on Human Amnion Epithelial Cells: Adenosinergic Pathways and Dichotomic Effects on Immune Effector Cell Populations

This study investigates the mechanism(s) underlying the immunoregulatory activities of placenta-derived human amnion epithelial cells (hAEC). The working hypothesis is that NAD⁺ and ATP, along with ectoenzymes involved in their metabolism, play a significant role in hAEC-mediated immune regulation. Proof of principle of the hypothesis was obtained by analyzing the interactions between hAEC and the main human leukocyte populations. The results obtained indicate that hAEC constitutively express a unique combination of functional ectoenzymes, driving the production of adenosine (ADO) via canonical (CD39, CD73) and alternative (CD38, CD203a/PC-1, CD73) pathways. Further, the picture is completed by the observation that hAEC express A1, A2a, and A2b ADO receptors as well as ADO deaminase, the enzyme involved in ADO catabolism. The contribution of the purinergic mediator to immunomodulation was confirmed by exposing in vitro different immune effector cells to the action of primary hAECs. B cells showed an enhanced proliferation and diminished spontaneous apoptosis when in contact with hAEC. T cell proliferation was partially inhibited by hAEC through ADO production, as confirmed by using specific ectoenzyme inhibitors. Further, hAEC induced an expansion of both T and B regulatory cells. Last, hAEC inhibited NK cell proliferation. However, the involvement of ADO-producing ectoenzymes is less apparent in this context. In conclusion, hAEC exert different in vitro immunoregulatory effects, per se, as a result of interactions with different populations of immune effector cells. These results support the view that hAEC are instrumental for regenerative medicine as well as in therapeutic applications for immune-related diseases.

Negative regulation of B cell responses and self-tolerance to RNA-related lupus self-antigen

The antibody response to RNA-related antigens such as Sm/RNP requires the endosomal RNA sensor TLR7, and this process is crucial in the development of systemic lupus erythematosus at least in animal models. The inhibitory B cell receptor CD72 is unique because it recognizes Sm/RNP and specifically inhibits the activation of Sm/RNP-reactive B cells by activating SH2-containing protein tyrosine phosphatase 1 (SHP-1). In the normal immune system, Sm/RNP-reactive B cells are tolerized by a unique mechanism that probably involves SHP-1. These self-reactive B cells appear in the peripheral lymphoid organs, differentiate into marginal zone B cells, and then undergo apoptosis without further maturation into plasma cells. Thus, CD72 is involved in the suppression of TLR7-mediated response to RNA in complexes with nuclear proteins that are resistant to nucleases, whereas free RNAs are degraded by nucleases before they encounter the endosomal RNA sensor.

Increased CD40 Ligation and Reduced BCR Signalling Leads to Higher IL-10 Production in B Cells From Tolerant Kidney Transplant Patients

BACKGROUND

An increased percentage of peripheral transitional B cells producing IL-10 has been observed in patients tolerant to kidney allografts. In healthy volunteers, the balance between the CD40 and B-cell receptor (BCR) signalling modulated IL-10 production by B cells, with stimulation via the BCR decreasing CD40-mediated IL-10 production. In this study, we evaluate whether in tolerant kidney transplant patients, the increased IL-10 production by B cells was due to an altered CD40 and/or BCR signalling.

METHODS

B cells obtained from a new cohort of tolerant renal transplant recipients and those from age- and sex-matched healthy volunteers were activated via CD40 and BCR, either alone or in combination.

RESULTS

In tolerant patients, we observed higher percentages of B cells producing IL-10 after CD40 ligation and higher expression of CD40L on activated T cells compared with healthy controls. Furthermore, B cells from tolerant recipients had reduced extracellular signal-regulated kinase signalling after BCR-mediated activation compared with healthy controls. In keeping with this, combining BCR signalling with CD40 ligation did not reduce IL-10 secretion as was observed in healthy control transitional B cells.

CONCLUSIONS

Altogether, our data suggest that the altered response of B cells in tolerant recipients may contribute to long-term stable graft acceptance.

Reduced human transitional B cell T1/T2 ratio is associated with subsequent deterioration in renal allograft function

