Allospecific CD4+ T cells retain effector function and are actively regulated by Treg cells in the context of transplantation tolerance

Although donor‐specific transfusion (DST) plus CD154 blockade represents a robust protocol for inducing transplantation tolerance, the underlying mechanisms are incompletely understood. In a murine T‐cell adoptive transfer model, we have visualized alloantigen‐specific, TCR‐transgenic for H2‐A^b/H2‐K^d_54–68 epitope (TCR75) CD4⁺ T cells with indirect allospecificity during the course of tolerance induction. Three main observations were made. First, although the majority of TCR75 CD4⁺ T cells were deleted following DST plus CD154 blockade, the surviving TCR75 CD4⁺ T cells were capable of making IL‐2, upregulating CD44, and undergoing cell division, suggesting that they were functionally active. Indeed, residual TCR75 CD4⁺ T cells reisolated from the primary recipients given DST plus CD154 blockade were fully capable of rejecting allografts upon secondary transfer. Second, in tolerant mice, TCR75 CD4⁺ T cells were not induced to express Foxp3 in the graft‐draining lymph node. TCR75 CD4⁺ T cells were also absent in accepted graft tissues in which endogenous Treg cells were enriched. Finally, DST plus CD154 blockade resulted in an abortive expansion of TCR75 CD4⁺ T cells, a process that required the presence of endogenous Treg cells. Collectively, surviving TCR75 CD4⁺ T cells are immunocompetent but kept in check by an endogenous immunosuppressive network induced by DST plus CD154 blockade.