Aspergillus fumigatus is an opportunistic fungal pathogen which produces spores found widely in the environment (Figure 1). This fungus is well equipped to deal with environmental stresses. For example, it is heavily melanised (and therefore appears green/black) which helps protect it from UV rays.
The spores produced by this species, known as conidia, are inhaled easily and it is estimated that we breathe several hundreds to thousands of conidia daily. When we inhale conidia, resident immune cells in our lungs phagocytose and destroy them − preventing them from causing an infection. Alveolar macrophages in the lung are the primary line of defence against conidia.
As with many fungal pathogens, when the immune system becomes impaired we become highly susceptible to A. fumigatus infections, which are associated with mortality rates of up to 80%. A. fumigatus is a major concern for patients undergoing solid organ transplants (and who are therefore usually receiving immunosuppressive drugs) who become susceptible to invasive pulmonary aspergillosis (IPA).
IPA is a lung infection whereby A. fumigatus conidia start to swell and form filamentous forms called germ tubes (short tubes) and hyphae (long tubes). These elongated cells can penetrate host cells and cause damage, and allow A. fumigatus to disseminate into the bloodstream. Our immune system also reacts more violently to these forms of the fungus which stimulate production of pro-inflammatory cytokines resulting in immunopathology.
Immunity to A. fumigatus
When conidia swell and start forming germ tubes/hyphae, the cell wall starts to change. This results in exposure of previously hidden pathogen-associated molecular patterns (PAMPs); key molecules recognised by receptors of the innate immune system (pattern recognition receptors, PRRs).
For example, β-glucans primarily make up the cell walls of hyphae but are shielded from the immune system in conidia by a proteinaceous layer. β–glucans are recognised by the PRR, Dectin-1, expressed by myeloid cells, which initiates inflammatory responses (Figure 2).
Dectin-1 mediates uptake and killing of A. fumigatus. Humans with a point mutation in the Dectin-1 gene are highly susceptible to A. fumigatus infection following transplants and this is cited as a risk factor.
Figure 2. Resting conidia have an outer protein layer ‘masking’ β–glucans from the immune system. Upon swelling, β-glucans become exposed and are able to be recognised by Dectin-1 on host leukocytes, resulting in the initiation of an immune response.
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