Further analyses of APRIL/APRIL-receptor/glycosaminoglycan interactions by biochemical assays linked to computational studies.

A proliferation-inducing ligand (APRIL) is a member of the tumor necrosis factor superfamily. APRIL is quite unique in this superfamily for at least for two reasons: (i) it binds to glycosaminoglycans (GAGs) via its positively charged N-terminus; (ii) one of its signaling receptor, the transmembrane activator and CAML interactor (TACI), was also reported to bind GAGs. Here, as provided by biochemical evidences with the use of an APRIL deletion mutant linked to computational studies, APRIL-GAG interaction involved other regions than the APRIL N-terminus. Preferential interaction of APRIL with heparin followed by chondroitin sulfate E was confirmed by in silico analysis. Both computational and experimental approaches did not reveal the heparan sulfate binding to TACI. Together, computational results corroborated experiments contributing with atomistic details to the knowledge on this biologically relevant trimolecular system. Additionally, a high-throughput rigorous analysis of the free energy calculations data was performed to critically evaluate the applied computational methodologies.

A proliferation-inducing ligand-mediated anti-inflammatory response of astrocytes in multiple sclerosis.

OBJECTIVE: The two related tumor necrosis factor members a proliferation-inducing ligand (APRIL) and B-cell activation factor (BAFF) are currently targeted in autoimmune diseases as B-cell regulators. In multiple sclerosis (MS), combined APRIL/BAFF blockade led to unexpected exacerbated inflammation in the central nervous system (CNS) of patients. Here, we investigate the role of the APRIL/BAFF axis in the CNS. METHODS: APRIL expression was analyzed in MS lesions by immunohistochemistry. The in vivo role of APRIL was assessed in the murine MS model, experimental autoimmune encephalitis (EAE). Functional in vitro studies were performed with human and mouse astrocytes. RESULTS: APRIL was expressed in lesions from EAE. In its absence, the disease was worst. Lesions from MS patients also showed APRIL expression upon infiltration of macrophages. Notably, all the APRIL secreted by these macrophages specifically targeted astrocytes. The upregulation of chondroitin sulfate proteoglycan, sometimes bearing chondroitin sulfate of type E sugar moieties, binding APRIL, in reactive astrocytes explained the latter selectivity. Astrocytes responded to APRIL by producing a sufficient amount of IL-10 to dampen antigen-specific T-cell proliferation and pathogenic cytokine secretion. Finally, an intraspinal delivery of recombinant APRIL before disease onset, shortly reduced EAE symptoms. Repeated intravenous injections of recombinant APRIL before and even at disease onset also had an effect. INTERPRETATION: Our data show that APRIL mediates an anti-inflammatory response from astrocytes in MS lesions. This protective activity is not shared with BAFF. ANN NEUROL 2019;85:406-420.

Abundant a proliferation-inducing ligand (APRIL)-producing macrophages contribute to plasma cell accumulation in immunoglobulin G4-related disease.

BACKGROUND: This study aimed to investigate the contribution of a proliferation-inducing ligand (APRIL), a member of the tumor necrosis factor (TNF) superfamily implicated in plasma cell survival, to the development of plasma cell-rich lesions in immunoglobulin G4-related disease (IgG4-RD). METHODS: We performed immunohistochemical staining for APRIL with Stalk-1 and Aprily-8 antibodies specifically recognizing APRIL-producing cells and secreted APRIL, respectively, in renal and submandibular lesions of IgG4-RD in comparison with those of Sjögren's syndrome and sialolithiasis. RESULTS: Numerous Stalk-1-positive APRIL-producing cells were detectable in lesions of IgG4-RD. These cells, identified as CD163-positive M2 macrophages, secreted APRIL that distributed close to and even on infiltrating plasma cells. In contrast, APRIL-producing cells and the secreted form of APRIL were rarely detectable in lesions of Sjögren's syndrome or sialolithiasis. Notably, APRIL expression decreased concomitantly with the level of plasma cell infiltration after successful glucocorticoid treatment. CONCLUSIONS: Abundant infiltration into tissue lesions of APRIL-producing M2 macrophages and retention of secreted APRIL in plasma-cell-rich areas support a role for APRIL in the pathogenesis of plasma cell-rich lesions in IgG4-RD.

