Family 18 of glycosyl hydrolases encompasses chitinases and so-called chi-lectins lacking enzymatic activity due to amino acid substitutions in their active site. Both types of proteins widely occur in mammals although these organisms lack endogenous chitin. Their physiological function(s) as well as evolutionary relationships are still largely enigmatic. An overview of all family members is presented and their relationships are described. Molecular phylogenetic analyses suggest that both active chitinases (chitotriosidase and AMCase) result from an early gene duplication event. Further duplication events, followed by mutations leading to loss of chitinase activity, allowed evolution of the chi-lectins. The homologous genes encoding chitinase(-like) proteins are clustered in two distinct loci that display a high degree of synteny among mammals. Despite the shared chromosomal location and high homology, individual genes have evolved independently. Orthologs are more closely related than paralogues, and calculated substitution rate ratios indicate that protein-coding sequences underwent purifying selection. Substantial gene specialization has occurred in time, allowing for tissue-specific expression of pH optimized chitinases and chilectins. Finally, several family 18 chitinase-like proteins are present only in certain lineages of mammals, exemplifying recent evolutionary events in the chitinase protein family.

Man has been found to produce highly conserved chitinases. The most prominent is the phagocyte-derived chitotriosidase, the plasma levels of which are markedly elevated in some pathological conditions. Here, we report that both polymorphonuclear neutrophils (PMNs) and macrophages (m) are a source of chitotriosidase. The enzyme is located in specific granules of human PMNs and secreted following stimulation with granulocyte macrophage colony-stimulating factor (GM-CSF). In addition, GM-CSF induces expression of chitotriosidase in m that constitutively secrete the enzyme and partly accumulate it in their lysosomes. Studies with recombinant human chitotriosidase revealed that the enzyme targets chitin-containing fungi. These findings are consistent with earlier observations concerning anti-fungal activity of homologous plant chitinases and beneficial effects of GM-CSF administration in individuals suffering from invasive fungal infections. In conclusion, chitotriosidase should be viewed as a component of the innate immunity that may play a role in defence against chitin-containing pathogens and the expression and release of which by human phagocytes is highly regulated.

S U M M A R Y Two distinct chitinases have been identified in mammals: a phagocyte-specific enzyme named chitotriosidase and an acidic mammalian chitinase (AMCase) expressed in the lungs and gastrointestinal tract. Increased expression of both chitinases has been observed in different pathological conditions: chitotriosidase in lysosomal lipid storage disorders like Gaucher disease and AMCase in asthmatic lung disease. Recently, it was reported that AMCase activity is involved in the pathogenesis of asthma in an induced mouse model. Inhibition of chitinase activity was found to alleviate the inflammation-driven pathology. We studied the tissue-specific expression of both chitinases in mice and compared it to the situation in man. In both species AMCase is expressed in alveolar macrophages and in the gastrointestinal tract. In mice, chitotriosidase is expressed only in the gastrointestinal tract, the tongue, fore-stomach, and Paneth cells in the small intestine, whereas in man the enzyme is expressed exclusively by professional phagocytes. This species difference seems to be mediated by distinct promoter usage. In conclusion, the pattern of expression of chitinases in the lung differs between mouse and man. The implications for the development of anti-asthma drugs with chitinases as targets are discussed.