29 In contrast to CD47−/− mice, these animals also showed a reduced level of total intestinal IgA. A defect in extravasation from blood vessels into the intestine and GALT, as suggested above for OVA-specific plasma cells, could be applied to all leucocytes and could explain the decreased number of total cells in CD47−/− mice. Maintained levels of total intestinal

IgA in CD47−/− mice could be the result of a homeostatic mechanism in place to ensure normal levels of IgA, possibly through generation of IgA-producing cells directly in the intestinal LP.30 We have previously shown that DC are required for activation of CD4+ T cells after antigen feeding.4 In this study, we show a significant reduction CT99021 cell line in the frequency of CD11b+ cells among CD103+ and CD103− DC in the MLN of CD47−/− mice. We additionally confirm that removal of MLN completely abrogates the capacity to induce oral tolerance.3 It is the CD103+ MLN DC that exclusively present orally administered antigen to T cells ex vivo,21 and this subset has been shown to be gut-derived.23 check details Furthermore, migration of DC from the gut to the MLN is crucial for the initiation of oral tolerance, as CCR7-deficient mice fail to generate this response.3 However, although CD47−/− mice have reduced cell numbers in their GALT, reduced DC frequencies in MLN, a reduced proportion of CD103+ CD11b+ DC in the LP and MLN, and decreased activation

of antigen-specific CD4+ T cells following antigen feeding, their capacity to induce oral tolerance is still maintained. Additionally, in preliminary experiments the capacity to generate OVA-specific FoxP3 regulatory T cells following feeding of OVA was not different between CD47−/− and Aurora Kinase WT mice (data not shown). These results indicate that the remaining CD11b+ and/or CD11b− DC are sufficient for the induction of oral tolerance in CD47−/− mice. Alternatively, DC are not completely necessary. We have recently

shown that feeding high doses of antigen can result in efficient proliferation of CD4+ T cells in DC-depleted mice.4 However, even when a 10-fold lower antigen dose was given orally, the CD47−/− mice were efficiently tolerized. Our study demonstrates reduced numbers of gut-derived CD11b+ CD172a+ DC and a blunted capacity to expand CD4+ T cells following oral immunization in CD47−/− mice. Importantly, these impairments do not influence the capacity to induce oral tolerance. This shows that decreased T cell proliferation does not necessarily equate to reduced T cell-mediated function. However, CD47−/− mice have a gut-specific defect in total immune cell numbers, and following oral immunization they show reduced levels of antigen-specific intestinal IgA but normal systemic IgA and IgG. Replacing the haematopoietic compartment with CD47-expressing cells does not restore cellularity or the capacity to produce intestinal IgA.