SARS-CoV-2 Infection of Salivary Glands Compromises Oral Antifungal Innate Immunity and Predisposes to Oral Candidiasis.

ABSTRACT

Saliva contains antimicrobial peptides considered integral components of host innate immunity, and crucial for protection against colonizing microbial species. Most notable is histatin-5 which is exclusively produced in salivary glands with uniquely potent antifungal activity against the opportunistic pathogen Candida albicans. Recently, SARS-CoV-2 was shown to replicate in salivary gland acinar cells eliciting local immune cell activation. In this study, we performed mechanistic and clinical studies to investigate the implications of SARS-CoV-2 infection on salivary histatin-5 production and Candida colonization. Bulk RNA-sequencing of parotid salivary glands from COVID-19 autopsies demonstrated statistically significant decreased expression of histatin genes. In situ hybridization, coupled with immunofluorescence for co-localization of SARS-CoV-2 spike and histatin in salivary gland cells, showed that histatin was absent or minimally present in acinar cells with replicating viruses. To investigate the clinical implications of these findings, salivary histatin-5 levels and oral Candida burden in saliva samples from three independent cohorts of mild and severe COVID-19 patients and matched healthy controls were evaluated. Results revealed significantly reduced histatin-5 in SARS-CoV-2 infected subjects, concomitant with enhanced prevalence of C. albicans. Analysis of prospectively recovered samples indicated that the decrease in histatin-5 is likely reversible in mild-moderate disease as concentrations tended to increase during the post-acute phase. Importantly, salivary cytokine profiling demonstrated correlations between activation of the Th17 inflammatory pathway, changes in histatin-5 concentrations, and subsequent clearance of C. albicans in a heavily colonized subject. The importance of salivary histatin-5 in controlling the proliferation of C. albicans was demonstrated using an ex vivo assay where C. albicans was able to proliferate in COVID-19 saliva with low histatin-5, but not with high histatin-5. Taken together, the findings from this study provide direct evidence implicating SARS-CoV-2 infection of salivary glands with compromised oral innate immunity, and potential predisposition to oral candidiasis.