The uncertain glory of APRIL

Soluble Immune Factor Profiles in Blood and CSF Associated with LRRK2 Mutations and Parkinson's Disease

Background and Objectives

Mutations in the Leucine-rich repeat kinase 2 (LRRK2) gene are one of the most common genetic causes of Parkinson's disease (PD) and are linked to immune dysregulation in both the central nervous system and periphery. However, peripheral and central profiles of soluble immune factors associated with LRRK2 mutations and PD have not been comprehensively characterized. Using serum and CSF samples from the LRRK2 Cohort Consortium (LCC), this study aimed to probe a broad range of soluble immune biomarkers associated with LRRK2 mutations and PD.

Methods

We investigated the levels of soluble immune regulators in the serum (n=651) and cerebrospinal fluid (CSF, n=129) of LRRK2 mutation carriers and non-carriers, both with and without PD. A total of 65 cytokines, chemokines, growth factors, and soluble receptors were assessed by Luminex immunoassay. A multivariable robust linear model was used to determine levels associated with LRRK2 mutations and PD status, adjusting for age, sex, and sample cohort. Correlations were assessed using the Spearman correlation coefficient. LRRK2 G2019S knock-in mice were used to validate the associations identified in the LCC.

Results

In this extensive discovery cohort, we identified several elevated serum immune regulatory factors associated with LRRK2 mutations. In particular, serum stromal cell-derived factor-1 alpha (SDF-1 alpha) levels, as supported by findings in LRRK2 G2019S knock-in mice, and tumor necrosis factor receptor II (TNF-RII) were significantly increased after multiple comparison adjustment. In contrast, LRRK2 mutations were associated with reduced soluble immune markers, including BAFF, CD40-Ligand, I-TAC, MIP-3 alpha, NGF beta, and IL-27 in CSF. Those with clinically diagnosed PD, with or without LRRK2 mutations, did not show strong signals in serum but reduced inflammatory analytes in CSF, including MIF, MMP-1, CD30, Tweak, and SDF-1 alpha. In addition, we found that the serum levels of these soluble immune factors display varied correlations with their corresponding CSF levels.

Discussion

This study highlights distinct immune profiles associated with LRRK2 mutations and PD in the periphery and CNS. Serum levels of SDF-1alpha and TNF-RII were elevated in LRRK2 mutation carriers, while CSF immune markers were reduced. In PD, irrespective of LRRK2 status, reduced CSF inflammatory analytes and weak serum signals were observed. These results provide insight into immune dysregulation linked to LRRK2 mutations. If replicable in independent datasets, they offer potential avenues for biomarker and therapeutic exploration.

Exosome-based strategy against colon cancer using small interfering RNA-loaded vesicles targeting soluble a proliferation-inducing ligand

BACKGROUND

Recent advancements in nanomedicine have highlighted the potential of exosome (Ex)-based therapies, utilizing naturally derived nanoparticles, as a novel approach to targeted cancer treatment.

AIM

To explore the targetability and anticancer effectiveness of small interfering peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 RNA (siPIN1)-loaded soluble a proliferation-inducing ligand (sAPRIL)-targeted Exs (designated as tEx[p]) in the treatment of colon cancer models.

METHODS

tEx was generated by harvesting conditioned media from adipose-derived stem cells that had undergone transformation using pDisplay vectors encoding sAPRIL-binding peptide sequences. Subsequently, tEx[p] were created by incorporating PIN1 siRNA into the tEx using the Exofect kit. The therapeutic efficacy of these Exs was evaluated using both in vitro and in vivo models of colon cancer.

RESULTS

The tEx[p] group exhibited superior anticancer effects in comparison to other groups, including tEx, Ex[p], and Ex, demonstrated by the smallest tumor size, the slowest tumor growth rate, and the lightest weight of the excised tumors observed in the tEx[p] group (P < 0.05). Moreover, analyses of the excised tumor tissues, using western blot analysis and immunohistochemical staining, revealed that tEx[p] treatment resulted in the highest increase in E-cadherin expression and the most significant reduction in the mesenchymal markers Vimentin and Snail (P < 0.05), suggesting a more effective inhibition of epithelial-mesenchymal transition tEx[p], likely due to the enhanced delivery of siPIN1.

CONCLUSION

The use of bioengineered Exs targeting sAPRIL and containing siPIN1 demonstrated superior efficacy in inhibiting tumor growth and epithelial-mesenchymal transition, highlighting their potential as a therapeutic strategy for colon cancer.

Past and present discovery of the BAFF/APRIL system - A bibliometric study from 1999 to 2023

Cytokines from the Tumour Necrosis Factor (TNF) family are important regulators of both physiological and pathological processes. The discovery of novel TNF ligands and receptors, BAFF and APRIL, have opened up new possibilities for scientists to explore the effect of these cytokines on the human immune system. The role of BAFF/APRIL system in B lymphocytes is particularly important for survival and maintenance of homeostasis. Aberrant expression of the system is associated with various immunological disorders. Hence, this study provides a comprehensive overview of the past and present BAFF/APRIL system research development in a bibliometric perspective. To our best knowledge, this is the first ever bibliometric analysis conducted focusing on the BAFF/APRIL system. A total of 1055 relevant documents were retrieved from WoSCC. Microsoft Excel, VOSviewer, and Biblioshiny of R studio were bibliometric tools used to analyse the scientific literature. From 1999, the annual publications showed an upward trend, with Journal of Immunology being the most productive journal. USA leads the race for BAFF/APRIL system research developments. Pascal Schneider, a senior researcher affiliated with University of Lausanne, Switzerland was recognised as the most productive author and institution in the BAFF/APRIL system research field. The research focus transitioned from focusing on the role of the system in B cell biology, to immunological disorders and finally to development of BAFF/APRIL targeting drugs. Despite several studies elucidating briefly the pathway mechanism of BAFF/APRIL system in B-cell selection, substantial research on the mechanism of action in disease models and T cell activation and development of immunomodulating drugs from natural origins remains largely unexplored. Therefore, future research focusing on these areas are crucial for the deeper understanding of the system in disease manifestations and progression allowing a better treatment management for various immunological disorders.

