The epigenome as a putative target for skin repair: the HDAC inhibitor Trichostatin A modulates myeloid progenitor plasticity and behavior and improves wound healing

The molecular pathways that drive bone marrow myeloid progenitors (BMMP) development are very well understood and include a tight controlled multi-stage gene hierarch. Monocytes are versatile cells that display remarkable plasticity and may give rise to specific subsets of macrophages to proper promote tissue homesostasis upon an injury. However, the epigenetic mechanisms that underlie monocyte differentiation into the pro-inflammatory Ly6C^high or the repairing Ly6C^low subsets are yet to be elucidated. We have previously shown that Epigenetic mechanisms Histone Deacetylase (HDAC) dependent are crucial for monocyte behavior and plasticity and in this work, we propose that this same mechanism underlies BMMP plasticity upon an inflammatory challenge in vivo.

BMMP were culture in the presence of GM-CSF alone or in combination with HDAC inhibitor (iHDAC) and phenotyped by flow cytometry, immune staining or western blot. iHDAC was topically added to skin wounds for 7 consecutive days and wound healing was monitored by flow cytometry and histopathological analysis.

When BMMP were cultured in the presence of iHDAC, we showed that the CD11b^low/Ly6C^low subset was the specific target of iHDAC that underwent chromatin hyperacetylation in vitro. Upon 13 days in the presence of iHDAC, BMMP gave rise to very elongated macrophages, that in turn, displayed a remarkable plasticity in a HDAC activity dependent fashion. HDAC-dependent cell shape was tight related to macrophage behavior and phenotype through the control of iNOS protein levels, showing that chromatin remodeling is a key component of macrophage plasticity and function. We then hypothesized that iHDAC would modulate the inflammatory response and favor tissue repair in vivo. To test this hypothesis, we topically added iHDAC to skin wounds during 7 consecutive days and followed tissue repair dynamics. In fact, iHDAC treated skin wounds presented an increase in wound closure at day 5 that was correlated to an enrichment in the CD11b^low/Ly6C^low subset and in very elongated F4/80 positives macrophages in vivo, fully recapitulating the behavior previously observed in vitro.

Our work provides the biological basis that connects chromatin remodeling to phenotypic plasticity, which in turn, may become a tractable therapeutic strategy in further translational studies.