IgA and the gut-vagina axis

Kazuhide Takada ^*

• Division of Microbiology, Department of Pathology and Microbiology, Nihon University School of Medicine, Tokyo, Japan

The microbiome in the reproductive tract influences both women's and their offspring's health [1][2][3][4]. A highly diverse gut microbiota is considered healthy [5]. However, the vaginal microbiota of a healthy reproductive-aged woman is often dominated by only one or two species of Lactobacillus, such as L. crispatus, L. gasseri, L. iners, and L. jensenii [6]. Interestingly, this property is observed only in humans and not in other primates [7][8][9]. Although some hypotheses have been proposed, the mechanism by which Lactobacillus dominates the human vaginal microbiota remains unknown [7,10,11].Various treatments have been used to modify the vaginal microbiota to a Lactobacillusdominant state against dysbiotic conditions. Probiotic treatment of the vaginal microbiota is promising [12]. However, the indigenous vaginal microbiota frequently surpasses the colonization of the probiotic L. crispatus strain (CTV-05 strain) from the vaginal source [13]. After 24 weeks, approximately 50% of patients who received this probiotic did not retain CTV-05 [12]. Consequently, understanding the mechanism to regulate the vaginal microbiota by the host is crucial for developing novel therapies, including probiotics, to address conditions related to vaginal dysbiosis.Immunoglobulins play important roles in regulating homeostasis and microbiota at mucosal sites. In the intestinal tract, immunoglobulin A (IgA) selectively attaches to microbes that have a close relationship with the host mucosa [14]. IgA appears to serve a dual and context-sensitive function, acting to exclude pathogens while facilitating the colonization of beneficial commensals [14]. Immunoglobulin G (IgG) plays a crucial role in promoting mucosal homeostasis in addition to regulating both non-invasive and invasive mucosal bacteria [15]. In contrast to the gastrointestinal tract and other mucosal tissues, the antibodies found in the vagina are primarily IgG instead of IgA [15]. In vagina, IgA, IgG, and IgM participate in immune defense [16]. Although the function of vaginal IgG remains partially understood, it has been noted to capture viruses and protect the host against viral infections [17].Recent advances in IgA-seq have revealed that the prevalence of IgA-coated vaginal bacteria is elevated in L. crispatus-dominant microbiota compared to other microbiota compositions [18]. Another study also reported that the levels of microbial IgA and IgG coating were lowest in individuals with diverse microbiota, particularly among women from ethnic minority groups [19]. Accumulating evidence suggests that IgA-producing cells in the vagina originate from the intestine [20,21]. Furthermore, although IgG levels in cervicovaginal fluid were higher than those of IgA, it was IgA that predominantly coated the bacteria [22]. Therefore, similar to the gastrointestinal tract, I hypothesize that IgA may regulate the composition of the vaginal microbiota in addition to its role in pathogen clearance. In this article, I discuss the possible regulation of the vaginal microbiota by the gut microbiota via IgA. Here, it is hypothesized that IgA induction for Lactobacillus occurs in the small intestine. Subsequently, Lactobacillus-reactive memory/effector cells migrate from the intestine to the vagina and produce Lactobacillus-specific IgA, increasing the number of IgA-coated Lactobacillus. Finally, these IgA-coated Lactobacillus strains promote stable vaginal colonization, highlighting IgA's role in the gut-vagina axis (Figure 1).Bacterial colonization in host organs is regulated by both direct and indirect mechanisms. Direct mechanisms include spatial nutrients or space competition, active antagonism, and metabolite inhibition [23]. The direct mechanisms in the vagina are beyond the scope of this study and have been discussed elsewhere [11]. Indirect mechanisms include mucus barrier function, oxygen limitation, and microbiota-mediated immune responses [23]. In the gut, some bacterial symbionts stimulate the synthesis of polyreactive low-affinity IgA antibodies that may exhibit cross-reactivity with antigens from other bacterial species, which can affect