Tolerance to food antigen manifests in the absence and/or suppression of antigen-specific immune responses locally in the gut but also systemically, a phenomenon known as oral tolerance. Oral tolerance is thought to originate in the gut-draining lymph nodes, which support the generation of FoxP3(+) regulatory T (Treg) cells. Here we use several mouse models to show that Treg cells, after their generation in lymph nodes, need to home to the gut to undergo local expansion to install oral tolerance. Proliferation of Treg cells in the intestine and production of interleukin-10 by gut-resident macrophages was blunted in mice deficient in the chemokine (C-X3-C motif) receptor 1 (CX3CR1). We propose a model of stepwise oral tolerance induction comprising the generation of Treg cells in the gut-draining lymph nodes, followed by migration into the gut and subsequent expansion of Treg cells driven by intestinal macrophages.

De novo induction of Foxp3⁺ regulatory T cells (Tregs) is particularly efficient in gut-draining mesenteric and celiac lymph nodes (mLN and celLN). Here we used LN transplantations to dissect the contribution of stromal cells and environmental factors to the high Treg-inducing capacity of these LN. After transplantation into the popliteal fossa, mLN and celLN retained their high Treg-inducing capacity, whereas transplantation of skin-draining LN into the gut mesenteries did not enable efficient Treg induction. However, de novo Treg induction was abolished in the absence of dendritic cells (DC), indicating that this process depends on synergistic contributions of stromal and DC. Stromal cells themselves were influenced by environmental signals as mLN grafts taken from germ-free donors and celLN grafts taken from vitamin A-deficient donors did not show any superior Treg-inducing capacity. Collectively, our observations reveal a hitherto unrecognized role of LN stromal cells for the de novo induction of Foxp3⁺ Tregs.

Immunotherapy generally fails to induce tumour regression in spontaneously arising tumours. Failure is attributed to both tumour-related factors and an ineffective immune response. As a model of tumour immunotherapy, without the confounding effects of potential tumour-determined mechanisms of immune evasion, we studied the requirements for rejection of skin grafts expressing a neo-self antigen in somatic cells and not in antigen-presenting cells. When antigen expression was restricted to somatic cells, both CD4 + and CD8 + effector cells were required for graft rejection. Although freshly placed grafts were spontaneously rejected, healed grafts established under the cover of T cell depletion were not rejected even after T cell numbers recovered to a level where freshly placed grafts on the same animal were rejected, suggesting that healed skin grafts expressing a neo-self antigen only in somatic cells could not be rejected by primed recipients with functional effector T cells. Local TLR7 ligation induced inflammatory responses and rejection of healed grafts exposed to the TLR agonist but did not induce rejection of untreated healed grafts on the same animal. Thus, local proinflammatory signalling via TLR7 can promote effector T cell function against skin cells displaying their nominal antigen. IntroductionTumours often express minor variants of self proteins. When these are antigenic in an in vivo or in vitro assay, they are designated as tumour-specific antigens (TSA), and are selected as potential targets for specific immunotherapy. TSA are held to be immunologically equivalent to minor transplantation antigens (MTA) as these are also minor allelic variants of self proteins, though such allelic variants are only effective MTA when directly presented by professional APC in a transplant model [1]. TSA and most viral antigens, in contrast, are immunogenic only following cross-presentation. In skin transplantation experiments involving MTA, antigen presentation occurs in a pro-inflammatory environment, which precludes examination of the effects of inflammation on effector T cell function. Chronic viral infections, associated with tumour formation, persist in the presence of seemingly adequate cell-mediated immune responses to viral antigens, and immunotherapy fails to induce tumour regression in the majority of spontaneously arising tumours. This apparent failure of We wished to study the role of pro-inflammatory stimuli on effector T cell-mediated elimination of antigen-expressing somatic cells, without interference from tumour-determined immunoregulatory mechanisms or from direct antigen presentation by local APC. We established a model in which a neo-self antigen, human growth hormone (hGH), which is around 70% homologous with mouse growth hormone, is expressed as a transgene in keratinocytes, driven from the keratin 14 (K14) promoter, and skin from transgenic animals is transplanted to a syngeneic non-transgenic recipient [6,7]. Our previous study showed that the majority of skin grafts expressing hGH as an MT...

The secondary humoral immune response is characterized by plasma B cells secreting isotype-switched and affinity-matured antibodies. The efficient generation of plasma B cells in the GC depends on the presence of follicular helper T (T FH ) cells, a cell type thought to arise from naive CD4-positive T cells by a hitherto unresolved differentiation pathway. Mice deficient for CD155, an adhesion receptor of the immunoglobulin superfamily, are impaired to mount a secondary humoral immune response upon oral administration of antigen, while the primary IgM response is unaffected. Here, we show that mice lacking CD155 harbor significantly reduced numbers of T FH cells in their Peyer's patches. This was paralleled by a decreased frequency of T FH cells in the GC. Moreover, the CD155 ligand CD226, which is involved in T-cell activation, is down-regulated during T FH cell differentiation, resulting in a complete absence of CD226 on those T FH cells residing in the GC. Concurrently, the expression of TIGIT/WUCAM, a newly discovered CD155 ligand, is induced in T FH cells. Thus, these cells replace an activating by a putative inhibitory CD155-binding partner during their differentiation.Key words: CD155 . CD226 . Follicular helper T cells . GC . TIGIT/WUCAM Introduction CD155 represents an Ig-like glycoprotein [1] and is the founder of a subfamily of adhesion receptors comprising also the related nectins 1-4. CD155/nectins take part in the complex regulation of cell growth, differentiation and motility [2,3], thereby in part explaining their identification as tumor markers [4][5][6][7]. Current evidence suggests that in contrast to nectins, CD155 acquired immunologically relevant functions. This apparent, yet not absolute, functional dichotomy in the CD155 family may relate to the observation that CD155 belongs to the category of rapidly evolving genes [8] that may promote the occurrence of new receptor/ligand interactions. In contrast to nectins, CD155 lacks self-adhesion [8]. Instead, CD155 binds to nectin-3 [9], vitronectin [10], CD96 [11,12], CD226 [13] as well as the most recently discovered TIGIT/WUCAM [14,15]. CD155 is broadly expressed among immune cells [16], whereas the expression patterns of the activating receptor CD226 and the potentially inhibitory TIGIT/WUCAM are more restricted. In the T lineage, CD155 is present on T cells of virtually all stages of differentiation, ranging from early thymocytes to the antigen-experienced memory T cells [16]. CD226 is first observed on single positive thymocytes [17] and its subsequent expression is subject to regulation depending on the differentiation path following antigen challenge [18]. Inversely, TIGIT/WUCAM is absent from resting human PBMC and is only expressed upon stimulation [14,15]. Interestingly, immature DC up-regulate CD155 upon stimulation [16], suggesting a role of this receptor in T-cell priming. CD226 was assigned a co-stimulatory capacity [19][20][21] and its interaction with CD155 is pivotal in cytotoxic T/NKdirected lysis of targets such as immature DC...