Mycoplasma genitalium Infection Activates Cellular Host Defense and Inflammation Pathways in a 3-Dimensional Human Endocervical Epithelial Cell Model

Sexually transmitted bacterial infections are among the most commonly reported notifiable diseases in the United States [1]. Mycoplasma genitalium is an emerging cause of male and female urogenital inflammation and has been implicated in male urethritis [2] and in several lower and upper reproductive tract syndromes of women, including cervicitis, pelvic inflammatory disease, and tubal-factor infertility [3, 4]. M. genitalium can persist for extended periods at urogenital sites, including the endocervix [5–8]. However, mechanistic evidence for establishment and persistence of infection remains scarce. Importantly, the host innate and adaptive immune responses are largely unknown and serve as impediments to our understanding of the pathogenesis of M. genitalium infection. Several lines of clinical evidence exist for M. genitalium as an independent cause of cervical inflammation [3, 4]. First, M. genitalium is a common endocervical infection, with prevalence values on par with those of other sexually transmitted infections (STIs), and has been associated significantly with cervicitis in pooled calculations of risk from 14 studies of high- and low-risk populations (odds ratio, 2.2; 95% confidence interval, 1.6–2.9) [4]. In addition, persistent cervical infection appears to be common, with an incidence of chronic M. genitalium infection of >20% in one longitudinal study [6]. Last, although the primary site(s) of infection is unknown, the mechanisms of disease are intriguing because M. genitalium has the smallest genome of any known human bacterial pathogen [9] and, consequently, represents a minimal bacterium to investigate mucosal pathogenesis. Experimental evidence also supports the hypothesis that M. genitalium infection can lead to mucosal inflammation at female urogenital sites. First, the organism can survive and replicate in cultures of human vaginal, cervical, and endometrial epithelial cells [10–12], resulting in acute and persistent proinflammatory responses [10, 11]. Among lower reproductive tract sites, human endocervical epithelial cells are most responsive to M. genitalium infection [11], during which secretion of proinflammatory cytokines and chemokines appears to be mediated in part by activation of Toll-like receptors (TLRs) [13–15]. Intracellular localization is hypothesized to be an important strategy for evasion of the host immune response and has been observed clinically [16]. However, outside of focused evaluations of acute and persistent cytokine responses, the pathologic consequences of endocervical M. genitalium infection and host response are almost entirely unknown. We used a well-characterized and physiologically relevant model of human endocervical epithelial cells [17] to study interactions of M. genitalium with the host cell. This organotypic model effectively recapitulates physical structural and barrier properties necessary for studying host-microbe interactions [18]. Previously, microbial products have been shown to elicit a specific endocervical innate immune signature that included antimicrobial peptide and mucin expression [19]. Using next-generation sequencing and human transcriptome analysis, we report key cellular pathways that are activated during acute M. genitalium infection. Comparative analyses provide unique insight into the mechanisms of reproductive tract inflammation induced by M. genitalium.