Mechanisms of immune tolerance to food antigens in humans

T cell-mediated immunoregulation in the gastrointestinal tract

In the intestinal tract, only a single layer of epithelial cells separates innate and adaptive immune effector cells from a vast amount of antigens. Here, the immune system faces a considerable challenge in tolerating commensal flora and dietary antigens while preventing the dissemination of potential pathogens. Failure to tightly control immune reactions may result in detrimental inflammation. In this respect, 'conventional' regulatory CD4(+) T cells, including naturally occurring and adaptive CD4(+) CD25(+) Foxp3(+) T cells, Th3 and Tr1 cells, have recently been the focus of considerable attention. However, regulatory mechanisms in the intestinal mucosa are highly complex, including adaptations of nonhaematopoietic cells and innate immune cells as well as the presence of unconventional T cells with regulatory properties such as resident TCRgammadelta or TCRalphabeta CD8(+) intraepithelial lymphocytes. This review aims to summarize the currently available knowledge on conventional and unconventional regulatory T cell subsets (Tregs), with special emphasis on clinical data and the potential role or malfunctioning of Tregs in four major human gastrointestinal diseases, i.e. inflammatory bowel diseases, coeliac disease, food allergy and colorectal cancer. We conclude that the clinical data confirms some but not all of the findings derived from experimental animal models.

Plant-produced vaccines: promise and reality

Plant-produced vaccines are a much-hyped development of the past two decades, whose time to embrace reality may have finally come. Vaccines have been developed against viral, bacterial, parasite and allergenic antigens, for humans and for animals; a wide variety of plants have been used for stable transgenic expression as well as for transient expression via Agrobacterium tumefaciens and plant viral vectors. A great many products have shown significant immunogenicity; several have shown efficacy in target animals or in animal models. The realised potential of plant-produced vaccines is discussed, together with future prospects for production and registration.

Perspectives on mucosal vaccines: is mucosal tolerance a barrier?

Mucosal administration of Ags induces specific Abs in external secretions and systemic unresponsiveness termed oral or mucosal tolerance. The dominant response depends on the species studied, the nature, dose, frequency, route of Ag application, and the use of adjuvants. The temporal sequence of Ag exposure determines the quality of the ensuing immune response; although initial mucosal Ag exposure results in systemic T cell hyporesponsiveness, pre-existing systemic responses are refractory to the tolerizing effects of mucosal Ag encounter. Mucosal and systemic humoral responses may be induced concomitantly with diminished systemic T cell responses, thereby permitting Ab-mediated containment of mucosal Ags without stimulation of the systemic immune compartment. B cell Ig isotype switching and differentiation toward IgA production share common regulatory mechanisms with the suppression of T cells. Optimization of mucosal vaccination strategies has the potential for enhancing protective immune responses and suppressing systemic responses to autoantigens desirable for the treatment of autoimmune diseases.

Epicutaneous immunization with type II collagen inhibits both onset and progression of chronic collagen-induced arthritis

Epicutaneous immunization is a potential non-invasive technique for antigen-specific immune-modulation. Topical application of protein antigens to barrier-disrupted skin induces potent antigen-specific immunity with a strong Th2-bias. In this study, we investigate whether the autoimmune inflammatory response of chronic collagen-induced arthritis (CCIA) in DBA/1-TCR-β Tg mice can be modified by epicutaneous immunization. We show that epicutaneous immunization with type II collagen (CII) inhibited development and progression of CCIA and, importantly, also ameliorated ongoing disease as indicated by clinical scores of disease severity, paw swelling and joints histology. Treated mice show reduced CII-driven T cell proliferation and IFN-γ production, as well as significantly lower levels of CII-specific IgG2a serum antibodies. In contrast, CII-driven IL-4 production and IgE antibody levels were increased consistent with skewing of the CII response from Th1 to Th2 in treated mice. IL-4 production in treated mice was inversely correlated with disease severity. Moreover, T cells from treated mice inhibited proliferation and IFN-γ production by T cells from CCIA mice, suggesting induction of regulatory T cells that actively inhibit effector responses in arthritic mice. The levels of CD4⁺CD25⁺ T cells were however not increased following epicutaneous CII treatment. Together, these results suggest that epicutaneous immunization may be used as an immune-modulating procedure to actively re-programme pathogenic Th1 responses, and could have potential as a novel specific and simple treatment for chronic autoimmune inflammatory diseases such as rheumatoid arthritis.

