Programmed cell death ligand 1 mediates antigen presentation, apoptosis, necrosis, and inflammatory response in Klebsiella pneumoniae-infected macrophages

Klebsiella pneumoniae (Kp) infection has high global complication and mortality rate. Programmed cell death ligand 1 (PD-L1) is important for immune evasion in tumorigenesis, however, with unclear mechanism in Kp infection. This study aims to explore the role and potential mechanisms of PD-L1 in Kp-infected mouse mononuclear macrophages RAW264.7 cells. Here, RAW264.7 cells were infected with classical Kp (cKp) and highly virulent Kp (hvKp), and transfected with PD-L1 knockdown. Subsequently, to investigate the effect of PD-L1 on the activation of CD4⁺ T cells, a co-culture system of RAW264.7 and CD4⁺ T cells was established. In RAW264.7 cells infected with Kp, PD-L1 knockdown reduced apoptosis and necrosis, with lower Bax, Cleaved Caspase 3, p-RIPK1/RIPK1, p-RIPK3/RIPK3, and p-MLKL/MLKL expression. Meanwhile, phagocytic activity and phagocytic index were enhanced, with increased Kp count. Furthermore, PD-L1 knockdown led to the activation of RAW264.7 cells, which participated in immune regulation, accompanied by higher levels of IL-1β, IL-6, TNF-α, MHC II, CD80, and CD86. Following co-culture of RAW264.7 and CD4⁺ T cells, PD-L1 knockdown reversed the effect of Kp infection to promote CD4⁺ T cell activation, as evidenced by decreased apoptosis and elevated IFN-γ, TNF-α, and IL-2 levels. This result preliminarily demonstrated that PD-L1 expression may be involved in antigen presentation in RAW264.7 cells with Kp infection. In conclusion, in Kp-infected RAW264.7 cells, PD-L1 mediates the increased apoptosis, necrosis, inflammatory responses, and CD4⁺ T cell activation, as well as inhibition of phagocytosis, and may be involved in antigen presentation, offering new therapeutic targets for Kp infection.