The natural protective barrier provided by the vagina is vitally important to women’s health, as evidence indicates that the loss of this barrier has been linked to increased risk of human immunodeficiency virus (HIV) infection. In addition to this critical role of protection against pathogens, there are implications for drug delivery and mucosal vaccine development, both of which may be affected by changes in the vaginal epithelium because of hormonal fluctuations from the menstrual cycle or hormonal contraceptives.

The epithelial layer is an important component of the protective vaginal barrier, which also includes microflora, vaginal fluid, lamina propria, and immune cells. Changes in the integrity of the vaginal epithelium occur under the influence of hormones, inflammation, and infection, therefore being able to noninvasively characterize components of the vaginal barrier under various conditions would provide insight into enhancing its protective effect or identifying women at greater risk of infection because of diminished barrier function. A dramatic illustration of the hormonal effects on vaginal epithelium is seen in the macaque, which has large fluctuations in vaginal epithelial thickness during the menstrual cycle, with thicker epithelium in the follicular phase, and thinner epithelium in the luteal phase. In humans, epithelial thickness variations are also present, however, less pronounced, with epithelial thickness variations occurring during the menstrual cycle and with use of hormonal contraceptives. Both vaginal cell layer and epithelial thickness decrease in the luteal phase, with decreased epithelial thickness after depomedroxyprogesterone acetate (DMPA) treatment similar to that in the luteal phase.

Vaginal epithelial thickness may play a role in acquisition of sexually transmitted infections (STI). Genital ulcerative disease, with focal loss of the epithelial barrier, has been linked to an increased risk of HIV. Macaques are more likely to acquire simian-human immunodeficiency virus (SHIV) in the late luteal phase, and treatment with progestins, mimicking the luteal phase, leads to thinning of the vaginal epithelium and increased infection with simian immunodeficiency virus (SIV). In contrast, treatment with estrogen, which increases the vaginal epithelial thickness, is protective against SHIV infection. Clinical studies have shown a link between increased HIV incidence and the use of DMPA, a contraceptive that may thin the vaginal epithelium. Nonoxynol-9 (N-9), a spermicide with in vitro action against HIV that was tested for prevention of HIV in clinical trials, actually increased HIV acquisition, a finding that may be due in part to epithelial disruption and thinning. In addition to providing protection from STIs, the vagina is used as a route for drug or vaccine delivery; vaginal epithelial integrity and thickness can impact drug delivery and determine whether effects are local or systemic.

Historically, vaginal administration of drugs has been underused, but with new drug delivery mechanisms such as advances in intravaginal rings, the numbers of papers and patents related to vaginal drug delivery for local and systemic drug release have recently increased dramatically. Some current vaginally administered treatments for local or systemic effects include hormone replacement, contraception, infertility treatment, treatment of local infections, and, more recently topical microbicides, medications designed to prevent the acquisition of HIV and other STIs. The vagina is also being explored as a route for vaccine delivery. As the number of vaginally administered drugs continues to increase, it is increasingly important to understand drug and vaccine effects on vaginal physiology and microstructure. Drug delivery in the vagina relies on a variety of parameters, including drug properties, vaginal pH, characteristics of vaginal fluid and cervical mucus, and vaginal epithelial thickness. Vaginal epithelial thickness can be affected by local administration of drugs, as seen after treatment with N-9, with epithelial thinning considered a marker of drug toxicity. A noninvasive measure of vaginal epithelial thickness could be an important part of drug development to predict uptake of drug as well as drug safety.

Optical coherence tomography (OCT) uses a noninvasive probe-based imaging system to achieve resolutions of 15-20 μm with depths of up to 1.5 mm, sufficient for visualization of the epithelial layer and underlying lamina propria. Its use has been explored in gynecology to image the lower reproductive tract to evaluate for dysplasia and to evaluate the epithelial response to drug treatment. These studies emphasized the use of visual-based scoring systems to describe changes in morphology of the epithelium and epithelial-stromal interface. In the toxicity studies, OCT was shown to be more sensitive when compared with standard method of colposcopy for detection of epithelial disruption.

In a sheep model characterized as a good model for the human vagina, we explored the use of OCT to measure vaginal epithelial thickness after the application of vaginal products and noninvasively monitored changes in the vaginal epithelial thickness over time. Whereas prior studies have focused on developing a scoring system for noninvasive assessment of epithelial disruption, the present study focuses on noninvasive longitudinal measurement of epithelial thickness, comparing OCT with histology findings. In this study, we used a compound known to cause epithelial thinning to show the ability of OCT to measure drug-induced epithelial thickness changes.