Pathobiological Studies of Zoonotic Blastocystis Subtypes using In Vitro Model Systems

Abstract

Blastocystis is a ubiquitous enteric parasite found in the intestinal tract of humans and a wide range of animals. Accumulating evidence over the last decade suggests association of Blastocystis with gastrointestinal disorders. Despite new knowledge of Blastocystis cell biology, genetic diversity, life cycle and epidemiology; several key questions concerning its clinical relevance remain unanswered. Numerous clinical and epidemiological studies either implicated or exonerate the parasite as a cause of intestinal disease. Clinical and experimental studies have associated Blastocystis with intestinal inflammation and it has been shown that Blastocystis has potential to modulate host immune response. Blastocystis is also considered an opportunistic pathogen and high prevalence is reported in immunocompromised HIV patients. However, nothing is known about the parasitic virulence factors and early events following host-parasite interactions. A large number of asymptomatic carriers and frequent reports of treatment failure also cast doubts over pathogenic potential of Blastocystis. Subtype-dependent variations in Blastocystis pathobiology have been proposed to be the primary reasons for these controversies, but direct experimental evidence is lacking. The aim of this study was to investigate the pathogenic potential of Blastocystis, by studying the interactions of zoonotic Blastocystis isolates belonging to subtype-4 and subtype-7 with human intestinal epithelial cell line Caco-2. This study reports that a pathogenic strain of Blastocystis evades antiparasitic host nitric oxide response by suppression of epithelial iNOS, making intestinal environment more conducive to its colonization. Following immune evasion, Blastocystis cysteine proteases rearrange F-actin and tight junction distribution, decrease transepithelial resistance, and increases epithelial permeability in Caco-2 monolayers. A strain-to-strain and not subtype-dependent variation in Blastocystis virulence was observed. Only one of the four isolates tested in this study suppressed Caco-2 iNOS and compromised epithelial barrier function. In addition, it was demonstrated that effects of Blastocystis on transepithelial electrical resistance and epithelial permeability were significantly abrogated by inhibition of epithelial Rho kinase. Additionally high-throughput drug susceptibility assays were developed for Blatocystis. Antibiotic susceptibility studies on these assays suggested that pathogenic strain of the parasite is resistant to metronidazole, the treatment of choice against Blastocystis infections. A subtype-dependent variation in susceptibility to a range of antimicrobial agents was also observed, suggesting that although subtyping of Blastocystis might not be useful in identification of virulent strains, but it does help in predicting treatment outcomes. Lastly, epithelial barrier dysfunction induced by pathogenic and metronidazole-resistant Blastocystis was significantly inhibited by inhibition of epithelial Rho kinase and HMG CoA reductase by Fasudil and Simvastatin respectively. Both these agents are clinically useful and could provide alternative or adjunctive treatment options against antibiotic resistant Blastocystis infections. These findings will certainly help to understand pathobiology of a poorly studied parasite whose public health importance is increasingly recognized. Our findings also suggest novel treatment options against antibiotic resistant and potentially virulent Blastocystis strains.