The Role of TLR4 Inducing Macrophage Pyroptosis in the Pathogenesis of Severe Aplastic Anemia

Introduction：To investigate the expression level of macrophage pyroptosis in patients with severe aplastic anemia (SAA) and its effect on downstream effector T cells, the study explored the mechanism of Toll like receptor 4 (TLR4) inducing macrophage pyroptosis and maintaining immune homeostasis of SAA through pyruvate kinase M2 (PKM2) at cellular and molecular levels, which provided a theoretical basis for the targeted therapy of macrophages in SAA. Three groups of bone marrow macrophages of new diagnosed SAA patients, remission patients and healthy controls were induced and cultured in vitro. qRT-PCR and Western Blot were used to detect pyroptosis-related indicators IL-1β, IL-18, NLRP3, Caspase-1 and GSDMD expression level; ELISA to detect the concentration of IL-1β and IL-18 in the culture supernatant.

Results: 1. The mRNA and protein levels of macrophages IL-1β, IL-18, NLRP3, Caspase-1, and GSDMD in the new diagnosed SAA group were significantly higher than those in healthy controls, and the IL-1β and IL-18 concentration in the macrophage culture supernatant increased significantly. The relative mRNA expression levels of macrophages IL-1β, IL-18, NLRP3, Caspase-1, and GSDMD in the new diagnosed group were significantly negatively correlated with WBC, RBC, Hb and PLT. 2. The relative expression level of TLR4 mRNA in macrophages in the new diagnosed group was significantly higher than that in the remission group and healthy controls, and the protein level of TLR4 was significantly increased; at the same time, the relative expression level of TLR4 mRNA was positively correlated with IL-1β, IL-18, NLRP3, Caspase-1 and GSDMD in macrophages. After knocking down TLR4 or adding TLR4 inhibitors, the mRNA and protein expression levels of IL-1β, IL-18, NLRP3, Caspase-1 and GSDMD were significantly lower than those in healthy controls, and the concentration of IL-1β and IL-18 in the culture supernatant reduced significantly. 3. The expression rate of PKM2 in macrophages of new diagnosed group by flow cytometry was significantly higher than that in the remission group and heathy controls, and the levels of PKM2 mRNA and protein were significantly increased. At the same time, the relative expression level of PKM2 mRNA was positively correlated with the expression levels of IL-1β, IL-18, NLRP3, Caspase-1, and GSDMD. After knocking down PKM2, the mRNA and protein expression levels of IL-1β, IL-18, NLRP3, Caspase-1, GSDMD were significantly lower than those healthy controls, and the concentration of IL-1β and IL-18 in the culture supernatant was also significantly reduced. After knocking down TLR4 or adding TLR4 inhibitor, PKM2 mRNA and protein expression levels in macrophages were significantly lower than those in healthy controls. 4. When CD8+ T cells were co-cultured with macrophages which were knocked down TLR4 or added with inhibitors, the expressions of perforin and granzyme B in CD8+ T cells were significantly reduced. CD8+ T cells were further co-cultured with K562 and the apoptosis level of K562 was reduced.

Summary：The level of macrophage pyroptosis in SAA patients increased, and the expression levels of macrophages TLR4 and PKM2 increased and positively correlated with the level of pyroptosis. Inhibiting the expression of TLR4 and PKM2, respectively, reduced the level of macrophage pyroptosis, and at the same time inhibiting TLR4 expression reduced the expression of PKM2. Macrophages that inhibited TLR4 expression reduced the killing function of CD8+ T cells. Therefore, TLR4 may induce pyroptosis of macrophages in SAA patients through PKM2 and enhance the killing function of CD8+ T cells.