Women account for a substantial majority of HIV infections in endemic regions, where women are also infected at a much younger age than men. Part of this epidemiological skewing is due to socio-cultural factors, but it is clear that biological factors enhance the susceptibility of women – particularly young women – to HIV acquisition after sexual exposure. These factors, including important differences in mucosal immunology at the site of genital HIV exposure, are the focus of this concise review. Compared to heterosexual men, women have an increased surface area of mucosal HIV exposure, increased mucosal expression of the HIV co-receptor CCR5 and a greater probability of virus exposure on the rectal mucosa. Differences that are specific to young women include a pro-inflammatory immune environment and a proportionate increase in single-cell, columnar genital epithelium. These important biological reasons for enhanced HIV susceptibility in young women highlight the need for targeted HIV prevention within this vulnerable population.
There are 33 million people living with HIV globally, and women account for over half of these infections. Within HIV-endemic countries, where heterosexual sex is the predominant mode of virus transmission, this female predominance is even more striking, with women accounting for almost 60% of HIV infections. Furthermore, when one looks at HIV prevalence according to age and gender, a striking observation is that women tend to acquire HIV at a much younger age than do men, with young women aged 15–24 years being eight times more likely than men to be HIV infected. Various factors, both behavioural and biological, appear to predispose young women to acquire HIV. In this concise review, we will briefly describe possible behavioural associations, before discussing the potential biological reasons for this profound disparity in more detail.
Socio-behavioural associations of HIV in young women
HIV is a sexually transmitted infection (STI) that is transmitted through unprotected sex. As such, there is no question that sexual behaviour and patterning are very important determinants of the disproportionately high HIV incidence in young women, although this is not the focus of this review. Women have the same or later age of sexual debut compared to men, but young women more often enter into a sexual relationship with older partners,[4-6] and an age differential between partners renders the younger partner at an increased risk of HIV acquisition because HIV is a life-long infection with an increased prevalence in older individuals. In addition, a less experienced partner may be unable to successfully negotiate condom use with her partner. In keeping with this, HIV rates can be high within young women reporting only a single lifetime partner and very few episodes of sexual intercourse. Indeed, this may also be one reason for the startlingly high per-act HIV incidence of up to 3% that was observed within women from the CAPRISA 004 study, although data regarding male partners in that study are not (or are not yet) available.
Other behavioural factors may also play a role. For example, douching has been associated with altered vaginal flora and an increased risk of HIV acquisition[8, 9]; rates of various intra-vaginal practices may be higher in young women, both in the general population[10, 11] and among female commercial sex workers.[12, 13] While a full review of all behavioural associations is beyond the scope of this review, we do not believe that they can fully explain the increased HIV prevalence and per-act HIV incidence that is seen within young women.[4, 5]
Sexually transmitted co-infections
Both classical bacterial STIs such as Chlamydia trachomatis and life-long viral infections such as herpes simplex virus 2 (HSV-2), are associated with increased HIV acquisition.[14, 15] For instance, HSV-2 seropositivity is associated with a threefold increase in HIV acquisition, due in part to the herpes-associated recruitment of putative HIV target cells to the mucosal site of subsequent HIV exposure.[17-19] The prevalence of C. trachomatis in the USA is higher among women than men, and the prevalence in women is substantially increased at younger ages (14–19 years). Similar gender differences are seen for HSV-2, with approximately 50% of women and only 25% of men being infected in urban centres in Africa, and the US HSV-2 prevalence also being almost twice as high among women as men. Again, these gender-associated differences in HSV-2 prevalence are particularly striking among young women.
In summary, the increased probability of a pre-existing genital co-infection at the time of HIV exposure increases a young woman's per-act risk of HIV acquisition in comparison with an age-matched man. While behaviour is an important determinant of the increased STI prevalence among younger women, other biological factors such as the lack of acquired anti-chlamydia immunity and differences in the mucosal immune environment (see below) may also be important.
Mucosal immunology and HIV acquisition in women and men
It is generally accepted that women have a higher per-act risk of HIV acquisition after virus exposure than men, as indicated by a number of serodiscordant couples studies,[25-28] although several well-performed studies have found similar seroconversion rates for men and women in discordant relationships.[29-31] One important factor in this increased risk, and also in the discrepant results of studies from different cohorts, is a simple matter of surface area. The surface area of the cervico-vaginal mucosa, the site of initial HIV exposure during heterosexual vaginal sex, is considerably larger than that of the penis and foreskin, with the latter being the site of most HIV acquisition in uncircumcised men. Indeed, elegant studies have shown a direct association between the foreskin surface area and the risk of HIV acquisition, and the cohort studies that show men to be at an equivalent (rather a lower) per-act risk compared to women have generally been performed in regions where men are uncircumcised. In addition, increased HIV mucosal exposure time within the female genital tract may play a role, because semen remains within the female genital tract for up to 3 days post-coitus.
The mucosal immune environment is an important determinant of HIV transmission, as well as these simple physical characteristics and the duration of exposure. Increased innate immune activation has been associated with HIV acquisition, and lower levels of immune activation in the blood correlate with reduced HIV incidence.[36-38] Furthermore, reduced genital immune activation among women has been associated with lower HIV susceptibility. Although a direct male–female comparison of mucosal immune activation has not been performed, systemic innate immune activation is higher in women than men, suggesting another mechanism for enhanced susceptibility. Our own studies have examined the expression of the HIV receptor CCR5 by CD4+ T cells isolated from the foreskin of 46 HIV-uninfected Ugandan men and from endocervical cytobrushes from 27 HIV-uninfected Kenyan women. In keeping with the concept that CD4+ target cells may be more susceptible in the female genital tract, the expression of the HIV co-receptor CCR5 was much higher on cervical cells (median, 81.5%) than on foreskin cells (median, 41.7%; Mann–Whitney P < 0.0001; Fig. 1).