Intestinal and pulmonary mucosal T cells: local heroes fight to maintain the status quo

Mucosal immunity in the lung and intestine is controlled by complex multifaceted systems. While mucosal T cells are essential for protection against invading pathogens owing to their proximity to the outside world, powerful systems must also be in place to harness ongoing inflammatory processes. In each site, distinct anatomical structures play key roles in mounting and executing both protective and deleterious mucosal T cell responses. Although analogies can be drawn regarding the immune systems of these two organs, there are substantial dissimilarities necessitated by unique physiologic constraints. Here, we discuss how T cell activation and effector function are generated in the mucosae.

Oral tolerance: lessons on treatment of food allergy

Oral tolerance is the active non-response by the immune system to an antigen administered through the oral route. It is postulated that food hypersensitivity results from a breakdown in oral tolerance induction, and the importance of oral tolerance in food hypersensitivity can be traced back to classic experiments from 1911 in which guinea pigs were protected from anaphylaxis by prior feeding of antigen. Host and antigenic factors play a role in determining the pathways and mechanisms to which a fed antigen can gain tolerance. Recent studies have demonstrated the potential of using oral tolerance to treat food allergies, and additional studies are necessary to further our understanding of mechanisms of oral tolerance induction.

Oral tolerance

Multiple mechanisms of tolerance are induced by oral antigen. Low doses favor active suppression, whereas higher doses favor clonal anergy/deletion. Oral antigen induces T-helper 2 [interleukin (IL)-4/IL-10] and Th3 [transforming growth factor (TGF)-beta] T cells plus CD4+CD25+ regulatory cells and latency-associated peptide+ T cells. Induction of oral tolerance is enhanced by IL-4, IL-10, anti-IL-12, TGF-beta, cholera toxin B subunit, Flt-3 ligand, and anti-CD40 ligand. Oral (and nasal) antigen administration suppresses animal models of autoimmune diseases including experimental autoimmune encephalitis, uveitis, thyroiditis, myasthenia, arthritis, and diabetes in the non-obese diabetic (NOD) mouse, plus non-autoimmune diseases such as asthma, atherosclerosis, graft rejection, allergy, colitis, stroke, and models of Alzheimer's disease. Oral tolerance has been tested in human autoimmune diseases including multiple sclerosis (MS), arthritis, uveitis, and diabetes and in allergy, contact sensitivity to dinitrochlorobenzene (DNCB), and nickel allergy. Although positive results have been observed in phase II trials, no effect was observed in phase III trials of CII in rheumatoid arthritis or oral myelin and glatiramer acetate (GA) in MS. Large placebo effects were observed, and new trials of oral GA are underway. Oral insulin has recently been shown to delay onset of diabetes in at-risk populations, and confirmatory trials of oral insulin are being planned. Mucosal tolerance is an attractive approach for treatment of autoimmune and inflammatory diseases because of lack of toxicity, ease of administration over time, and antigen-specific mechanisms of action. The successful application of oral tolerance for the treatment of human diseases will depend on dose, developing immune markers to assess immunologic effects, route (nasal versus oral), formulation, mucosal adjuvants, combination therapy, and early therapy.

Immune response versus mucosal tolerance to mucosally administered antigens

The enhancement of protective mucosal responses to infectious agents sought by vaccinologists, and the desired suppression of immune responses to autoantigens, allergens and food antigens induced by mucosal exposure to relevant antigens, may seem paradoxical. However, such outcomes are not mutually exclusive because of a hierarchy in the quality of immune responses: mucosal immunity manifested by the appearance of secretory antibodies and systemic tolerance manifested by diminished cell-mediated responses may be induced concomitantly. We have demonstrated that the ingestion, or nasal application, of a neo-antigen-keyhole limpet hemocyanin (KLH)-primes human volunteers for subsequent humoral mucosal and systemic responses induced by systemic immunization but suppresses cell-mediated responses (T cell proliferation and delayed-type hypersensitivity). Furthermore, extended ingestion of selected food antigens diminished, but did not totally suppress, cell-mediated immunity and levels of serum antibodies while salivary antibodies were easily detectable. Most importantly, the humoral and cellular immune responses induced by systemic immunization with KLH could not be suppressed by subsequent extended ingestion of large doses of the same antigen. Thus, ongoing systemic responses are refractory to the induction of mucosal tolerance in humans. This finding is of considerable importance for the development of and sequence of immunization with mucosally administered vaccines.