Human transitional B cells express relatively high IL-10 and low TNF-α levels, which correlate with B regulatory activity in vitro. Herein, we aim to further define B regulatory phenotype and determine whether B regulatory activity can serve as a prognostic marker for renal allograft dysfunction (graft loss or 2-fold fall in estimated glomerular filtration rate). Transitional B cells can be divided into T1 and T2 subsets based on surface phenotype. T1 cells express a significantly higher ratio of IL-10 to TNF-α than T2 cells or other B subsets. When analyzed in 45 kidney transplant recipients at the time of late for-cause biopsy, the T1/T2 ratio was independently associated with allograft dysfunction over the next 5 years. Next, the T1/T2 ratio was examined in an independent set of 97 clinically stable kidney transplant recipients 2 years after transplant. Again, the T1/T2 ratio was strongly and independently associated with allograft dysfunction over the ensuing 5 years. In these clinically quiescent patients, a low T1/T2 ratio identified a 41-patient subgroup in which 35% developed allograft dysfunction, with 25% losing their allografts. However, none of the 56 patients with a high ratio developed graft dysfunction. In both the initial study and validation groups, the T1/T2 ratio was a much stronger predictor of graft dysfunction than donor-specific antibodies or the estimated glomerular filtration rate. Thus, the T1/T2 ratio, a relative measure of expressing an anti-inflammatory cytokine profile, is a novel prognostic marker that might inform individualized immunosuppression.

HAX1 deletion impairs BCR internalization and leads to delayed BCR-mediated apoptosis

Deletion of HAX1 in mice causes a severe reduction in the numbers of lymphocytes in the bone marrow and in the spleen. Additionally, B220(+) B progenitor cells in the bone marrow are reduced, suggesting an important function of HAX1 in B cell development. HAX1 is thought to play a protective role in apoptotic processes; therefore, we investigated the role of HAX1 in bone marrow B progenitor cells and splenic B cells. We did not observe an effect on the survival of Hax1(-/-) bone marrow cells but detected enhanced survival of splenic Hax1(-/-) B cells upon in vitro starvation/growth-factor withdrawal. To explain this apparent inconsistency with previous reports of HAX1 function, we also studied the B cell receptor (BCR)-induced apoptosis of IgM-stimulated splenic naïve B cells and found that apoptosis decreased in these cells. We further found impaired internalization of the BCR from Hax1(-/-) splenic B cells after IgM crosslinking; this impaired internalization may result in decreased BCR signaling and, consequently, decreased BCR-mediated apoptosis. We measured HAX1 binding to the cytoplasmic domains of different Ig subtypes and identified KVKWI(V)F as the putative binding motif for HAX1 within the cytoplasmic domains. Because this motif can be found in almost all Ig subtypes, it is likely that HAX1 plays a general role in BCR-mediated internalization events and BCR-mediated apoptosis.

The Role of EBV in the Pathogenesis of Diffuse Large B Cell Lymphoma

Epstein-Barr virus (EBV) infection is a common feature of B cell lymphoproliferative disorders (LPDs), including diffuse large B cell lymphoma. Approximately 10 % of DLBCLs are EBV-positive, with the highest incidence in immunocompromised and elderly patients. Here, we review the clinical, genetic, and pathologic characteristics of DLBCL and discuss the molecular role of EBV in lymphoma tumorigenesis. Using EBV-positive DLBCL of the elderly as a model, we describe the key features of EBV-positive DLBCL. Studies of EBV-positive DLBCL of the elderly demonstrate that EBV-positive DLBCL has a distinct biology, related to both viral and host factors. The pathogenic mechanisms noted in EBV-positive DLBCL of the elderly, including enhanced NFκB activity, are likely to be a generalizable feature of EBV-positive DLBCL. Therefore, we review how this information might be used to target the EBV or its host response for the development of novel treatment strategies.

Exome sequencing of lymphomas from three dog breeds reveals somatic mutation patterns reflecting genetic background

Lymphoma is the most common hematological malignancy in developed countries. Outcome is strongly determined by molecular subtype, reflecting a need for new and improved treatment options. Dogs spontaneously develop lymphoma, and the predisposition of certain breeds indicates genetic risk factors. Using the dog breed structure, we selected three lymphoma predisposed breeds developing primarily T-cell (boxer), primarily B-cell (cocker spaniel), and with equal distribution of B- and T-cell lymphoma (golden retriever), respectively. We investigated the somatic mutations in B- and T-cell lymphomas from these breeds by exome sequencing of tumor and normal pairs. Strong similarities were evident between B-cell lymphomas from golden retrievers and cocker spaniels, with recurrent mutations in TRAF3-MAP3K14 (28% of all cases), FBXW7 (25%), and POT1 (17%). The FBXW7 mutations recurrently occur in a specific codon; the corresponding codon is recurrently mutated in human cancer. In contrast, T-cell lymphomas from the predisposed breeds, boxers and golden retrievers, show little overlap in their mutation pattern, sharing only one of their 15 most recurrently mutated genes. Boxers, which develop aggressive T-cell lymphomas, are typically mutated in the PTEN-mTOR pathway. T-cell lymphomas in golden retrievers are often less aggressive, and their tumors typically showed mutations in genes involved in cellular metabolism. We identify genes with known involvement in human lymphoma and leukemia, genes implicated in other human cancers, as well as novel genes that could allow new therapeutic options.

