B cells drive iBALT formation in a mouse model of chronic lung allograft dysfunction

We developed a mouse model of chronic lung allograft dysfunction (CLAD) after lung transplantation (LTx), characterized by persistent inflammation and extensive iBALT. We aimed to characterize CLAD-associated iBALT and the molecular determinants of its formation, in order to prevent its onset. Left lungs from C57BL/6J (syngeneic group) or mice on a C57BL/6J background carrying a single human HLA-A2.1 molecule (mismatched, CLAD group) were orthotopically transplanted into C57BL/6J mice. Left lungs from HLA-A2.1 mice were transplanted into B cell-deficient (muMT-/-) and EBI2-/- mice to understand the role of B cells in iBALT formation. Compared to syngeneic, the mismatched HLA grafts presented decreased lung function and histological signs resembling human lymphocytic bronchiolitis, characteristic of CLAD. The mismatched grafts further exhibited strong iBALT formation, associated with pathological alterations of the bronchial epithelium. B cells in iBALT were active, as the markers CD69, CD80, MHCII and GL7 were upregulated. The formation of iBALT was disrupted in mismatched grafts transplanted to muMT-/- mice, showing that B cells are necessary for iBALT development. The GPCR EBI2 has lately been shown as a key player for B cell organization in secondary and tertiary lymphoid organs. In line with this background, transplantation of mismatched lung grafts into EBI2-/- mice resulted in iBALT of decreased size, as well as treatment with a pharmacological EBI2 inhibitor. The development of iBALT in mouse CLAD relies on the presence of B cells and of the immune receptor EBI2, which further represents a potential target to improve the outcomes after LTx.