Interaction of the opportunistic pathogen, Candida albicans with the host coagulation protein, factor XII

Candida albicans is an opportunistic pathogen that lives as part of the mammalian commensal flora within mucosal passages, such as the gastrointestinal tract. C. albicans is a polymorphic fungus that can switch between yeast, true hyphal and pseudohyphal morphologies. Hyphae are associated with C. albicans virulence and possess hypha-specific proteins, such as Als3p, a multifunctional adhesin that is able to manipulate its own uptake by host cells and aids in the progression of C. albicans infections. C. albicans infections can range from superficial infections such as thrush to life threatening nosocomial bloodstream infections in immunocompromised patients, known as systemic candidiasis. There are more than 250,000 cases of systemic candidiasis worldwide each year with a mortality rate of 40%, even following administration of antifungal treatment. Haemostatic proteins have been demonstrated to bind to the surface of C. albicans. However, the implications of these binding interactions in respect to bloodstream infections, remain to be understood. Coagulation protein factor XII (FXII) is one of the main proteins which comprise the contact activation system. Once activated, FXII (FXIIa) is able to initiate several downstream pathways including the intrinsic coagulation cascade, fibrinolysis, the bradykinin pathway and the complement pathway. FXII is dispensable for haemostasis but contributes to the development of thrombosis. Research from the last decade has shown that FXII can increase immune responses including increasing monocyte and macrophage activity and phagocytosis as well as neutrophil migration. However, it is not known if FXII retains these attributes when bound to C. albicans. The current study aimed to identify the effect of FXII binding to C. albicans in the progression of systemic candidiasis. I have demonstrated that FXII binds specifically to hyphal morphologies, but not to yeast cells in both purified and plasma conditions. One of the main risk factors for mortality in systemic candidiasis is biofilm formation on catheters, giving a constant route for C. albicans escape into the bloodstream. I have demonstrated that FXII promotes biofilm formation on catheter-like surfaces and in contrast to this, biofilm formation was reduced in the presence of FXII deficient plasma. This suggests a role for FXII in biofilm formation. My investigations using C. albicans mutant strains has identified Als3p as a likely FXII binding ligand. Interestingly, FXII does not require zinc to bind to the hyphal surface despite it being well understood that FXII requires zinc for its activation. FXII also becomes locked in its zymogen form upon binding to the hyphal surface, even when in the presence of a number of its known activators. FXIIa also binds C. albicans hyphae, which retains its activity on the hyphal surface and can still be inhibited once bound. Collectively these data suggest that the hyphal surface is unable to mediate FXII activity or act as a constant 'on' switch for activation of FXII's downstream pathways. This differs from our understanding of bacterial interactions with FXII, suggesting FXII plays different roles between fungal and bacterial infections. Around 46% of plasma FXII binds hyphae but does not affect plasma clot lysis time, suggesting that FXII binding to hyphae does not impact normal clot degradation. Furthermore, this interaction augments macrophage engulfment time of C. albicans preincubated with FXII and FXIIa, in comparison to C. albicans without preincubation. Data from this project has shown that platelets do not aggregate on the hyphal surface, conversely, hyphae preincubated with FXIIa induced platelet aggregation. The immune data gathered in this study indicate a potential role for FXII and FXIIa in the immune response against C. albicans. I have identified several crucial points during the progression of systemic candidiasis where FXII could potentially impact the progression of C. albicans infection. Overall, this study has not only increased our current knowledge on the interactions of plasma proteins and C. albicans, but also highlighted an urgency to understand more on these binding interactions. I have shown that FXII binding to catheters can be beneficial to C. albicans in respect to the promotion of biofilms and therefore being detrimental to the host. In contrast, I have demonstrated that FXII may be increasing the immune response once bound to the hyphal surface, being favourable to the host. This study has identified a putative link between haemostasis and C. albicans bloodstream infections, influencing not only immune recognition but also biofilm formation and platelet aggregation, highlighting an urgent need to continue this field of investigation.