Oral and intrathymic exposure to antigen can each induce systemic antigen-specific immune tolerance, but the mechanisms have not been well defined. We studied the effects that the route of exposure to antigen has on the mechanisms of tolerance in vivo using synthetic polymorphic class II major histocompatibility complex (MHC) peptides in a skin delayed-type hypersensitivity (DTH) response model. Lewis rats were immunized by injection in the footpad with synthetic peptides (RT1.Bu and/or RT1.Du) derived from the hypervariable domain of MHC class II beta chain of the Wistar-Furth rat in complete Freund's adjuvant and challenged 2 weeks later by injection in the ear with the MHC peptides. An "oral" group received the peptide mixture by gavage (100 micrograms/day for 5 days) 3 days before immunization, and an "intrathymic" group received a single intrathymic injection of 100 micrograms of peptides 48 hours before immunization. Oral therapy reduced the DTH response to 23 +/- 7%, and intrathymic exposure reduced the DTH response to 26 +/- 6% (P < 0.001) as compared with control DTH responses of unmodified Lewis animals. Immunohistological evaluation of DTH skin lesions showed that oral and intrathymic therapy each decreased mononuclear cell infiltration, fibrin deposition, and endothelial activation when compared with that seen in control rats. In contrast, while both protocols markedly reduced interleukin (IL-2) and interferon-gamma expression, they had differing effects on local expression of IL-4, transforming growth factor-beta, IL-2R, and CD45RC (a possible discriminant between Th1 and Th2 cells in rats). Oral therapy was associated with increased expression of IL-4 and preservation of transforming growth factor-beta expression by residual IL-2R+, CD45RC- mononuclear and endothelial cells, whereas thymic exposure suppressed essentially all features of immune activation including IL-2R induction and cytokine expression. Our data a) document the detailed pattern of cytokine expression and mononuclear and endothelial cell activation markers during DTH responses and b) confirm that oral tolerance is associated with immune deviation to a predominance of Th2 cell function, whereas intrathymic tolerance may be mediated by T-cell anergy or clonal deletion.