Proinflammatory mediators elicit secretion of the intracellular B-lymphocyte stimulator pool (BLyS) that is stored in activated neutrophils: implications for inflammatory diseases

FcRY, an Fc receptor related gene differentially expressed during B lymphocyte development and activation

A bioinformatics approach has lead to the identification of FcRY, a new Fc receptor related gene. FcRY is predicted to encode a protein with three immunoglobulin (Ig) domains followed by a mucin-like domain containing a proline-rich stalk and a C-terminal leucine rich region. The predicted protein lacks a hydrophobic domain for insertion into the plasma membrane, suggesting that FcRY is an intracellular or secreted protein. This feature is shared with the product of the FcRX/FCRL/FREB gene that is closely linked to FcRY in both human and mouse genomes. Fcry transcripts are first detectable among mouse B lineage cells at the pre-B cell stage. Splenic B cells of the newly formed, follicular, and marginal zone subsets express Fcry, as do germinal center B cells to a lesser extent. FcRY is also expressed in subpopulations of human B cells. A consistent characteristic of FcRY in both species is low level gene expression, which can be further downregulated in normal mouse B cells by signaling through the B cell receptor (BCR) or CD40, thereby suggesting a correlation between cell cycle entrance and diminished FcRY expression. Fcry is upregulated by short-term treatment with BAFF/BLyS, which promotes B cell survival rather than proliferation. LPS induces very rapid but transient enhancement. We observed a pronounced upregulation of Fcry expression in WEHI 231 cells induced by BCR crosslinking to undergo cell cycle arrest prior to apoptosis, consistent with the possible regulation of Fcry expression by cell cycle status.

Development of an immunoassay kit for detecting the alteration of serum B cell activating factor in thermally injured mice

B cell activating factor (BAFF), a member of the family of tumor necrosis factor (TNF) ligands, is essential for the development of peripheral mature, long lived B lymphocytes. Previous studies were almost related to the function or mechanism of BAFF protein and there is little report about BAFF expression in thermally injured animals. Here, we developed a special ELISA kit to study the change of BAFF expression in thermally injured mouse model. It was shown that BAFF expression changed in double-phase. Peripheral BAFF level dropped rapidly after thermal injury and at 24 h of scald it was only 1/4 compared with that of the control group, 24 h later it began to rise up slowly and then returned to the normal level comparable to the control at 120 h.

TACItly changing tunes: farewell to a yin and yang of BAFF receptor and TACI in humoral immunity?

PURPOSE OF REVIEW

The complex system of the tumour necrosis factor ligands BAFF and APRIL and their three receptors BCMA, TACI and BAFF receptor and its role in B-cell development and function is the objective of extensive research. Whereas the importance of BAFF/BAFF receptor interactions for B-cell survival could be clearly demonstrated, TACI is believed to counteract BAFF activity as a negative regulator in the murine model. The primarily immunodeficient phenotype of human TACI deficiency, however, claims a distinct function of this receptor in human peripheral B-cell development, class switch recombination and terminal differentiation.

RECENT FINDINGS

Common variable immunodeficiency comprises a heterogeneous group of antibody deficiency syndromes characterized by impaired terminal B-cell differentiation. By means of molecular genetics common variable immunodeficiency is still ill-defined, but the description of the deficiency of the inducible costimulator in a small subgroup of common variable immunodeficiency patients set the starting point for the molecular dissection of this disease entity. The recent discovery of genetic defects in the tumour necrosis factor receptor superfamily members TACI and BAFF receptor in patients with common variable immunodeficiency denotes further advances in this field.

SUMMARY

In this review we will discuss recent progress made in the understanding of the BAFF/APRIL-TACI/BCMA/BAFF receptor system in relation to the recent discovery that mutations in human TACI cause a primary humoral immunodeficiency. This suggests a refined role for TACI in human B-cell biology.

