The 74-kilodalton Immunodominant Antigen of the Pathogenic Oomycete Pythium Insidiosum is a Putative Exo-1,3-beta-glucanase

The oomycetous, fungus-like, aquatic organism Pythium insidiosum is the causative agent of pythiosis, a life-threatening infectious disease of humans and animals living in tropical and subtropical areas of the world. Common sites of infection are the arteries, eyes, cutaneous/subcutaneous tissues, and gastrointestinal tract. Diagnosis of pythiosis is time-consuming and difficult. Radical excision of the infected organs is the main treatment for pythiosis because conventional antifungal drugs are ineffective. An immunotherapeutic vaccine prepared from P. insidiosum crude extract showed limited efficacy in the treatment of pythiosis patients. Many pythiosis patients suffer lifelong disabilities or die from an advanced infection. Recently, we identified a 74-kDa major immunodominant antigen of P. insidiosum which could be a target for development of a more effective serodiagnostic test and vaccines. Mass spectrometric analysis identified two peptides of the 74-kDa antigen (s74-1 and s74-2) which perfectly matched a putative exo-1,3-ss-glucanase (EXO1) of Phytophthora infestans. Using degenerate primers derived from these peptides, a 1.1-kb product was produced by PCR, and its sequence was found to be homologous to that of the P. infestans exo-1,3-ss-glucanase gene, EXO1. Enzyme-linked immunosorbent assays targeting the s74-1 and s74-2 synthetic peptides demonstrated that the 74-kDa antigen was highly immunoreactive with pythiosis sera but not with control sera. Phylogenetic analysis using part of the 74-kDa protein-coding sequence divided 22 Thai isolates of P. insidiosum into two clades. Further characterization of the putative P. insidiosum glucanase could lead to new diagnostic tests and to antimicrobial agents and vaccines for the prevention and management of the serious and life-threatening disease of pythiosis.