Not only may immune cells in the female genital mucosa be more activated and express higher levels of HIV co-receptors and therefore be more able to sustain HIV replication, but the virus may have greater access to these cells. Young women are more prone than men to genital microabrasions during sex, providing a portal for HIV entry. This is likely to be related to the nature of the exposed surface: this is a relatively impermeable keratinized skin epithelium on the foreskin and penis, while in women there is a combination of a simple columnar epithelium (the endocervix) and a stratified squamous epithelium (the vagina and ectocervix), which lacks tight junctions. In women, the relative contribution of each of these types of mucosa to HIV acquisition remains unclear. The single-cell columnar epithelium of the endocervix has immune characteristics compatible with HIV susceptibility, and macaque transmission studies suggest that this is the site of most virus entry, but HIV can be acquired in the congenital absence of a cervix or when using a physical cervical barrier such as the diaphragm. Furthermore, the relative surface area of the vaginal mucosa exceeds that of the endocervix by more than 15-fold,[49, 50] and the distribution of HIV surrogates after simulated vaginal intercourse was predominantly in the peri-cervical region of the vagina. Some studies suggest that HIV acquisition is also feasible across the columnar epithelium of the endometrium, but the bulk of the evidence points to the lower genital tract as the site of most acquisition in vivo [summarized in].
Finally, the role of the recto-sigmoid mucosa in differential heterosexual male–female HIV susceptibility is rarely considered. The sigmoid mucosa is particularly rich in HIV target cells,[52, 53] and virus exposure during receptive anal sex is much more likely than insertive to result in HIV acquisition. Anal sex is commonly practiced by heterosexual couples, albeit less commonly reported in couples from sub-Saharan Africa, and in this context, it is only the female rectal mucosa that will be exposed to HIV. Therefore, this may be another important factor that increases the relative HIV risk in women.
Overall, there are a number of important biological differences regarding the mucosal site of exposure in men versus women that provide plausible mechanisms for a higher per-act acquisition rate in women than men.
Age and HIV susceptibility in women
Not only are women more susceptible to HIV acquisition on a per-act basis, but young women seem to be at a particularly high risk, with many infected after just a few sexual experiences. As discussed above, there are important socio-behavioural reasons for this, but biology is also likely to play an important role. Indeed, there are a number of very relevant differences between the genital mucosa of adolescent and older women.
Perhaps the most prominent biological discrepancy is the nature of the cervico-vaginal epithelium.[56, 57] In sexually mature women, the endocervix is composed of a single-cell, simple columnar epithelium, while the ectocervix and vagina are lined with a stratified squamous epithelium. However, cervical ectopy, defined as the extension of the columnar epithelium beyond the endocervix to include the ectocervix, is the norm in adolescent women.[56, 58] As women mature, a process known as squamous metaplasia causes the columnar epithelium of the ectocervix to transform into squamous epithelium. However, as a simple columnar epithelium is more susceptible to HIV and other STIs, such as C. trachomatis and Neisseria gonorrhoeae,[46, 56, 57] this means that in younger women, a much greater proportion of the virus-exposed genital mucosa is highly HIV susceptible.
In addition, the genital secretions of adolescent women have increased levels of pro-inflammatory cytokines, and young women also have more inflammatory immune cells present in cervicovaginal lavage (CVL) fluid.[58, 60] An inflammatory genital milieu has consistently been associated with an increased risk of HIV acquisition in prospective studies,[61, 62] providing another biological mechanism for enhanced HIV susceptibility in younger women.
Finally, the menstrual cycle itself may be associated with significant variation in mucosal immunology and HIV susceptibility. Carefully controlled studies have shown that macaques are more susceptible to SIV vaginal challenge during the luteal (progesterone dominant) phase of the menstrual cycle, perhaps due to increased CD4+ T cell and CCR5 expression density in the ectocervical mucosa, and in high doses progesterone is well demonstrated to thin the vaginal epithelium and to enhance vaginal SIV acquisition. While the issue of menstrual variability in HIV susceptibility is harder to define in human studies, the use of injectable progestins doubled HIV acquisition rates in HIV-uninfected women. This is in keeping with the hypothesis that the luteal phase of the menstrual cycle may represent a ‘window of vulnerability’ not only because possible target cells increased, but because components of all arms of the immune system (innate, humoral and cellular immunity) are suppressed by sex hormones to enhance the chances of successful implantation and reproduction.
Women account for most HIV infections globally, and young women are at particularly high risk, even after a very small number of exposures. Part of this increased risk is due to socio-cultural factors, but biology also plays a key role. There are important differences in mucosal immunology at the site of genital HIV exposure between men and women, as well as between younger and older women, that would be expected to enhance HIV susceptibility. These biological factors enhancing HIV susceptibility in young women highlight the need for targeted HIV prevention within this vulnerable population.
This work was supported in part by the Canadian Institutes of Health Research (HET-85518, MOP-89983).