Dietary antigen-specific T-cell responses: switch from an interleukin-10-dominated response in normal mice to a T-helper 1 cytokine profile in Galphai2-deficient mice prior to colitis

We investigated dietary antigen-specific T-cell responses in mesenteric lymph nodes (MLN) and Peyer's patches (PP) in noncolitic control mice as well as in colitis-prone mice prior to onset of histological active colitis. T cells were restimulated in vitro with constituents isolated from the mouse diet. Interestingly, MLN T cells of littermate G(alpha)i2+/- control mice responded to soya with high production of interleukin (IL)-10, but did not produce proinflammatory T-helper 1 (Th1) cytokines. Recall dietary antigen stimulation of G(alpha)i2+/- PP T cells did not result in increased IL-10 production above the spontaneous production in the absence of antigenic stimulation. In strong contrast, MLN T cells from precolitic G(alpha)i2-/- mice produced high levels of interferon-gamma (IFN-gamma) upon restimulation with soya, which could be abolished using a major histocompatibility complex class II-blocking antibody. In conclusion, the present study demonstrates that MLN T lymphocytes in normal healthy mice respond with a significantly increased production of the regulatory cytokine IL-10 on re-encounter with dietary proteins in vitro. In marked contrast precolitic G(alpha)i2-/- mice respond to dietary antigens with a Th1-dominated cytokine response in the mucosa, prior to onset of colitis, with excessive IFN-gamma production. These results suggest that aberrant immune responses to dietary antigens could contribute as a potential pathogenic mechanism in the onset of colitis in G(alpha)i2-deficient mice.

Oral Tolerance in Humans: Failure to Suppress an Existing Immune Response by Oral Antigen Administration

Orally administered antigens can induce systemic tolerance. In animal models, oral tolerance can both prevent and treat experimental autoimmune diseases. The induction of oral tolerance to human autoantigens has been envisioned as a potential treatment for human autoimmune conditions. The results of previous human studies from our laboratory have provided evidence that oral administration of a model antigen, keyhole limpet hemocyanin (KLH), prior to systemic immunization, can decrease the magnitude of subsequent T cell proliferative and skin test responses to KLH. The present study was designed to test the hypothesis that orally administered KLH could attenuate a preexisting immune response to KLH. Thus, human subjects (n = 8) were primed subcutaneously with KLH without adjuvant prior to a 42-day feeding regimen of 100 mg of KLH per day. At the end of the feeding regimen, the subjects were boosted with KLH, again without adjuvant. Eight control subjects were immunized as above with KLH, but fed ovalbumin. The measurement of antigen-driven T cell proliferation, serum and salivary antibody, and dermal delayed hypersensitivity responses to KLH failed to reveal significant differences between subjects fed KLH and those fed ovalbumin. These results indicate that the KLH dose and feeding regimen used in this study failed to attenuate the primary response or to prevent the secondary response to KLH. Therefore, some form of immunomodulation greater than that provided by oral administration of antigen alone is required in humans for suppression of an existing immune response.

Evaluation of the immunogenicity of dietary proteins in cats and the influence of the canning process

OBJECTIVE

To characterize the antigen-specific immune response to dietary proteins in cats and evaluate whether there was a qualitative or quantitative difference between the responses to dietary proteins when those proteins were fed unprocessed or as part of a canned diet.

ANIMALS

14 healthy domestic shorthair cats.

PROCEDURE

Cats were fed 2 dietary proteins (soy and casein) either as unprocessed aqueous suspensions or as part of canned diets for 21 days. Serum IgG and IgA and salivary IgA were assayed by indirect ELISA, and antigen-specific proliferation of mesenteric lymph node-derived lymphocytes was determined.

RESULTS

Robust serum IgG and IgA responses to dietary proteins were elicited, irrespective of the form in which they were fed. Salivary IgA responses to unprocessed proteins were not detected. However, a significant salivary IgA response to the protein isolated from the canned casein diet was observed in cats fed canned casein but not in those fed unprocessed casein. Lymphocyte proliferation to the antigens was slight, and there were no significant differences between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that cats develop robust serum IgG and IgA responses to dietary proteins when fed as either aqueous suspensions or as part of canned diets. For certain proteins, there may be an increase and a qualitative difference in the immunogenicity of canned diets, compared with unprocessed proteins. Canned diets may not be ideal for management of cats with enteritis.