Biallelic loss-of-function mutation in NIK causes a primary immunodeficiency with multifaceted aberrant lymphoid immunity

Primary immunodeficiency disorders enable identification of genes with crucial roles in the human immune system. Here we study patients suffering from recurrent bacterial, viral and Cryptosporidium infections, and identify a biallelic mutation in the MAP3K14 gene encoding NIK (NF-κB-inducing kinase). Loss of kinase activity of mutant NIK, predicted by in silico analysis and confirmed by functional assays, leads to defective activation of both canonical and non-canonical NF-κB signalling. Patients with mutated NIK exhibit B-cell lymphopenia, decreased frequencies of class-switched memory B cells and hypogammaglobulinemia due to impaired B-cell survival, and impaired ICOSL expression. Although overall T-cell numbers are normal, both follicular helper and memory T cells are perturbed. Natural killer (NK) cells are decreased and exhibit defective activation, leading to impaired formation of NK-cell immunological synapses. Collectively, our data illustrate the non-redundant role for NIK in human immune responses, demonstrating that loss-of-function mutations in NIK can cause multiple aberrations of lymphoid immunity.

Protein phosphatase 6 controls BCR-induced apoptosis of WEHI-231 cells by regulating ubiquitination of Bcl-xL

Crosslinking BCR in the immature B cell line WEHI-231 causes apoptosis. We found that Bcl-xL was degraded by polyubiquitination upon BCR crosslinking and in this study explored the mechanism that controls the degradation of Bcl-xL. Ser(62) of Bcl-xL was phosphorylated by JNK to trigger polyubiquitination, and this was opposed by serine/threonine protein phosphatase 6 (PP6) that physically associated with Bcl-xL. We show BCR crosslinking decreased PP6 activity to allow Ser(62) phosphorylation of Bcl-xL. CD40 crosslinking rescues BCR-induced apoptosis, and we found PP6 associated with CD40 and PP6 activation in response to CD40. Our data suggest that PP6 activity is regulated to control apoptosis by modulating Ser(62) phosphorylation of Bcl-xL, which results in its polyubiquitination and degradation.

Inflammatory mechanisms in the pathogenesis of pulmonary arterial hypertension

Inflammation is a prominent feature of human and experimental pulmonary hypertension (PH) as suggested by infiltration of various inflammatory cells and increased expression of certain cytokines in remodeled pulmonary vessels. Macrophages, T and B lymphocytes, and dendritic cells are found in the vascular lesions of idiopathic pulmonary arterial hypertension (PAH) as well as in PAH associated with connective tissue diseases or infectious etiologies such as HIV. In addition, PAH is often characterized by the presence of circulating chemokines and cytokines, increased expression of growth (such as VEGF and PDGF) and transcriptional (e.g., nuclear factor of activated T cells or NFAT) factors, and viral protein components (e.g., HIV-1 Nef), which directly contribute to further recruitment of inflammatory cells and the pulmonary vascular remodeling process. These inflammatory pathways may thus serve as potential specific therapeutic targets. This article provides an overview of inflammatory pathways involving chemokines and cytokines as well as growth factors, highlighting their potential role in pulmonary vascular remodeling and the possibility of future targeted therapy.

CD72 Stimulation Modulates Anti-IgM Induced Apoptotic Signaling through the Pathway of NF-κB, c-Myc and p27Kip1

Engagement of mIgM induces G1 arrest and apoptosis in immature B cells. The biochemical mechanism(s) regulating the cell death process are poorly understood. Cross-linking of CD72 (a B cell co-receptor) with anti-CD72 antibody was shown to protect B cells from apoptosis. We investigated the molecular mechanism involved in apoptosis preventing signaling mediated by CD72 ligation using a derivative (WEHIdelta) of the WEHI231 cell line which is representative of immature B cells. Apoptotic WEHIdelta cells following cross-linking of mIgM demonstrate a dramatic loss of c-Myc protein after transient up-regulation. In contrast, pre-ligation of CD72 was able to sustain c-Myc expression after transient up-regulation. Cross-linking of mIgM of WEHIdelta cells causes accumulation of the Cdk inhibitor, p27(Kip1). CD72 pre-ligation was shown to inhibit the accumulation of p27(Kip1) protein. Moreover, NF-kappaB activity was not suppressed in WEHIdelta cells after mIgM cross-linking when the cells were pre-treated with anti-CD72 antibody. These results strongly suggest that the apoptosis preventing signal evoked by CD72 ligation is delivered through the pathway of NF-kappaB, c-Myc, p27(Kip1) and cyclin.

