**3. The host: Immunological and genetic factors, age and hormonal status, and sexual behavior**

#### **3.1. Immunological and genetic factors**

An important issue that contributes to CT pathogenesis is its remarkable ability to avoid the host immune system. Several strategies are displayed by these bacteria to prevent immune degradation, such as its intracellular lifestyle, its ability to escape from phagolysosomal pathway, its resistance to interferon gamma (IFN-γ), among others lesser known bacterial molecules. Actually, host immune response has opposite results on chlamydial infection outcome. A low immune response generates a suitable environment for pathogen colonization, while a strong immune response could lead to excessive inflammation and tissue damage. Pathogens own characteristics in conjunction with host genetic susceptibility are important in determining the severity of the illness.

At the site of invasion, an intense inflammation occurs, attracting different types of cells, such as macrophages, neutrophils, T and B lymphocytes, natural killers, and dendritic cells. Locally, there is an increased production of reactive oxygen species (ROS) that produces oxidative DNA damage, lipid peroxidation, energy depletion, modulation of gene expression, and proteins synthesis. Oxidative stress provokes pathologic changes that harm reproductive tissues. In addition, a broad collection of pro- and anti-inflammatory cytokines are released including IFN-γ, tumor necrosis factor (TNF-α), interleukin (IL) IL-1β, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12, IL-13, IL-17, IL-22, vascular endothelial growth factor (VEGF), granulocytemacrophage colony-stimulating factor (GM-CSF), and lactoferrin. These mediators trigger cellular inflammatory responses that mediate direct damage to the tissues [74,75]. Most of them are involved in the pathophysiology of tubal infertility, birth defects, and miscarriage [76].

In addition to the inflammatory response, CT infection evokes vigorous local and systemic humoral and cell-mediated immune responses. Several CT-specific antibodies, IgG isotype ratherthan secretory IgA, are found in circulation and in the cervicovaginal fluid of the female genital tract. These antibodies can neutralize chlamydial antigens; nevertheless, they do not assure resolution of the infection. Antibodies toward the bacterial MOMP protein are prevalent in primary chlamydial infections, whereas antibodies against CT-hsp10 and hsp60 are present in recurrent or persistent infections and correlate with severe sequelae such as tubal infertility, ectopic pregnancy, and PID [77–79]. Regarding the cell-mediated immune response, the T-helper lymphocytes type 1 (Th1) secrete IFN-γ, IL-2, and IL-12 and play a role in the resolution of infection [80,81]. On the other hand, the T-helper lymphocytes type 2 (Th2), which support the humoral immune response, produces IL-4, IL-5, IL-6, and IL-10 and participates in the developing tubal scarring [82]. The inflammatory response occurs, at the same extent, in both initial and repeat infections, whereas T-cell responses are predomi‐ nant in the latter ones [80,83].

Actually, host immune response is considered as one of the most important determinants in chlamydial infection outcome. A delicate balance between pro- and anti-inflammatory cytokines is needed to clear infection, avoiding tissue injuring. At this point, host genetic predisposition is a major player of pathology and CT-related infertility development [84].

Host genetic polymorphisms may encode aberrant or dysfunctional toll-like receptors (TLR) and nucleotide-binding oligomerization domain proteins (NOD) that do not appropriately recognize CT. These individuals have an impaired bacterial clearance and a high risk to develop an aberrant immune response, favoring CT persistence [85–88]. Identifying host genetic factors and bacterial virulence factors involved in immunoevasion and immunopa‐ thology remains a major priority in research for preventing chlamydial infection and its sequelae. Furthermore, the understanding of local innate and adaptive immune responses and their actors along the genital tract will be crucial for designing new therapeutic approaches and for developing a protective vaccine.

#### **3.2. Age and hormonal status**

cytokine responses [56]. Chlamydial TTSS effectors mediate the interaction with the host as they are injected to the cytoplasm and alter host cell functioning [57–59]. Important TTSS effectors are the inclusion (Inc) proteins that are bacterial proteins present at the inclusion membrane. For instance, IncA promotes the fusion of individual CT-containing vesicles to form a single inclusion [60,61]. Natural IncA bacterial mutants are associated with reduced virulence [62]. Another TTSS effector is TARP, mentioned in the previous section, as a bacterial protein that favors CT internalization via an actin recruiting mechanism [34,63]. Additionally, a chlamydial cytotoxin glycosylates the eukaryotic protein Rac1, and thereby induces actin reorganization and promotes the invasion of host cell [64,65]. Chlamydial glycolipid exoanti‐ gens [66] and the lipopolysaccharide [67] may constitute additional virulence factors. Other proteins encoded by the cryptic plasmid or related to the ability of the bacteria to survive under stressful metabolic conditions such as iron or tryptophan deprivation are thought to increase virulence and pathogenicity [68,69]. Chlamydial stress proteins, GroEL and GroES, may activate toll-like receptors and trigger a potent inflammatory response, injuring host repro‐

