New research in Antimicrobial Agents and Chemotherapy presents evidence that tear lipocalin (TL) could contribute to innate immunity by scavenging siderophores and thereby inhibiting bacterial and fungal growth.

The cornea acts as a physical barrier to protect the rest of the eye. As the cornea contains no blood vessels, nutrients are provided by tear fluid, which also contains a variety of small molecules with antibacterial functions, including human tear lysozyme. TL is another component of human tear fluid, but until now its exact function has been unknown.

TL is a member of a large protein family known as the lipocalins, secreted molecules that are found in a wide variety of species and which are known to bind small hydrophobic molecules. It had previously been suggested that TL binding to these molecules could have a general protective function. In this work, Fluckinger et al. wanted to have a more detailed look at the function of TL.

Fluckinger et al. first analysed the binding of TL to different siderophores using a competitive displacement assay. There are two main groups of bacterial siderophores: the catecholates such as Escherichia coli enterobactin and the hydroxamates such as Streptomyces deferroxamine. Both these groups were analysed, along with fungal siderophores including Aspergillus nidulans triacetylfusarinine C. The results showed that TL bound with high affinity to all the siderophores tested with the exception of Pseudomonas aeruginosa pyoverdine — the authors suggest that this reflects the fact that pyoverdine has an additional chromophore residue that precludes binding in the TL active site.

Fluckinger et al. went on to look at the effects of TL on bacterial and fungal growth. Under non-iron-limiting conditions, the presence of TL had no effect on E. coli growth. However, under iron-limiting conditions, the presence of TL severely inhibited E. coli growth and this inhibition was reversed by the addition of free iron. In agreement with previous results, the presence of TL had no effect on P. aeruginosa growth. For fungi, the authors examined the effect of TL on the growth of a mutant strain of A. nidulans that is deficient in siderophore biosynthesis and showed that the presence of TL significantly inhibited growth.

One member of the lipocalin family, neutrophil gelatinase-associated lipocalin (NGAL), was already known to scavenge siderophores. This paper shows that TL also mops up siderophores, thereby inhibiting bacterial growth; unlike NGAL, however, which only binds to bacterial catecholate siderophores, TL binds to both catecholate and hydroxamate siderophores, and can inhibit both bacterial and fungal growth.