Myeloid cell populations and fibrogenic parameters in bleomycin- and HOCl-induced fibrosis

Mouse models resembling systemic sclerosis can be chemically induced by application of bleomycin or hypochloric acid (HOCl). To date, little is known about inflammatory cells and their potential role in scleroderma (Scl)-related fibrosis. Therefore, we compared both Scl models to define the early immune cell subsets in relation to fibrosis-related parameters. Both agents induced a significant increase in dermal thickness and collagen deposition after 4 weeks, as hallmarks of Scl. However, clinical skin thickness, densely packed, sirius red-stained collagen bundles and collagen cross-links were more pronounced in HOCl-induced Scl. In parallel, there was a significant upregulation of procollagen α1(I), α-SMA and TGF-β transcripts in HOCl animals, whereas IL-1β and MMP-13 mRNA levels were significantly increased in bleomycin-treated mice. Flow cytometric analysis of the Scl skin demonstrated an early cellular infiltrate containing mainly CD19⁺ B cells, CD4⁺ T cells, CD11c⁺ DC and CD11b⁺ myeloid cells, the latter ones being significantly more prominent after HOCl injection. Subanalysis revealed that Scl mice exhibited a significant increase of inflammatory myeloid CD11b⁺  Ly6C^low-high  CD64^low-high cells (HOCl>bleomycin). In particular, in the HOCl model, activated dermal macrophages (CCR2^low  MHCII^high ) and monocyte-derived DC (CCR2^high MHCII^high ) predominated over less activated CD11b⁺ myeloid cells. In conclusion, the two models differ in certain aspects of the murine and human scleroderma but in the HOCl model, myeloid CD11b⁺  MHCII^high cells correlate with some fibrosis-related parameters. Therefore, analysis of both models is suggested to cover a comprehensive profile of Scl symptoms but with focus on the HOCl model when the role of early myeloid immune cells will be evaluated.