A diametric role for OX40 in the response of effector/memory CD4+ T cells and regulatory T cells to alloantigen

OX40 is a member of the TNFR superfamily that has potent costimulatory properties. Although the impact of blockade of the OX40-OX40 ligand (OX40L) pathway has been well documented in models of autoimmune disease, its effect on the rejection of allografts is less well defined. In this article, we show that the alloantigen-mediated activation of naive and memory CD4(+) T cells results in the induction of OX40 expression and that blockade of OX40-OX40L interactions prevents skin allograft rejection mediated by either subset of T cells. Moreover, a blocking anti-OX40 had no effect on the activation and proliferation of T cells; rather, effector T cells failed to accumulate in peripheral lymph nodes and subsequently migrate to skin allografts. This was found to be the result of an enhanced degree of cell death among proliferating effector cells. In clear contrast, blockade of OX40-OX40L interactions at the time of exposure to alloantigen enhanced the ability of regulatory T cells to suppress T cell responses to alloantigen by supporting, rather than diminishing, regulatory T cell survival. These data show that OX40-OX40L signaling contributes to the evolution of the adaptive immune response to an allograft via the differential control of alloreactive effector and regulatory T cell survival. Moreover, these data serve to further highlight OX40 and OX40L as therapeutic targets to assist the induction of tolerance to allografts and self-Ags.

Excess CD40L does not rescue anti-DNA B cells from clonal anergy

CD40L, a member of the tumor necrosis factor (TNF) ligand family, is overexpressed in patients with systemic lupus erythematosus and in lupus mouse models. Previously, we demonstrated that B cells producing pathogenic anti-Sm/RNP antibodies are deleted in the splenic marginal zone (MZ), and that MZ deletion of these self-reactive B cells is reversed by excess CD40L, leading to autoantibody production. To address whether excess CD40L also perturbs clonal anergy, another self-tolerance mechanism of B cells whereby B cells are functionally inactivated and excluded from follicles in the peripheral lymphoid tissue, we crossed CD40L-transgenic mice with the anti-DNA H chain transgenic mouse line 3H9, in which Ig λ1+ anti-DNA B cells are anergized. However, the percentage and localization of Ig λ1+ B cells in CD40L/3H9 double transgenic mice were no different from those in 3H9 mice. This result indicates that excess CD40L does not perturb clonal anergy, including follicular exclusion. Thus, MZ deletion is distinct from clonal anergy, and is more liable to tolerance break.

Autophagy and the immune system

Stressors ranging from nutrient deprivation to immune signaling can induce the degradation of cytoplasmic material by a process known as autophagy. Increasingly, research on autophagy has begun to focus on its role in inflammation and the immune response. Autophagy acts as an immune effector that mediates pathogen clearance. The roles of autophagy bridge both the innate and adaptive immune systems and include functions in thymic selection, antigen presentation, promotion of lymphocyte homeostasis and survival, and regulation of cytokine production. In this review, we discuss the mechanisms by which autophagy is regulated, as well as the functions of autophagy and autophagy proteins in immunity and inflammation.

Apoptotic marginal zone deletion of anti-Sm/ribonucleoprotein B cells

CD40L is excessively produced in both human and murine lupus and plays a role in lupus pathogenesis. To address how excess CD40L induces autoantibody production, we crossed CD40L-transgenic mice with the anti-DNA H-chain transgenic mouse lines 3H9 and 56R, well-characterized models for studying B-cell tolerance to nuclear antigens. Excess CD40L did not induce autoantibody production in 3H9 mice in which anergy maintains self-tolerance, nor did it perturb central tolerance, including deletion and receptor editing, of anti-DNA B cells in 56R mice. In contrast, CD40L/56R mice restored a large number of marginal zone (MZ) B cells reactive to Sm/ribonucleoprotein (RNP) and produced autoantibody, whereas these B cells were deleted by apoptosis in MZ of 56R mice. Thus, excess CD40L efficiently blocked tolerance of Sm/RNP-reactive MZ B cells, leading to production of anti-Sm/RNP antibody implicated in the pathogenesis of lupus. These results suggest that self-reactive B cells such as anti-Sm/RNP B cells, which somehow escape tolerance in the bone marrow and migrate to MZ, are tolerized by apoptotic deletion in MZ and that a break in this tolerance may play a role in the pathogenesis of lupus.

