PDCD1LG2 and pneumonitis: A potential mechanism behind this finding is that PD-1 inhibitors block the binding of PD-1 to PD-L2, which, in turn, enhances PD-L2 binding to the receptor-repulsive guidance molecule B. This leads to local T-cell clonal expansion, disrupting the balance of respiratory tolerance and thereby increasing the risk of pneumonitis [1].