Differential effect of cholera toxin on CD45RA+ and CD45RO+ T cells: specific inhibition of cytokine production but not proliferation of human naive T cells

Mechanisms of Cholera Toxin in the Modulation of TH17 Responses

Numerous studies have shown that TH17 cells and their signature cytokine IL-17A are critical to host defense against various bacterial and fungal infections. The protective responses mediated by TH17 cells and IL-17A include the recruitment of neutrophils, release of antimicrobial peptides and chemokines, and enhanced tight junction of epithelial cells. Due to the importance of TH17 cells in infections, efforts have been made to develop TH17-based vaccines. The goal of vaccination is to establish a protective immunological memory. Most currently approved vaccines are antibody-based and have limited protection against stereotypically different strains. Studies show that T-cell-based vaccines may overcome this limitation and protect hosts against infection of different strains. Two main strategies are used to develop TH17 vaccines: identification of TH17-specific antigens and TH17-skewing adjuvants. Studies have revealed that cholera toxin (CT) induces a potent Th17 response following vaccination. Antigen vaccination along with CT induces a robust TH17 response, which is sometimes accompanied by TH1 responses. Due to the toxicity of CT, it is hard to apply CT in a clinical setting. Thus, understanding how CT modulates TH17 responses may lead to the development of successful TH17-based vaccines.

Cholera toxin directly enhances IL-17A production from human CD4+ T cells

The significance of Th17 cells and IL-17A signaling in host defense and disease development has been demonstrated in various infection and autoimmune models. Additionally, the generation of Th17 cells is highly influenced by microbes. However, the specific bacterial components capable of shaping Th17 responses have not been well defined. The goals of this study were to understand how a bacterial toxin, cholera toxin (CT), modulates Th17-dominated response in isolated human CD4(+) T cells, and what are the mechanisms associated with this modulation. CD4(+) cells isolated from human peripheral blood were treated with CT. The levels of cytokine production and specific Th cell responses were determined by ELISA, Luminex assay, and flow cytometry. Along with the decreased production of other proinflammatory cytokines (IFN-γ, TNF-α, and IL-2), we found that CT could directly enhance the IL-17A production through a cAMP-dependent pathway. This enhancement is specific for IL-17A but not for IL-17F, IL-22, and CCL20. Interestingly, CT could increase IL-17A production only from Th17-committed cells, such as CCR6(+)CD4(+) T cells and in vitro-differentiated Th17 cells. Furthermore, we also demonstrated that this direct effect occurs at a transcriptional level because CT stimulates the reporter activity in Jurkat and primary CD4(+) T cells transfected with the IL-17A promoter-reporter construct. This study shows that CT has the capacity to directly shape Th17 responses in the absence of APCs. Our findings highlight the potentials of bacterial toxins in the regulation of human Th17 responses.

Toxins-useful biochemical tools for leukocyte research

Leukocytes are a heterogeneous group of cells that display differences in anatomic localization, cell surface phenotype, and function. The different subtypes include e.g., granulocytes, monocytes, dendritic cells, T cells, B cells and NK cells. These different cell types represent the cellular component of innate and adaptive immunity. Using certain toxins such as pertussis toxin, cholera toxin or clostridium difficile toxin, the regulatory functions of Gα_i, Gαs and small GTPases of the Rho family in leukocytes have been reported. A summary of these reports is discussed in this review.

Inhibition of T cell proliferation by cholera toxin involves the modulation of costimulatory molecules CTLA-4 and CD28

Cholera toxin (CT) is known to inhibit the proliferation of murine and human T lymphocytes. In this study we have analysed the mechanisms underlying the inhibitory effect of CT on subpopulations of human CD4+ and CD8+ T lymphocytes. We show that CT dramatically prevents the activation of resting T lymphocytes, whereas it has a minor effect on cells that have been previously activated. Analysis of DNA content of the CT-treated T cells showed an arrest in the G(0)/G(1) phase and this correlated with high expression of the cyclin-dependent kinase inhibitor p27(kip). Moreover, we show that CT up-regulates the expression of the inhibitory molecule CTLA-4 in naïve, effector and memory resting CD4+ T cells and in resting CD8+ T lymphocytes. The regulation of CTLA-4 expression by CT is at the transcriptional level. Indeed, in cells treated with CT we observed an increase of two mRNA variants coding for the membrane and the soluble CTLA-4 molecules. In parallel with the up-regulation of the inhibitory CTLA-4, CT down-modulates the costimulatory molecule CD28 on CD4+ and CD8+ resting T cells. The increased expression of CTLA-4 played a role in controlling T cell activation and function as blocking anti-CTLA-4 F(ab')(2) mAbs partially inhibited anti-CD3 mAbs induced proliferation. These findings show that the inhibition of T cell proliferation by CT affects early stages of the T cell activation and involves the modulation of costimulatory molecules CTLA-4 and CD28 on resting T cells.

Recombined CC chemokine ligand 2 into B16 cells induces production of Th2-dominant [correction of dominanted] cytokines and inhibits melanoma metastasis

This study is aimed to verify whether CCL2 can induce Th2 polarization in vivo and subsequently inhibit tumor metastasis. B16 cells (a murine melanoma cell line) highly expressing CCL2 (CCL2-B16 cells) were obtained by transfection with recombinant plasmid CCL2-pcDNA3. Primary thymocytes were co-cultured with CCL2-B16 cells and STAT-6-mediated Th2 polarization was noticed after co-culture. Caudal vein injection of CCL2-B16 cells effectively inhibited pulmonary metastasis in C57BL/6 mice, but not in nude mice, indicating that T cells play a role in CCL2-induced inhibition of tumor metastasis. We found that high level of CCL2 up-regulated the expression of Th2-related cytokine (IL-4) in tumor microenvironment and increased CD4+, CD8+, and CD45RB+ cells in the peripheral blood and tumor tissues. We also demonstrated that inoculation of mice with CCL2-B16 cells prolonged mice survival time when they were reinjected with wildtype B16 cells, implying that CCL2 can activate immuno-memory in mice. It is concluded that high expression of CCL2 can induce Th2 polarization in tumor microenvironment and can effectively inhibit tumor metastasis, which casts new lights on the role of chemokines in reconstruction of immune surveillance in patients suffering from tumors.