The role of APRIL - A proliferation inducing ligand - In autoimmune diseases and expectations from its targeting.

Autoimmunity occurs when an adaptive immune response is directed against a self-antigen. As such, autoimmune reactions associated with the production of autoantibodies are common. These autoantibodies may either be pathogenic by inducing the initial damage to self, or exacerbate the reaction secondarily to the initial damage. In both cases, the pathway(s) leading to exposure of the immune system to the self-antigen inducing the production of autoantibodies is largely unknown. The latter is largely complicating the setting of putative prophylactic treatments. As a consequence, one possible way to control these diseases is to eliminate the cells producing antibodies. We will see that this approach is not yet part of any treatment in autoimmunity. Indeed, all the currently available non-specific immunosuppressive treatments do not target directly quiescent antibody-producing plasma cells. However, treatments aimed at depleting precursors of plasma cells, mature B-lymphocytes and/or antigen-experienced B cells not yet fully differentiated into plasma cells, are emerging. Such strategies were recently proven to be highly successful in several autoimmune disorders by two independent ways. The first way is by induction of B-cell cytotoxicity with an antibody directed against the surface antigen CD20. The second way is by antagonism of a key B-cell survival factor, the B-cell activation factor from the TNF superfamily (BAFF). In the present review, we will focus on the current knowledge regarding the role of a molecule related to BAFF, a proliferation-inducing ligand (APRIL), in autoimmune diseases, which acts on antibody-producing plasma cells. We will discuss expectations deriving from APRIL targeting in autoimmune diseases.

Toll-Like Receptor 9 Stimulation Induces Aberrant Expression of a Proliferation-Inducing Ligand by Tonsillar Germinal Center B Cells in IgA Nephropathy.

The TNF family member a proliferation-inducing ligand (APRIL; also known as TNFSF13), produced by myeloid cells, participates in the generation and survival of antibody-producing plasma cells. We studied the potential role of APRIL in the pathogenesis of IgA nephropathy (IgAN). We found that a significant proportion of germinal centers (GCs) in tonsils of patients with IgAN contained cells aberrantly producing APRIL, contributing to an overall upregulation of tonsillar APRIL expression compared with that in tonsils of control patients with tonsillitis. In IgAN GC, antigen-experienced IgD-CD38+/-CD19+ B cells expressing a switched IgG/IgA B cell receptor produced APRIL. Notably, these GC B cells expressed mRNA encoding the common cleavable APRIL-α but also, the less frequent APRIL-δ/ζ mRNA, which encodes a protein that lacks a furin cleavage site and is, thus, the uncleavable membrane-bound form. Significant correlation between TLR9 and APRIL expression levels existed in tonsils from patients with IgAN. In vitro, repeated TLR9 stimulation induced APRIL expression in tonsillar B cells from control patients with tonsillitis. Clinically, aberrant APRIL expression in tonsillar GC correlated with greater proteinuria, and patients with IgAN and aberrant APRIL overexpression in tonsillar GC responded well to tonsillectomy, with parallel decreases in serum levels of galactose-deficient IgA1. Taken together, our data indicate that antibody disorders in IgAN associate with TLR9-induced aberrant expression of APRIL in tonsillar GC B cells.

Progression of fibrosis in patients with chronic viral hepatitis is associated with IL-17(+) neutrophils.

BACKGROUND & AIMS: The pro-inflammatory cytokine IL-17 plays a crucial role in liver diseases associated with hepatic fibrosis and increased risk of cancer development. Nevertheless, the cellular source of this cytokine has never been characterized in patients with liver fibrosis. METHODS: In this study, we investigated liver biopsies from 49 patients with chronic viral hepatitis at different stages of liver fibrosis. We monitored IL-17 production by intracellular flow cytometry, immunofluorescence and immunohistochemical in situ stainings, allowing a precise quantification, characterization and localization of IL-17(+) cells. RESULTS: Density of IL-17(+) cells increased with the stage of liver fibrosis specifically in fibrotic septa and portal areas (correlation coefficient r = 0.7373; P < 0.0001). Data clearly show that the frequency of intrahepatic IL-17(+) lymphocytes (including T, NKT and NK cells) was independent on stage of liver fibrosis, and we observed no statistical differences in number of IL-17(+) macrophages during progression of fibrosis. On the other hand, the number of IL-17(+) neutrophils in fibrotic septa and portal areas strongly correlated with the stages of fibrosis (correlation coefficient r = 0.6986; P < 0.0001), contributing significantly to total IL-17 production in liver tissue. CONCLUSIONS: Our data indicate that neutrophils represent an important source of IL-17 in the human liver, especially in late fibrosis stages. Inhibition of this specific harmful subset of neutrophils may offer therapeutic opportunities in fibrotic liver.

