Background.

Intravenous injection of soluble antigen has been reported to induce immunologic tolerance through a variety of mechanisms including T-cell deletion, anergy, and suppression. To clarify the reported discrepancies, we studied mechanisms of intravenous tolerance to a defined transgenic minor transplantation antigen in mice.

Methods.

Wild-type C57BL/6 (B6) mice or congenic B6 B-cell knockout mice were made tolerant to β-galactosidase (β-gal). Clinical tolerance was assessed by placement of B6 β-gal transgenic (tg) and third-party skin grafts. In vitro analysis of T-and B-cell immunity and in vivo treatment with anti-TGFβ antibodies were used to define mechanisms of induced tolerance.

Results.

Intravenous injection of β-gal induced true immunologic tolerance to β-gal tg skin in wild-type but not in B-cell–deficient recipients, suggesting that antigen presentation by B cells was required for the effect. The tolerogenic manipulation primed a population of CD4+, β-gal-specific, TGFβ-producing T cells. Although evidence for both anergy and suppression were observed, subsequent data demonstrated that TGFβ was a critical immunoregulatory mediator of the tolerant state: neutralizing anti-TGFβ antibodies fully prevented the induction of tolerance to B6 β-gal tg skin grafts. Second male β-gal tg grafts placed onto female recipients that were previously made tolerant to female β-gal tg skin were rapidly rejected, however, suggesting that this TGFβ-induced tolerance could not be linked to additional antigenic determinants.

Conclusions.

The studies demonstrate a critical role for TGFβ in mediating tolerance after intravenous injection of antigen but additionally raise concerns about the stability of this tolerant state.