Mucosal FOXP3-expressing CD4+ CD25high regulatory T cells in Helicobacter pylori-infected patients

Helicobacter pylori chronically colonizes the stomach and duodenum and causes peptic ulcers or gastric adenocarcinoma in 10 to 20% of infected individuals. We hypothesize that the inability of patients to clear H. pylori infections is a consequence of active suppression of the immune response. Here we show that H. pylori-infected individuals have increased frequencies of CD4(+) CD25(high) T cells in both the stomach and duodenal mucosa compared to uninfected controls. These cells have the phenotype of regulatory T cells, as they express FOXP3, a key gene for the development and function of regulatory T cells, as well as high levels of the cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) protein. In contrast, mucosal CD4(+) CD25(low) and CD4(+) CD25(-) cells express little FOXP3 mRNA and low levels of the CTLA-4 protein. Mucosal CD4(+) CD25(high) T cells are present in individuals with asymptomatic H. pylori infections as well as in duodenal ulcer patients. The frequencies of CD4(+) CD25(high) cells are also increased in the stomachs of H. pylori-infected patients with gastric adenocarcinoma, particularly in cancer-affected tissues. These findings suggest that regulatory T cells may suppress mucosal immune responses and thereby contribute to the persistence of H. pylori infections.

Phorbol esters and cAMP differentially regulate the expression of CD4 and CD8 in human thymocytes

BACKGROUND

Intrathymic development and selection of the T lymphocyte repertoire is restricted by the interactions of the T cell antigen receptor and CD4 or CD8 co-receptors with self major histocompatibility complex molecules. Positive or negative selection depends on a tight regulatory control of CD4 and CD8 expression. Determining the intracellular signals that differentially regulate the expression of CD4 and CD8 is important to understand the mechanisms that are implicated in selection of single positive CD4+CD8- or CD4-CD8+.

RESULTS

The present study shows that stimulation of human thymocytes by phorbol esters or cAMP result in a differential regulation of CD4 and CD8 expression, both at the mRNA and cell surface glycoprotein level.

CONCLUSIONS

The differential regulation of CD4 and CD8 gene expression suggests that the selective activation of protein kinase C (PKC) and cAMP-dependent protein kinases (PKA) may be required for the selection of single positive CD4+CD8- and CD4-CD8+ cells during Intrathymic differentiation.

Modulation of human lymphocyte proliferative response with aging

Previously, we have demonstrated age-associated alterations in transmembrane signaling. One of the most reproducible alterations found in the immune response with aging is the decrease of lymphocyte proliferation on stimulation with various different mitogens. Here, we confirm that proliferative responses to stimulation with phytohaemagglutin (PHA), recombinant human IL-2, or anti-CD3 monoclonal antibody are all greater in the young (20-25 years) than old (60-87 years) population. We attempted to modulate the proliferative response using various agents acting at different levels of transmembrane signaling (pertussis toxin, cholera toxin, isoproterenol, PMA, Ca ionophore A23187), as well as at the level of the lymphocyte plasma membrane (methyl-beta-cyclodextrin, MBCD), or by using antioxidant vitamins (Vitamin E or C). None of these agents was able to restore effectively the proliferative response of lymphocytes from the aged to the level of young subjects. Even the combination of A23187 and PMA acting directly on calcium metabolism and protein kinase C activity was insufficient to restore the decreased mitogenic capacity of T cells from elderly subjects. Cyclodextrin, which decreases the cholesterol content of the membrane, increased the proliferative response of lymphocytes of elderly subjects, but not to the level of the young. Vitamin E had a very strong inhibitory effect on lymphocyte stimulation in both the age groups, except in combination with MBCD in T cells of the elderly, while Vitamin C had no significant modulatory effect. MAPK ERK and p38 activation was found to be decreased with aging in T cells after anti-CD3 mAb stimulation. Vitamin E but not Vitamin C strongly inhibited MAPK ERK or p38 activation. The direct activation of certain molecules or the modulation of the cholesterol content of the membrane seems to be effective immunomodulatory interventions with aging.

CD45: new jobs for an old acquaintance

Identified as the first and prototypic transmembrane protein tyrosine phosphatase (PTPase), CD45 has been extensively studied for over two decades and is thought to be important for positively regulating antigen-receptor signaling via the dephosphorylation of Src kinases. However, new evidence indicates that CD45 can function as a Janus kinase PTPase that negatively controls cytokine-receptor signaling. A point mutation in CD45, which appears to affect CD45 dimerization, and a genetic polymorphism that affects alternative CD45 splicing are implicated in autoimmunity in mice and multiple sclerosis in humans. CD45 is expressed in multiple isoforms and the modulation of specific CD45 splice variants with antibodies can prevent transplant rejections. In addition, loss of CD45 can affect microglia activation in a mouse model for Alzheimer's disease. Thus, CD45 is moving rapidly back into the spotlight as a drug target and central regulator involved in differentiation of multiple hematopoietic cell lineages, autoimmunity and antiviral immunity.