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Excerpt
β-galactosidase can behave as a minor transplantion antigen in C57BL/6 (B6, H-2b) mice as they reject congenic B6 β-galactosidase transgenic (β-gal tg) skin grafts by day 25. Recall immune responses after rejection reveal β-gal specific proliferation and type 1 cytokine production, cytotoxicity of β-gal tg cells and β-gal specific delayed type hypersensitivity (DTH) responses. We induced immunologic tolerance to β-gal tg skin grafts by pretreatment of the B6 recipients with i.v. injection of soluble β-gal. B6 β-gal tg skin graft survival was prolonged for > 5 months, while third party grafts were rejected within 14 days. Recall responses in tolerized mice revealed an absence of β-gal-specific IFNγ, IL-2, IL-4 or IL-5 production (no type 2 immune deviation). Spleen cells obtained from the tolerant animals were unable to mediate CTL activity or DTH responses. In an effort to discern whether clonal deletion, anergy and/or suppression were active mechanisms of the tolerance, we next tested whether we could recover β-gal specific immunity from tolerant animals by in vitro incubation of tolerized immune cells in the presence of exogenous IL-2. Although the cells were unable to respond to β-gal in primary recall responses, incubation with IL-2 resulted in recovery of β-gal specific, IFNγ - producing T cells, thus excluding clonal deletion as an operative mechanism. Furthermore, spleen cells obtained from tolerized mice acted as suppressor cells, in that co-injection of these spleen cells inhibited ovalbumin (OVA) specific DTH responses normally produced by OVA-primed T cells. Finally, β-gal stimulation of immune cells from the tolerant animals induced significant upregulation of TGFβ message. These results suggest that the i.v. immunization resulted in expansion of a population of β-gal specific, TGFβ producing T cells capable of suppressing the pathogenic, anti-skin graft immune response. Taken together these data demonstrate the feasibility of inducing graft tolerance to a transgenic minor antigen and implicate TGFβ mediated suppression and/or anergy as mechanisms of such tolerance. Lessons obtained from this experimental model may translate into approaches for inducing graft tolerance in human organ transplant recipients.