Anti-inflammatory role of APRIL by modulating regulatory B cells in antigen-induced arthritis

APRIL (A Proliferation-Inducing Ligand), a member of the TNF superfamily, was initially described for its ability to promote proliferation of tumor cells in vitro. Moreover, this cytokine has been related to the pathogenesis of different chronic inflammatory diseases, such as rheumatoid arthritis. This study aimed to evaluate the ability of APRIL in regulating B cell-mediated immune response in the antigen-induced arthritis (AIA) model in mice. AIA was induced in previously immunized APRIL-transgenic (Tg) mice and their littermates by administration of antigen (mBSA) into the knee joints. Different inflammatory cell populations in spleen and draining lymph nodes were analyzed using flow cytometry and the assay was performed in the acute and chronic phases of the disease, while cytokine levels were assessed by ELISA. In the acute AIA, APRIL-Tg mice developed a less severe condition and a smaller inflammatory infiltrate in articular tissues when compared with their littermates. We also observed that the total cellularity of draining lymph nodes was decreased in APRIL-Tg mice. Flow cytometry analysis revealed an increase of CD19+IgM+CD5+ cell population in draining lymph nodes and an increase of CD19+CD21hiCD23hi (B regulatory) cells in APRIL-Tg mice with arthritis as well as an increase of IL-10 and CXCL13 production in vitro.

A PRoliferation-Inducing Ligand (APRIL) in the Pathogenesis of Immunoglobulin A Nephropathy: A Review of the Evidence

A PRoliferation-Inducing Ligand (APRIL), the thirteenth member of the tumor necrosis factor superfamily, plays a key role in the regulation of activated B cells, the survival of long-lived plasma cells, and immunoglobulin (Ig) isotype class switching. Several lines of evidence have implicated APRIL in the pathogenesis of IgA nephropathy (IgAN). Globally, IgAN is the most common primary glomerulonephritis, and it can progress to end-stage kidney disease; yet, disease-modifying treatments for this condition have historically been lacking. The preliminary data in ongoing clinical trials indicate that APRIL inhibition can reduce proteinuria and slow the rate of kidney disease progression by acting at an upstream level in IgAN pathogenesis. In this review, we examine what is known about the physiologic roles of APRIL and evaluate the experimental and epidemiological evidence describing how these normal biologic processes are thought to be subverted in IgAN. The weight of the preclinical, clinical, and genetic data supporting a key role for APRIL in IgAN has galvanized pharmacologic research, and several anti-APRIL drug candidates have now entered clinical development for IgAN. Herein, we present an overview of the clinical results to date. Finally, we explore where more research and evidence are needed to transform potential therapies into clinical benefits for patients with IgAN.

Role of B-Cell Activating Factor (BAFF) in Inflammatory Bowel Disease

As early commencement of inflammatory bowel disease (IBD) treatment has been shown to substantially improve outcomes, it is of utmost importance to make a timely diagnosis of this disease. Despite undisputed sensitivity of fecal calprotectin, the most widely accepted IBD biomarker, in discriminating between irritable bowel syndrome (IBS) and IBD, as well as recognized role in monitoring disease activity and response to therapy, perhaps the biggest setback of calprotectin use in IBD is lack of specificity. Therefore, an additional biomarker in IBD is warranted. B-cell activating factor (BAFF), a member of the tumor necrosis factor (TNF) superfamily, recently emerged as a viable candidate for this role. So far, overproduction of BAFF has been observed in various autoimmune diseases, most notably in systemic lupus erythematosus, where BAFF-inhibitor belimumab was approved for treatment. As BAFF levels were also shown to correlate with indices of IBD, in this review we aimed to summarize the current evidence with respect to the role of BAFF in diagnosis and assessing the activity of IBD, as well as putative therapeutic implications that may arise from exploring of this relation.

The TNFSF Members APRIL and BAFF and Their Receptors TACI, BCMA, and BAFFR in Oncology, With a Special Focus in Breast Cancer

Tumor necrosis factor (TNF) superfamily consists of 19 ligands and 29 receptors and is related to multiple cellular events from proliferation and differentiation to apoptosis and tumor reduction. In this review, we overview the whole system, and we focus on A proliferation-inducing ligand (APRIL, TNFSF13) and B cell-activating factor (BAFF, TNFSF13B) and their receptors transmembrane activator and Ca²⁺ modulator (CAML) interactor (TACI, TNFRSF13B), B cell maturation antigen (BCMA, TNFRSF17), and BAFF receptor (BAFFR, TNFRSF13C). We explore their role in cancer and novel biological therapies introduced for multiple myeloma and further focus on breast cancer, in which the modulation of this system seems to be of potential interest, as a novel therapeutic target. Finally, we discuss some precautions which should be taken into consideration, while targeting the APRIL–BAFF system.

