Evidence that non-deletional mechanisms are responsible for inducing and maintaining unresponsiveness after intrathymic injection of non-professionalantigen presenting cells

Introduction: Intrathymic inoculation of donor alloantigen and concomitant immunosuppressive treatment can induce immune unresponsiveness to alloantig en. To examine the role of non-deletional mechanisms in the development of unresponsiveness, fractionated splenocytes were injected into only 1 lobe o f the thymus. Methods and Results: Untreated CBA (H2(k)) mice or controls pre-treated wit h anti-CD4 monoclonal antibody alone (on Day -28 and -27 relative to transp lantation) acutely rejected C57BL/10 (H2(b)) cardiac allografts. Intrathymi c inoculation of unfractionated splenocytes, resting B (rB) cells, or dendr itic cells into both thymic lobes with the antibody resulted in indefinite survival of cardiac allografts. In contrast, when donor rB cells or dendrit ic cells were delivered into a single lobe of the thymus with the antibody, only rB cells induced indefinite prolongation of graft survival; unfractio nated splenocytes or dendritic cells were markedly less effective. Mice tha t had 1 of the 2 thymic lobes removed were able to reject grafts even when treated with the antibody 27 days before transplantation. Therefore, T-cell export from 1 thymic lobe was sufficient to induce graft rejection. Finall y, adoptive transfer of splenocytes from mice with long-term surviving prim ary grafts resulting from the intrathymic injection of rB cells significant ly prolonged a graft from the same donor strain in a naive syngeneic recipi ent. Conclusion: Taken together, these data suggest that regulatory mechanisms g enerated by intrathymic injection of a non-professional antigen presenting cell, in this study donor rB cells, suppressed the rejection response media ted by T cells exported from the uninjected lobe.