Lipocalin 2 mediates an innate immune response to bacterial infection by sequestrating iron Original paper

What was studied?

This study examined lipocalin 2 sequestrates bacterial siderophores as a host innate immune mechanism that restricts bacterial iron acquisition and limits systemic infection. Using mouse models and in vitro growth assays, the authors tested whether Toll-like receptor (TLR) signaling induces lipocalin 2 (also called NGAL/siderocalin/24p3) and whether lipocalin 2 directly inhibits bacterial growth by binding iron-loaded catecholate siderophores, particularly enterochelin/enterobactin. The work links pathogen nutrient-scavenging chemistry (siderophores) to an inducible host countermeasure within the acute phase response, defining a molecular “nutritional immunity” pathway relevant to microbes commonly represented in microbiome datasets (notably Enterobacteriaceae).

Who was studied?

The primary experimental subjects were mice, including C57BL/6 wild-type controls, TLR4-deficient mice, and genetically engineered lipocalin-2–deficient (Lcn2 knockout) mice. Mice were challenged intraperitoneally with lipopolysaccharide (LPS) to probe TLR-dependent induction of lipocalin 2, or infected with a clinical Escherichia coli strain (H9049) to model sublethal and lethal Gram-negative sepsis. For specificity controls, mice were also infected with Staphylococcus aureus (ATCC 25923). Complementary in vitro assays exposed bacteria to iron-restricted media or acute phase serum with recombinant lipocalin 2 and/or purified siderophores (enterochelin, ferrichrome, aerobactin, rhizoferrin) to determine which microbial iron-scavenging strategies were vulnerable to lipocalin 2.

Most important findings

Lipocalin 2 was strongly induced by TLR stimulation: LPS drove a marked rise in lipocalin 2 mRNA and protein in a TLR4-dependent manner, indicating that innate sensing pathways rapidly upregulate this iron-sequestration effector during infection. Functionally, lipocalin-2–deficient mice developed substantially higher bacteremia and tissue burdens after E. coli challenge, with clinical signs of sepsis tracking with bacterial load; at higher inocula, knockouts showed dramatically increased mortality. Mechanistically, lipocalin 2 bound catecholate-type siderophores (including enterochelin/enterobactin) but not hydroxamate or polycarboxylate siderophores (such as ferrichrome, aerobactin, rhizoferrin), and recombinant lipocalin 2 inhibited E. coli growth in iron-restricted conditions in an enterochelin-dependent manner. This specificity mattered in vivo: supplying ferrichrome provided an alternative iron route that bypassed lipocalin-2–mediated restriction and increased lethality in wild-type mice, while S. aureus infection (lipocalin-2–independent iron acquisition) was not worsened by lipocalin 2 deficiency. Collectively, the key microbiome-relevant association is that Enterobacteriaceae relying on enterobactin-like catecholate siderophores are selectively constrained by host lipocalin 2, a relationship that may influence pathogen expansion signals in host-associated microbial community data.

Key implications

For clinicians and microbiome researchers, this paper frames lipocalin 2 as an inducible host “gatekeeper” against enterobactin-producing taxa, offering a mechanistic lens for interpreting shifts in Enterobacteriaceae abundance during inflammation and sepsis risk. Because lipocalin 2 acts by neutralizing catecholate siderophores, pathogens (or community members) that switch siderophore classes or access exogenous hydroxamate siderophores may escape this defense—an evolutionary logic that can help explain why certain infections progress despite robust inflammatory signaling. Clinically, the findings also support lipocalin 2 as both a biomarker of acute inflammation and a potential therapeutic lever in bloodstream infections where siderophore-mediated iron piracy is dominant, while emphasizing that efficacy would be organism- and siderophore-specific rather than broadly antibacterial.

Citation

Flo TH, Smith KD, Sato S, et al. Lipocalin 2 mediates an innate immune response to bacterial infection by sequestrating iron. Nature. 2004;432(7019):917-921. doi:10.1038/nature03104