| Cholera toxin (CT and the closely related heat-labile enterotoxin (HLT) from Escherichia coli (LT-I) have been extensively studied and employed as mucosal adjuvants. However, information concerning the inmunostimulatory properties of the structurally related but antigenically distinct type II heat-labile toxins is lacking. While CT, LT-IIa, and LT-IIb holotoxins were all found to possess potent ad uvant properties, we observed CT induced a Predominant Th2-associated immune response, whereas LT-IIa, and LT-IIb elicited a more balanced Th1/Th2 profile with significantly higher levels of immunoglobulin G2a (IgG2a) and interferon gamma production than that observed in CT-immunized mice. Additional studies addressing the Immunomodulatory effects of the nontoxic A2/B subunits of CT and LT-IIa genetically coupled to saliva-binding region (SBR) demonstrated that the CT-based chimera (SBR-CTA2/B) induced significantly higher mucosal and systemic antibodies in mice compared to that observed in mice immunized with LT-IIa-based chimera (SBR-LT-IIaA2/B). The potent adjuvant properties of (SBR-CTA2/B) were associated with a selective ability to upregulate B7-2 expression on mucosal and systemic B cells. The costimulatory activity of SBR-CTA2/B-treated cells revealed a significant enhancement in anti-CD3-stimulated CD4+T cell proliferative responses Old was significantly reduced by treatment with anti-B7-2 antibodies. We next identified the cellular mechanisms involved in the distinct cytokine profiles induced by CT, LT-IIa, and LT-IIb. Thus, we initially analyzed the cytokine production from anti-CD3-stimulated human peripheral blood mononuclear cells in the presence or absence of CT, LT-IIa, or LT-IIb. Similar to our observations in the mouse model, we observed that, unlike LT-IIa and LT-IIb. CT selectively suppressed Th1-associated cytokines, including interleukin (IL)-2, IL-12, and tumor necrosis factor alpha (TNF-alpha). Moreover, CT-treated cultures exhibited a significant, reduction in both T cell activation and subsequent upregulation of CD40 ligand (CD40L) compared to LT-IIa- or LT-IIb-treated cultures. Using an in vitro co-culture system, CT-treated CD4+T cells induced significantly less TNF-alpha and IL-12 p70 production by both autologous monocytes and dendritic cells compared to LT-IIa- or LT-IIb-treated T cells. These studies establish the adjuvant properties of the serogroup II HLT and aid in understanding the cellular mechanisms involved in the distinct immune responses observed with serogroup I and serogroup II HLT. |