In addition to CT serovars and virulence molecules, other bacterial factors may be involved in the pathogenicity and chlamydial infection outcome, such as the pathogen load, route of infection, bacterial ability to enter persistent state, ascension capacity and strength to colonize genital upper tract, resistance to antibiotic treatment, and so on. Further studies are needed to determine the contribution of each bacterial factor to the development of severe damage on

**3. The host: Immunological and genetic factors, age and hormonal status,**

An important issue that contributes to CT pathogenesis is its remarkable ability to avoid the host immune system. Several strategies are displayed by these bacteria to prevent immune degradation, such as its intracellular lifestyle, its ability to escape from phagolysosomal pathway, its resistance to interferon gamma (IFN-γ), among others lesser known bacterial molecules. Actually, host immune response has opposite results on chlamydial infection outcome. A low immune response generates a suitable environment for pathogen colonization, while a strong immune response could lead to excessive inflammation and tissue damage. Pathogens own characteristics in conjunction with host genetic susceptibility are important in

At the site of invasion, an intense inflammation occurs, attracting different types of cells, such as macrophages, neutrophils, T and B lymphocytes, natural killers, and dendritic cells. Locally, there is an increased production of reactive oxygen species (ROS) that produces oxidative DNA damage, lipid peroxidation, energy depletion, modulation of gene expression, and proteins synthesis. Oxidative stress provokes pathologic changes that harm reproductive tissues. In

ductive tissues [70–73].

140 Genital Infections and Infertility

the female reproductive system.

**3.1. Immunological and genetic factors**

determining the severity of the illness.

**and sexual behavior**

CT preferentially targets young women at reproductive age. It has been reported that the highest incidence occurs in women between 16 to 24 years [89]. Therefore, the impact on female reproductive health is very important. Notwithstanding younger women are at higher risk of contracting a chlamydial infection, the rates of developing PID increase with age, being more frequent in the 30 to 40 decade [90]. After menopause, the frequency of chlamydial infections decreases substantially [91].

Accordingly, estrogen and progesterone are important for the establishment of chlamydial infection. Furthermore, estrogen receptors have been involved in the internalization of CT [92,93]. Sex hormones affect the clinical outcome of chlamydial infections; thus, follicle stimulating hormone (FSH), luteinizing hormone (LH), estradiol (E2), progesterone (P4), and prolactin (PRL) are involved not only in the establishment of chlamydial infection but also in the development of sequelae on reproductive tissue [94,95]. Additionally, oral contraceptive use was found to be a risk factor for CT infection. Several studies have attempted to elucidate the relation between the hormonal status at the time of infection and its contribution to tubal occlusion in genital chlamydial infection. Unfortunately, the role of sex steroids in infection outcome is far from being fully understood. Sex hormones have been shown to modulate immune responses and to have antioxidant effects in the female genital tract [96–98]. However, the mechanisms by which sex hormones may benefit or impair the colonization of genital tissues by pathogenic microorganisms should be further investigated.

#### **3.3. Sexual behavior and other host factors**

Sexual initiation at young age and a higher number of sexual partners are associated with increased risk of CT infection [99]. In a similar manner, having sex without protection favors CT contagion. This sexual behavior is also associated with higher incidence of sexually transmitted pathogens such as *Neisseria gonorrhoeae*, *Candida albicans*, *Human Immunodeficiency Virus*, among others [100–103]. Taken together, unsafe and high-risk sexual conduct is linked to impairment of women reproductive health, and particularly to tubal infertility. As it has been previously mentioned, some women have a genotypic predisposition to develop an abnormal immune response and severe inflammation following CT infection. In these women, there is a high rate of tubal obstruction and CT-related infertility, independent of sexual behavior [84].

In general, multiple sexual partners and unsafe intercourse increase the risk of CT recurrent infections and coinfection with other sexually transmitted pathogens. Thereby, the high chance to suffer repeat and chronic infections that target women reproductive tissue raises the infertility-associated pathologies.
