S100A4 enhances the pro-fibrotic effect of macrophage to promote collagen capsule formation of Trichinella spiralis.

BACKGROUND: Trichinella spiralis is a zoonotic parasitic nematode capable of infecting humans and animals, causing trichinellosis. It poses a critical threat to human health, livestock industry and food safety. The muscle larva of T. spiralis is encased within a collagen capsule that is believed to protect it from host immunity. A strong macrophage-dominated inflammatory infiltrate accumulates around the capsule, yet how macrophages contribute to capsule formation remains unclear. Our objective was to investigate the effect of macrophage on collagen capsule formation. METHODOLOGY/PRINCIPAL FINDINGS: The results of immunofluorescence showed that S100A4 ⁺ cells accumulated in T. spiralis-infected muscle, and were composed predominantly of CD206 ⁺ macrophages. These macrophages were positioned in direct contact with α-SMA+ myofibroblasts. Moreover, numerous cells positive for both CD206 (M2 macrophage canonical marker) and α‑smooth muscle actin (α-SMA, biomarker of myofibroblasts) were observed. Western blot analysis revealed that S100A4 expression was restricted chiefly to M2 macrophage and its expression was markedly amplified by IL-4 together with excretory-secretory (ES) products from muscle larvae of T. spiralis. qPCR showed that inhibiting S100a4 transcription reduced Arg1 mRNA levels in macrophages activated by ES and IL-4. Immunofluorescence and western blot analysis showed the activated macrophages induced modest upregulation of α-SMA in fibroblasts. Immunohistochemistry revealed that inhibition of S100A4 by niclosamide reduced the expressions of CD206, TGF-β1 and collagen I in T. spiralis-infected muscle. Compared with albendazole alone, the combination of niclosamide and albendazole further reduced the larval burden in T. spiralis-infected mice. CONCLUSIONS/SIGNIFICANCE: In conclusion, infection with T. spiralis significantly upregulated S100A4 expression in skeletal muscle-infiltrating macrophages. S100A4 further promoted macrophage M2 polarization. The M2-like macrophages either drove fibroblast activation or acquired myofibroblast-like features, thereby fostering collagen capsule formation. Blocking S100A4 diminished macrophage M2 polarization, prevented collagen capsule formation, and represented a potential adjunctive strategy against trichinellosis.