MMP9 and major depressive disorder: In contrast, immune exhaustion and matrix dysregulation markers—PD-1 (ρ = 0.58, p < 0.001) and MMP-9 (ρ = 0.49, p < 0.001)—were strongly positively correlated with HAM-D scores, underscoring the relevance of immune checkpoint activation and extracellular matrix disruption in the pathogenesis of depression in COPD.