Biological effects of Pseudomonas aeruginosa exotoxin A: lymphoproliferation of T lymphocytes in athymic mice

Revisiting the virulence hallmarks of Pseudomonas aeruginosa: a chronicle through the perspective of quorum sensing

Pseudomonas aeruginosa is an opportunistic pathogen and the leading cause of mortality among immunocompromised patients in clinical setups. The hallmarks of virulence in P. aeruginosa encompass six biologically competent attributes that cumulatively drive disease progression in a multistep manner. These multifaceted hallmarks lay the principal foundation for rationalizing the complexities of pseudomonal infections. They include factors for host colonization and bacterial motility, biofilm formation, production of destructive enzymes, toxic secondary metabolites, iron-chelating siderophores and toxins. This arsenal of virulence hallmarks is fostered and stringently regulated by the bacterial signalling system called quorum sensing (QS). The central regulatory functions of QS in controlling the timely expression of these virulence hallmarks for adaptation and survival drive the disease outcome. This review describes the intricate mechanisms of QS in P. aeruginosa and its role in shaping bacterial responses, boosting bacterial fitness. We summarize the virulence hallmarks of P. aeruginosa, relating them with the QS circuitry in clinical infections. We also examine the role of QS in the development of drug resistance and propose a novel antivirulence therapy to combat P. aeruginosa infections. This can prove to be a next-generation therapy that may eventually become refractory to the use of conventional antimicrobial treatments.

Bacterial toxins as immunomodulators

Bacterial toxins are the causative agent at pathology in a variety of diseases. Although not always the primary target of these toxins, many have been shown to have potent immunomodulatory effects, for example, inducing immune responses to co-administered antigens and suppressing activation of immune cells. These abilities of bacterial toxins can be harnessed and used in a therapeutic manner, such as in vaccination or the treatment of autoimmune diseases. Furthermore, the ability of toxins to gain entry to cells can be used in novel bacterial toxin based immuno-therapies in order to deliver antigens into MHC Class I processing pathways. Whether the immunomodulatory properties of these toxins arose in order to enhance bacterial survival within hosts, to aid spread within the population or is pure serendipity, it is interesting to think that these same toxins potentially hold the key to preventing or treating human disease.

Effect of Pseudomonas aeruginosa exotoxin A on IFN-gamma synthesis: expression of costimulatory molecules on monocytes and activity of NK cells

The aim of the study was (1) to evaluate the effect of Pseudomonas aeruginosa Exotoxin A (P-ExA) on the production of IFN-gamma in anti-CD3 induced human peripheral blood mononuclear cells (PBMC) and (2) to establish the effect of P-ExA on the IFN-gamma dependent cellular activities such as the expression of costimulatory molecules on monocytes and cytotoxicity of NK cells. The toxin in a high dose (100 ng/ml) inhibited IFN-gamma synthesis. Inhibitory effect of P-ExA was abolished by IL-1alpha which in a combination with P-ExA exerted a strong synergistic effect on IFN-gamma synthesis. Other monokines such as IL-1beta, IL-6, TNF-alpha neither reversed the inhibitory effect of P-ExA nor induced production of IFN-gamma. P-ExA also inhibited IFN-gamma-induced cellular events: (1) expression of costimulatory molecules on monocytes (CD80, CD86, ICAM-1, HLA-DR); (2) cytotoxic activity of NK cells. Inhibition of NK cells activity by P-ExA was not reversed by cytokines such as IL-2, IFN-alpha and TNF-alpha, which are known to enhance effector functions of NK cells. From these results we conclude that: (1) inhibition of IFN-gamma synthesis, as well as IFN-gamma-induced expression of costimulatory molecules and NK-cell effector functions may lead to suppression of specific and non-specific defense mechanisms, respectively, which are necessary for elimination of PA bacteria; (2) enhancement of IFN-gamma synthesis induced by P-ExA in a combination with IL-1alpha may cause harmful, Th1 cells dependent, inflammatory reactions of the host (septic shock, tissue damage) during infection with Pseudomonas aeruginosa.

GM-CSF is required for the Pseudomonas exotoxin A-induced proliferation of immature T cells in athymic mice

Previous studies have demonstrated that Pseudomonas exotoxin A stimulated the proliferation of immature T lymphocytes within the splenocytes of athymic mice. These studies were performed to determine which lymphokines were involved in the proliferation of the immature T cells. The results of this study indicate that exotoxin A does not induce the production of interleukin-2 or tumor necrosis factor from B cell-depleted splenotypes from athymic mice. However, exotoxin A does induce the production of granulocyte-macrophage colony-stimulating factor (GM-CSF) from B cell-depleted splenocytes. Furthermore, the GM-CSF was shown to be produced by a Thy1+, CD4-, CD8- T lymphocyte. The addition of anti-GM-CSF antibody abrogates the exotoxin A-induced proliferation of B cell-depleted splenocytes from athymic mice. Thus, these data indicate that exotoxin A induces the production of GM-CSF from immature T lymphocytes within the splenocytes of athymic mice and the exotoxin A-induced proliferation of these immature T cells is dependent on the presence of GM-CSF.

Immunosuppressive effects of Pseudomonas aeruginosa exotoxin A on human B-lymphocytes

In this study we investigated the effects of exotoxin A on proliferation and differentiation of human B-cells in vitro. Exotoxin A at a concentration of 1 microgram/ml inhibited the proliferation of B-cells preactivated by insolubilized anti-IgM antibody or by formalinized Staphylococcus aureus particles, plus IL-2 or IL-4. B-cell blasts obtained after preactivation of tonsillar B-cells produce IgG and IgM in culture supernatants, and this Ig production is enhanced by IL-2 or IL-4. Exotoxin A inhibited the production of IgG and IgM by the B-blasts at the concentration of 1 microgram/ml.

Induction of interleukin-1 from murine peritoneal macrophages by Pseudomonas aeruginosa exotoxin A

Pseudomonas exotoxin A, an ADP-ribosylating toxin produced by Pseudomonas aeruginosa, has been shown to stimulate the proliferation of murine thymocytes, which requires the participation of accessory cells. This requirement for accessory cells can be replaced by supernatant from adherent peritoneal exudate cells that have been stimulated with exotoxin A. Antibody to exotoxin A inhibits the induction of the thymocyte mitogenic activity from adherent peritoneal macrophages. However, antibody to exotoxin A had no effect on the thymocyte proliferation if the antibody was added to supernatant which contained thymocyte mitogenic activity. The thymocyte mitogenic activity was associated with a protein or protein complex with a molecular mass of greater than 10,000 daltons. D10 bioassays indicated the presence of interleukin-1 (IL-1) in the supernatant. Antibody to IL-1 inhibited the ability of supernatant to induce thymocytes to proliferate. Therefore, these data suggest that Pseudomonas exotoxin A can stimulate the production of IL-1 from adherent peritoneal cells, which induces murine thymocytes to proliferate.