Aspergillus fumigatus conidial melanin modulates host cytokine response

Abstract

Melanin biopigments have been linked to fungal virulence. Aspergillus fumigatus conidia are melanised and are weakly immunogenic. We show that melanin pigments on the surface of resting Aspergillus fumigatus conidia may serve to mask pathogen-associated molecular patterns (PAMPs)-induced cytokine response. The albino conidia induced significantly more proinflammatory cytokines in human peripheral blood mononuclear cells (PBMC), as compared to melanised wild-type conidia. Blocking dectin-1 receptor, Toll-like receptor 4 or mannose receptor decreased cytokine production induced by the albino but not by the wild type conidia. Moreover, albino conidia stimulated less potently, cytokine production in PBMC isolated from an individual with defective dectin-1, compared to the stimulation of cells isolated from healthy donors. These results suggest that β-glucans, but also other stimulatory PAMPs like mannan derivatives, are exposed on conidial surface in the absence of melanin. Melanin may play a modulatory role by impeding the capability of host immune cells to respond to specific ligands on A. fumigatus.

Figure 1

Human PBMC response to wild-type (WT) melanised and albino A. fumigatus conidia (10⁷/ml). Interleukin-6 (IL-6), tumour necrosis factor alpha (TNF) (Figure 1a) and interleukin 10 (IL-10), interferon-gamma (IFNγ) levels as shown (Figure 1b). A. fumigatus conidia were poorly immunogenic at the concentrations of 10⁵ to 10⁶ /ml (data not shown). Data was pooled from 3 sets of experiments and expressed as mean ± SEM; n = 6 subjects. * p < 0.05 in respective cytokine production between WT conidia and albino conidia.

Figure 2

PBMC pre-incubated with laminarin (blocks dectin-1; Figures 2a & b), Bartonella LPS (blocks TLR4; Figures 2c & d) and mannose (blocks mannose receptor pathway; Figures 2e & f) and stimulated with albino and wild-type (WT) Aspergillus conidia. Data pooled from 3 sets of experiments and expressed as mean ± SEM, n = 6 subjects. * p < 0.05 as compared to respective control (without specified inhibitor).

Figure 3

PBMC response to albino and wild-type (WT) strains of A. fumigatus conidia in dectin-1-deficient homozygous subject versus 3 control subjects. As compared to controls, the dectin-1 deficient subject displayed a trend towards less cytokine production with the albino conidia (Fig 3a & c) than WT conidia. Positive control using Candida albicans blastoconidia, heat-killed with exposed surface β-glucans with and without dectin-1 inhibitor, laminarin (Fig 3b & d). Data pooled from 2 sets of experiments and expressed as mean ± SEM.