The role of the S-1 and B-oligomer components of pertussis toxin in its adjuvant properties for Th1 and Th2 cells

Pertussis toxin (PT) is a major virulence factor of Bordetelfa pertussis which exerts a range of biological effects on the immune system. The toxin is a heterohexameric protein with an A-B toxin structure [ 11. The A-promoter comprises a single S1 subunit with ADP-ribosyltransferase activity which regulates signal transduction in mammalian cell membranes. The Boligomer is composed of five subunits S2, S3, S4 and S5 and mediates binding of the toxin to eukaryotic cells as well as inducing mitogenic T cell activation. It has been reported that PT, like other A-B toxins, such as cholera toxin and E. coli enterotoxin, possesses adjuvant properties. I t has been demonstrated that co-injection of antigens with PT enhances IgE production, DTH reactions and experimental autoimmune disease in mice [ I , 31. Paradoxically PT appears to influence immune responses mediated by reciprocally regulated subpopulations of CD4+ Th cells that secreted IFNy. IL-2 or IL-4 and IL-5, termed Thl and Th2 cells respectively. However the mechanisms by which PT exerts its adjuvanticity remain to be defined. In this study we have examined the capacity of PT to augment cytokine secretion by antigen-specific T cells when coinjected with a foreign antigen. Mice were immunized with 20pg of KLH alone or with 1.Opg of active PT at week 0 and week 4. Spleen cells were isolated on week six and stimulated with a range of concentrations of KLH to determine Th cell responses. We found that PT can potentiate antigen-specific CD4' T cell proliferation and the secretion of IFNy, IL-2, IL-4 and IL-5 when injected with the antigen (Fig 1 A & B and data not shown). A chemically detoxified PT (PTd), or a genetic mutant with substitutions/de\etions in the S1 and B-oligomer components, that abrogate enzymatic activity and receptor binding [3], were found to be devoid of the adjuvant properties. In contrast, a non-toxic S-1 mutant devoid of enzymatic activity, but still capable of receptor binding, had a modest effect on IFN-y and IL-2 secretion, but lost its ability to enhance IL-4 or IL-5 production. These findings demonstrate that PT can potentiate the activation of both Thl and Th2 subpopulations of T cells. Although we have evidence that enhanced T cell responses may be associated with increased expression of the co-stimulatory molecules B7-1 and B7-2, the adjuvant effect of PT also appears to reflect distinct effects of the B-oligomer and the S-1 subunit on T cell cytokine secretion, through their stimulation of alternative signal transduction pathways. In recent studies it has been shown that the activation of the adenylate cyclase pathway upregulates IL-4 and IL-5 production by activated CD4+ T cells, the ADPribosyltransferase activity of the SI subunit may allow it to selectively induce the secretion of Th2 type cytokines by T cells [4]. Conversely activation of the inositol phosphate pathway, the signal transduction pathway of the B-oligomer, induces the production of new mRNA for IFNy and IL-2 [5 ] , suggesting that the B-oligomer may selectively potentiate Thl responses. Recent evidence suggests that Th 1 cells play a key role in protection against B. pertussis infection. Using a murine aerosol challenge model we have shown that infection with B. pertussis