Analysis of the CD40 and CD28 pathways on alloimmune responses by CD4(+) Tcells in vivo

Background. Blockade of the CD40 and CD28 pathways is a powerful strategy t o inhibit CD4-mediated alloimmune responses. In this study, we examine the relative roles of the CD40 and CD28 pathways on CD4-mediated allograft reje ction responses, and further characterize the role of these pathways on CD4 (+) T-cell activation, priming for cytokine production, and cell proliferat ion in response to alloantigen in vivo. Methods. BALB/c skin allografts were transplanted onto C57BL/6 Rag 1-/- rec ipients reconstituted with CD4 cells from CD28-/- or CD40L-/- donors. The p opliteal lymph node assay was used to study the role of these pathways on C D4-cell activation and priming in vivo. To investigate the role of CD40 and CD28 blockade on CD4-cell proliferation, the fluorescein dye carboxyfluore scein diacetate succinimidyl. ester was used. We performed heterotopic card iac transplantation using CD40-/- mice to evaluate the role of CD40 on dono r versus recipient cells in CD4-mediated rejection. Results. B6 Rag 1-/- recipients reconstituted with CD28-/- CD4(+) T cells a cutely rejected allografts (median survival time 15 days), whereas recipien ts reconstituted with CD40L-/- CD4(+) T cells had significantly prolonged s urvival of BALB/c skin grafts (MST 71 days). CD40L blockade was equivalent to or inferior to CD28 blockade in inhibition of in vivo CD4-cell activatio n, priming for cytokine production, and proliferation responses to alloanti gen. BALB/c recipients depleted of CD8 cells promptly rejected donor B6 CD4 0-/- cardiac allografts, whereas B6 CD40-/- recipients depleted of CD8 cell s had significantly prolonged survival of BALB/c wild-type cardiac allograf ts. Conclusions. The CD40/CD40L pathway, but not the CD28/B7 pathway, is critic al for CD4-mediated rejection responses, however, the responsible mechanism s remain unclear.