Background: Renal fibrosis is the common final pathway of progressive renal diseases, in which the heterogeneity of macrophages in the renal microenvironment plays an important role. Regardless of etiology, elevated TGF-β levels are highly correlated with activated pro-fibrotic.
Methods: ELISA was used to detect CD5 molecule-like (CD5L) in serum samples from end-stage renal disease (ESRD) and healthy controls, as well as in mice serum with unilateral ureteral obstruction (UUO). Recombinant CD5L (rCD5L) was injected into UUO mice to assess renal injury, fibrosis, and macrophage infiltration, including macrophage polarization and the proportion of Ly6Clow (pro-fibrotic) macrophages. In addition, rCD5L was added to bone marrow-derived macrophages (BMDMs) to investigate the activation of TGFβ. Finally, the potential of the CD5L antibody as a therapeutic target to prevent renal fibrosis and its mechanism was investigated.
Results: The expression of CD5L was significantly upregulated in the serum of patients with ESRD and UUO mice. A negative correlation between the content of CD5L and the glomerular filtration rate (eGFR) was confirmed in patients. Histological analysis showed that rCD5L-treated UUO mice had more severe renal injury and fibrosis than the control group. Furthermore, rCD5L promoted the phenotypic transfer of monocytes from Ly6Chigh to LyC6low, and rCD5L also promoted M2-type macrophage polarization. Interestingly, in BMDMs, rCD5L promoted TGF-β signaling pathway activation by promoting Smad2/3 phosphorylation. In vivo experiments showed the same trend. Moreover, we used Co-IP to identify HSPA5 in macrophages that interact with CD5L and then used immunofluorescence localization to demonstrate that the binding of CD5L to HSPA5 on the cell membrane in macrophages could inhibit the formation of the Cripto/hSPA5 complex, release TGF-β receptors, and promote the activation of the ERK/TGF-β signaling pathway in macrophages. The CD5L antibody could also reduce the degree of renal fibrosis in UUO mice, suggesting that CD5L blockade holds promise as a potential therapeutic approach for renal fibrosis.
Conclusion: CD5L may serve as a reference marker for renal fibrosis, contributing to its progression by activating the TGF-β signaling pathway and influencing macrophage phenotype and function. By blocking CD5L, the M2 polarization of macrophages in the fibrotic kidney could be inhibited, attenuating renal fibrosis. Therefore, blocking CD5L may be a new therapeutic target.