Bacteria colonize the lower genital tract of most women and the predominant species of bacteria in healthy women is lactobacilli. Commonly found vaginal lactobacillus species include Lactobacillus crispatus, L. gasseri, L. jensenii and L. iners 12. Bacterial vaginosis (BV) is characterized by an alteration of genital tract flora such that the predominant bacteria are no longer lactobacilli, but instead consist of polymicrobial communities of multiple genera of gram positive and gram negative organisms 3. Gardnerella vaginalis, Prevotella sp.,Bacteroides sp., Peptostreptococcus sp., Mycoplasma hominis and Mobiluncus sp. as well as other recently described bacteria are commonly found in BV 23. Lactobacilli, usually L. iners, are also frequently present in BV, but make up a relatively small proportion of the total flora 24. BV has been noted to be the most prevalent vaginal disorder in adult women worldwide with the frequency depending on the group that is studied 5. BV is found in 24% to 37% of women attending STD clinics but seen at lower rates in women that are not sexually active.

BV is associated with an increased risk of infections by HIV and some other organisms as discussed below, as well as with increased risk of preterm birth, which is a leading cause of infant death in the United States 678. Treatment of BV can reduce preterm birth in high risk cases 79. BV is also associated with miscarriage and pelvic inflammatory disease 101112.

Diagnosis of BV is commonly made by examination of four criteria: vaginal fluid pH (BV results in a pH >4.5); presence of clue cells (bacteria-coated epithelial cells); a homogenous discharge; and production of an amine odor when KOH is added to vaginal fluid 13. Gram stains of vaginal fluid can also aid in diagnosis of BV 14.

Oral or intravaginal antibiotic treatment with metronidazole or clindamycin cures BV in most women, but BV can resolve spontaneously in nearly a third of subjects 15161718. However, BV recurs in a significant fraction of treated women.

Several cross-sectional studies performed in Thailand 19, Uganda 20, and Malawi 2122 showed that women with BV had an increased incidence of HIV infection. While suggestive, these studies do not prove a cause and effect relationship between BV and HIV infection. However, a prospective study in Kenya 23 showed that the presence of BV and the absence of lactobacilli or absence of hydrogen peroxide-producing lactobacilli upon examination were all significantly associated with acquisition of HIV infection at follow-up.

There is also evidence, some of it through cross-sectional studies, that the presence of BV increases the risk of infection with several other sexually transmitted infections (STI), including herpes simplex virus type 2 (HSV-2), gonorrhea, Trichomonas vaginalis and Chlamydia trachomatis 232425. All of these STI have been suggested to increase susceptibility of women to sexual transmission of HIV 26, and so BV may both directly increase HIV susceptibility and indirectly increase it by increasing the number of women with these other STI.

While the above studies suggest that BV can influence susceptibility of women to HIV infection, other recent studies suggest that BV increases expression of HIV in the lower genital tract of women that are already infected with HIV. Thus, the levels of HIV, as assessed by HIV RNA, and the detection frequency of HIV in the genital tract are significantly higher in the genital tract of women with BV when compared to women without BV 2728. HIV levels were inversely correlated with levels of lactobacilli but positively correlated with Mycoplasma hominis 27. An additional study showed that women with lower levels of vaginal lactobacilli had higher genital tract HIV 29.

A number of mechanisms have been suggested that could account for the increase in susceptibility to HIV and/or increased expression of HIV in the genital tract (Table 1) 2630. These include decreased levels of hydrogen peroxide-producing lactobacilli, production by BV flora of enzymes (e.g. mucinases) that degrade protective mechanisms such as mucous, or production by BV flora of stimulatory substances that increase influx of target cells, HIV expression or infection of cells (see below). In fact, BV is associated with increased levels of pro-inflammatory cytokines such as IL-1β and IL-8 [Reviewed in 31]. IL-1β can induce the production of other pro-inflammatory cytokines, and IL-8 is known to recruit immune cells, thus possibly increasing the number of cellular targets for HIV infection 32.

