Candida albicans, the primary causative agent of fungal vaginitis, will affect 75% of all women of reproductive age at least once in their lifetime. Long believed to result from immunodeficiency, a growing body of evidence strongly suggests that vaginitis is now considered to be an immunopathology, in which the host response actually drives disease symptoms. Lack of a comprehensive understanding of the host and fungal factors that initiate symptomatic disease high remains a barrier to progress in better treating and managing this most prevalent human fungal infection. Guided by strong preliminary data, we have identified the newly described fungal toxin Candidalysin as the major virulence factor governing vaginitis pathogenesis. Therefore, the objective of this proposal is to determine if variation in expression or activity of Candidalysin in clinical isolates governs C. albicans pathogenicity at the vaginal mucosa. These aims will test our central hypothesis that variation in Candidalysin activity partly underlies the variable symptomatology of Candida vaginal colonization and identification of host signaling engaged by Candidalysin may elucidate new molecular therapeutic targets. Under the first aim, we will determine amino acid residues that contribute to reduced pathogenicity of a variant Candidalysin observed in clinical isolates. We will also determine differential host responses to wild-type and variant Candidalysin at the vaginal mucosa. The second aim will focus on identifying prevalent genetic mechanisms (promoter sequences, Kex proteases) that control Candidalysin expression and function. We will confirm these by functional level by allelic transfers between strong and weak Candidalysin expressing strains. The third aim seeks to identify host factors elicited by Candidalysin that may drive immunopathology in vivo and determine if blockade of these signaling mechanisms can alleviate symptomatic disease. The outcomes of this project will provide foundational information regarding function of Candidalysin isoforms, genetic checkpoints of Candidalysin expression, and identify promising new pathways that may be exploited for the clinical management of vaginitis.