CHAPTER 2: Literature Review
2.3 Mechanisms for increased HIV-1 risk in women using injectable HCs
2.3.3 Influence of injectable HC use on innate and soluble factors in the genital tract
a central role in determining viral kinetics, influence recruitment of HIV-1 target cells to the mucosa, and development of anti-viral immunity following infection (Stacey et al., 2009;
Roberts et al., 2010; Roberts et al., 2012; Bebell et al., 2008). In addition, pre-existing inflammatory responses, both in the genital mucosa and systemically, are also thought to influence risk of HIV-1 infection in women before they become infected (Masson et al., 2015; Naranbhai et al., 2014).
Several different, partially overlapping, functional classes of cytokines have been described (Abbas and Lichtman, 2007; Charo and Ransohoff, 2006; Connolly et al., 2005). Pro- inflammatory cytokines, such as TNF-β, L-12p40, IL-12p70, IL1α, IL-6, TNF-α, and IL-1β, are involved in recruitment of immune cells from blood into tissue, stimulating their differentiation and activation. Anti-inflammatory cytokines, including IL-1Ra and IL-10, regulate excessive inflammatory reactions, and are produced to counteract the actions of inflammatory cytokines. Chemokines are chemotactic cytokines, including eotaxin, monocyte chemoattractant protein (MCP)-1, macrophage-derived chemokine (MDC), Fractalkine, macrophage inflammatory protein (MIP)-1α, MCP-3, interferon-γ-induced protein (IP)-10, growth related oncogene (GRO), MIP-1β, IL-8, and RANTES, are responsible for recruitment of immune cells to inflammatory sites and damaged tissue. Growth factors, such as interferon (IFN)-α, transforming growth factor (TGF)-α, vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF)-AB/BB, and fibroblast growth factors (FGF)-2, are involved in cell growth and proliferation.
Hematopoietic cytokines, such as IL-9, FMS-like tyrosine kinase-3 (Flt3L), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage (GM)-CSF, IL-7, and IL-3, are involved in hematopoesis of immune cells from stem cell progenitors. Adaptive cytokines, such as IL-15, IL-5, IL-17, soluble CD40 ligand (sCD40L), IFN-γ, IL-2, IL-4, IL-13, sIL- 2Rα, and IFN-α, stimulate growth, differentiation, activation and survival of immune cells.
Changes in hormones during the menstrual cycle, including changes induced by HC use, has been suggested to alter cytokine production in both the upper and lower female reproductive tract (Huijbregts et al., 2013). Use of DMPA has been shown to reduce resistance to microbial invasion by modulating soluble innate mucosal defenses (Figure 2.3, number 3).
DMPA was found to inhibit secretion of IFN-γ, IL-2, IL-4, IL-6, IL-12, TNF-α and MIP-1α by activated PBMCs in vitro (Huijbregts et al., 2013). DMPA has also been shown to have glucocorticoid receptor (GR) agonistic activity that inhibits the expression of GR-regulated genes (Bamberger et al., 1999; Koubovec et al., 2005; Koubovec et al., 2004). For example, DMPA represses the gene encoding IL-2 in normal human lymphocytes via the GR, which suggests that the GR might mediate the immunosuppressive effects of DMPA in vivo (Bamberger et al., 1999; Koubovec et al., 2005; Koubovec et al., 2004). Cervical cell lines stimulated with both DMPA and TNF-α produced lower levels of RANTES and increased levels in IL-8, while NET-EN stimulation had no effect (Africander et al., 2011). Govender et al. (2014) showed that endocervical cells treated with DMPA showed decreased inflammatory responses, marked by lower expression of IL-6, IL-8 and RANTES mRNA, while NET-EN did not (Govender et al., 2014).
In contrast to these studies showing dampening of inflammatory responses, others have suggested that DMPA use may increase expression of RANTES, although decreasing levels of other protective innate factors like the secretory leukocyte protease inhibitor (SLPI) and biodefensin-2 (BD-2) in genital secretions (Morrison et al., 2014). Importantly, this increase in genital RANTES and decreased concentrations of SPLI was also associated with increased risk of HIV-1 acquisition (Morrison et al., 2014).
Others have suggested that DMPA use may dampen immune responses to other common infectious diseases in women, such as tuberculosis. PBMCs from women with tuberculosis who were using DMPA produced lower amounts of IL-1α, IL-12p40, IL-10, IL-13 and G- CSF following stimulation with the mycobacterial antigen BCG compared to non-HC users (Kleynhans et al., 2011). In addition, DMPA using women also had decreased frequencies of circulating monocytes, a key correlate of protection against tuberculosis, compared to non- HC users (Kleynhans et al., 2011). In mice infected with tuberculosis, treatment with DMPA also resulted in down-regulated plasma cytokine levels of TNF-α, IFN-γ, G-CSF, IL-6, IL-10, IL-17 and MCP-1 and increased secretion of IP-10 compared to control mice (Kleynhans et al., 2013).
Matrix metalloproteases (MMPs) are a family of zinc dependent proteases that are required during normal reproductive processes for degradation of specific components of the extra cellular matrix and tissue remodeling in the endometrial compartment of the female genital tract during normal menstruation (Birkedal-Hansen, 1995; Cawston, 1995; Lockwood and Schatz, 1996; Rodgers et al., 1994; Rodgers et al., 1993). MMPs comprise collagenases (enzymes that break down peptide bonds in collagen that includes MMP-1), gelatinases (proteolytic enzymes that hydrolyze gelatin that include MMP-2 and MMP-9), stromelysins (enzymes involved in breakdown of the extracellular matrix that include MMP-3 and MMP- 10) and matrilysins (uterine metalloproteinases that are also involved in extracellular matrix degradation that include MMP-7). MMPs are regulated by multiple ways, being activated by other proteases, at the level of gene transcription and by hormones (Vincent et al., 2000).
MMPs are inhibited by specific endogenous tissue inhibitors of metalloproteinases (TIMPs)
(Fernandez-Catalan et al., 1998; Gomis-Ruth et al., 1997). Regulation of TIMP expression occurs during tissue development and remodelling (Fernandez-Catalan et al., 1998; Gomis- Ruth et al., 1997). Injectable HCs have been shown to suppress production of epithelial cell- specific MMP-7 and stromal-epithelial specific MMP-3 (Bruner et al., 1995; Osteen et al., 1994; Schatz et al., 1997; Singer et al., 1997). Women using injectable HCs were also found to have decreased TIMP concentrations in their upper reproductive tracts (Vincent et al., 2002). Although not fully understood, changes in the local ratio of MMPs:TIMPs may influence tissue remodeling processes in the reproductive tract, and degradation of extracellular matrix components such as collagen, elastin, fibronectin and laminin by MMP- 1, MMP-2 and MMP-9 thus affecting epithelial barrier function upon HIV-1 exposure (Li et al., 2007; Osteen et al., 1994; Vincent et al., 2000, 2002; Zhang et al., 1998; Zhang, 1998).
2.3.4 Susceptibility to bacterial vaginosis (BV) and stability of the vaginal microbiome