A number of in vitro studies show that genital tract fluids from women with BV are highly stimulatory for immune cells and can up-regulate expression of HIV. Thus, incubation of the chronically-HIV-infected monocytic cell line U1 with genital fluid from women with BV substantially increased HIV expression 33343536. In contrast, genital fluid collected from women without BV did not induce HIV expression. HIV expression was also induced in T cell lines and in peripheral blood mononuclear cells by genital fluids from women with BV 3537. The substances in genital fluids that stimulated HIV expression in cells were found to function through activation of NF-kB 37.

Bacteria from BV have also been tested for their ability to stimulate HIV expression in cells. Gardnerella vaginalis, the bacterium most frequently isolated in BV, significantly induced HIV expression in U1 cells 3839. Lysozyme treatment reduced U1 activation suggesting a cell wall component of G. vaginalis was involved in stimulation of the U1 cells. Anaerobes Peptostreptococcus asaccharolyticus and Prevotella bivia also stimulated HIV expression 40 as did non-anaerobic bacteria Mycoplasma hominis and Streptococcus 39. In contrast, other bacteria found in genital samples including Bacteroides ureolyticus, Peptostreptococcus anaerobius, and Lactobacillus acidophilus did not stimulate HIV expression 40.

Taken together, many of the above studies suggested that the HIV-stimulatory activity in genital fluids acted through Toll-like receptors (TLR). For example, genital fluids stimulated HIV expression through the NF-kB pathway 41, and stimulation of cells through TLR is well documented to activate NF-kB 42. Also, many of the ligands for TLR are bacterial products 42 and BV mucosal fluids would be expected to contain such products. A recent study using the 293 cell line modified to express either TLR2, TLR4 or control cells expressing no functional TLR, directly determined whether mucosal fluids from women with BV stimulated cells through TLR 41. The results showed that genital fluids from women with BV stimulated cells predominantly through TLR2, while surprisingly there was relatively little stimulation through TLR4. In contrast, fluids from women without BV stimulated cells relatively little through either TLR2 or TLR4. Further, the TLR2-positive cells supported higher levels of expression of the HIV promoter when exposed to genital secretions from women with BV, suggesting that HIV-infected cells in the genital tract might express higher levels of HIV during episodes of BV. Other studies showed that genital tract fluid from women with BV can stimulate lymphocytes and other cells to express higher levels of TLR4 and TNF-α 43.

Dendritic cells (DC), cells important for antigen processing and presentation to the immune system, are found in the lower genital tract and are known to express both TLR2 and TLR4. DC are suggested to be one of the first cells that take up HIV during sexual transmission 4445. DC are also potent antigen presenting cells whose function would be important for vaccination against a number of mucosal pathogens, including HIV. We have investigated the hypothesis that genital tract secretions from women with BV might substantially affect either DC antigen presenting function or DC uptake and infection by HIV. We observed that secretions from women with BV potently stimulate secretion of IL-12 by monocyte-derived dendritic (MDDC) (Figure 1) 46. Genital fluid from women with BV also increased MDDC secretion of IL-23 and p40 and upregulated cell surface HLA-DR, CD40 and CD83. Further, BV fluids decreased MDDC endocytic ability (a marker of stimulation and maturation of DC) and increased proliferation of T cells in an allogeneic MLR with MDDC as the antigen presenting cells 46. Genital fluids from women without BV had much lower or no stimulatory activity for MDDC. These studies suggest that BV may substantially affect local DC antigen presenting function in women.

Since the above studies showed that BV genital secretions potently stimulate DC, we hypothesized that this stimulation might increase infection of DC or enhance the ability of DC to transfer HIV to T cells. However, our studies to date do not show BV enhancing HIV infection of DC (Fig. 2) or transfer of HIV by DC to T cells (Fig. 3). In fact, BV genital secretions appear to suppress HIV transfer to T cells (Fig. 3). While our studies currently do not support a role for direct effects of BV genital secretions on DC in enhancing HIV transmission, these findings do not rule out the possibility that BV promotes HIV transmission by altering DC function or trafficking in vivo.