Metal Sequestration and Antimicrobial Activity of Human Calprotectin are pH-Dependent.

Human calprotectin (CP, S100A8/S100A9 oligomer) is an abundant innate immune protein that sequesters transition metal ions in the extracellular space to limit nutrient availability and the growth of invading microbial pathogens. Our current understanding of the metal-sequestering ability of CP is based on biochemical and functional studies performed at neutral or near-neutral pH. Nevertheless, CP can be present throughout the human body and is highly expressed at infection and inflammation sites that tend to be acidic. Here, we evaluate the metal-binding and antimicrobial properties of CP in the pH 5.0-7.0 range. We show that Ca(II)-induced tetramerization, an important process for the extracellular functions of CP, is perturbed by acidic conditions. Moreover, low pH impairs the antimicrobial activity of CP against some bacterial pathogens, including Staphylococcus aureus and Salmonella enterica serovar Typhimurium. At mildly acidic pH, CP loses the ability to deplete Mn from microbial growth medium, indicating that Mn(II) sequestration is attenuated under acidic conditions. Evaluation of the Mn(II)-binding properties of CP at pH 5.0-7.0 indicate that mildly acidic conditions lower the Mn(II)-binding affinity of the His6 site. Lastly, CP is less effective at preventing capture of Mn(II) by the bacterial solute-binding proteins MntC and PsaA at low pH. These results indicate that acidic conditions compromise the ability CP to sequester Mn(II) and thereby starve microbial pathogens of this nutrient. Broadly, this work highlights the importance of considering the chemical complexity of biological sites when describing the interplay between CP and microbes in host-pathogen interactions.