Lung transplantation (LTx) causes an alloimmune response of the innate and adaptive immune system. We aimed to map the diversity and tissue localization of involved immune cells post-LTx.

We performed murine isograft (C57BL/6 to C57BL/6) and allograft (Balbc to C57BL/6) orthotopic left LTx and administered daily immunosuppression. Allografts (n=3/timepoint) and isografts (n=1/timepoint) were assessed at post-operative day (POD) 7 or 35. Regions of interest (ROIs) were selected within each graft, focusing on airway, artery, vein, and parenchyma (total 12 ROIs), on which spatial proteomics was performed (Nanostring GeoMx).

In allografts, the expression of innate and adaptive immune cell markers is significantly elevated, mainly in the arterial ROI, indicating an increased presence of T-cells (CD45, CD4, CD44 and PD-1+/PD-L1-), B-cells (CD19) and dendritic cells (CD11c). Compatible with chronic rejection, these immune cell markers were increased at POD 7 and rose further by POD 35 (not shown). Additionally, the T-regulatory cell marker (FOXP3) is decreased in all ROIs, indicating impaired immune modulation. The endothelial marker CD31 is significantly decreased in allograft veins and parenchyma, but surprisingly not in arterial ROIs.

As opposed to past belief, the arteries instead of the airways appear to be the main site of initial immune activation in murine allograft lungs, with particular involvement of T-cells, B-cells and dendritic cells.