Production of the plasma-cell survival factor a proliferation-inducing ligand (APRIL) peaks in myeloid precursor cells from human bone marrow.

The bone marrow (BM) is an organ extremely efficient in mediating long-term survival of plasma cells (PCs), ensuring an immune humoral memory. This implies that the BM must provide continuously key PC survival factors. Our results show that the BM is an organ constitutively rich in a proliferation-inducing ligand (APRIL), a member of the tumor necrosis factor superfamily implicated in PC survival. APRIL production is induced during hematopoiesis in myeloid cells by non-lineage-committing factors such as stem cell factor, thrombopoietin, IL-3, and FMS-like tyrosine kinase 3 ligand. Notably, APRIL production, both in the human and mouse systems, peaks in myeloid precursor cells, before dropping in fully mature granulocytes. Myeloid cells secrete APRIL that circulates freely in BM plasma to act on PCs, usually at distance from APRIL production sites. Selective APRIL in vivo antagonism and in vitro coculture experiments further demonstrated that myeloid precursor cells mediates PC survival in an APRIL-dependent manner Thus, APRIL production by myeloid precursor cells shows that the 2 main BM functions, hematopoiesis and long-term PC survival, are linked. Such constitutive and high APRIL production may explain why BM mediates long-term PC survival.

Spatial heterogeneity for APRIL production by eosinophils in the small intestine.

Eosinophils may reside in the lower intestine to play several homeostatic functions. Regulation of IgA+ plasma-cell (PC) homeostasis is one of these functions. Here, we assessed regulation of expression for a proliferation-inducing ligand (APRIL), a key factor from the TNF superfamily for PC homeostasis, in eosinophils from the lower intestine. We observed a strong heterogeneity, since duodenum eosinophils did not produce APRIL at all, whereas a large majority of eosinophils from the ileum and right colon produced it. This was evidenced both in the human and mouse adult systems. At these places, the human data showed that eosinophils were the only cellular sources of APRIL. The number of IgA+ PCs did not vary along the lower intestine, but ileum and right colon IgA+ PC steady-state numbers significantly diminished in APRIL-deficient mice. Use of blood cells from healthy donors demonstrated that APRIL expression in eosinophils is inducible by bacterial products. Use of germ-free and antibiotics-treated mice confirmed the dependency on bacteria for APRIL production by eosinophils from the lower intestine. Taken together, our study shows that APRIL expression by eosinophils is spatially regulated in the lower intestine with a consequence on the APRIL dependency for IgA+ PC homeostasis.

Heterogeneity of antibody-secreting cells infiltrating autoimmune tissues.

The humoral response is frequently dysfunctioning in autoimmunity with a frequent rise in total serum immunoglobulins, among which are found autoantibodies that may be pathogenic by themselves and/or propagate the inflammatory reaction. The infiltration of autoimmune tissues by antibody-secreting cells (ASCs) constitutes another dysfunction. The known high dependency of ASCs on the microenvironment to survive combined to the high diversity of infiltrated tissues implies that ASCs must adapt. Some tissues even within a single clinical autoimmune entity are devoid of infiltration. The latter means that either the tissue is not permissive or ASCs fail to adapt. The origin of infiltrated ASCs is also variable. Indeed, ASCs may be commonly generated in the secondary lymphoid organ draining the autoimmune tissue, and home at the inflammation site under the guidance of specific chemokines. Alternatively, ASCs may be generated locally, when ectopic germinal centers are formed in the autoimmune tissue. Alloimmune tissues with the example of kidney transplantation will also be discussed own to their high similarity with autoimmune tissues. It should also be noted that antibody production is not the only function of ASCs, since cells with regulatory functions have also been described. This article will review all the phenotypic variations indicative of tissue adaptation described so for at the level of ASC-infiltrating auto/alloimmune tissues. The aim is to potentially define tissue-specific molecular targets in ASCs to improve the specificity of future autoimmune treatments.

