The B pentamer of LT-IIa, a type II heat-labile enterotoxin of Escherichia coli enhances dendritic cell functions
Lee, Chang Hoon
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The mucosal membranes of the respiratory, gastric, and genital tracts are the major sites in which most human pathogens invade and/or colonize. Thus, it is highly desirable to develop agents that can enhance protective immune responses on those surfaces. Mucosal vaccines are one of the most effective methods for protecting the mucosa against invading/colonizing pathogens. However, due to endogenous immunesuppressing mechanisms in the highly organized mucosal immune system, it is difficult to evoke strong immune responses to antigens on those surfaces. Thus, almost all trials which evaluated an experimental mucosal immunization have failed to provide protection against infection. To circumvent the immune-suppressing mechanisms, strong mucosal adjuvants are employed. Heat-labile enterotoxins (HLT) have been shown to be potent mucosal adjuvants. Yet, HLT are toxic and the mechanisms by which HLT evoke mucosal immune responses in vivo are poorly characterized. In this study, we demonstrated that LT-IIa-B 5 , the non-toxic B pentameric subunit of LT-IIa, a type II heat-labile enterotoxin, is an effective mucosal adjuvant. Intranasal (i.n.) immunization of mice with the model antigen (Ag), ovalbumin (OVA) in the presence of LT-IIa-B 5 , enhanced uptake of Ag by dendritic cells (DC) in the nasal-associated lymphoid tissue (NALT) in vivo and in bone-marrow derived dendritic cells (BMDC) in vitro. We also demonstrated that LT-IIa-B5 induces maturation of BMDC in vitro. Using a syngeneic adoptive transfer model, i.n. administration of LT-IIa-B 5 increased Ag-specific DO11.10 CD4 + T cell proliferation in the CLN of wt, but not in the CLN of TLR2-deficient mice. Furthermore, we showed that LT-IIa-B5 enhanced the migration of DC from the NALT to the cervical lymph nodes (CLN) in a manner that required cellular expression of TLR2 and the C-C chemokine receptor 7 (CCR7). LT-IIa-B 5 triggers increased expression of the costimulatory molecules CD80 and CD86 in NALT-derived DCs in the CLN. Moreover, when used as a mucosal adjuvant, LT-IIa-B 5 dramatically increased Ag-specific salivary IgA in TLR2-dependent manner. These immunopotentiating effects, however, were not limited to the proximal mucosa. LT-IIa-B 5 also induced enhanced Agspecific immune responses in the sera of intranasally-immunized mice. These data indicated that the mucosal adjuvant properties of LT-IIa-B 5 depend, in part, on the capacity to promote activation of DC in mucosal tissues. The capacity of LT-IIa-B 5 to functionally interact with TLR2, to enhance uptake of Ag, and to stimulate Ag-specific CD4 + T cell proliferation, and to promote Ag-specific mucosal and systemic immune responses are distinctive immunomodulatory characteristics of LT-IIa-B 5 which may be exploitable when incorporated as an adjuvant in future mucosal vaccinesvelopment. Overall, the non-toxic nature of the pentamer, in combination with its strong immunomodulating characteristics, makes this type II HLT an intriguing candidate as a new, safe, and effective mucosal adjuvant.