A review of the human vs. porcine female genital tract and associated immune system in the perspective of using minipigs as a model of human genital Chlamydia infection

The porcine immune system is well characterized and highly resembles that of humans
11],36], although there are some differences. One of the differences is the anatomy of the
lymph nodes, which are inverted in pigs 38]. The inverted lymph node structure only affects the lymphocyte migration through
the lymph node. Porcine lymphocytes mainly leave the lymph node through high endothelial
venules instead of efferent lymph vessels, as they do in humans 21],38],39]. Otherwise the physiology and immunologic reactions of the B and T cell areas in
the lymph nodes do not differ 21],38].

Most of the protein mediators of the immune system are present with the same structure
and function in humans and pigs and most of the immune cells identified in both species
are similar 36],40]. The distribution of leukocytes in the blood is very similar in pigs and humans with
a high percentage of neutrophils 41], however, within the lymphocyte populations, pigs have a higher proportion of CD4
⁺
CD8
⁺
double positive T cells and ?? T cells in the blood. Otherwise the distribution of
the different lymphocyte populations in pigs and humans is quite similar 11],36],40],42] as summarized in Table 2.

Table 2. Lymphocyte subsets and antibodies in serum in humans and pigs

The major histocompatibility complex (MHC) system in pigs, called the swine leukocyte
antigen (SLA) system is very similar to the human leukocyte antigen system, in terms
of polymorphic loci, haplotypes and differentiated expression on different cell populations
11],43]. However, resting porcine T lymphocytes can express MHCII before activation 11],43], whereas human T cells only express MHCII when activated 44].

All the cytokines in the human Th1/Th2/Th17/Treg paradigm have porcine orthologs 36], however, it is suggested that IL-4 might play a different role in pigs 45]. The expression and frequency of immunoglobulins are quite similar (Table 2) except that IgD has not been demonstrated in pigs. Similar to humans, pigs have
at least five IgG subclasses: IgG1, IgG2a, IgG2b, IgG3 and IgG4 11]. Humans have two IgA heavy constant region genes (C?) and therefore two subtypes
of IgA designated IgA1 and IgA2 46], whereas pigs only have one C? gene and therefore only one class of IgA 46]-48]. Circulating IgA is mostly bone marrow derived and monomeric in humans 49], while circulatory IgA in pigs is half dimeric IgA and half monomeric IgA 50]. The dimeric proportion of circulating IgA in the pig is, however, primarily derived
from the intestinal synthesis and lymph. Due to the hepatic pIgR-mediated transcytosis
of polymeric IgA (pIgA) to the bile, the dimeric IgA is thought to be relatively short-lived
in the circulation 50]. The hepatic polymeric immunoglobulin receptor (pIgR)-mediated transcytosis of pIgA
happens in both humans and pigs 50].

In women, IgA2 is known to be the predominant isotype subclass in the genital secretions
51] while this distinction cannot be made in the porcine FGT secretions.

When modeling genital infections and evaluating vaccine responses, the toll-like receptors
(TLR) play a crucial role in recognition of the pathogens and induction of and controlling/directing
the immune response. It has been shown that the porcine TLR system is very similar
to that of humans 41]. In terms of cytokines such as the neutrophil chemokine IL-8, the coding gene carried
by humans and pigs is an ortholog 41]. Furthermore, human- and porcine macrophages produce indoleamine 2,3-dioxygenase
(IDO) in response to lipopolysaccharide (LPS) and Interferon gamma (IFN-?) stimulation
36],41].

7.1 6.1. The genital mucosal immune response

The genital mucosal immune responses are of specific importance when using the pig
as a model of human genital C. trachomatis infections. The genital immune response is challenged in the sense that it has to
tolerate sperm, the semi-allogeneic conceptus and the commensal vaginal flora, while
it must mount defense responses against sexually transmitted pathogens in order to
eliminate them 52].

The genital immune system consists of both innate and adaptive factors. The innate
system is primarily built by the epithelial barrier, the production of antimicrobial
agents and cytokines by the epithelial cells and the innate immune cells 40],53]. Both innate and adaptive humoral mediators and immune cells in the genital immune
system are regulated by progesterone and estradiol and therefore fluctuate through
the menstrual or estrous cycles 53].