Lymph node tumor metastases: more susceptible than primary tumors to CD8(+) T-cell immune destruction.

Hematopoietic cells, and more particularly, dendritic cells are so called "professional" antigen-presenting cells, which prime CD8(+) T-cell responses. They achieve this by taking up antigens and presenting them to CD8(+) T cells in the draining lymph nodes. This process is called cross-presentation (XP). For most developing tumors, XP of tumor antigens results in CD8(+) T-cell tolerance. In addition to XP, direct presentation by any kind of cell can also occur in lymph nodes. We discuss here how a non-hematopoietic cell can efficiently prime CD8(+) T cells by direct presentation in lymph nodes. Such a T-cell activation pathway is likely to be of importance for the control of cancer metastases that use the lymphatic system to spread.

Identification of proteoglycans as the APRIL-specific binding partners.

B cell activating factor of the tumor necrosis factor (TNF) family (BAFF) and a proliferation-inducing ligand (APRIL) are closely related ligands within the TNF superfamily that play important roles in B lymphocyte biology. Both ligands share two receptors--transmembrane activator and calcium signal--modulating cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA)--that are predominantly expressed on B cells. In addition, BAFF specifically binds BAFF receptor, whereas the nature of a postulated APRIL-specific receptor remains elusive. We show that the TNF homology domain of APRIL binds BCMA and TACI, whereas a basic amino acid sequence (QKQKKQ) close to the NH2 terminus of the mature protein is required for binding to the APRIL-specific "receptor." This interactor was identified as negatively charged sulfated glycosaminoglycan side chains of proteoglycans. Although T cell lines bound little APRIL, the ectopic expression of glycosaminoglycan-rich syndecans or glypicans conferred on these cells a high binding capacity that was completely dependent on APRIL's basic sequence. Moreover, syndecan-1-positive plasma cells and proteoglycan-rich nonhematopoietic cells displayed high specific, heparin-sensitive binding to APRIL. Inhibition of BAFF and APRIL, but not BAFF alone, prevented the survival and/or the migration of newly formed plasma cells to the bone marrow. In addition, costimulation of B cell proliferation by APRIL was only effective upon APRIL oligomerization. Therefore, we propose a model whereby APRIL binding to the extracellular matrix or to proteoglycan-positive cells induces APRIL oligomerization, which is the prerequisite for the triggering of TACI- and/or BCMA-mediated activation, migration, or survival signals.

APRIL secreted by neutrophils binds to heparan sulfate proteoglycans to create plasma cell niches in human mucosa.

The bone marrow constitutes a favorable environment for long-lived antibody-secreting plasma cells, providing blood-circulating antibody. Plasma cells are also present in mucosa-associated lymphoid tissue (MALT) to mediate local frontline immunity, but how plasma cell survival there is regulated is not known. Here we report that a proliferation-inducing ligand (APRIL) promoted survival of human upper and lower MALT plasma cells by upregulating expression of the antiapoptotic proteins bcl-2, bcl-xL, and mcl-1. The in situ localization of APRIL was consistent with such a prosurvival role in MALT. In upper MALT, tonsillar epithelium produced APRIL. Upon infection, APRIL production increased considerably when APRIL-secreting neutrophils recruited from the blood infiltrated the crypt epithelium. Heparan sulfate proteoglycans (HSPGs) retained secreted APRIL in the subepithelium of the infected zone to create APRIL-rich niches, wherein IgG-producing plasma cells accumulated. In lower MALT, neutrophils were the unique source of APRIL, giving rise to similar niches for IgA-producing plasmocytes in villi of lamina propria. Furthermore, we found that mucosal humoral immunity in APRIL-deficient mice is less persistent than in WT mice. Hence, production of APRIL by inflammation-recruited neutrophils may create plasma cell niches in MALT to sustain a local antibody production.

CD56bright NK cells after hematopoietic stem cell transplantation are activated mature NK cells that expand in patients with low numbers of T cells.

