**Felix Hempel 1,**†**, Martin Roderfeld 1,**†**, Rajkumar Savai 2,3, Akylbek Sydykov 3, Karuna Irungbam 1, Ralph Schermuly 3, Robert Voswinckel 4,5, Kernt Köhler 6, Yury Churin 1, Ladislau Kiss 3, Jens Bier 3, Jörn Pons-Kühnemann <sup>7</sup> and Elke Roeb 1,\***


Received: 20 September 2019; Accepted: 5 October 2019; Published: 7 October 2019

**Abstract:** Bone marrow-derived fibrocytes (FC) represent a unique cell type, sharing features of both mesenchymal and hematopoietic cells. FC were shown to specifically infiltrate the injured liver and participate in fibrogenesis. Moreover, FC exert a variety of paracrine functions, thus possibly influencing the disease progression. However, the overall contribution of FC to liver fibrosis remains unclear. We aimed to study the effect of a specific FC depletion, utilizing a herpes simplex virus thymidine kinase (HSV-TK)/Valganciclovir suicide gene strategy. Fibrosis was induced by oral thioacetamide (TAA) administration in C57BL/6J mice. Hepatic hydroxyproline content was assessed for the primary readout. The HSV-TK model enabled the specific depletion of fibrocytes. Hepatic hydroxyproline content was significantly reduced as a result of the fibrocyte ablation (−7.8%; 95% CI: 0.7–14.8%; *p* = 0.033), denoting a reduced deposition of fibrillar collagens. Lower serum alanine transaminase levels (−20.9%; 95% CI: 0.4–36.9%; *p* = 0.049) indicate a mitigation of liver-specific cellular damage. A detailed mode of action, however, remains yet to be identified. The present study demonstrates a relevant functional contribution of fibrocytes to chronic toxic liver fibrosis, contradicting recent reports. Our results emphasize the need to thoroughly study the biology of fibrocytes in order to understand their importance for hepatic fibrogenesis.

**Keywords:** fibrocytes; liver fibrosis; bone marrow; myofibroblasts; thioacetamide (TAA); HSV-TK