Global, Survival, and Apoptotic Transcriptome during Mouse and Human Early Embryonic Development

Survival and cell death signals are crucial for mammalian embryo preimplantation development. However, the knowledge on the molecular mechanisms underlying their regulation is still limited. Mouse studies are widely used to understand preimplantation embryo development, but extrapolation of these results to humans is questionable. Therefore, we wanted to analyse the global expression profiles during early mouse and human development with a special focus on genes involved in the regulation of the apoptotic and survival pathways. We used DNA microarray technology to analyse the global gene expression profiles of preimplantation human and mouse embryos (metaphase II oocytes, embryos at the embryonic genome activation stage, and blastocysts). Components of the major apoptotic and survival signalling pathways were expressed during early human and mouse embryonic development; however, most expression profiles were species-specific. Particularly, the expression of genes encoding components and regulators of the apoptotic machinery were extremely stable in mouse embryos at all analysed stages, while it was more stage-specific in human embryos. CASP3, CASP9, and AIF were the only apoptosis-related genes expressed in both species and at all studied stages. Moreover, numerous transcripts related to the apoptotic and survival pathway were reported for the first time such as CASP6 and IL1RAPL1 that were specific to MII oocytes; CASP2, ENDOG, and GFER to blastocysts in human. These findings open new perspectives for the characterization and understanding of the survival and apoptotic signalling pathways that control early human and mouse embryonic development.

Toward Small-Molecule Inhibition of Protein-Protein Interactions: General Aspects and Recent Progress in Targeting Costimulatory and Coinhibitory (Immune Checkpoint) Interactions

Protein-Protein Interactions (PPIs) that are part of the costimulatory and coinhibitory (immune checkpoint) signaling are critical for adequate T cell response and are important therapeutic targets for immunomodulation. Biologics targeting them have already achieved considerable clinical success in the treatment of autoimmune diseases or transplant recipients (e.g., abatacept, belatacept, and belimumab) as well as cancer (e.g., ipilimumab, nivolumab, pembrolizumab, atezolizumab, durvalumab, and avelumab). In view of such progress, there have been only relatively limited efforts toward developing small-molecule PPI inhibitors (SMPPIIs) targeting these cosignaling interactions, possibly because they, as all other PPIs, are difficult to target by small molecules and were not considered druggable. Nevertheless, substantial progress has been achieved during the last decade. SMPPIIs proving the feasibility of such approaches have been identified through various strategies for a number of cosignaling interactions including CD40-CD40L, OX40-OX40L, BAFFR-BAFF, CD80-CD28, and PD-1-PD-L1s. Here, after an overview of the general aspects and challenges of SMPPII-focused drug discovery, we review them briefly together with relevant structural, immune-signaling, physicochemical, and medicinal chemistry aspects. While so far only a few of these SMPPIIs have shown activity in animal models (DRI-C21045 for CD40-D40L, KR33426 for BAFFR-BAFF) or reached clinical development (RhuDex for CD80-CD28, CA-170 for PD-1-PD-L1), there is proof-of-principle evidence for the feasibility of such approaches in immunomodulation. They can result in products that are easier to develop/ manufacture and are less likely to be immunogenic or encounter postmarket safety events than corresponding biologics, and, contrary to them, can even become orally bioavailable.

Monoclonal Antibody: A New Treatment Strategy against Multiple Myeloma

2015 was a groundbreaking year for the multiple myeloma community partly due to the breakthrough approval of the first two monoclonal antibodies in the treatment for patients with relapsed and refractory disease. Despite early disappointments, monoclonal antibodies targeting CD38 (daratumumab) and signaling lymphocytic activation molecule F7 (SLAMF7) (elotuzumab) have become available for patients with multiple myeloma in the same year. Specifically, phase 3 clinical trials of combination therapies incorporating daratumumab or elotuzumab indicate both efficacy and a very favorable toxicity profile. These therapeutic monoclonal antibodies for multiple myeloma can kill target cells via antibody-dependent cell-mediated cytotoxicity, complement-dependent cytotoxicity, and antibody-dependent phagocytosis, as well as by direct blockade of signaling cascades. In addition, their immunomodulatory effects may simultaneously inhibit the immunosuppressive bone marrow microenvironment and restore the key function of immune effector cells. In this review, we focus on monoclonal antibodies that have shown clinical efficacy or promising preclinical anti-multiple myeloma activities that warrant further clinical development. We summarize mechanisms that account for the in vitro and in vivo anti-myeloma effects of these monoclonal antibodies, as well as relevant preclinical and clinical results. Monoclonal antibody-based immunotherapies have already and will continue to transform the treatment landscape in multiple myeloma.

Identification of the sAPRIL binding peptide and its growth inhibition effects in the colorectal cancer cells

BACKGROUND

A proliferation-inducing ligand (APRIL) is a member of the tumor necrosis factor (TNF) super family. It binds to its specific receptors and is involved in multiple processes during tumorigenesis and tumor cells proliferation. High levels of APRIL expression are closely correlated to the growth, metastasis, and 5-FU drug resistance of colorectal cancer. The aim of this study was to identify a specific APRIL binding peptide (BP) able to block APRIL activity that could be used as a potential treatment for colorectal cancer.

METHODS

A phage display library was used to identify peptides that bound selectively to soluble recombinant human APRIL (sAPRIL). The peptides with the highest binding affinity for sAPRIL were identified using ELISA. The effects of sAPRIL-BP on cell proliferation and cell cycle/apoptosis in vitro were evaluated using the CCK-8 assay and flow cytometry, respectively. An in vivo mouse model of colorectal cancer was used to determine the anti-tumor efficacy of the sAPRIL-BP.

