Background/Purpose: Monocytes and monocyte-derived cells play a crucial role during homeostasis and also during the development of various inflammatory diseases including skin fibrosis. Ly6C^hi inflammatory monocytes formed in the bone marrow enter the circulation as mature C-C Chemokine Receptor 2 (CCR2⁺) cells and exert their function by their ability to differentiate into macrophages and other myeloid cell types in the peripheral tissues. Our lab previously showed upregulation of monocytes and monocyte derived macrophages followed by loss of adipose-derived stromal cells (ADSC) in bleomycin-induced skin fibrosis. CCR2 is implicated to have a pathogenic role in multiple fibrosis models.  Here, we hypothesized that CCR2⁺ monocytes that infiltrate the skin following bleomycin treatment play an important role in modulating ADSC maintenance and bleomycin-induced skin fibrosis changes.

Methods: C57BL/6J wild type (WT) and CCR2 knockout (CCR2KO) mice were injected with either PBS (control) or 100ug of bleomycin subcutaneously on the lower back skin for indicated time periods. Skin was assessed for cellular changes by flow cytometry and for fibrotic changes by H&E staining followed by dermal and dermal white adipose tissue (DWAT) thickness measurements.

Results: Consistent with the idea that monocyte accumulation in skin is dependent on CCR2 and that the macrophages are monocyte-derived, we observed a robust increase in monocytes and monocyte-derived macrophages in WT but not CCR2KO mice at day 21 after bleomycin treatment. Contrary to expectations, however, while bleomycin-treated WT mice showed ~55% reduction in ADSC numbers compared to PBS control, CCR2KO mice under the same conditions showed further severity in ADSC loss (~80% reduction). CCR2 KO mice also showed increased dermal thickness without further loss of DWAT compared to bleomycin-treated WT mice. These results suggest a role for CCR2⁺ cells, potentially monocytes and monocyte-derived macrophages, in maintaining ADSC numbers and limiting fibrosis in bleomycin-induced skin fibrosis.

Conclusion: Our results thus far suggest a protective role for CCR2, potentially by mediating the accumulation of monocytes and macrophages and maintaining ADSC numbers during bleomycin-induced skin fibrosis. Further studies will focus on establishing the role of CCR2-dependent monocytes versus CCR2 on other cell populations as well as on understanding the mechanism by which CCR2 regulates ADSC populations. These results could thus help in better understanding and therapeutic targeting of the disease.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/ccr2-circulating-monocytes-contribute-to-the-survival-of-adsc-in-bleomycin-induced-skin-fibrosis/