Heterosexual transmission of HIV-1 is the predominant mode of infection worldwide. The transmission of HIV-1 has been shown to increase with contraceptive use, although the mechanism of action is unclear. Hormonal contraceptives are efficient at preventing pregnancy; however, the associated changes that occur to the genital microenvironment can influence the risk of HIV-1 infection, as well as transmission from HIV-1-infected women to seronegative partners. Here CCR5 upregulation with hormonal use is briefly discussed in the context of HIV transmission.
The identification of biological co-factors for HIV-1 transmission may point towards novel avenues for controlling HIV-1 dissemination. Various forms of contraception have been shown to be associated with an increased risk of acquiring HIV-1, especially hormonal contraceptive use1. Previous studies have demonstrated increased HIV-1 DNA shedding from the cervix of women using oral contraceptives, a dose–response relationship was observed when women using no contraceptive, low and high-dose OCs were compared2,3. With the continued rapid spread of HIV-1, an understanding of the factors that govern transmission rates remains of critical importance, particularly in women of a reproductive age. Understanding how contraceptive choice may modify the risk of acquiring HIV-1 is essential.
Heterosexual transmission of HIV-1 is the predominant mode of infection in most parts of the world, although HIV-1 is not efficiently transmitted by this route4. Several co-factors have been identified that may increase the risk of acquiring HIV-1. The use of OCs is one such factor that can alter epithelial cell integrity, reduce epithelial thickness, alter genital microflora, change luminal pH and increase the likelihood of cervical ectopy5–7.
Several potential routes of HIV-1 infection have been shown, able to occur in the lower and upper female genital tract8. The mechanisms involving the infection of CD4 cells have been proposed to involve antigen-presenting cells (APC), T cells and epithelial cells; these can act independently or in tandem to establish infection. The current understanding of the early events surrounding HIV-1 transmission via this mucosal route is that resident APC acquire HIV-1, and subsequently present virus to T lymphocytes before systemic spread. This can occur either in the cervical epithelium or in the lamina propria after transcytosis9. Two principal types of APC are at the genital mucosa; macrophages and dendritic cells. Dendritic cells are thought to be preferential targets for HIV-1 infection because of the expression of CCR510. These cells are either directly infected11 or simply ‘carry’ the virus bound to a lectin receptor, DC-SIGN12. CD4 T lymphocytes are also potential targets for HIV-1, as the activation phenotype of cervical T lymphocytes differs to that observed in blood13, thus rendering them more susceptible to HIV-1 infection14.
The individual effects of estrogen and progesterone has been shown to influence genital morphology and the functional capacity of cells15. Further studies in humans were undertaken to investigate the underlying mechanisms by which combined OCs may enhance HIV-1 transmission or persistence in the female genital tract. Thus analysis of T-lymphocyte activation markers and CCR5/CXCR4 chemokine co-receptors in women using no contraceptive method and those using a medium-dose OC pill was performed.
Methods and Results
A fine cervical cytobrush (Medscand AB, Sweden) was placed within the cervical canal and rotated once. In the absence of blood contamination, dislodged endocervical cells were transferred to the laboratory with matched peripheral blood samples16. All women recruited tested negative for STI and showed minimal inflammation in the cervix. Cervical and blood preparations were surface-stained and analysed using FACS Calibur software (Becton Dickinson, UK). A typical forward scatter versus side scatter profile is shown in Figure 1; specific gates are also shown to be able to characterize individual cellular subpopulations. T lymphocytes were identified using anti-CD3 and anti-CD8 antibodies. Monocytes/macrophages were characterized as staining for anti-CD14 and anti-HLA-DR. Dendritic cells stained negative for the dendritic cell cocktail (anti-CD3, anti-CD14, anti-CD16, anti-CD19 and anti-CD56) and positive for anti-HLA-DR.
A comparative number of CD4 and CD8 T lymphocytes were observed within the intraepithelial layer of the cervix and blood of investigated subjects. An analysis of the percentage of T cells expressing activation markers revealed much higher levels in the cervix than in peripheral blood; this was not influenced by contraceptive use13. The higher levels of T-cell activation in the cervical epithelium may represent the level of antigenic exposure at this site. Similarly, lamina propria CD4 T cells of the intestine are highly activated compared with blood, and have been shown to be susceptible to productive infection by HIV-1 in the absence of exogenous stimuli, suggesting that CD4 T cells in the cervix may also permit productive infection17.
Chemokines direct inflammation through a series of receptors attracting T lymphocytes and monocytes/macrophages. During inflammation, chemokine production of regulated upon activation: normal T-cell expressed/secreted (RANTES), macrophage inflammatory protein (MIP) 1α and MIP-1β is increased causing a T helper cell 1 type microenvironment that is further able to recruit leukocytes to the site of injury18. Chemokines and their receptors have been shown to play a decisive role in HIV-1 transmission, directing the preferential transfer of macrophage-tropic (CCR5-utilizing) strains of HIV-1 after sexual transmission. This process can be influenced by production of local chemokines that can competitively bind the CCR5 (via RANTES, MIP-1α, MIP-1β) or CXCR4 (stromal-derived factor type 1) co-receptors. However, in HIV-1-seropositive individuals the cell surface expression and density of chemokine co-receptors have previously been shown to correlate with disease, particularly CCR5 expression19. We thus sought the expression of CCR5 and CXCR4 in healthy women using COC. Cell surface expression of CCR5 increased with oral contraceptive use only on cervical CD4 T cells (P < 0.05 using Student's t-test) but not CD8 T cells. This upregulation of CCR5 on CD4 T cells was only associated with the early activation marker (CD69+), but not the latter stage markers (CD25+, HLA-DR+). Further investigations measured the density of chemokine co-receptors on cervical T-cell subsets and was shown to increase in women using OC (CCR5 expression: CD4 P < 0.05, CD8 P < 0.05 using Student's t-test) compared with those using no contraceptive20. Comparative studies in peripheral blood showed similar levels of chemokine co-receptor expression in both groups analysed.
These studies demonstrate that the use of OC is associated with increased expression of CCR5 and CXCR4 chemokine co-receptors. These findings may provide a possible mechanism to explain the epidemiological link between OC use and increased HIV-1 incidence. Moreover, in HIV-1-infected women the use of such OCs may influence viral load, disease progression and transmission. Contraceptive use is being promoted worldwide, including in areas where the incidence of HIV-1 is increasing; further knowledge regarding the effect of oral contraception and other non-barrier contraceptives on HIV-1 transmission is imperative.
This work was supported by the St. Stephens Trust at Chelsea & Westminster Hospital and by the Booth Research Fund at Northwick and St. Marks Hospitals.
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