RESULTS

Three candidate peptides were characterized from eight phage clones with high binding affinity for sAPRIL. The peptide with the highest affinity was selected for further characterization. The identified sAPRIL-BP suppressed tumor cell proliferation and cell cycle progression in LOVO cells in a dose-dependent manner. In vivo in a mouse colorectal challenge model, the sAPRIL-BP reduced the growth of tumor xenografts in nude mice by inhibiting proliferation and inducing apoptosis intratumorally. Moreover, in an in vivo metastasis model, sAPRIL-BP reduced liver metastasis of colorectal cancer cells.

CONCLUSIONS

sAPRIL-BP significantly suppressed tumor growth in vitro and in vivo and might be a candidate for treating colorectal cancers that express high levels of APRIL.

TNF superfamily protein-protein interactions: feasibility of small- molecule modulation

The tumor necrosis factor (TNF) superfamily (TNFSF) contains about thirty structurally related receptors (TNFSFRs) and about twenty protein ligands that bind to one or more of these receptors. Almost all of these cell surface protein-protein interactions (PPIs) represent high-value therapeutic targets for inflammatory or immune modulation in autoimmune diseases, transplant recipients, or cancers, and there are several biologics including antibodies and fusion proteins targeting them that are in various phases of clinical development. Small-molecule inhibitors or activators could represent possible alternatives if the difficulties related to the targeting of protein-protein interactions by small molecules can be addressed. Compounds proving the feasibility of such approaches have been identified through different drug discovery approaches for a number of these TNFSFR-TNFSF type PPIs including CD40-CD40L, BAFFR-BAFF, TRAIL-DR5, and OX40-OX40L. Corresponding structural, signaling, and medicinal chemistry aspects are briefly reviewed here. While none of these small-molecule modulators identified so far seems promising enough to be pursued for clinical development, they provide proof-of-principle evidence that these interactions are susceptible to small-molecule modulation and can serve as starting points toward the identification of more potent and selective candidates.

siRNA interference with a proliferation-inducing ligand gene in the Sgr-7901 gastric carcinoma cell line

OBJECTIVE

The present study aimed to investigate the influence of siRNA interference with a proliferation- inducing ligand (APRIL) gene on gastric carcinoma sgr-7901 cell apoptosis. Correlations between APRIL silencing and tyrosine kinase (trka) expression were also explored.

METHODS

Two APRIL-silencing siRNA vectors were constructed, and transfected into human gastric carcinoma sgr-7901 cells, expression before and after transfection being detected using RT-PCR and western blot analyses. The expression of 15 trka genes was detected using RT- PCR and apoptotic rates of sgr-7901 were assessed by flow cytometry.

RESULTS

The expression levels of receptor trka genes were significantly decreased, and the apoptotic rate of sgr-7901 was significantly increased after transfection (P < 0.05).

CONCLUSION

APRIL gene silencing can increase the apoptotic rate of gastric carcinoma cells, and inhibit the expression of receptor trka genes. There is a correlation between the signaling pathways of APRIL and trka.

The TNF family member APRIL promotes colorectal tumorigenesis

The tumor necrosis factor (TNF) family member APRIL (A proliferation inducing ligand) is a disease promoter in B-cell malignancies. APRIL has also been associated with a wide range of solid malignancies, including colorectal cancer (CRC). As evidence for a supportive role of APRIL in solid tumor formation was still lacking, we studied the involvement of APRIL in CRC. We observed that ectopic APRIL expression exacerbates the number and size of adenomas in Apc(Min) mice and in a mouse model for colitis-associated colon carcinogenesis. Furthermore, knockdown of APRIL in primary spheroid cultures of colon cancer cells and both mouse and human CRC cell lines reduced tumor clonogenicity and in vivo outgrowth. Taken together, our data therefore indicate that both tumor-derived APRIL and APRIL produced by non-tumor cells is supportive in colorectal tumorigenesis.

Development and characterization of APRIL antagonistic monoclonal antibodies for treatment of B-cell lymphomas

APRIL (A proliferation-inducing ligand) is a TNF family member that binds two TNF receptor family members, TACI and BCMA. It shares these receptors with the closely related TNF family member, B-cell activating factor (BAFF). Contrary to BAFF, APRIL binds heparan sulfate proteoglycans (HSPGs), which regulates cross-linking of APRIL and efficient signaling. APRIL was originally identified as a growth promoter of solid tumors, and more recent evidence defines APRIL also as an important survival factor in several human B-cell malignancies, such as chronic lymphocytic leukemia (CLL). To target APRIL therapeutically, we developed two anti-human APRIL antibodies (hAPRIL.01A and hAPRIL.03A) that block APRIL binding to BCMA and TACI. Their antagonistic properties are unique when compared with a series of commercially available monoclonal anti-human APRIL antibodies as they prevent in vitro proliferation and IgA production of APRIL-reactive B cells. In addition, they effectively impair the CLL-like phenotype of aging APRIL transgenic mice and, more importantly, block APRIL binding to human B-cell lymphomas and prevent the survival effect induced by APRIL. We therefore conclude that these antibodies have potential for further development as therapeutics to target APRIL-dependent survival in B-cell malignancies.