MicroRNAs regulate B-cell receptor signaling-induced apoptosis

Apoptosis induced by B-cell receptor (BCR) signaling is critical for antigen-driven selection, a process critical to tolerance and immunity. Here, we examined the roles of microRNAs (miRNAs) in BCR signaling-induced apoptosis using the widely applied WEHI-231 model. Comparison of miRNA levels in BCR-stimulated and -unstimulated cells revealed that 39 miRNAs were differentially expressed upon stimulation of the BCR. Importantly, stimulation in the presence of anti-CD40 antibodies, which rescues cells from BCR-induced apoptosis, prevented most changes in miRNA expression. Ectopic expression of mir-150 and mir-181a1b1, miRNAs that were upregulated upon BCR stimulation, resulted in inhibition of cell growth. Finally, we showed that ectopic expression of mir-150, mir-181a1b1 and mir-17∼92 sensitized cells to anti-IgM stimulation-induced growth inhibition. Together, these results demonstrate that miRNAs are involved in BCR signaling, suggesting that they may have important roles in the regulation of B cell-mediated tolerance and immunity.

HIF-1α signaling by airway epithelial cell K-α1-tubulin: role in fibrosis and chronic rejection of human lung allografts

Long term survival of the human lung allografts are hindered by chronic rejection, manifested clinically as bronchiolitis obliterans syndrome (BOS). We previously demonstrated significant correlation between the development of antibodies (Abs) to K-α1-tubulin (Kα1T) and BOS. In this study, we investigated the molecular basis for fibrinogenesis mediated by ligation of Kα1T expressed on airway epithelial cells by its specific Abs. Using RT-PCR we demonstrate that normal human bronchial epithelial (NHBE) cells upon ligation of Kα1T with specific Abs caused upregulation of pro-fibrotic growth factors. Western blot analysis of NHBE incubated with Kα1T Abs increased hypoxia inducible factor (HIF-1α). Kα1T Ab-mediated growth factor expression is dependent on HIF-1α as inhibition of HIF-1α returned fibrotic growth factor expression to basal levels. In conclusion, we propose that HIF-1α -mediated upregulation of fibrogenic growth factors induced by ligation of Kα1T Abs is critical for development of fibrosis leading to chronic rejection of lung allograft.

Amplified B lymphocyte CD40 signaling drives regulatory B10 cell expansion in mice

BACKGROUND

Aberrant CD40 ligand (CD154) expression occurs on both T cells and B cells in human lupus patients, which is suggested to enhance B cell CD40 signaling and play a role in disease pathogenesis. Transgenic mice expressing CD154 by their B cells (CD154(TG)) have an expanded spleen B cell pool and produce autoantibodies (autoAbs). CD22 deficient (CD22(-/-)) mice also produce autoAbs, and importantly, their B cells are hyper-proliferative following CD40 stimulation ex vivo. Combining these 2 genetic alterations in CD154(TG)CD22(-/-) mice was thereby predicted to intensify CD40 signaling and autoimmune disease due to autoreactive B cell expansion and/or activation.

METHODOLOGY/PRINCIPAL FINDINGS

CD154(TG)CD22(-/-) mice were assessed for their humoral immune responses and for changes in their endogenous lymphocyte subsets. Remarkably, CD154(TG)CD22(-/-) mice were not autoimmune, but instead generated minimal IgG responses against both self and foreign antigens. This paucity in IgG isotype switching occurred despite an expanded spleen B cell pool, higher serum IgM levels, and augmented ex vivo B cell proliferation. Impaired IgG responses in CD154(TG)CD22(-/-) mice were explained by a 16-fold expansion of functional, mature IL-10-competent regulatory spleen B cells (B10 cells: 26.7×10(6)±6 in CD154(TG)CD22(-/-) mice; 1.7×10(6)±0.4 in wild type mice, p<0.01), and an 11-fold expansion of B10 cells combined with their ex vivo-matured progenitors (B10+B10pro cells: 66×10(6)±3 in CD154(TG)CD22(-/-) mice; 6.1×10(6)±2 in wild type mice, p<0.01) that represented 39% of all spleen B cells.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate for the first time that the IL-10-producing B10 B cell subset has the capacity to suppress IgG humoral immune responses against both foreign and self antigens. Thereby, therapeutic agents that drive regulatory B10 cell expansion in vivo may inhibit pathogenic IgG autoAb production in humans.

Role of peroxisome proliferator-activated receptor gamma and its ligands in the treatment of hematological malignancies

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a multifunctional transcription factor with important regulatory roles in inflammation, cellular growth, differentiation, and apoptosis. PPARgamma is expressed in a variety of immune cells as well as in numerous leukemias and lymphomas. Here, we review recent studies that provide new insights into the mechanisms by which PPARgamma ligands influence hematological malignant cell growth, differentiation, and survival. Understanding the diverse properties of PPARgamma ligands is crucial for the development of new therapeutic approaches for hematological malignancies.

