Molecular immune evasion strategies of the opportunistic fungal pathogen Candida tropicalis

Candida, part of the human microbiota, can cause opportunistic infections that are either localised or systemic candidiasis in immunocompromised patients. Emerging resistance to the standard antifungal drugs is a big factor in increased mortality rate associated with invasive Candida infections. While there are several species of Candida, an increasing number of Candida tropicalis isolates have been recently reported from patients with invasive candidiasis or inflammatory bowel diseases. In order to establish infections, C. tropicalis has to adopt several strategies to escape the host immune attack, but these have not been characterised yet. Understanding the immune evasion strategies is of utmost importance as these can be exploited as novel therapeutic targets. This study investigates the molecular immune evasion properties of two C. tropicalis proteins: cell surface and secreted pH related antigen 1 (CtPra1) and secreted aspartic protease 1 (Sapt1). Both CtPra1 and Sapt1 were recombinantly produced using a Kluyveromyces lactis yeast expression system. CtPra1, was shown to bind human C3 and C3b, central molecules of the complement pathways that are important components of the innate immune system. CtPra1 displayed weak proteolytic activity against C3 and was also found to bind human complement regulatory proteins C4BP and Factor-H. Sapt1 efficiently cleaved human mannose-binding lectin and collectin-11, initiating molecules of the lectin pathway of complement. Sapt1 also appeared to possess weak proteolytic activity against C3b and C1q, the initiating molecule of classical pathway. As a result, CtPra1 and Sapt1 inhibited functional complement activation with different effects on classical, lectin and alternative pathways. In addition, Sapt1 cleaved DC-SIGN, the receptor on antigen presenting dendritic cells. RT-PCR experiments using C. tropicalis clinical isolates revealed relatively higher levels of expression of both CtPRA1 and SAPT1 genes. Thus C. tropicalis appears to be a master of immune evasion by using Pra1 and Sapt1 proteins. Preliminary bioinformatics studies using molecular modelling and docking have identified a panel of inhibitors that can potentially inhibit Sapt1. Further investigation using in-vivo models will help to identify if these proteins can be novel therapeutic targets.