Cervicovaginal epithelial barrier integrity is maintained by intercellular junctions that prevent the invasion of microbes, with the exception of certain pathogenic organisms that have developed strategies to breech the epithelial barrier. Many other factors further fortify this barrier. Mucus produced by cervical and vaginal epithelial cells forms a glycocalyx on the epithelial surface that retains immunological mediators, including immunoglobulins and antimicrobial peptides [20
]. Furthermore, a variety of leukocytes migrate into and through the epithelium to conduct immunosurveillance [23
]. The purpose of the present study was to characterize cervical and vaginal epithelial junctions to better understand their role in STI pathogenesis and immune defense of the lower female genital tract.
Our electron-microscopy studies indicate that classical tight junctions comprise the principal intracellular junctions between epithelial cells in the endocervix, in accordance with the current knowledge of the structure of simple columnar epithelia [25
]. These tight junctions formed a barrier that was impermeable to Cy3-labeled IgG. In contrast, the uppermost layers of the stratified squamous ectocervical epithelium were devoid of organized intracellular junctions, and the apical layers were permeable to Cy3-labeled IgG. Exclusionary junctions were observed directly beneath this layer, and IgG did not penetrate beyond this point.
The present study also surveyed the expression of discrete junctional molecules representing the different types of intracellular junctions in both the columnar epithelium of the endocervix and the stratified squamous epithelium of the ectocervix and vagina. The results from this investigation indicate that F11R, E-cadherin, occludin, claudin-1, and TJP1 are abundant junctional molecules in the human endocervix. JAM3, a desmosomal junctional molecule, was not detected in the endocervix. Claudin-1 expression was found in distinct foci, whereas the other junctional molecules were expressed uniformly at sites of cellular contact throughout the epithelium. Previous studies of cervical cancer biomarkers have described select junctional proteins in human cervical tissue. E-cadherin was previously described in the endocervical mucosa [26
], and images published by Lee et al. [13
] depicted irregular punctate claudin-1 staining localized to the basal cervical epithelial cells similar to that observed in the present study.
All of the intracellular junctional proteins surveyed in the present study, including JAM3, were detected in ectocervical and vaginal stratified squamous epithelia. The ectocervix is structurally a part of the vaginal wall and shares a continuous, morphologically identical mucosal layer with vaginal tissue [19
]. Therefore, it was not unexpected that the distribution of F11R, JAM3, claudin-1, TJP1, and E-cadherin was similar in these two tissues. Each of these molecules displayed a spiderweb-like distribution in the basal and suprabasal layers consistent with their functions as mediators of cell-cell adhesion. Little or no staining was observed in the most apical layers, where the epithelial cells gradually lose cell-cell contacts and are eventually sloughed into the lumen. Claudin-1, occludin, TJP1, and E-cadherin expression has been previously described in the human ectocervix [14
]. Claudin-4 and claudin-7 have been also detected in ectocervical and vaginal epithelial cells [14
]. To our knowledge, F11R expression has not been reported in cervical/vaginal epithelia, but it has been observed at other mucosal sites, such as the intestine and nasal epithelium [11
]. JAM3 expression has not been well studied in stratified squamous epithelia and has not been detected previously in the female mucosal epithelium, but it has been observed in the endothelium and retinal epithelium [18
]. The structure and distribution of adhesion molecules in the endocervical columnar epithelium and cervicovaginal squamous epithelium, as revealed by the present study, are diagrammed in .
FIG. 6. Localization of selected interepithelial adhesion molecules in the female lower genital tract. A) The endocervical epithelium contains classic tripartite junctions. The tight junctions are located near the apical surface; they seal the epithelium and (more ...)
Some pathogens are known to affect the integrity of epithelial junctions to facilitate transmission across the mucosal surface. In the context of the female reproductive tract, Nazli et al. [16
] showed that exposing female genital epithelial cells to free HIV virions or gp-120 envelope glycoprotein resulted in increased permeability. This correlated with increased production of the proinflammatory cytokine tumor necrosis factor-alpha and disruption of the tight junctional molecules TJP1, occludin, and claudin-1, -2, and -4 [16
]. Of interest, many junctional molecules also serve as leukocyte adhesion receptors, so they may play a role in the recruitment of CD4+ HIV target cells to the cervicovaginal lumen. F11R is a known ligand of lymphocyte-associated antigen 1 expressed on T cells, macrophages, and neutrophils; it may provide a foothold for migratory leukocytes [17
]. Similarly, E-cadherin is a receptor for the lymphocyte adhesion molecule alphaEbeta7 integrin on T cells [30
In contrast to tight and adherens junctions, little evidence indicates that desmosomal structure is altered by pathogen invasion or inflammation [31
]. However, the desmosomal molecule JAM3 is a ligand for the macrophage-1 receptor on macrophages and neutrophils, and JAM3 regulates the influx of leukocytes, particularly neutrophils, in response to inflammatory stimuli [18
]. For a leukocyte to migrate between epithelial cells, the epithelial junctional bonds must be disrupted. Permeability to infiltrating leukocytes is largely regulated by secreted proinflammatory cytokines and chemokines [32
A definitive understanding about the composition of cervical and vaginal epithelial junctions also provides an important foundation for future studies on pathogen transmission. For infections such as HIV, preventing epithelial barrier breach by cell-free or cell-associated virus is of the utmost importance [33
]. Results from the present study indicate that the uppermost layers of the stratified squamous epithelium covering the vagina and ectocervix may not comprise a physical barrier against STIs but, rather, a potential zone for interactions with immunological mediators that may be retained at this site. Mapping the normal expression of key molecular regulators of barrier resistance is an important first step in elucidating how microbes take advantage of these mechanisms to infect a host.