A heterodimer of S100A8 and S100A9 calcium-binding proteins that constitutes approximately 60% of the cytosolic protein content of neutrophils. Calprotectin is a central effector of nutritional immunity, sequestering essential transition metals from invading pathogens at sites of infection and inflammation.
Structure and Metal Binding
Multi-Metal Sequestration
Calprotectin possesses two distinct metal-binding sites:
- Site I (His3Asp): binds Zn(II) with high affinity.
- Site II (hexahistidine, His6): unique among S100 proteins. This site binds Zn(II), Mn(II), Fe(II), AND Ni(II). Critically, the hexahistidine site coordinates Ni(II) preferentially over Zn(II), making calprotectin the first identified host protein with preferential nickel-sequestering activity [maier 2019 nickel microbial pathogenesis].
Functional Implications
- Calprotectin simultaneously restricts multiple metals at infection sites, creating a multi-metal "desert" that pathogens must overcome to establish infection.
- The preferential Ni coordination is significant because it means calprotectin can selectively target nickel-dependent virulence factors (urease, [NiFe]-hydrogenase) even in the presence of higher zinc concentrations.
Release and Distribution
- Released by neutrophils at infection sites during NETosis (neutrophil extracellular trap formation) and degranulation.
- Also expressed by monocytes, macrophages, and epithelial cells under inflammatory conditions.
- Found at high concentrations in abscess cavities, mucosal surfaces, and the gut lumen during inflammation.
- Constitutes the dominant antimicrobial protein in neutrophil abscesses surrounding staphylococcus aureus infections.
Pathogen Targets
Staphylococcus aureus
- Calprotectin sequesters Zn, Mn, and Ni from S. aureus in abscess environments, directly inhibiting urease activity and Mn-SOD function [maier 2019 nickel microbial pathogenesis].
- S. aureus counters with staphylopine, a broad-spectrum metallophore that scavenges metals even in the calprotectin-rich abscess environment [maier 2019 nickel microbial pathogenesis].
Streptococcus pneumoniae
- Calprotectin Mn sequestration limits streptococcal Mn-dependent virulence, including Mn-SOD activity [akbari 2022 metal homeostasis streptococci].
Klebsiella pneumoniae
- Calprotectin inhibits urease activity by restricting nickel availability to K. pneumoniae [maier 2019 nickel microbial pathogenesis].
Fecal Calprotectin as a Biomarker
Calprotectin shed into the gut lumen is measurable in stool as fecal calprotectin (FC), which has become a standard non-invasive biomarker:
- IBD: Fecal calprotectin distinguishes inflammatory bowel disease from irritable bowel syndrome and monitors disease activity [amerikanou 2022 ibd biomarkers trace metals].
- NEC: Elevated in necrotizing enterocolitis in preterm infants, where it reflects neutrophil infiltration and mucosal damage [pendergrass 2026 nickel nec preterm gut].
- Elevated FC reflects neutrophil-driven inflammation anywhere in the GI tract.
The Dual Role
Calprotectin exemplifies the tension between host defense and collateral damage:
- Protective: restricts metals from pathogens, limiting virulence.
- Potentially harmful: metal restriction also affects commensal bacteria, potentially worsening dysbiosis. Chronic calprotectin elevation in inflammatory conditions may contribute to the metal-depleted mucosal environment that favors pathobionts with superior metal acquisition systems.
Connections
- nutritional immunity -- calprotectin is a primary effector of host metal sequestration
- nickel -- preferential Ni binding at the hexahistidine site
- zinc, manganese -- traditional calprotectin targets
- staphylococcus aureus -- best-characterized calprotectin-pathogen interaction
- inter kingdom metal shielding -- biofilms may physically exclude calprotectin
- lactoferrin -- complementary metal-sequestering protein (iron-focused)