Macrophages and neutrophils as major source of B-cells activating factors in H. pylori associated diseases
Year: 2012
Type: dissertation
Abstract: The cytokine APRIL (A proliferation inducing ligand) and the cytokine BLyS (B lymphocyte simulator) are two recently discovered factors that belong to the TNF-(tumor necrosis factor) family. Since their discovery, these cytokines have been intensively studied for their role in B cells biology. Indeed, APRIL and BLyS are involved in the maturation of B cells, but, most importantly, they increase the survival and proliferation of these cells. However, the effect of these cytokines is amplified in neoplastic B cells in lymphomas, and self-reactive B cells in autoimmune diseases. In particular, it has been demonstrated that APRIL is over-expressed in different types of lymphoma, whereas BLyS is very abundant in serum and tissues of patients with different types of autoimmune diseases, such as, rheumatoid arthritis or systemic lupus erythematous. In our study, we evaluated the role of these cytokines in two diseases caused by the bacterium Helicobacter pylori (H. pylori): MALT lymphoma (I) and autoimmune gastritis (AIG) (II). (I) the development of lymphoid follicles with germinal centers and an acquired MALT (mucosa-associated lymphoid tissue) are characteristic of H. pylori-induced gastritis. These MALT represents a pre-neoplastic condition associated with infection, which can evolve into a B-cell lymphoma. It is currently unclear which cytokines or soluble factors promote B cell activation and the genesis of lymphoma during H. pylori infection. In our study, we have demonstrated that gastric MALT lymphoma is characterized by high expression of APRIL. Furthermore, APRIL is over-expressed also in a pre-MALT situation, suggesting that this cytokine may be involved in the induction, as well as in the proliferation of the lymphoma. Further analysis allowed us to identify the macrophages as the main source of APRIL in this type of cancer. We confirmed this observation by in vitro experiments, suggesting that H. pylori is able to induce the secretion of APRIL by macrophages. We have also shown that the contribution to APRIL secretion is supported also by tumour infiltrating T cells specific for H. pylori. Thus, we have demonstrated that H. pylori is able to induce APRIL secretion by macrophage in a direct way, following in vitro infection, but even in an undirect way, mediated by H. pylori-specific T lymphocytes. Our data represent the first evidence of the involvement of APRIL in the development of MALT lymphoma in H. pylori infected patients. Moreover, we identified macrophages as key cells in this process. (II) Among the diseases caused by H. pylori there is also AIG. It has recently been reported that the cytokine BLyS plays a pivotal role in the pathogenesis of two different models of autoimmune disease (collagen induced arthritis and experimental autoimmun encephalomyelitis); in these works, it has been demonstrated that BLyS induce naive T cells to differentiate into the Th17 phenotype. These effector T cells, produce IL-17 and they play a crucial role in the pathogenesis and maintenance of several autoimmune diseases. Thus, given that it is established that H. pylori infection is associated with a Th17 response, we wondered to investigate whether the BLyS/Th17 axis is involved in the development of AIG following H. pylori-infection. To test this hypothesis, we have considered H. pylori-induced chronic gastritis as a model for our analysis because it is the typical inflammatory context that might evolve AIG in susceptible patients. We have demonstrated that patients with chronic gastritis not only present, as expected, a Th17-enriched cellular infiltrate in gastric mucosa, but also that in these biopses there is a significant expression of BLyS. Noteworthy, after the eradication of the bacterium there is a marked reduction of both BLyS and IL-17 cytokines in gastric mucosa, underlining the crucial role of H. pylori in the axis BLyS/Th17. We have also shown that BLyS is able to activate in vitro monocytes and macrophages, inducing the secretion of IL-1β, IL-6, TGF-β and IL-23 by these cells: all these cytokines are fundamental for Th17 cells differentiation. In addition, we identified neutrophils as a possible source of BLyS in gastric mucosa. Indeed, H. pylori-stimulated neutrophils secrete a great amount of BLyS in vitro. Accordingly, there are evidence that neutrophils represent a great percentage of cellular infiltrate during H. pylori-sustained inflammation. Therefore, we hypothesized a model for AIG development of following H. pylori-infection: the bacterium stimulates gastric mucosa-infiltrating neutrophils to release BLyS. BLyS, in turn could on one hand lead to the survival of autoreactive B cells (characterized by auto-antibodies production), but on the other hand, it might also acts on monocytes/macrophages (which are also abundant in the inflammatory infiltrate) for the induction of pro-Th17 cytokines. The Th17-pro-inflammatory profile may increase inflammation and worsen the tissue damage which is typically found in patients with AIG. However, this study is still in progress and many things remain to be understood to validate this model. We first of all need to identify BLyS-expressing cells in chronic gastritis; to this aim different double-immunohistochemistry staining with specific markers for different immune cell types are currently underway. In addition, the results of immunohistochemical analysis will be corroborated with in vitro experiments aimed to: i) confirm the ability of cells, identified by immunohistochemistry, to secrete BLyS following stimulation with H. pylori, ii) confirm the role of BLyS in the differentiation of Th17 cells. Although this second study is currently ongoing, we have demonstrated that H. pylori might create a cytokine microenvironment which can drive the development of MALT lymphoma or AIG. In particular, we have shown that H. pylori is able to induce the production and release of BLyS and APRIL by macrophages and neutrophils, respectively, in two diseases characterized by dysfunction of B cells. These data may suggest that targeting these cytokines may be a new additional therapeutic approach for the treatment of AIG and MALT lymphoma
Author: Munari, Fabio
Language: English
Subfield: Gastroenterology
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W89048862