Search

Gastric epithelial cells (GECs) are the primary target for Helicobacter pylori infection and may act as APCs regulating local T cell responses. We previously reported that H. pylori infection of GECs induces the expression of the T cell coinhibitory molecule B7-H1 on GECs. This process contributes to the hyporesponsiveness of CD4+ effector T cells and accumulation of regulatory T cells. In the present study, we investigated the impact of H. pylori cytotoxin-associated gene A (CagA) on the modulation of the expression of the T cell costimulator B7-H2 by GECs. B7-H2 is involved in promoting Th17 type responses. H. pylori infection downregulates B7-H2 expression by GECs in a CagA-dependent manner. IFN-γ, which is increased in the H. pylori–infected gastric mucosa, synergizes with H. pylori in downregulating B7-H2 expression by GECs. CagA-mediated modulation of B7-H2 on GECs involves p70 S6 kinase phosphorylation. The CagA-dependent B7-H2 downregulation in GECs correlates with a decrease in Th17 type responses in vitro and in vivo. Furthermore, CagA-dependent modulation of Th17 responses was inversely correlated with the H. pylori colonization levels in vivo. Our data suggest that CagA contributes to the ability of H. pylori to evade Th17-mediated clearance by modulating expression of B7-H2 and, thus, to the establishment of the H. pylori chronic infection.

The human gastrointestinal mucosa is exposed to a diverse normal microflora and dietary Ags and is a common site of entry for pathogens. The mucosal immune system must respond to these diverse signals with either the initiation of immunity or tolerance. APCs are important accessory cells that modulate T cell responses which initiate and maintain adaptive immunity. The ability of APCs to communicate with CD4+ T cells is largely dependent on the expression of class II MHC molecules by the APCs. Using immunohistochemistry, confocal microscopy, and flow cytometry, we demonstrate that α-smooth muscle actin+, CD90+ subepithelial myofibroblasts (stromal cells) constitutively express class II MHC molecules in normal colonic mucosa and that they are distinct from professional APCs such as macrophages and dendritic cells. Primary isolates of human colonic myofibroblasts (CMFs) cultured in vitro were able to stimulate allogeneic CD4+ T cell proliferation. This process was dependent on class II MHC and CD80/86 costimulatory molecule expression by the myofibroblasts. We also demonstrate that CMFs, engineered to express a specific DR4 allele, can process and present human serum albumin to a human serum albumin-specific and DR4 allele-restricted T cell hybridoma. These studies characterize a novel cell phenotype which, due to its strategic location and class II MHC expression, may be involved in capture of Ags that cross the epithelial barrier and present them to lamina propria CD4+ T cells. Thus, human CMFs may be important in regulating local immunity in the colon.

CD74, or the class II MHC-associated invariant chain, is best known for the regulation of Ag presentation. However, recent studies have suggested other important roles for this protein in inflammation and cancer studies. We have shown that CD74 is expressed on the surface of gastric cells, and Helicobacter pylori can use this receptor as a point of attachment to gastric epithelial cells, which lead to IL-8 production. This study investigates the ability of H. pylori to up-regulate one of its receptors in vivo and with a variety of gastric epithelial cell lines during infection with H. pylori. CD74 expression was increased dramatically on gastric biopsies from H. pylori-positive patients and gastric cell lines exposed to the bacteria. Gastric cells exposed to H. pylori-conditioned medium revealed that the host cell response was responsible for the up-regulation of CD74. IL-8 was found to up-regulate CD74 cell surface expression because blocking IL-8Rs or neutralizing IL-8 with Abs counteracted the increased expression of CD74 observed during infection with H. pylori. These studies demonstrate how H. pylori up-regulates one of its own receptors via an autocrine mechanism involving one of the products induced from host cells.

During H. pylori infection, T cells are recruited to the gastric mucosa, but the host response is unable to clear the infection. We have previously shown that during infection H. pylori induces B7-H1 on the gastric epithelium, and interacting T cells are thus anegized. Recently, T regulatory cells (Treg) with a CD4+ CD25high FoxP3+ phenotype were found at an increased frequency in the H. pylori-infected gastric mucosa. Since B7-H1 promotes T cell anergy during H. pylori infection and Treg cells are present in the infected gastric mucosa, we hypothesized that B7-H1 expression by gastric epithelial cells during H. pylori infection promotes development of T regulatory cells. In order to investigate this, we examined by flow cytometry the induction of the Treg phenotype when naïve T cells were incubated with H. pylori-exposed gastric epithelial cells. The number of CD4+CD25+FoxP3+ cells increased together with the production of IL-10 and TGF-β in the cultures. The frequency of Treg cells along with IL-10 and TGF-β production was markedly decreased when B7-H1 was blocked with monoclonal antibodies. The function of these Treg cells was investigated by sorting them for use in co-cultures with activated T cells, which effectively decreased proliferation of the activated T cells. B7-H1 may thereby play a crucial role in the development of CD4+CD25+FoxP3+ Treg cells and the inadequate T cell response leading to chronic infection.

CD4+ T cells are recruited to the gastric mucosa during H. pylori infection, but are generally hyporesponsive. CD4+CD25+FoxP3+ regulatory T cells (Treg) are present during infection and impact the CD4+ T cell response. We have previously shown that epithelial cell responses to H. pylori play an important role in inhibiting CD4+ T cell responses via the local induction of Treg cells from naïve CD4+ T cells. As TGF-β plays a role in the development of Tregs, we hypothesized that gastric epithelial cells (GECs) produce TGF-β during H. pylori infection that contributes to Treg development and suppresses local CD4+ T cell effector function. In order to investigate this, we examined GEC production of TGF-β after exposure to H. pylori and found them to produce significant levels of TGF-β that were partially dependant on the cag PAI virulence factor of H. pylori. TGF-β produced by GEC in response to H. pylori increased the local development of CD4+CD25+FoxP3+ Treg cells from naïve T cells in co-culture with GEC and decreased proliferation of activated CD4+ T effector cells in co-culture with GEC. These events were decreased when TGF-β neutralizing antibodies were present. The observations presented here introduce a novel mechanism used by H. pylori via gastric epithelial cells leading to inhibition of CD4+ T cells that may contribute to chronicity of infection.