14-3-3sigma regulates B-cell homeostasis through stabilization of FOXO1

14-3-3σ regulates cytokinesis and cell cycle arrest induced by DNA damage but its role in the immune system is unknown. Using gene-targeted 14-3-3σ-deficient (i.e., KO) mice, we studied the role of 14-3-3σ in B-cell functions. Total numbers of B cells were reduced by spontaneous apoptosis of peripheral B cells. Upon B-cell antigen receptor engagement in vitro, KO B cells did not proliferate properly or up-regulate CD86. In response to T cell-independent antigens, KO B cells showed poor secretion of antigen-specific IgM. This deficit led to increased lethality of KO mice after vesicular stomatitis virus infection. KO B cells showed elevated total FOXO transcriptional activity but also increased FOXO1 degradation. Coimmunoprecipitation revealed that endogenous 14-3-3σ protein formed a complex with FOXO1 protein. Our results suggest that 14-3-3σ maintains FOXO1 at a consistent level critical for normal B-cell antigen receptor signaling and B-cell survival.

B-lymphocyte homeostasis and BLyS-directed immunotherapy in transplantation

Current strategies for immunotherapy after transplantation are primarily T-lymphocyte directed and effectively abrogate acute rejection. However, the reality of chronic allograft rejection attests to the fact that transplantation tolerance remains an elusive goal. Donor-specific antibodies are considered the primary cause of chronic rejection. When naive, alloreactive B-cells encounter alloantigen and are activated, a resilient "sensitized" state, characterized by the presence of high-affinity antibody, is established. Here, we will delineate findings that support transient B-lymphocyte depletion therapy at the time of transplantation to preempt sensitization by eliminating alloreactive specificities from the recipient B-cell pool (ie, "repertoire remodeling"). Recent advances in our understanding of B-lymphocyte homeostasis provide novel targets for immunomodulation in transplantation. Specifically, the tumor necrosis factor-related cytokine BLyS is the dominant survival factor for "tolerance-susceptible" transitional and "preimmune" mature follicular B-cells. The transitional phenotype is the intermediate through which all newly formed B-cells pass before maturing into the follicular subset, which is responsible for mounting an alloantigen-specific antibody response. Systemic BLyS levels dictate the stringency of negative selection during peripheral B-cell repertoire development. Thus, targeting BLyS will likely provide an opportunity for repertoire-directed therapy to eliminate alloreactive B-cell specificities in transplant recipients, a requirement for the achievement of humoral tolerance and prevention of chronic rejection. In this review, the fundamentals of preimmune B-cell selection, homeostasis, and activation will be described. Furthermore, new and current B-lymphocyte-directed therapy for antibody-mediated rejection and the highly sensitized state will be discussed. Overall, our objective is to propose a rational approach for induction of humoral transplantation tolerance by remodeling the primary B-cell repertoire of the allograft recipient.

Augmented antibody response with premature germinal center regression in CD40L transgenic mice

Although CD40 signaling is required for activation and differentiation of B cells, including germinal center (GC) formation and generation of memory B cells, in vivo generation of CD40 signaling augments plasma cell differentiation but disrupts GCs. Thus, CD40 signaling is thought to direct B cells to extrafollicular plasma cell fate rather than GC formation. In this study, we analyzed CD40L transgenic (CD40LTg) mice that constitutively express CD40L on B cells. After immunization, activation of B cells, but not dendritic cells, was augmented, although dendritic cells can be activated by CD40 ligation. Bone marrow chimera carrying CD40LTg and nontransgenic B cells showed increased Ab production from transgenic, but not from coexisting nontransgenic, B cells, suggesting that CD40L on a B cell preferentially stimulates the same B cell through an autocrine pathway, thereby augmenting Ab production. Although GCs rapidly regressed after day 5 of immunization and failed to generate late-appearing high-affinity Ab, CD40LTg mice showed normal GC formation up to day 5, as well as normal generation of long-lived plasma cells and memory B cell responses. This observation suggests that CD40 signaling does not block GC formation or differentiation of GC B cells, but it inhibits sustained expansion of GC B cells and augments B cell differentiation.

B cell abnormality and autoimmune disorders

Various abnormalities have been described in B cells from patients with systemic autoimmune diseases such as systemic lupus erythematosus (SLE) and lupus-prone mice. Many of the abnormalities do not appear to be connected with the pathogenesis of the disease. However, various animal models developing lupus-like disease including both spontaneous mutans such as (NZB x NZW)F1 and MRL/lpr and mice generated by transgenic or knockout technology such as Bim-deficient and CD40L-transgenic mice show defect in apoptosis of mature B cells induced by ligation of the B cell antigen receptor (BCR). BCR-mediated apoptosis appears to be involved in deletion of self-reactive B cells. Thus, defect in BCR-mediated apoptosis is a widely observed B cell abnormality in lupus-prone mice and may play a role in the pathogenesis of systemic autoimmune diseases by abrogating deletion of self-reactive B cells.