We studied early NK-cell recovery in 29 allografted patients undergoing different lymphoreductive regimens. Already at 2 wk after graft take, the number of NK cells had reached (supra)normal levels but NK-cell subsets were skewed. The number of CD56(dim) CD16(bright) NK cells was low and correlated strongly with the level of hematopoiesis, whereas the number of the more abundant NK cells expressing high levels of CD56 did not. Post-transplant CD56(bright) NK cells (ptCD56(bright)) differed from CD56(bright) NK cells in normal controls (CD56(bright)) in being HLA-DR- and perforin-positive, CCR7(-), CD27(-), CD127(-) and mostly c-kit(-). CD56(bright) from normal controls stimulated by IL-15 in vitro (NK(IL-15)) acquired all the characteristics distinguishing CD56(bright) from ptCD56(bright). IL-2 exerted similar effects. Moreover, when cultured without cytokines, ptCD56(bright), CD56(bright) and NK(IL-15) responded similarly by upregulating CD127 and c-kit but not CCR7. IL-12 stimulated IFN-γ production in ptCD56(bright), whereas CD56(bright) responded only to IL-12 plus IL-15. Hence, ptCD56(bright) have all the features of cytokine-stimulated CD56(bright). Because only patients with low numbers of T cells had high numbers of ptCD56(bright), we conclude that ptCD56(bright) are activated CD56(bright) that expand while competing with T cells for the elevated post-transplant level of IL-15.

Absence of up-regulation for a proliferation-inducing ligand in Sjögren's sialadenitis lesions.

OBJECTIVE: To determine whether a proliferation-inducing ligand (APRIL) has a role in the survival of plasma cells infiltrating salivary glands from SS patients. METHODS: We performed immunological staining for APRIL in minor salivary glands from SS with a pair of antibodies specifically recognizing APRIL-producing cells and secreted APRIL. RESULTS: Despite high leucocyte infiltration, APRIL-producing cells, identified as neutrophils, were rare in SS salivary glands. Keratinocytes from the adjacent oral epithelium also produced APRIL, but we never detected significant levels of secreted APRIL in SS salivary glands. We obtained similar results with B-cell lymphomas associated with SS. In fact, there was no significant difference in APRIL production and the level of secreted APRIL in these pathological samples compared with normal corresponding tissues. CONCLUSION: The combined observation that APRIL production is not up-regulated in lesions from SS patients, and that secreted APRIL is not retained in these lesions, indicates that plasma cells frequently present in SS lesions may not rely on APRIL for survival, as they do in other rheumatic diseases.

Inhibition of Chondroitin Sulfate Proteoglycans by APRIL.

Chondroitin sulfate proteoglycans (CSPGs) are major constituents of the extracellular matrix and well-established obstacles to regeneration in the central nervous system. As such, they are promising targets for therapy in neurological pathologies where repair is needed, such as spinal cord injuries, and multiple sclerosis. Since CSPGs mediate their inhibitory functions by interacting with signaling protein partners through their variably sulfated chondroitin sulfate glycosaminoglycan (CS-GAG) chains, blocking these epitopes presents a path to promoting repair. A member of the tumor necrosis factor (TNF) superfamily, a proliferation-inducing ligand (APRIL) has been shown to bind to CSPGs. Here we describe in vitro methods to evaluate APRIL's ability to block CSPGs from interacting with their partner proteins and promote neuronal growth.

Case Report: In Situ Expression of a Proliferation-Inducing Ligand in Neuromyelitis Optica.

A proliferation inducing ligand (APRIL) mediates a key role in the generation and survival of antibody-inducing plasmocytes. Based on this, APRIL has been targeted in autoimmune diseases including multiple sclerosis (MS) and optic neuritis (ON). In MS lesions, APRIL has a new cellular target, the reactive astrocyte and mediates an immunosuppressive activity. Here, we analyzed APRIL expression in a case of neuromyelitis optica (NMO), another autoimmune neurodegenerative disease, showing selective aquaporin-4 depletion in the spinal cord, complement deposition and infiltration of polymorphonuclear cells. We analyzed by immunohistochemistry the presence of APRIL-producing cells, plasmocytes, astrocytes and the localization of secreted APRIL in a lesion from NMO. Plasmocytes were present close to APRIL-producing cells in meninges. However, our main observation was that APRIL targets reactive astrocytes in this lesion of NMO similarly to MS.

