937
ß-galactosidase can behave as a minor transplantation antigen in
C57BL/6 (B6, H-2b) mice as they reject congenic B6
ß-galactosidase transgenic (ß-gal tg) skin grafts by day 25. Recall
immune responses after rejection reveal ß-gal specific proliferation and
type 1 cytokine production, cytotoxicity by ß-gal tg cells and
ß-gal specific delayed type hypersensitivity (DTH) responses. We induced
immunologic tolerance to ß-gal tg skin grafts by pretreatment of the B6
recipients with i.v. injection of soluble ß-gal. B6 ß-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 ß-gal-specific IFN_, 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 ß-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
ß-gal in primary recall responses, incubation with IL-2 resulted in
recovery of ß-gal specific, IFN
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, ß-gal stimulation of immune cells from the tolerant animals
induced significant upregulation of TGFß message. These results suggest
that the i.v. immunization resulted in expansion of a population of ß-gal
specific, TGFß 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 TGFß 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.