Caspofungin Treatment of Aspergillus fumigatus Results in ChsG-Dependent Upregulation of Chitin Synthesis and the Formation of Chitin-Rich Microcolonies

Treatment of Aspergillus fumigatus with echinocandins such as caspofungin inhibits the synthesis of cell wall β,1-3 glucan which triggers a compensatory stimulation of chitin synthesis. Activation of chitin synthesis can occur in response to sub-MIC concentrations of caspofungin and to CaCl2 and CFW, agonists of the PKC and Ca2+-calcineurin signalling pathways. The response of a range of ΔAfchs (chitin synthase) mutants to these agonists was tested to determine the chitin synthase enzymes that were required for the compensatory up-regulation of chitin synthesis. Only the ΔAfchsG mutant was hypersensitive to caspofungin and all other ΔAfchs mutants tested remained capable of increasing their chitin content in response to treatment with CaCl2 and CFW and caspofungin. The resulting increase in cell wall chitin content correlated with reduced susceptibility to caspofungin in both the wild-type and all ΔAfchs mutants tested, with the exception of ΔAfchsG, which remained sensitive to caspofungin. In vitro exposure to the chitin synthase inhibitor, nikkomycin Z along with caspofungin demonstrated synergistic efficacy which was again AfChsG-dependent. Dynamic imaging using microfluidic perfusion chambers demonstrated that treatment with sub-MIC caspofungin resulted initially in hypha tip lysis. However, thickened hyphae emerged that formed aberrant microcolonies in the continued presence of caspofungin. In addition, intra-hyphal hyphae were formed in response to echinocandin treatment. These in vitro data demonstrate that A. fumigatus has the potential to survive echinocandin treatment in vivo by AfChsG-dependent upregulation of chitin synthesis. Chitin-rich cells may therefore persist in human tissues and act as the focus for breakthrough infections.