4-Hydroxybutyrate Promotes Endogenous Antimicrobial Peptide Expression in Macrophages

Naturally occurring antimicrobial peptides (AMPs) are effector molecules secreted by several cell types but especially by cells of the innate immune system such as macrophages. AMPs largely avoid resistance acquired by bacteria to man-made antibiotics. Different short chain fatty acids (SCFAs) have proven to be efficient inducers of AMP secretion. The SCFA butyrate is an endogenous histone deacetylase inhibitor that has been shown to induce the expression of the AMP cramp (cathelicidin LL-37) in cells of the immune system and mucosal epithelium, which provides protection against pathogens. Clinical applications of butyrate, however, are limited due to its cytotoxic effects. Hydroxylated derivatives of butyrate (2-hydroxybutyrate [2HB], 3-hydroxybutyrate [3HB], and 4-hydroxybutyrate [4HB]) are also endogenous molecules, but their capability of inducing AMP expression has been unexplored. 4HB has been used for the production of polymeric surgical meshes intended for soft tissue repair. This study evaluated the ability of hydroxylated derivatives of butyrate to induce the upregulation of AMPs in murine bone marrow-derived macrophages in vitro. Noncytotoxic effects and increased cramp and -defensin-4 were found to be induced by 4HB. An in vivo increased resistance to deliberate bacterial contamination was shown by a surgical mesh composed of a polymer of 4HB compared with polypropylene surgical mesh. Impact Statement This study evaluated the biological activity of hydroxylated derivatives of butyrate as inductors of antimicrobial peptides (AMPs) in murine bone marrow-derived macrophages in vitro. A differential modulation of AMP expression by the hydroxylated derivatives of butyrate is shown. The ability of sodium 4-hydroxybutyrate to upregulate AMP expression through a histone deacetylase inhibitory-independent mechanism, and to promote increased resistance to bacterial contamination in vivo are also shown. The findings provide an alternative for prevention of bacterial contamination of implanted biomaterials. Functionalization of biomaterials with hydroxylated derivatives of butyrate can enhance the endogenous antimicrobial activity of the immune system through increased production of AMPs by host cells, thus providing protection against bacterial contamination.