Interaction of Lipocalin 2, Transferrin, and Siderophores Determines the Replicative Niche of Klebsiella pneumoniae during Pneumonia

Pathogenic bacteria require iron for replication within their host. Klebsiella pneumoniae and other Gram-negative pathogens produce the prototypical siderophore enterobactin (Ent) to scavenge iron in vivo. In response, mucosal surfaces secrete lipocalin 2 (Lcn2), an innate immune protein that binds Ent to disrupt bacterial iron acquisition and promote acute inflammation during colonization. A subset of K. pneumoniae isolates attempt to evade Lcn2 by producing glycosylated Ent (Gly-Ent, salmochelin) or the alternative siderophore yersiniabactin (Ybt). However, these siderophores are not functionally equivalent and differ in their abilities to promote growth in the upper respiratory tract, lungs, and serum. To understand how Lcn2 exploits functional differences between siderophores, isogenic mutants of an Ent(+) Gly-Ent(+) Ybt(+) K. pneumoniae strain were inoculated into Lcn2(+/+) and Lcn2(−/−) mice, and the pattern of pneumonia was examined. Lcn2 effectively protected against the iroA ybtS mutant (Ent(+) Gly-Ent(−) Ybt(−)). Lcn2(+/+) mice had small foci of pneumonia, whereas Lcn2(−/−) mice had many bacteria in the perivascular space. The entB mutant (Ent(−) Ybt(+) Gly-Ent(−)) caused moderate bronchopneumonia but did not invade the transferrin-containing perivascular space. Accordingly, transferrin blocked Ybt-dependent growth in vitro. The wild type and the iroA mutant, which both produce Ent and Ybt, had a mixed phenotype, causing a moderate bronchopneumonia in Lcn2(+/+) mice and perivascular overgrowth in Lcn2(−/−) mice. Together, these data indicate that Lcn2, in combination with transferrin, confines K. pneumoniae to the airways and prevents invasion into tissue containing the pulmonary vasculature.