Abstract
Most microbial pathogens have a metabolic iron requirement, necessitating the acquisition of this nutrient in the host. In response to pathogen invasion, the human host limits iron availability. Although canonical examples of nutritional immunity are host strategies that limit pathogen access to Fe(III), little is known about how the host restricts access to another biologically relevant oxidation state of this metal, Fe(II). This redox species is prevalent at certain infection sites and is utilized by bacteria during chronic infection, suggesting that Fe(II) withholding by the host may be an effective but unrecognized form of nutritional immunity. Here, we report that human calprotectin (CP; S100A8/S100A9 or MRP8/MRP14 heterooligomer) inhibits iron uptake and induces an iron starvation response in Pseudomonas aeruginosa cells by sequestering Fe(II) at its unusual His6 site. Moreover, under aerobic conditions in which the Fe(III) oxidation state is favored, Fe(II) withholding by CP was enabled by (i) its ability to stabilize this redox state in solution and (ii) the production and secretion of redox-active, P. aeruginosa-produced phenazines, which reduce Fe(III) to Fe(II). Analyses of the interplay between P. aeruginosa secondary metabolites and CP indicated that Fe(II) withholding alters P. aeruginosa physiology and expression of virulence traits. Lastly, examination of the effect of CP on cell-associated metal levels in diverse human pathogens revealed that CP inhibits iron uptake by several bacterial species under aerobic conditions. This work implicates CP-mediated Fe(II) sequestration as a component of nutritional immunity in both aerobic and anaerobic milieus during P. aeruginosa infection.
Highlights
Most microbial pathogens have a metabolic iron requirement, necessitating the acquisition of this nutrient in the host
We discovered that human calprotectin (CP;4 S100A8/S100A9 or MRP8/MRP14 oligomer), a metal-sequestering protein well-known for withholding Mn(II) and Zn(II), coordinates Fe(II) with high affinity and has the capacity to inhibit microbial iron acquisition under reducing conditions [9]
We observed that CP-Ser and WT CP caused an ϳ1.5-fold reduction in cell-associated iron for PAO1 and PA14 compared with the untreated control, in agreement with previously reported inhibition of P. aeruginosa iron uptake by CP-Ser (Fig. 1A and Fig. S2) [9]
Summary
Nolan‡3 From the ‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 and §Department of Pharmaceutical Sciences, School of Pharmacy and ¶Department of Microbiology and Immunology, School of Medicine, University of Maryland, Baltimore, Maryland 21201
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
