PDCD1 and depressive symptom measurement: PD-1 (ρ = 0.58, p < 0.001) and MMP-9 (ρ = 0.49, p < 0.001) also positively correlated with HAM-D scores, further implicating immune exhaustion and blood–brain barrier (BBB) compromise in the pathogenesis of depression in AECOPD.