Solid organ transplant recipients (SOTr) need a lifelong maintenance immunosuppressive therapy (ISt), combining 2 to 3 drugs targeting mainly T-cell activation, the key cellular subtype of adaptive immunity involved in graft rejection. Surprisingly, SOTr exhibit an increased risk of Pseudomonas aeruginosa (Pa)-mediated acute airway infection (AAI) with a mortality of up to 35% [1Su H., Ye Q., Wan Q., Zhou J. Predictors of mortality in abdominal organ transplant recipients with Pseudomonas aeruginosa infections Ann Transplant 2016 ;  21 : 86-93 [cross-ref]

Cliquez ici pour aller à la section Références], while identified risk factors are neutropenia and epithelial barrier impairment, effectors belonging to the innate immunity [2Di Pasquale M.F., Sotgiu G., Gramegna A., et al. Prevalence and Etiology of Community-acquired Pneumonia in Immunocompromised Patients Clin Infect Dis 2019 ;  68 : 1482-1493 [cross-ref]

Cliquez ici pour aller à la section Références]. Thus, ISt-induced inhibition of adaptive immunity alone may not explain this increased susceptibility to Pa. Although the effects on adaptive immunity are extensively described, those on the airway epithelial and mucosal immunity remain unclear. Our project aims at characterizing ISt-induced modifications of innate immunity, especially on epithelial barrier, during Pa-mediated AAI. We screened combinations of tacrolimus, mycophenolic acid (MPA) or everolimus, and prednisone. We specifically focused on MPA and its mechanism of guanosine triphosphate (GTP) depletion through inhibition of inosine monophosphate dehydrogenase (IMPDH).

We treated human bronchial cells (BEAS-2B) with mycophenolic acid (MPA 10μM) for 24h, and then performed a 4h-infection by Pa (strain PAO1, multiplicity of infection 1) followed by 20h-persistance. We assessed cytotoxicity by LDH release and IL-6 production by ELISA on supernatants at 24h of stimulation. When indicated, GTP 100μM was added to the culture medium simultaneously to MPA. Kruskall-Wallis and Dunn's tests were performed for multiple comparisons (significant if P<0.05).

Treatment by MPA 10μM enhanced PAO1-induced epithelial cytotoxicity of approximately 80% (medians 15,6% vs. 29,0%, P<0.0001) (Fig. 1A). MPA also diminished pro-inflammatory cytokine IL-6 production of 85% (medians 1261 vs. 186pg/mL, P<0.0001) at 24h and 80% at 4h (medians 179 vs. 36pg/mL, P<0.01) (Fig. 1B-C). Both these modifications were at least partially reversible with adjunction of GTP.

Our screening of ISt combinations reveals that MPA alters respiratory epithelial integrity and dampens its innate immune response, two major risk factors of Pa-mediated AAI. These alterations seem linked to the known mechanism of inhibition of IMPDH by MPA, since they are reversed by GTP supplementation. Mechanisms underlying these results are still to be explored and assessed in an in vivo model. Describing an additional effect of ISt on respiratory innate and mucosal immunity may eventually allow to both adapt these drug regimens during AAI and develop new therapeutic assets like immunomodulators against Pa.