Depletion of folate receptor β-expressing macrophages alleviates bleomycin-induced experimental skin fibrosis.

Objectives: Folate receptor β (FRβ)-expressing macrophages have been identified as activated macrophages. Here, we investigated the infiltration of FRβ-expressing macrophages in a murine model of bleomycin (BLM)-induced skin fibrosis and assessed the antifibrotic effects of depletion of FRβ-expressing macrophages in this model using a recombinant immunotoxin to FRβ.
Methods: A recombinant immunotoxin (anti-FRβ-PE38) was prepared by conjugating the Fv portion of the anti-mouse FRβ heavy chain with truncated Pseudomonas exotoxin A (VH-PE38) and the Fv portion of the anti-mouse FRβ light chain. BLM-induced skin fibrosis mice were intravenously treated with either anti-FRβ-PE38 or VH-PE38 as a control protein. Skin fibrosis was evaluated by the change of skin thickness and hydroxyproline content on Day 29. The TGFβ1 mRNA levels in the treated skin were assessed by quantitative real-time RT-PCR on Day 9.
Results: Numbers of FRβ-expressing macrophages increased in BLM-injected skin. Anti-FRβ-PE38 treatment led to a dramatic reduction in the number of FRβ-expressing macrophages. Additionally, skin thickness and hydroxyproline content, were markedly reduced. TGFβ1 mRNA levels were also down-regulated after the treatment. TGFβ1 expression was enriched in FRβ-expressing macrophages compared with FRβ-negative macrophages.
Conclusion: These results indicated that anti-FRβ-PE38 treatment efficiently depleted FRβ-expressing macrophages and consequently alleviated BLM-induced skin fibrosis.