Human Polymorphonuclear Leukocytes Inhibit Aspergillus fumigatus Conidial Growth by Lactoferrin-Mediated Iron Depletion

Abstract

Aspergillus fumigatus, a common mold, rarely infects humans, except during prolonged neutropenia or in cases of chronic granulomatous disease (CGD), a primary immunodeficiency caused by mutations in the NADPH oxidase that normally produces fungicidal reactive oxygen species. Filamentous hyphae of Aspergillus are killed by normal, but not CGD polymorphonuclear leukocytes (PMN); however, the few studies on PMN-mediated host defenses against infectious conidia (spores) of this organism have yielded conflicting results, some showing that PMN do not inhibit conidial growth, with others showing that they do, most likely using reactive oxygen species. Given that CGD patients are exposed daily to hundreds of viable A. fumigatus conidia, yet considerable numbers of them survive years without infection, we reasoned that PMN use ROS-independent mechanisms to combat Aspergillus. We show that human PMN from both normal controls and CGD patients are equipotent at arresting the growth of Aspergillus conidia in vitro, indicating the presence of a reactive oxygen species-independent factor(s). Cell-free supernatants of degranulated normal and CGD neutrophils both suppressed fungal growth and were found to be rich in lactoferrin, an abundant PMN secondary granule protein. Purified iron-poor lactoferrin at concentrations occurring in PMN supernatants (and reported in human mucosal secretions in vivo) decreased fungal growth, whereas saturation of lactoferrin or PMN supernatants with iron, or testing in the presence of excess iron in the form of ferritin, completely abolished activity against conidia. These results demonstrate that PMN lactoferrin sequestration of iron is important for host defense against Aspergillus.

Induction of lactoferrin gene expression by innate immune stimuli in mouse mammary epithelial HC-11 cells

“…It is abundant in the secondary granules of the neutrophils and in biological fluids such as tears, saliva, seminal plasma and secretions of nasal, pancreatic, gastrointestinal, bronchial and uterine tissue (see review and reference therein [1]). Lactoferrin serves as a barrier with well established roles in anti-bacterial, anti-viral (review and reference therein [2]) and anti-fungal [3] infections. It is also known to modulate immune responses during infections (review and reference therein [4]).…”

Mendelian Genetics of Human Susceptibility to Fungal Infection

“…This fungus-specific difference in infection susceptibility may relate to the greater phagocyte dependence on nonoxidative mechanisms for killing Candida and Rhizopus over Aspergillus (Diamond and Clark 1982). Yet, nonoxidative killing mechanisms are also operational against Aspergillus (Zarember et al 2007), explaining why 65% of CGD patients never develop IA despite the ubiquitous exposure to environmental Aspergillus conidia during their lifetime. Not surprisingly, consonant with the lack of heightened susceptibility of neutropenic patients to mucosal yeast infections, cryptococcosis and dimorphic fungi, these infections do not occur in CGD.…”

A Polysaccharide Virulence Factor of a Human Fungal Pathogen Induces Neutrophil Apoptosis via NK Cells

“…The morbidity and mortality associated with the latter remain high despite recent advances in prevention, diagnosis, and treatment (8,9). Long recognized as critical to host defense against Aspergillus species, particularly when conidia escape macrophage phagocytosis (10), PMNs provide essential anticonidia defense and prevent conidial germination by releasing proteolytic enzymes and rapidly producing ROS in the so-called oxidative burst (11,12).…”