Phagocytosis of Staphylococcus aureus by macrophages exerts cytoprotective effects manifested by the upregulation of antiapoptotic factors

It is becoming increasingly apparent that Staphylococcus aureus are able to survive engulfment by macrophages, and that the intracellular environment of these host cells, which is essential to innate host defenses against invading microorganisms, may in fact provide a refuge for staphylococcal survival and dissemination. Based on this, we postulated that S. aureus might induce cytoprotective mechanisms by changing gene expression profiles inside macrophages similar to obligate intracellular pathogens, such as Mycobacterium tuberculosis. To validate our hypothesis we first ascertained whether S. aureus infection could affect programmed cell death in human (hMDMs) and mouse (RAW 264.7) macrophages and, specifically, protect these cells against apoptosis. Our findings indicate that S. aureus-infected macrophages are more resistant to staurosporine-induced cell death than control cells, an effect partly mediated via the inhibition of cytochrome c release from mitochondria. Furthermore, transcriptome analysis of human monocyte-derived macrophages during S. aureus infection revealed a significant increase in the expression of antiapoptotic genes. This was confirmed by quantitative RT-PCR analysis of selected genes involved in mitochondria-dependent cell death, clearly showing overexpression of BCL2 and MCL1. Cumulatively, the results of our experiments argue that S. aureus is able to induce a cytoprotective effect in macrophages derived from different mammal species, which can prevent host cell elimination, and thus allow intracellular bacterial survival. Ultimately, it is our contention that this process may contribute to the systemic dissemination of S. aureus infection.

TCL1A expression delineates biological and clinical variability in B-cell lymphoma

The assembly of a collection of gene-expression signatures of the major types of B-cell non-Hodgkin's lymphoma has identified increased T-cell leukemia/lymphoma 1A (TCL1) expression in multiple lymphoma types and cases, and has enabled the investigation of the functional and clinical importance of TCL1 expression. Specifically, Burkitt's lymphoma cases show a homogeneously strong expression of TCL1, whereas diffuse large B-cell lymphoma, follicular lymphoma, mantle cell lymphoma, chronic lymphocytic leukemia, nodal marginal zone lymphoma, and splenic marginal zone lymphoma display a striking variability in the intensity of TCL1 staining. This was validated in two independent series. A Gene-Set Enrichment Analysis of the genes correlated with TCL1A expression found that variation in the level of expression of TCL1A was significantly associated with some of the most important gene signatures recognizing B-cell lymphoma pathogenesis and heterogeneity, such as germinal center, B-cell receptor, NF-kappaB (and its target genes), death, MAP kinases, TNFR1, TOLL, and IL1R. Additionally, TCL1 expression was correlated with shorter time to treatment in chronic lymphocytic leukemia cases and shorter lymphoma-specific survival in mantle cell lymphoma series, thus indicating the clinical and biological significance of TCL1 expression, and suggesting TCL1A as a potential therapeutic target.

TRAF proteins in CD40 signaling

The tumor necrosis factor receptor (TNFR) superfamily molecule CD40 is expressed by a wide variety of cell types following activation signals, and constitutively on B lymphocytes, macrophages, and dendritic cells. CD40 signals to cells stimulate kinase activation, gene expression, production of a antibody and a variety of cytokines, expression or upregulation of surface molecules, and protection or promotion of apoptosis. Initial steps in CD40-mediated signal cascades involve the interactions of CD40 with various members of the TNFR-associated factor (TRAF) family of cytoplasmic proteins. This review summarizes current understanding of the nature of these interactions, and how they induce and regulate CD40 functions.

Treating autoimmune demyelination by augmenting lymphocyte apoptosis in the central nervous system

The elimination of autoreactive T cells from the central nervous system (CNS) by apoptosis plays an important role in switching off autoimmune attack. B-cell apoptosis in the CNS probably also has a key role in downregulating autoimmunity. Augmenting lymphocyte apoptosis in the CNS is a potential strategy for treating autoimmune CNS diseases such as multiple sclerosis. These strategies involve modulation of the physiological pro-apoptotic and anti-apoptotic pathways that control lymphocyte fate in the CNS. In the case of T cells, apoptosis can be augmented by enhancing activation-induced T-cell apoptosis through the CD95 (Fas) pathway and by inhibiting costimulation-induced anti-apoptotic pathways mediated through BCL-2 and BCL-X L.

