Calprotectin

Precise definition

Calprotectin is a heterodimer of two S100 family proteins, S100A8 (calgranulin A, MRP8) and S100A9 (calgranulin B, MRP14). It is exceptionally abundant in neutrophils, accounting for roughly 40-60% of their soluble cytosolic protein, and is also expressed by monocytes and activated macrophages. The functional antimicrobial unit is typically a heterotetramer (S100A8/S100A9)2 that assembles in the presence of calcium.

Each S100 subunit contains EF-hand calcium-binding motifs, and the assembled complex forms transition-metal-binding sites at the interface between subunits. Binding of Ca2+ increases the protein's affinity for transition metals and promotes tetramerization, coupling calcium sensing to metal sequestration. Because it scavenges metals to restrict microbial growth, calprotectin is a central effector of nutritional immunity and a member of the broader group of host metal-withholding proteins alongside lactoferrin, transferrin, and lipocalin-2.

How it works

The antimicrobial activity of calprotectin comes from two transition-metal-binding sites formed where the S100A8 and S100A9 subunits meet. Site 1 is a His3Asp (histidine-rich) site, and site 2 is an unusual His6 site composed of six histidine residues contributed by both subunits. The His6 site binds Mn2+ (and also Fe2+ and Ni2+) with high affinity, while both sites can bind Zn2+; this His6 arrangement is what allows calprotectin, uniquely among S100 proteins, to sequester manganese.

By binding these metals with picomolar-to-nanomolar affinities, calprotectin lowers the free concentration of Mn and Zn in the extracellular space below what microbes need to grow. Manganese starvation is especially damaging because Mn is a required cofactor for enzymes such as superoxide dismutase (SodA) and central-metabolism and glycolytic enzymes; zinc limitation impairs the many zinc-dependent proteins microbes rely on. Calcium binding at the EF-hands allosterically tightens the metal sites, so calprotectin is most active in the high-calcium extracellular environment it encounters after neutrophil release or NETosis.

Calprotectin is deployed at inflammatory sites through several routes: passive release from dying or degranulating neutrophils, active secretion, and incorporation into neutrophil extracellular traps (NETs). Once outside the cell it also acts as a damage-associated molecular pattern (DAMP), signaling through Toll-like receptor 4 (TLR4) and RAGE to amplify inflammation, in addition to its direct metal-withholding role.

Concrete examples

In the mammalian abscess, calprotectin chelates manganese and zinc to inhibit Staphylococcus aureus; mice lacking S100A9 (and therefore functional calprotectin) show increased staphylococcal growth in abscesses, and S. aureus counters this pressure with high-affinity Mn (MntABC/MntH) and Zn (ZnuABC) import systems. Similar contests occur with Acinetobacter baumannii, Klebsiella pneumoniae, Salmonella Typhimurium, and Pseudomonas aeruginosa, each of which upregulates metal-import machinery under calprotectin-imposed starvation.

The effect extends beyond bacteria. Calprotectin restricts the fungal pathogens Candida albicans and Aspergillus fumigatus by withholding zinc and manganese, limiting hyphal growth. In the gut, calprotectin shed from infiltrating neutrophils shapes the metal landscape of the inflamed mucosa and can influence which members of the microbiota thrive, an example of the metal-microbiome-disease axis in action.

The same protein is exploited diagnostically. Fecal (stool) calprotectin is a routine noninvasive test used to distinguish inflammatory bowel disease (Crohn's disease and ulcerative colitis) from non-inflammatory conditions such as irritable bowel syndrome, and to monitor disease activity and response to therapy. In the blood, serum calprotectin (often reported as S100A8/A9 or MRP8/14) is studied as a marker of inflammation in rheumatoid arthritis, sepsis, and other inflammatory disorders.

Why it matters

Calprotectin is one of the clearest molecular illustrations of nutritional immunity: rather than attacking microbes directly, the host restricts access to essential trace metals. Understanding its mechanism explains why so many pathogens invest heavily in manganese and zinc uptake systems, and why those importers are attractive antibacterial and antifungal targets. It also frames metals not just as nutrients but as contested resources at the host-microbe interface.

Clinically, calprotectin bridges basic metallobiology and everyday medicine. Because it is stable in feces and rises with mucosal neutrophil infiltration, fecal calprotectin has become a standard, inexpensive way to gauge gut inflammation and reduce unnecessary endoscopies. Within microbial metallomics, calprotectin is a keystone concept linking host immunity, trace-metal homeostasis (metallostasis), and disease, and a reminder that the outcome of infection can turn on the availability of a single trace metal such as manganese or zinc.