The number 13 of the family: a proliferation inducing ligand.

The TNF superfamily member a proliferation inducing ligand (APRIL, TNFSF13) plays a late role in humoral immunity at the level of antibody-producing plasmocytes. The recent characterization of the first immunodeficient patient with an inactivating mutation in the APRIL gene provided the last piece of functional data lacking in the human system. Based on this function, APRIL has been considered as a valuable target to dampen unwanted antibody production. After reviewing the late data acquired on the physiological function of APRIL in humoral immunity, we will here review the state of the art regarding APRIL targeting in autoimmune diseases.

Advanced Molecular Dynamics Approaches to Model a Tertiary Complex APRIL/TACI with Long Glycosaminoglycans.

Glycosaminoglycans (GAGs) are linear anionic periodic polysaccharides participating in a number of biologically relevant processes in the extracellular matrix via interactions with their protein targets. Due to their periodicity, conformational flexibility, pseudo-symmetry of the sulfation pattern, and the key role of electrostatics, these molecules are challenging for both experimental and theoretical approaches. In particular, conventional molecular docking applied for GAGs longer than 10-mer experiences severe difficulties. In this work, for the first time, 24- and 48-meric GAGs were docked using all-atomic repulsive-scaling Hamiltonian replica exchange molecular dynamics (RS-REMD), a novel methodology based on replicas with van der Waals radii of interacting molecules being scaled. This approach performed well for proteins complexed with oligomeric GAGs and is independent of their length, which distinguishes it from other molecular docking approaches. We built a model of long GAGs in complex with a proliferation-inducing ligand (APRIL) prebound to its receptors, the B cell maturation antigen and the transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI). Furthermore, the prediction power of the RS-REMD for this tertiary complex was evaluated. We conclude that the TACI-GAG interaction could be potentially amplified by TACI's binding to APRIL. RS-REMD outperformed Autodock3, the docking program previously proven the best for short GAGs.

Polarized Secretion of APRIL by the Tonsil Epithelium Upon Toll-Like Receptor Stimulation.

In mucosa such as tonsil, antibody-producing plasmocytes (PCs) lie in sub-epithelium space, which is thought to provide a suitable environment for their survival. A proliferation inducing ligand (APRIL) is one key survival factor for PCs present in this area. According to in situ staining, apical epithelial cells produced APRIL, and the secreted product had to migrate all through the stratified surface epithelium to reach basal cells. A similar process also occurred in the less-organized crypt epithelium. Tonsil epithelial cells captured secreted APRIL, thanks to their surface expression of the APRIL coreceptor, either syndecan-1 or -4 depending on their differentiation stage. In the most basal epithelial cells, secreted APRIL accumulated inside secretory lamp-1+ vesicles in a polarized manner, facing the sub-epithelium. The tonsil epithelium upregulated APRIL production by apical cells and secretion by basal cells upon Toll-like receptor stimulation. Furthermore, LPS-stimulated epithelial cells sustained in vitro PC survival in a secreted APRIL-dependent manner. Taken together, our study shows that the tonsil epithelium responds to pathogen sensing by a polarized secretion of APRIL in the sub-epithelial space, wherein PCs reside.

The microenvironment of DLBCL is characterized by noncanonical macrophages recruited by tumor-derived CCL5.

Tissue invasion by tumor cells induces a host inflammatory response that variably impacts tumorigenesis. This has been well documented for tumor-associated macrophages (TAMs) that could play a pro/M2- or an anti/M1-tumoral function. TAMs frequently infiltrate diffuse large B-cell lymphoma (DLBCL), an aggressive neoplasm arising from germinal center-experienced B cells. However, the pathway leading to the presence of TAMs in DLBCL remains unknown, and their impact is unclear. Here, we show that some DLBCL tumor cells expressed the chemokine CCL5, enabling the differential recruitment of blood monocytes through their expression of CCR1 and CCR5. CCL5 expression by DLBCL was not related to molecular subtypes, and healthy tonsillar B cells did not produce this chemokine, implying a posttransformation event. A single-cell analysis revealed that most DLBCL TAMs had a noncanonical gene signature with the concomitant expression of M1 and M2 genes. The presence of noncanonical TAMs may explain the lack of impact of macrophages on DLBCL development reported in some survival studies.