Macrophages prevent the differentiation of autoreactive B cells by secreting CD40 ligand and interleukin-6

Activation of the innate immune system promotes polyclonal antibody secretion to eliminate invading pathogens. Inherent in this process is the potential to activate autoreactive B cells and induce autoimmunity. We showed previously that TLR-stimulated dendritic cells and macrophages regulate B cell tolerance to Smith antigen, in part through the secretion of interleukin-6 (IL-6). In this manuscript, we show that neutralization of IL-6 fails to abrogate macrophage-mediated repression and identify soluble CD40 ligand (CD40L) as a second repressive factor secreted by macrophages. CD40L selectively repressed Ig secretion by chronically antigen-experienced (anergic) immunoglobulin transgenic and nontransgenic B cells but not by transiently stimulated B cells. The importance of macrophages in maintaining B cell tolerance was apparent in lupus-prone MRL/lpr mice. Compared with C57BL/6 mice, macrophages from MRL/lpr mice were significantly less efficient at repressing immunoglobulin secretion coincident with diminished IL-6 and CD40 ligand production. These data indicate that macrophages regulate autoreactive B cells by secreting repressive factors that prohibit terminal differentiation of B cells. The regulation of autoreactive B cells by macrophages is diminished in lupus-prone mice suggesting a role in autoimmunity.

Macrophages prevent the differentiation of autoreactive B cells by secreting CD40 ligand and interleukin-6

Activation of the innate immune system promotes polyclonal antibody secretion to eliminate invading pathogens. Inherent in this process is the potential to activate autoreactive B cells and induce autoimmunity. We showed previously that TLR-stimulated dendritic cells and macrophages regulate B cell tolerance to Smith antigen, in part through the secretion of interleukin-6 (IL-6). In this manuscript, we show that neutralization of IL-6 fails to abrogate macrophage-mediated repression and identify soluble CD40 ligand (CD40L) as a second repressive factor secreted by macrophages. CD40L selectively repressed Ig secretion by chronically antigen-experienced (anergic) immunoglobulin transgenic and nontransgenic B cells but not by transiently stimulated B cells. The importance of macrophages in maintaining B cell tolerance was apparent in lupus-prone MRL/lpr mice. Compared with C57BL/6 mice, macrophages from MRL/lpr mice were significantly less efficient at repressing immunoglobulin secretion coincident with diminished IL-6 and CD40 ligand production. These data indicate that macrophages regulate autoreactive B cells by secreting repressive factors that prohibit terminal differentiation of B cells. The regulation of autoreactive B cells by macrophages is diminished in lupus-prone mice suggesting a role in autoimmunity.

Interdomain A is crucial for ITAM-dependent and -independent regulation of Syk

Non-receptor type protein tyrosine kinase (PTK) Syk is essential for the signaling via the B cell antigen receptor (BCR). Upon BCR crosslinking, Syk is recruited via its tandem SH2 domains to tyrosine-phosphorylated Ig-alpha/Ig-beta constituting components of BCR, and is then activated. The interdomain A lying between the two SH2 domains is highly conserved among different species of Syk and between Syk and ZAP-70. The mutant Syk carrying a deletion in the interdomain A (Delta140-159) became phosphorylated regardless of BCR ligation and did not induce Ca2+ mobilization upon crosslinking of BCR. Furthermore, in vitro binding assay revealed that deletion of a part of the interdomain A abolished its binding activity to phosphorylated Ig-alpha/Ig-beta. These results indicate that the interdomain A of Syk is required for activation of Syk by binding to the phosphorylated Ig-alpha/Ig-beta upon BCR ligation and inhibition of spontaneous activation at the resting state.

Aiolos controls gene conversion and cell death in DT40 B cells

The Ikaros family transcription factor Aiolos is important for B cell function, since B cells of Aiolos-null mutant mice exhibit an activated phenotype, enhanced B-cell receptor (BCR) signalling response and develop a systemic lupus erythematosus (SLE) type autoimmune disease. Aiolos has also been reported to interact with anti-apoptotic Bcl-2 and Bcl-x(L) in T cells, but whether Aiolos regulates cell death has not been studied in B cells. Here we show that the disruption of Aiolos in the DT40 B cell line induces a cell death sensitive phenotype, as the Aiolos(-/-) cells are more prone to apoptosis by nutritional stress, BCR cross-linking, UV- or gamma-irradiation. Furthermore, the Aiolos(-/-) cells have defective Ig gene conversion providing evidence that Aiolos is needed for the somatic diversification of the BCR repertoire. The re-expression of DNA-binding isoform Aio-1 was able to restore the gene conversion defect of the Aiolos-deficient cells, whereas the introduction of dominant negative isofom Aio-2 had no effect on gene conversion, thus demonstrating the functional importance of alternative splicing within Ikaros family. Although the Aiolos(-/-) cells exhibit reduced expression of activation-induced cytidine deaminase (AID), ectopic AID overexpression did not restore the gene conversion defect in the Aiolos(-/-) cells. Our findings indicate that Aiolos may regulate gene conversion in an AID independent manner.