The epithelial cells in the FGT with interconnecting tight junctions play an important
role in the immune protection by providing a strong physical barrier, transporting
antibodies to the mucosal surface, secreting antibacterial compounds and by recruiting
immune cells 54],55]. The sex hormones regulate the structural changes in the epithelium during the cycle.
Under the influence of estrogen, the integrity and strength of tight junctions in
the epithelial barrier, is significantly weakened in women 54],56]. The secretion of antimicrobial compounds is also suppressed during the midcycle
in women 53],57].

To preserve an intact protective barrier, the genital mucosal immune response is often
non-inflammatory to avoid inflammation-mediated injuries usually caused by phagocytic
activity and complement activation 55]. Most of the antigens in the FGT are therefore met with mucosal tolerance 55].

7.1.1 6.1.1. Distribution of immune cells in the genital tract tissue

The genital mucosa does not have immune inductive sites such as the nasal-associated
lymphoid tissue or intestinal Peyer’s patches 55]. Thus, the genital mucosa lacks an organized center to disseminate antigen-stimulated
B and T lymphocytes to the distinct sites of the mucosa. However, lymphoid aggregates
(LA) are present in the female genital mucosa of both pigs 35] and humans 55] and leukocytes are dispersed throughout the mucosa of the FGT 58] as illustrated in Figure 2.

The LA are located in the basal layer of the endometrium close to the base of the
uterine epithelial glands and built by a core of B cells surrounded by T cells and
an outer layer of macrophages 58]. The T cells in the LA are primarily CD8
⁺
T cells, however, CD4
⁺
T cells are also present in variable numbers in the LA 58]. Both CD4
⁺
and CD8
⁺
T cells are found as intraepithelial lymphocytes and dispersed throughout the subepithelial
tissue 58]. Aggregates of NK cells can also be found in the endometrium but they are placed
in close contact with the luminal epithelium 58].

The leukocytes present in the FGT covers macrophages, dendritic cells, NK cells, neutrophils,
B cells and T cells 53],59],60] with lymphocytes being the predominant immune cell type in both pigs and women 35],61],62]. The number of immune cells and the size of LA are under strong hormonal influence
and fluctuate through the cycle 55],58] as summarized in Table 3.

Table 3. Fluctuations in immune cells and antibody levels in the female genital tract during
the hormonal cycles. Both women and pigs show regional differences in the hormonal regulation of the genital
immune system. The antibody fluctuations seem similar in women and pigs but the influx
of neutrophils during estrus is specific for pigs. It should be noted that the porcine
studies are rather old and only including few animals. LGT – Lower genital tract,
UGT – upper genital tract

7.1.2 6.1.2. The humoral genital immune response

The immunoglobulins found in the FGT either have been locally produced by subepithelial
plasma cells, or derived from the circulation 63]. Although IgG producing plasma cells can be found in the FGT 64], genital IgG is mainly derived from the circulation 63],65]-67] and transported to the mucosal surface by mechanisms such as passive leakage, paracellular
diffusion or receptor-mediated transport 63],65]. In contrast, genital IgM and IgA are primarily derived from the subepithelial plasma
cells 65],68]-70] with up to 95% of the porcine IgA being locally produced 71] and up to 70% of the IgA being locally produced in women 55]. When produced locally, the polymeric secretory IgA (sIgA) is actively transported
across the mucosal epithelia cells by the polymeric immunoglobulin receptor (pIgR)
65],66]. The secretion of sIgA primarily takes place in the cervix due to the focused pIgR
localization in the cervix in women 72]. The pIgR is also expressed in the uterus, but to a lesser extent and in variable
levels due to hormonal regulation 55].

Usually, sIgA is the predominant isotype found in mucosal secretions, such as the
intestinal fluid. However, in the secretions from the FGT, there is a greater proportion
of IgG compared to sIgA 65],73]-75].

The FGT humoral immune response is under strong hormonal influence during the menstrual
or estrous cycle 57],74]. The cyclic fluctuations in the antibody levels are compared in Table 3. The information on cycle-dependent variations in the level of antibodies in pigs
is sparse and more knowledge is needed within this area.

7.1.3 6.1.3. Immunological differences of relevance for a Chlamydia model

The most important immunological difference with potential influence on Chlamydia models is the slightly different influx of immune cells in the porcine FGT, characterized
by an increase in neutrophils during estrus. It should be kept in mind that this increased
innate response during estrus could influence the establishment of infection.