Expression and purification of APRIL by auto-induction

APRIL (A proliferation-inducing ligand) is a newly-identified member of the tumor necrosis factor family that induces pleiotropic biological responses, including immunological responses, IgA class switch and cell growth. It is associated with multiple diseases such as cancer and autoimmune diseases. High levels of APRIL mRNA can be detected in transformed cell lines and several malignant tumors; heparin sulfate proteoglycans (HSPG) are also involved in the APRIL tumor cell proliferation induction response. The interaction of APRIL and HSPG occurs through an N-terminal basic region on APRIL. We successfully expressed recombinant APRIL using an auto-induction system in Escherichia coli. By using in situ cleavage, we generated the mature form of APRIL, comprising its N-terminal basic region, which has full biological activity: receptor binding capability and proliferation induction activity.

APRIL (TNFSF13) regulates collagen-induced arthritis, IL-17 production and Th2 response

A proliferation-inducing ligand (APRIL or TNFSF13) shares receptors with B-cell activation factor of the TNF family (BAFF) on B and T cells. Although much is known about the function of APRIL in B cells, its role in T cells remains unclear. Blocking both BAFF and APRIL suggested that BAFF and/or APRIL contributed to collagen-induced arthritis (CIA); however, the role of APRIL alone in CIA remained unresolved. We show here that, in vitro, our newly generated APRIL(-/-) mice exhibited increased T-cell proliferation, enhanced Th2 cytokine production under non-polarizing conditions, and augmented IL-13 and IL-17 production under Th2 polarizing conditions. Upon immunization with OVA and aluminum potassium sulfate, APRIL(-/-) mice responded with an increased antigen-specific IgG1 response. We also show that in APRIL(-/-) mice, the incidence of CIA was significantly reduced compared with WT mice in parallel with diminished levels of antigen-specific IgG2a autoantibody and IL-17 production. Our data indicate that APRIL plays an important role in the regulation of cytokine production and that APRIL-triggered signals contribute to arthritis. Blockade of APRIL thus may be a valuable adjunct in the treatment of rheumatoid arthritis.

The BLyS family: toward a molecular understanding of B cell homeostasis

The B Lymphocyte Stimulator (BLyS) family of ligands and receptors regulates humoral immunity by controlling B lymphocyte survival and differentiation. Herein, we review the ligands and receptors of this family, their biological functions, and the biochemical processes through which they operate. Pre-immune B lymphocytes rely on BLyS signaling for their survival, whereas antigen experienced B lymphocytes generally interact more avidly with a homologous cytokine, A Proliferation Inducing Ligand (APRIL). The molecular basis for signaling via the three BLyS family receptors reveals complex interplay with other B lymphocyte signaling systems, affording the integration of selective and homeostatic processes. As our understanding of this system advances, molecular targets for manipulating humoral immunity in both health and disease should be revealed.

Regulation of B-cell-activating factor (BAFF)/B lymphocyte stimulator (BLyS) expression in human neutrophils

The expression and production of cytokines by cells of the innate immune system, including monocytes/macrophages, dendritic and NK cells, play a critical role not only in defensive and inflammatory but also in immunoregulatory and anti-/pro-tumoral processes. Studies performed in the last years have well ascertained that polymorphonuclear neutrophils can also be induced to express and produce chemokines, proinflammatory, anti-inflammatory, immunoregulatory, angiogenic and fibrogenic cytokines, as well as ligands belonging to the TNF superfamily. Among the latter group of molecules, B-cell-activating factor (BAFF)/B lymphocyte stimulator (BLyS), known to be essential for B lymphocyte homeostasis and related pathologies, has recently been identified as one of the factors potentially expressed by human neutrophils. The addition of this novel TNF superfamily member, and more recently also of the closely related "A Proliferation-Inducing Ligand" (APRIL), to the list of cytokines produced by neutrophils not only testifies to the continuous growth of this area of investigation, but also implies the involvement of neutrophils in B-cell-dependent autoimmune diseases and tumors.

Murine models of chronic lymphocytic leukaemia: role of microRNA-16 in the New Zealand Black mouse model

Mouse models are valuable tools in the study of human chronic lymphocytic leukaemia (CLL). The New Zealand Black (NZB) strain is a naturally occurring model of late-onset CLL characterized by B-cell hyperproliferation and autoimmunity early in life, followed by progression to CLL. Other genetically engineered models of CLL that have been developed include (NZB x NZW) F1 mice engineered to express IL5, mice expressing human TCL1A, and mice overexpressing both BCL2 and a tumour necrosis factor receptor-associated factor. The applicability to human CLL varies with each model, suggesting that CLL is a multifactorial disease. Our work with the de novo NZB model has revealed many similarities to the human situation, particularly familial CLL. In NZB, the malignant clones express CD5, zap-70, and have chromosomal instability and germline Ig sequence. We also identified a point mutation in the 3'-flanking sequence of Mirn16-1, which resulted in decreased levels of the microRNA, miR-16 in lymphoid tissue. Exogenous restoration of miR-16 to an NZB malignant B-1 cell line resulted in cell cycle alterations, suggesting that the altered expression of Mirn15a/16-1 is an important molecular lesion in CLL. Future studies utilizing the NZB mouse could ascertain the role of environmental triggers, such as low dose radiation and organic chemicals in the augmentation of a pre-existing propensity to develop CLL.

