4 Preterm birth Systemic infection Intrauterine ascension TNF-α-dependent TNF-α-independent Uterine NK cell Macrophaqe Dendritic cell T regulatory cell Figure 1: Roads to Preterm. Birth pathogens may enter the placental blood supply via systemic circulation due to previous or current onset of infection. Murine models where intraperitoneal injections are employed are utilized to mimic maternal systemic infection in order to study the ability of a pathogen to activate decidual immunity. Data presented in this paper demonstrate that systemic infections correlate to TNF-α -dependent immune responses that ultimately induce preterm birth. In contrast, intrauterine ascending infections occur when a pathogen ascends the uterine cavity via the vaginal tract. Surgical procedures in mice, rats, and rabbits have mimicked uterine cavity infections through intrauterine infusion of pathogens directly into the amniotic sac or between two placental units. Importantly, data summarized here demonstrate that pathogens introduced through intrauterine ascension do not tend to activate a TNF-α -driven axis to induce preterm birth. Furthermore, the presence of bacterial agents can be probed for by testing the amniotic ﬂuid for microbial invasion of the amniotic cavity (MIAC). Women undergoing preterm labor with premature rupture of membranes (PROM) show 34% positive MIAC pathology. Interestingly, at the time that these women deliver 75% show positive MIAC pathology. It has been suggested that the inﬂammatory atmosphere of preterm labor allows for increased bacterial proliferation [44, 51]. 5. Viral Entry Though data is relatively scarce, evidence suggests that viral entry into trophoblast cells induces trophoblast apoptosis and the resultant inﬂammatory events can lead to preterm birth [52]. The general route of entry in placental viral infec- tions is cell-speciﬁc as diﬀerent viruses enter trophoblasts that express their viral receptor [42, 52, 53]. Once viral uncoating or shedding occurs within a cell, intracellular TLRs are available as sentinels within endosomal pockets Infectious Diseases in Obstetrics and Gynecology where viruses may replicate. Intracellular TLRs, TLR3, TLR7/8, and TLR9 are speciﬁc for viral genomic motifs and, not surprisingly, are associated with several viruses that prove correlation with adverse pregnancy outcomes. 6. TLRs: Sentinels of Uterine Immunity Here, we summarize known pathogens, viral or bacterial, associated with induction of preterm birth. We highlight immunological pathways that demonstrate speciﬁc PAMPs that activate TLRs to induce preterm birth. Work in mouse models illustrates diﬀerential TLR-mediated cascades of inﬂammatory-based uterine immunity that can lead to preterm birth. Importantly, clinical correlates where amni- otic ﬂuid is tested or maternal/fetal membranes are probed begin to support these ﬁndings. 6.1. TLR2. TLR2 is one of the more promiscuous TLRs in that upon activation it can concomitantly signal with TLR1 and TLR6. In line with these ﬁndings, TLR2 is not only speciﬁc for gram positive bacteria due to its speciﬁcity for peptidoglycan, but also signals upon recognition of meningococcal porins, fungal, parasitic, and viral pathogen- associated molecular patterns [12]. Studies show that women who deliver preterm with the condition of chorioamnionitis show signiﬁcant upregulation of the TLR2 receptor as compared to controls, implicating a role for the receptor in preterm birth and infection [54]. Murine studies demon- strate preterm birth occurs when TLR2 is activated either through systemic or intrauterine delivery ([11], our unpub- lished data). Systemic activation induces activation of the NF-κB transcription factor, and ﬁrst trimester trophoblasts produce signiﬁcant amounts of IL-8 and IL-6 ultimately undergoing apoptosis [12]. In contrast, intrauterine delivery of TLR2 agonists did not induce increased TNF-α production or NF-κB activation [11] (Table 1). TLR2-speciﬁc human pathogens that correlate with increased incidence of preterm birth are ureaplasma ureatic- ulum, group B streptococcus, and cytomegalovirus (CMV) [55, 56]. Ureaplasma ureaticulum is one of three bacteria that are grouped together and termed bacterial vaginosis. While common genital infections such as Chlamydia trachomatis and neisseria gonorrhoea are rarely associated with aberrant pregnancy outcomes, high loads of bacterial vaginosis in pregnant women prove nine times more likely to lead to prematurity [47]. Thus, the activity of TLR2 in the presence of ureaplasma ureaticulum is an important correlation in the literature to follow at the clinical level. CMV shows the greatest correlation of human infection and adverse pregnancy outcomes as this herpes virus is present in 80% of the general population, but becomes active under conditions of immune suppression. Data demonstrate that human CMV may infect syncitiotrophobalsts or cytotrophoblasts and induce proinﬂammatory cascades such as IL-8 production [57, 58]. 6.2. TLR3. While TLR2 generally “sees” pathogens that are extracellular, TLR3 possesses speciﬁcity for double stranded RNA viral motifs and is found in endosomal pockets.