Evidence for a limited contribution of immune regulation to cardiac allograft acceptance.

We have used donor-reactive DTH responses to study the immune regulation that is displayed by C57BL/6 mice after they accept DBA/2 cardiac or renal allografts. This regulation is expressed by splenocytes from the allograft acceptors, and involves their alloantigen-induced production of the anti-inflammatory cytokines transforming growth factor beta (TGFbeta) and/or interleukin-10 (IL-10) at DTH challenge sites, and presumably within accepted allografts. In cardiac allograft acceptors the production of these cytokines depends on a population of CD25(+) splenocytes. During these studies, we have encountered several situations in which allograft acceptance does not correlate with DTH-detectable immune regulation: (1) splenocytes from cardiac or kidney allograft acceptors lose TGFbeta-mediated inhibition of donor-reactive DTH responses by 150 days post-transplant, although they retain ongoing allograft function; (2) cardiac allograft acceptors rapidly reject donor-matched skin allografts, retain good cardiac allograft function, but lose DTH-detectable immune regulation; (3) Balb/c mice accept C57BL/6 cardiac allografts when treated with anti-CD40L mAb (MR1), but fail to express DTH-detectable immune regulation; and (4) infusion of C57BL/6 mice with peritoneal exudate cells (PEC) that were educated ex vivo to DBA/2 alloantigens in the presence of IL-10 and TGFbeta, causes them to exhibit DTH-detectable immune regulation mediated by both TGFbeta and IL-10, but they fail to accept DBA/2 cardiac allografts. These observations suggest that the process of allograft acceptance, as it is studied in murine transplant models, is metastable, and does not necessarily reflect the achievement of allograft tolerance. Further, the development of allograft tolerance probably requires more than regulatory T cells, representing a coordinated evolution of multiple immune processes over a prolonged period of time.