BAFF and LPS cooperate to induce B cells to become susceptible to CD95/Fas-mediated cell death

Microorganisms with pathogen-associated molecular patterns (PAMP) activate B cells directly by binding to TLR and also indirectly by inducing APC to release cytokines such as BAFF that promote B cell survival. We found that murine B cells activated concomitantly with LPS (TLR-4 ligand) and BAFF are protected from spontaneous apoptosis, but are more susceptible to Fas/CD95-mediated cell death. This increased susceptibility to Fas-induced apoptosis is associated with a dramatic coordinated up-regulation of Fas/CD95 and IRF-4 expression through a mechanism mediated, at least in part, by inhibition of the MEK/ERK pathway. Up-regulation of Fas/CD95 by BAFF is restricted to B cells activated through TLR-4, but not through TLR-9, BCR or CD40. TLR ligands differ in the BAFF family receptors (R) they induce on B cells: BAFF-R is increased by the TLR4 ligand, LPS, but not by the TLR9 ligand, CpG-containing oligodeoxynucleotides, which, in contrast, strongly up-regulates transmembrane activator and CAML interactor (TACI). This suggests the up-regulation of Fas by BAFF is mediated by BAFF-R and not by TACI. Consistently, APRIL, which binds to TACI and B cell maturation antigen but not BAFF-R, did not enhance Fas expression on LPS-activated B cells. Increased susceptibility to Fas-mediated killing of B cells activated with LPS and BAFF may be a fail-safe mechanism to avoid overexpansion of nonspecific or autoreactive B cells.

Hodgkin's lymphoma: molecular targets and novel treatment strategies

The WHO classification of Hodgkin's lymphoma (HL) distinguishes between two major subtypes, classical and nodular lymphocyte predominant HL. Approximately 95% of patients with HL will have the classical HL histology, which is characterized by the presence of rare malignant Hodgkin's and Reed-Sternberg cells among an overwhelming number of benign reactive cells. In recent years, new studies have shed more light on the biological and molecular features of Hodgkin's and Reed-Sternberg cells, providing hope that new targeted therapy may be developed to enhance the cure rate and to reduce treatment-related toxicity. In this review, the current understanding of the pathology and biology of HL will be discussed, as well as the current treatment approaches for patients with classical HL. Future treatment strategies will also be discussed based on our understanding of HL biology.

Targeting Lymphoma Cells and Their Microenvironment with Novel Antibodies

Novel monoclonal antibodies are currently being evaluated and have been shown to have significantly influenced the treatment of Hodgkin's and non-Hodgkin's lymphoma. It is of the utmost importance to reduce treatment-related toxicity and, hopefully, improve the cure rate of lymphoma. This is possible by using novel therapies such as monoclonal antibodies, which target tumor cells while sparing normal cells. Investigational antibody therapies include those targeting malignant cells as well as those targeting the microenvironment. Continued investigation is encouraged to combine monoclonal antibodies with other targeted therapies and incorporate their use into standards of care in the treatment of lymphoma.

The role of the BAFF/APRIL system on T cell function

BAFF is a key factor controlling B cell survival and maturation and its over-expression promotes B cell-mediated autoimmune disorders and participates in the progression of B cell lymphomas. Yet, BAFF and a related ligand APRIL are expressed by T lymphocytes and modulate their functions. BAFF and APRIL promote T cell activation and survival. BAFF over-expression in transgenic (Tg) mice enhances T helper 1 (Thl)-driven delayed-type hypersensitivity (DTH), but inhibits T helper 2 (Th2) cell-mediated allergic airway inflammation in mice. Some of these effects are also dependent on BAFF-induced modification of the B cell compartment. Therefore, direct BAFF/APRIL signalling in T cells and/or T cell modulation in response to a BAFF-modified B cell compartment may play an important role in inflammation and immunomodulation.

BLyS receptor signatures resolve homeostatically independent compartments among naïve and antigen-experienced B cells

The BLyS family of receptors includes two cytokines, BLyS and APRIL; and three receptors, BR3, BCMA and TACI. Together, these regulate the size and composition of peripheral B cell pools. The multiplicity of ligand-receptor sets, in conjunction with differential receptor expression, alternative binding partners and disparate downstream signaling characteristics, affords the potential to establish independently regulated homeostatic niches among primary and antigen-experienced B cell subsets. Thus, BLyS signaling via BR3 is the dominant homeostatic regulator of primary B cell pools, whereas APRIL interactions with BCMA likely govern memory B cell populations. Short-lived antibody forming cell populations and their proliferating progenitors express a TACI-predominant signature. Further, within each niche, relative fitness to compete for available cytokine is determined by exogenous inputs via adaptive and innate receptor systems, affording intramural hierarchies that determine clonotype composition.

BLyS and APRIL in rheumatoid arthritis

The cytokines B lymphocyte stimulator (BLyS) and a proliferation-inducing ligand (APRIL) enhance autoimmune disease by sustaining B cell activation. In RA, B cells contribute to the formation of 3 functionally distinct types of lymphoid microarchitectures in the inflamed synovium: ectopic GCs; T cell-B cell aggregates lacking GC reactions; and unorganized, diffuse infiltrates. We examined 72 tissues representing the 3 types of synovitis for BLyS and APRIL production and for expression of APRIL/BLyS receptors. Biologic effects of BLyS and APRIL were explored by treating human synovium-SCID mouse chimeras with the APRIL and BLyS decoy receptor transmembrane activator and CAML interactor:Fc (TACI:Fc). GC+ synovitis had the highest levels of APRIL, produced exclusively by CD83+ DCs. BLyS was present in similar levels in all tissue types and derived exclusively from CD68+ macrophages. In GC+ synovitis, treatment with TACI:Fc resulted in GC destruction and marked inhibition of IFN-gamma and Ig transcription. In contrast, inhibition of APRIL and BLyS in aggregate and diffuse synovitis left Ig levels unaffected and enhanced IFN-gamma production. These differential immunomodulatory effects correlated with the presence of TACI+ T cells in aggregate and diffuse synovitis and their absence in GC+ synovitis. We propose that BLyS and APRIL regulate B cell as well as T cell function and have pro- and antiinflammatory activities in RA.

