Acute cellular rejection (ACR) is a common and important clinical complication following lung transplantation. While there is a clinical need for the development of novel therapies to prevent ACR, the regulation of allospecific effector T-cells in this process remains incompletely understood. Using the MHC-mismatched mouse orthotopic lung transplant model, we investigated the short-term role of anti-CD154 mAb therapy alone on allograft pathology and alloimmune T-cell effector responses. Untreated C57BL/6 recipients of BALB/c left lung allografts had high-grade rejection and diminished CD4+: CD8+ graft ratios, marked by predominantly CD8+>CD4+ IFN-γ+ allospecific effector responses at day 10, compared to isograft controls. Anti-CD154 mAb therapy strikingly abrogated both CD8+ and CD4+ alloeffector responses and significantly increased lung allograft CD4+: CD8+ ratios. Examination of graft CD4+ T-cells revealed significantly increased frequencies of CD4+CD25+Foxp3+ regulatory T-cells in the lung allografts of anti-CD154-treated mice and was associated with significant attenuation of ACR compared to untreated controls. Together, these data show that CD154/CD40 costimulation blockade alone is sufficient to abrogate allospecific effector T-cell responses and significantly shifts the lung allograft toward an environment predominated by CD4+ T regulatory cells in association with an attenuation of ACR.