BAFF is elevated in serum of patients with Wegener's granulomatosis

BAFF (B-cell activating factor of the TNF family) plays a crucial role in B-cell survival. Elevated BAFF serum levels have been linked to several autoimmune diseases in humans, and therapies targeting BAFF were successful in animal models of rheumatoid arthritis and systemic lupus erythematosus. Wagener's granulomatosis (WG), a chronic systemic vasculitis, is characterized by circulating autoantibodies (cANCA) targeting neutrophils, which can produce BAFF. To investigate whether BAFF is involved in WG pathology, BAFF serum levels were measured by ELISA in 46 WG patients and 62 healthy donors. We report the novel finding that in WG patients serum levels of BAFF were significantly increased (median 3.95 ng/ml, p=0.009) compared to healthy controls (median 2.38 ng/ml). The difference was even more pronounced when comparing controls with untreated WG patients (median 4.61 ng/ml, p=0.001). Treatment of WG patients with glucocorticoids was associated with lower BAFF levels. The serum BAFF level in treated WG patients was about the same as in the control group. We propose that BAFF might be a pathogenic factor in WG and that targeting BAFF may represent a new therapeutic strategy in a subset of chronically relapsing WG patients with elevated BAFF levels.

Interferon-activated neutrophils store a TNF-related apoptosis-inducing ligand (TRAIL/Apo-2 ligand) intracellular pool that is readily mobilizable following exposure to proinflammatory mediators

Neutrophils are versatile cells, which play a role, not only in inflammatory processes but also in immune and antitumoral responses. Recently, we have reported that interferon (IFN)-activated neutrophils are able to release biologically active tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/APO2 ligand), a molecule exerting selective, apoptotic activities toward tumor and virus-infected cells, as well as immunoregulatory functions on activated T lymphocytes. Herein, we show that only a minor fraction of the total TRAIL, newly synthesized by IFN-activated neutrophils within 24 h, is released outside, the rest being retained intracellularly, mainly in secretory vesicles and light membrane fractions. We demonstrate that the intracellular pool of TRAIL present in IFN-pretreated neutrophils is rapidly mobilizable to the cell surface and can be secreted following exposure to proinflammatory mediators such as TNF-alpha, lipopolysaccharide, formyl-methionyl-leucyl-phenylalanine, CXC chemokine ligand 8/interleukin-8, insoluble immunocomplexes, and heat shock protein Gp96. These various proinflammatory agonists functioned as effective secretagogue molecules only, in that they failed to augment TRAIL mRNA expression or TRAIL de novo synthesis in freshly isolated neutrophils or cultured with or without IFN. In addition, supernatants from IFN-treated neutrophils stimulated with proinflammatory mediators induced the apoptosis of target cells more effectively than supernatants from neutrophils activated with IFNs alone. Collectively, our results uncover a novel mechanism, whereby the release of soluble TRAIL by neutrophils can be greatly amplified and further reinforce the notion that neutrophils are important cells in tumor surveillance and immunomodulation.

Inflammation and the reciprocal production of granulocytes and lymphocytes in bone marrow

The coordinated production of leukocytes in bone marrow is crucial for innate and adaptive immunity. Inflammation alters normal leukocyte production by promoting granulopoiesis over lymphopoiesis, a response that supports the reactive neutrophilia that follows infection. Here we demonstrate that this specialization for granulopoiesis is determined by inflammation-induced reductions of growth and retention factors, most significantly stem cell factor and CXCL12, which act preferentially to inhibit lymphoid development. These hierarchical effects suggest that the normal equilibrium of leukocyte production in bone marrow is determined by lymphopoiesis' higher demand for specific growth factors and/or retention signals. Inflammation regulates this balance by reducing growth factors that have less impact on developing neutrophils than lymphocytes. We demonstrate that granulopoiesis and lymphopoiesis are coupled specifically in the bone marrow by development in a common niche and propose that the leukopoietic equilibrium is specified by limiting amounts of developmental resources.

The role of APRIL and BAFF in lymphocyte activation

The TNF family ligands BAFF (also called BLyS) and APRIL regulate lymphocyte survival and activation. BAFF binds to three receptors, BAFF-R, TACI and BCMA, whereas APRIL interacts with TACI, BCMA and proteoglycans. The contribution of BAFF and APRIL to B-cell and plasma-cell survival, CD154 (CD40L)-independent antibody isotype switching, germinal center maintenance, T-dependent and T-independent antibody responses, and T cell co-stimulation are relatively well understood. Constitutive BAFF produced by stromal cells determines the size of the peripheral B cell pool, whereas inducible BAFF produced by myeloid and other cells supports local survival of B lymphocytes and can be associated with development of autoimmunity when deregulated.