Heparan sulfate proteoglycan binding promotes APRIL-induced tumor cell proliferation

APRIL, a proliferation-inducing ligand, is a member of the tumor necrosis factor (TNF) family that is expressed by various types of tumors and influences their growth in vitro and in vivo. Two receptors, transmembrane activator and cyclophilin ligand interactor (TACI) and B-cell maturation antigen (BCMA), bind APRIL, but neither is essential for the tumor-promoting effects, suggesting that a third receptor exists. Here, we report that APRIL specifically binds to heparan sulfate proteoglycans (HSPG) on the surface of tumor cells. This binding is mediated by the heparin sulfate side chains and can be inhibited by heparin. Importantly, BCMA and HSPG do not compete, but can bind APRIL simultaneously, suggesting that different regions in APRIL are critical for either interaction. In agreement, mutation of three lysines in a putative heparin sulfate-binding motif, which is not part of the TNF fold, destroys interaction with HSPG, while binding to BCMA is unaffected. Finally, whereas interaction of APRIL with HSPG does not influence APRIL-induced proliferation of T cells, it is crucial for its tumor growth-promoting activities. We therefore conclude that either HSPG serve as a receptor for APRIL or that HSPG binding allows APRIL to interact with a receptor that promotes tumor growth.

B cells as a therapeutic target in autoimmune diseases

Historically, the pathogenic role of B cells in autoimmune disease has been attributed to the formation of autoantibodies which, as soluble immunoglobulins or immunocomplexes, can trigger cellular damage and initiate the inflammatory cascade. Recent results from clinical trials applying B cell-directed therapeutics in rheumatoid arthritis and systemic lupus erythematosus have challenged such traditional views and encouraged novel ideas about the disease involvement of B cells. Suppression of disease activity, often disconnected from effects on autoantibody titers, has supported the notion that B cells may promote autoimmune disease by serving as antigen-presenting cells that sustain T cell activation. Likewise, B cells have been implicated in supporting the process of ectopic lymphoid neogenesis, a mechanism that stabilises pathogenic immune responses in target tissues and thus contributes to disease chronicity. As a general rule, clinical effects of B cell-directed therapeutics have often been unanticipated and unpredicted by experimental models, emphasis-ing the need to explore and verify disease principles in the patient.

The BLyS family of ligands and receptors: an archetype for niche-specific homeostatic regulation

Discovery and characterization of the tumor necrosis factor (TNF) family member B-lymphocyte stimulator (BLyS) has opened a novel chapter in the role of TNF family members in the homeostatic control of lymphocyte populations. BLyS and its sister cytokine APRIL (a proliferation-inducing ligand) act primarily as soluble trimers and serve to regulate the steady-state numbers of nearly all B-cell compartments. This homeostatic regulation is accomplished through the regulation of B-cell production rates, selection thresholds, and lifespan. Differential expression of the three BLyS receptors during differentiation and activation provides related yet distinct homeostatic niches for follicular, marginal zone, and memory B-cell subsets.

Ligands working as receptors: reverse signaling by members of the TNF superfamily enhance the plasticity of the immune system

The inflammatory cytokine tumor necrosis factor (TNF), as well as most other ligand members of the TNF superfamily, exist both as classical soluble cytokines, but also in the form of type II transmembrane proteins. Both forms possess bioactivity, although some effects are distinct. In addition, an increasing body of evidence suggests that the membrane integrated ligands can receive signals, i.e. act as receptors which can transmit positive and negative feedback signals into the ligand bearing cell. Thus, reverse signaling enables a two-way communication in cell-to-cell signaling, and it is conceivable that this bi-directional signal exchange contributes to the plasticity of the ligand-receptor systems. Reverse signaling mainly has been observed in the immune system and within the TNF superfamily. Its function is only beginning to emerge warranting additional investigation, especially when it comes to therapeutic strategies involving cytokine modulation. This review provides an update of the literature about reverse signaling of transmembrane TNF family members and discusses its potential biological and clinical impact.

Selectively frequent expression of CXCR5 enhances resistance to apoptosis in CD8(+)CD34(+) T cells from patients with T-cell-lineage acute lymphocytic leukemia

We investigated CD4(+)CD34(+), CD8(+)CD34(+), CD4(+)CD34(-), and CD8(+)CD34(-) T cells from cord blood and from typical patients with T-cell-lineage acute lymphocytic leukemia and T-cell-lineage chronic lymphocytic leukemia in terms of expression and functions of CXCR5/CXCL13. We found that CXCR5 was selectively frequently expressed on T-cell-lineage acute (chronic) lymphocytic leukemia (T-ALL) CD8(+)CD34(+) T cells, but not on T-ALL CD4(+)CD34(+), CD4(+)CD34(-), and CD8(+)CD34(-) T cells. CXCR5 was rarely expressed on all types of CD34(+) and CD34(-) CB or T-CLL T cells. CXCL13/B cells attracting chemokine 1 induced significant resistance to TNF-alpha-mediated apoptosis in T-ALL CD8(+)CD34(+) T cells, instead of induction of chemotactic and adhesive responsiveness. A proliferation-inducing ligand expression in T-ALL CD8(+)CD34(+) T cells was upregulated by CXCL13/BCA-1 (B-cell attracting chemokine 1). The CXCR5/CXCL13 pair by means of activation of APRIL (A proliferation-inducing ligand) induced resistance to apoptosis in T-ALL CD8(+)CD34(+) T cells in livin-dependent manner. In this process, cell-cell contact in culture was necessary. Based on our findings, we suggested that there were differential functions of CXCR5/CXCL13 in distinct types of cells. Normal lymphocytes, especially naive B and T cells, utilized CXCR5/CXCL13 for migration, homing, maturation, and cell homeostasis, as well as secondary lymphoid tissue organogenesis. Meanwhile, certain malignant cells took advantages of CXCR5/CXCL13 for infiltration, resistance to apoptosis, and inappropriate proliferation.

