Immune Regulation of the Liver Through the PCSK9/CD36 Pathway During Heart Transplant Rejection

Background: PCSK9 (proprotein convertase subtilisin/kexin 9), which is mainly secreted by the liver, is not only a therapeutic target for hyperlipidemia and cardiovascular disease, but also has been implicated in the immune regulation of infections and tumors. However, the role of PCSK9 and the liver in heart transplant rejection (HTR) and the underlying mechanisms remain unclear.

Methods: We assessed serum PCSK9 expression in both murine and human recipients during HTR and investigated the effect of PCSK9 ablation on HTR by using global knockout mice and a neutralizing antibody. Moreover, we performed multiorgan histological and transcriptome analyses, and multiomics and single-cell RNA-sequencing studies of the liver during HTR, as well. We further used hepatocyte-specific Pcsk9 knockout mice to investigate whether the liver regulated HTR through PCSK9. Last, we explored the regulatory effect of the PCSK9/CD36 pathway on the phenotype and function of macrophages in vitro and in vivo.

Results: Here, we report that murine and human recipients have high serum PCSK9 levels during HTR. PCSK9 ablation prolonged cardiac allograft survival and attenuated the infiltration of inflammatory cells in the graft and the expansion of alloreactive T cells in the spleen. Next, we demonstrated that PCSK9 was mainly produced and significantly upregulated in the recipient liver, which also showed a series of signaling changes, including changes in the TNF-α (tumor necrosis factor α) and IFN-γ (interferon γ) signaling pathways and the bile acid and fatty acid metabolism pathways. We found mechanistically that TNF-α and IFN-γ synergistically promoted PCSK9 expression in hepatocytes through the transcription factor SREBP2 (sterol regulatory element binding protein 2). Moreover, in vitro and in vivo studies indicated that PCSK9 inhibited CD36 expression and fatty acid uptake by macrophages and strengthened the proinflammatory phenotype, which facilitated their ability to promote proliferation and IFN-γ production by donor-reactive T cells. Last, we found that the protective effect of PCSK9 ablation against HTR is dependent on the CD36 pathway in the recipient.

Conclusions: This study reveals a novel mechanism for immune regulation by the liver through the PCSK9/CD36 pathway during HTR, which influences the phenotype and function of macrophages and suggests that the modulation of this pathway may be a potential therapeutic target to prevent HTR.