Candida Infection Biology – fungal armoury, battlefields ... - FINSysB

Candida albicans Secreted Aspartic Proteases Cause
Inflammatory Response Irrespective of Their Proteolytic
Activity
Neelam Pandey 1 , Donatella Pietrella 1 , Lydia Schild 2 , Francesco Bistoni 1 ,
Bernhard Hube 2 and Anna Vecchiarelli 1
1 Department of Experimental Medicine and Biochemical Science, University of Perugia,
Perugia, Italy; 2 Department of Microbial Pathogenicity Mechanisms, Leibniz Institute for
Natural Product Research and Infection Biology, Hans Knoell Institute, Jena, Germany
Candida albicans normally inhabits humans as a commensal, but causes fatal infections
when the immune system of patients is suppressed. It has a strong armory of virulence
attributes which facilitates bypassing the immune system and causes mucosal,
cutaneous and systemic infections. These attributes are the ability to colonize, invade
the host tissue and secrete aspartic proteases. We here report that recombinant secreted
aspartic proteases (rSaps) including rSap1, rSap2, rSap3 and rSap6 induce inflammatory
cytokine production by human monocytes to different degrees. rSap1, rSap2 and rSap6
induced the secretion of IL-1beta, TNF-alpha and IL-6 significantly, while rSap3
stimulated the secretion of IL-1beta and TNF-alpha. All these rSaps induced Ca 2+ influx
in monocytes. Pepstatin A, a potential inhibitor of aspartic proteases, has no effect on
the cytokine secretion induced by these rSaps. This suggests that the inflammatory
response due to rSaps is not because of their proteolytic activity, a hypothesis further
supported by the observation that the ability of rSaps to induce inflammatory cytokine
secretion was independent of protease-activated receptor (PAR) activation, and of the
optimal pH for individual rSap activity. Moreover, rSaps stimulated Akt activation and
thus induced IkB-alpha phosphorylation which mediated translocation of NFkB into the
nucleus in human monocytes. These findings give evidence that rSap1, rSap2 and rSap3
cause an inflammatory response irrespective of their proteolytic activity and mediate the
inflammatory response via activation of Akt/NF-kB. The inflammatory response
terminates with inflammasome activation. This occurs through a process requiring Sap
internalization via a clathrin dependent mechanism, and caspase-1 activation, and it is
mediated by K + efflux and ROS production. By demonstrating that the recognized
virulence factors of C. albicans such as Sap2 and Sap6 are potent inducers of
inflammasome activation, our results represent a step forward in the comprehension of
the close relationship between fungus virulence and host response mechanisms.
We are extending our study by using an in vivo experimental model of murine candidiasis
to analyze the role of Sap2 and Sap6 in the inflammatory response in the vaginal
candidiasis, and to investigate whether specific mAbs are indeed able to affect this
process.
This study was funded by the Eurpoean Commission through the FINSysB Marie Curie Initial Training
Network (PITN-GA-2008-214004).
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