Structures of APRIL-Receptor Complexes

TACI is a member of the tumor necrosis factor receptor superfamily and serves as a key regulator of B cell function. TACI binds two ligands, APRIL and BAFF, with high affinity and contains two cysteine-rich domains (CRDs) in its extracellular region; in contrast, BCMA and BR3, the other known high affinity receptors for APRIL and BAFF, respectively, contain only a single or partial CRD. However, another form of TACI exists wherein the N-terminal CRD is removed by alternative splicing. We find that this shorter form is capable of ligand-induced cell signaling and that the second CRD alone (TACI_d2) contains full affinity for both ligands. Furthermore, we report the solution structure and alanine-scanning mutagenesis of TACI_d2 along with co-crystal structures of APRIL.TACI_d2 and APRIL.BCMA complexes that together reveal the mechanism by which TACI engages high affinity ligand binding through a single CRD, and we highlight sources of ligand-receptor specificity within the APRIL/BAFF system.

The BAFF/APRIL system: life beyond B lymphocytes

B cell activating factor belonging to the TNF family (BAFF) and a proliferation-inducing ligand (APRIL) are two members of the TNF ligand superfamily. Studies of BAFF, APRIL and their receptors have highlighted the importance of this ligand/receptor system in regulating B cell homeostasis, tolerance and malignancy. Neutralizing BAFF can inhibit disease progression in animal models of autoimmunity, possibly by reducing survival of autoreactive B cells. In addition, BAFF inhibitors also prevent B lymphoma cell survival and may be useful for the treatment of lymphoid cancers. Recent work suggests that BAFF is also important for T cell activation and differentiation, an aspect that may be critical for the progression of certain autoimmune diseases. Therefore, targeting the BAFF/APRIL system may protect against autoimmunity and lymphoid cancers through the inhibition of pathogenic B and T cell functions.

Potential role of APRIL as autocrine growth factor for megakaryocytopoiesis

A proliferation-inducing ligand (APRIL) is a new tumor necrosis factor family member implicated in tumor cell proliferation. We investigated the role of APRIL in megakaryocytopoiesis, a developmental hematopoietic process responsible for progenitor cell differentiation to megakaryoblasts and megakaryocytes, leading to platelet formation. APRIL is not expressed in CD34+ progenitor cells from healthy donors, but it is massively up-regulated during the proliferative phase of megakaryocytic cell differentiation. Exogenous APRIL expression in primary cells increases megakaryocytic cell growth, suggesting that APRIL acts as a proliferative factor in megakaryocytopoiesis. More importantly, neutralization of endogenous APRIL was able to dramatically reduce megakaryocyte expansion and platelet production. Thus, our data provide evidence that APRIL acts as a growth factor for terminal megakaryocytopoiesis and may promote physiologic platelet production.

The Crystal Structure of A Proliferation-inducing Ligand, APRIL

A proliferation-inducing ligand (APRIL) is a TNF-like cytokine that stimulates tumor cell growth. Within the TNF ligand superfamily, APRIL is most similar to B-cell activation factor (BAFF) with which it shares 30% sequence identity. APRIL binds the receptors B-cell maturation antigen (BCMA) and TACI with high affinity; both of these receptors have also been shown to bind BAFF, although BCMA has significantly higher affinity for APRIL than BAFF. Determination of the crystal structure of APRIL from three crystallization conditions at resolutions of 1.8-2.4A over a pH range from 5.0 to 8.5 reveals a compact trimeric ligand with a backbone fold very similar to that of BAFF (1.1A RMSD over 122 structurally equivalent Calpha atoms), with the exception of differences in the AA' and DE loop regions. Whereas BAFF has been shown to form 20-trimer assemblies under certain conditions, the molecular determinants required for BAFF-like assemblies are absent in the APRIL structure. No crystal packing suggestive of the formation of higher-order assemblies is seen in any of the crystal forms nor does the structure vary significantly between pH 5.0 and 8.5. Modeling of the APRIL-BCMA complex shows the resulting interface is in agreement with mutagenesis data.

APRIL promotes B-1 cell-associated neoplasm

A tumor-supporting role for the TNF-like ligand APRIL has been suggested. Here we describe that 9- to 12-month-old APRIL transgenic mice develop lymphoid tumors that originate from expansion of the peritoneal B-1 B cell population. Aging APRIL transgenic mice develop progressive hyperplasia in mesenteric lymph nodes and Peyer's patches, disorganization of affected lymphoid tissues, mucosal and capsular infiltration, and eventual tumor cell infiltration into nonlymphoid tissues such as kidney and liver. We detected significantly increased APRIL levels in sera of B cell chronic lymphoid leukemia (B-CLL) patients, indicating that APRIL promotes onset of B-1-associated neoplasms and that APRIL antagonism may provide a therapeutic strategy to treat B-CLL patients.