CD154 deficiency uncouples allograft CD8⁺ T-cell effector function from proliferation and inhibits murine airway obliteration

Obliterative bronchiolitis (OB) limits the long-term success of lung transplantation, while T-cell effector mechanisms in this process remain incompletely understood. Using the murine heterotopic tracheal transplant model of obliterative airway disease (OAD) to characterize airway allograft rejection, we previously reported an important role for CD8⁺ T cells in OAD. Herein, we studied the role of CD154/CD40 costimulation in the regulation of allospecific CD8⁺ T cells, as airway rejection has been reported to be CD154-dependent. Airway allografts from CD154^-/- recipients had significantly lower day 28 OAD scores compared to wild-type (WT) recipients, and adoptive transfer of CD8⁺ T cells from WT recipients, but not CD154^-/- recipients, were capable of airway rejection in fresh CD154^-/- allograft recipients. Intragraft CD8⁺ T cells from CD154^-/- mice showed similar expression of the surface markers CD69, CD62L^low CD44^high and PD-1, but markedly impaired IFN-γ and TNF-α secretion and granzyme B expression versus WT controls. Unexpectedly, intragraft and systemic CD8⁺ T cells from CD154^-/- recipients demonstrated robust in vivo expansion similar to WT recipients, consistent with an uncoupling of proliferation from effector function. Together, these data suggest that a lack of CD154/CD40 costimulation results in ineffective allospecific priming of CD8⁺ T cells required for murine OAD.