Evidence for the differential regulation of mucosal IgA immunity and oral tolerance. Studies in gene knockout and transgenic mice using the cholera toxin adjuvant system

Abstract

Despite the continuos exposure to a myriad of food and microbial antigens, inflammatory reactions in the intestinal mucosa are rare. Homeostasis in the intestinal mucosa is maintained by regulatory T cells, which can be tolerized or primed to provide efficient B cell help. Two types of responses can be elicited by mucosal antigen; active IgA immunity or oral tolerance. The latter being the dominant type of reaction to almost all soluble antigens, whereas active IgA immunity requires the use of effective adjuvants, such as cholera toxin (CT) or ISCOMS. This thesis explores the regulatory role of CD4+ and CD8+ T cells for oral tolerance induction and active IgA immunity. Moreover, we have addressed the question of whether oral tolerance and active IgA immunity are reciprocally regulated or can co-exist. The governing functions of IL-12 (Th1) or IL-4 (Th2) for induction of tolerance or active IgA immunity were also assessed using gene knockout mice and ISCOMS and CT-adjuvant. Finally, we experimentally addressed the hypothesis that the type of response elicited by oral antigen administration is determined at the antigen-presenting cell (APC) level. We found that mice lacking CD8+ T cells exhibited significantly stronger mucosal IgA responses and failed to develop local tolerance to fed antigens, suggesting a down-regulatory function of CD8+ T cells in the normal gut mucosa. By contrast, oral tolerance at the systemic level, was clearly independent of CD8+ T cells, but affected both Th1 and Th2 CD4+ T cell functions. Furthermore, already established tolerance could not be abrogated by CT, whereas this adjuvant and ISCOMS prevented the induction of tolerance in all mouse strains tested, including IL-4 and IL-12-deficient mice. Th2 cells /IL-4 was critical for CT-induced IgA immunity, while ISCOMS were dependent on Th1 cells/IL-12 activity, indicating differential regulatory requirements for induction of mucosal IgA immune responses. Oral tolerance was readily induced in both these mouse strains in the absence of adjuvant, suggesting also separate regulatory pathways for induction of oral tolerance and active IgA immunity. The reciprocal regulation of these response patterns was further indicated by the finding that antigen conjugated to CT, strongly promoted mucosal IgA responses even in IL-4 deficient mice, while oral tolerance was prevented by the conjugation. The ability to circumvent the requirement for IL-4 was dependent on the enzymatic activity of CT, acting on the APC, because conjugation to the enzymatically inactive, but cell-receptor binding CTB, failed to affect IgA immunity. Using OVA TCR transgenic mice we could demonstrate that clonal T cells can be either tolerized or induced to enhance active IgA immunity depending on the function of the APC in the gut and that fed antigen can simultaneously induce local IgA immunity and systemic CD4+ T cell tolerance, arguing for the existence of separate and distinct activation pathways of the two response patterns; oral tolerance and active IgA immunity, in the gut mucosa.