Dendritic cells of the oral mucosa and the induction of oral tolerance. A local affair

The oral mucosa is an important site to induce immunological tolerance to protein antigens. Previously we have established that oral contacts to allergen can lead to systemic tolerance in both humans and experimental animals. Because of the importance of tolerance induction as a possible way to modulate allergic reactivity, we wished to study the mechanisms involved in efficient tolerance induction via the oral mucosa. Dendritic Langerhans' cells in both skin and oral epithelium are the first cells to encounter antigen. Therefore, possible functional differences between Langerhans' cells from skin and oral mucosa were studied by migration and transfer experiments. It was found that dendritic cells derived from the oral mucosa were not able to transfer tolerance, but that they acted as antigen-presenting cells in sensu stricto irrespective of the source and route of antigen administration.

Oral induction of tolerance to nickel sensitization in mice

Antigen contact via the alimentary tract prior to sensitization may result in systemic immunologic unresponsiveness ("oral tolerance"). The induction of oral tolerance seems an attractive strategy to combat undesired immune responses, such as allograft rejection and autoimmune and allergic diseases. We describe clear and reproducible sensitization to nickel in mice reared under nickel-free conditions. Hypersensitivity was induced by injecting nickel sulfate intradermally into the flank skin and elicited by injecting the metal salt into the pinnae of the ears. The effectiveness of orally induced hyporesponsiveness could be inferred from a low degree of hypersensitivity obtained with mice raised and maintained in cages with nickel-releasing covers and water nipples. This mouse model for the assay of nickel hypersensitivity was used for oral tolerance studies by administrating non-toxic doses of nickel sulfate in drinking water or intragastrically prior to sensitization. In these animals, the development of delayed-type hypersensitivity was suppressed in a dose-dependent way, and the hyporesponsiveness could be transferred by CD8+ cells. The antigen specificity of this oral tolerance could be demonstrated by the concomitant use of sensitization and challenge procedures for nickel and chromium. The hypersensitivity assay described provides a versatile, highly reproducible experimental model to study immunoregulation of oral tolerance to clinically relevant metal allergens.

Reduced frequency of nickel allergy upon oral nickel contact at an early age

From animal studies we know that oral administration of T-dependent antigens before sensitization effectively induces systemic immune unresponsiveness. Such 'oral tolerance' is persistent, dose-dependent, antigen-specific and presumably T suppressor cell-mediated. Oral tolerance induction could be an effective way to prevent undesired T cell-mediated immune functions, such as playing a role in allograft reaction, autoimmune and allergic diseases. In the present study allergic contact hypersensitivity (ACH) to nickel, currently presenting the most frequent contact allergy in man, was chosen to establish the feasibility of oral prevention of undesired T cell-mediated immunity in man. Potentially tolerizing (oral nickel contacts via orthodontic braces) as well as sensitizing (ear piercing) events were studied retrospectively in 2176 patients attending nine European patch test clinics. Patients were interviewed by means of a confidential questionnaire. The results show that ear piercing strongly favoured development of nickel ACH. More importantly, patients having had oral contacts with nickel-releasing appliances (dental braces) at an early age, but only if prior to ear piercing, showed a reduced frequency of nickel hypersensitivity. Frequencies of other hypersensitivities, in particular to fragrance, were not affected. These results support our view that induction of specific systemic immunologic tolerance by timely oral administration of antigens is feasible in man.

Oral induction of immunological tolerance to chromium in the guinea pig

Metal alloys used in dentistry may elicit adverse side-effects. Contact allergic reactions to metals released from such alloys are among the most frequently encountered problems. In an earlier study, we observed that oral contacts with nickel or chromium salts did not sensitize, but rather decreased the risk of subsequent sensitization to these metals. In the present study, we focused on chromium allergy and extended our earlier observations by further dose-response studies. In addition, we compared different chromium valencies as to their potential oral tolerogenic effects. Development of immunological tolerance in chromium-fed guinea pigs was demonstrated by their inability to develop chromium hypersensitivity after a subsequent immunization attempt. For these studies, the techniques of immunization and skin testing were first improved. One feeding with a high dose of K2Cr2O7 (containing hexavalent chromium) was effective in full tolerance induction. In contrast, trivalent chromium (CrCl3) induced a distinctly lower degree of tolerance, whereas metallic chromium powder was not detectably tolerogenic after a limited number of feedings. Dose-frequency-response studies with K2Cr2O7 showed that full tolerance could also be induced by an increase in the number of feedings with sub-optimal tolerogenic doses. The present results therefore support our hypothesis that long-lasting oral contact with chromium-releasing metal alloys may ultimately result in strong immune tolerance to this metal in subjects without previous skin contact with it. This view is further supported by recent insights into the unique tolerogenicity of oral, as compared with gastro-intestinal, allergenic contacts.