Background: Modulation of P-glycoprotein (P-gp) activity in graft-infiltrating T cells may alter their susceptibility to immunosuppression.
Methods: P-gp activity was measured by rhodamine efflux in T-cell subsets from bronchoalveolar lavage (BAL) of five healthy volunteers and 27 lung allograft recipients. The effect of T-cell activation on P-gp activity was modeled by stimulation of peripheral blood mononuclear cells with staphylococcal enterotoxin B.
Results: Most BAL T cells expressed memory-effector markers. Patients had a lower proportion of CD4 T cells (P = 0.005), whereas control subjects had CD4-to-CD8 ratios similar to peripheral blood. In controls, basal P-gp activity was greatly increased in both CD4 (35% P-gp active) and CD8 (63%) lung T cells compared with peripheral T cells. Basal P-gp activity was elevated in patient BAL T cells but was lower than control BAL activity (CD4, P = 0.07; CD8, P = 0.03). Lung T cells from transplant patients had modest (CD4) or marked (CD8) increases in substrate-induced P-gp activity compared with normal lung, indicating that P-gp was not irreversibly inhibited. Patients with acute cellular rejection (ACR) had reduced P-gp activity in CD4, but not CD8, BAL T cells compared with patients without ACR (P = 0.004). To determine the relationship between T-cell activation on P-gp modulation, P-gp activity was measured in staphylococcal enterotoxin B-stimulated peripheral blood mononuclear cells. P-gp activity was abrogated in CD71 cycling cells but remained high in a persistent but minor population of resting naive T cells.
Conclusions: Lung T cells have increased in vivo P-gp activity and therefore may eliminate substrate drugs, resulting in local resistance to immunosuppressive therapy. However, P-gp function is reduced during T-cell activation, providing a window of susceptibility to treatment during ACR.