the composition of the microbiota. Indeed, a significant percentage of commensal gut bacteria are coated with IgA antibodies [23].Recently, IgA-seq has revealed that some microbes in the vagina are also coated with IgA and IgG [18,22,24]. IgA and IgG are present in the cervicovaginal secretions bound to L. crispatus, L. iners, Gardnerella vaginalis, and Prevotella bivia [24]. Interestingly, in a L. crispatus-dominant microbiota, the number of IgA-coated vaginal bacteria was found to be increased compared to other microbiota compositions [18]. Therefore, IgA may regulate the vaginal microbiota, although whether it is polyreactive or species-specific remains unclear.Next, where is IgA produced and how does it bind to Lactobacillus? In humans, IgA is classified into two subclasses, IgA1 and IgA2. The levels of IgA1 and IgA2 in the female genital tract secretions are approximately equal [25]. These observed equal ratios of IgA1 and IgA2, coupled with the predominance of polymeric IgA in cervical secretions, indicates that IgA is synthesized locally in the mucosa [25]. Notably, hysterectomy decreased immunoglobulin levels in the vagina, indicating that immunoglobulins generated locally and transferred from the bloodstream by uterine tissues, to some extent, contribute to humoral immunity in the vagina [26]. While B cells represent a small cell population across all female reproductive tract tissues, plasma cells that produce IgA and IgG are primarily located in the cervix and, to a lesser degree, in the vagina [27]. In vivo, initial infection with herpes simplex virus 2 (HSV2) fails to produce plasma cells within the lamina propria of the female reproductive tract. In contrast, upon secondary challenge with HSV2, circulating memory B cells that migrate into the female reproductive tract act as a source of rapid and substantial virus-specific IgG2b, IgG2c, and IgA secretion into the lumen of this tract [28]. CD4 tissue-resident memory T cells generate interferon-γ, which results in the expression of chemokines like CXCL9 and CXCL10. Circulating memory B cells are attracted to the vaginal mucosa via a CXCR3-dependent mechanism, where they generate virus-specific IgG2b, IgG2c, and IgA, which are subsequently released into the lumen [28]. However, these reactions have been observed under pathogenic conditions induced by HSV2. As vaginal Lactobacillus species are not pathogens, the mechanism of IgA induction for them under steady vaginal conditions remains obscure.Another possible site for IgA induction for Lactobacillus is the gastrointestinal tract, which is the basis for the present hypothesis. Recent studies indicate that the primary origin of IgA targeting is the small intestine [14]. Human gut-associated lymphoid tissue comprises multi-follicular Peyer's patches (PP). Human PP consists of numerous individual follicles that extend throughout the entire length of the small intestine. Their density is increased in the terminal ileum, culminating in the formation of a lymphoid ring at the ileocecal junction [29]. The PP serves as a location for adaptive immune priming and encompasses various specialized microanatomical niches that facilitate the effective initiation and propagation of immune responses [29]. In humans, the core small intestinal microbiota includes Streptococcus, Veillonella, Fusobacterium, Prevotella, and Haemophilus [30]. Notably, Lactobacillus is one of the segment-specific microbes found in the ileum [30]. In mice, Lactobacillus is one of the dominant bacterial genus in the small intestine [31,32]. Therefore, IgA induction for Lactobacillus may occur mainly in the PP of the ileum in humans.How do Lactobacillus-reactive immune cells migrate from the ileum to the female reproductive tract? Accumulating evidence from previous studies on orally administered human papillomavirus (HPV) vaccines based on genetically modified lactic acid bacteria provides us insights. In vivo, HPV16 L1-specific vaginal IgA was detected after oral administration of L. lactis transformed with two types of HPV16 L1-encoding plasmids [33]. L. lactis with HPV16 L2-encoding plasmids also induces HPV16 L1-specific vaginal IgA in mice [34]. In humans, oral administration of recombinant L. lactis expressing the HPV 16 E7 oncogene also induces the vaginal secretion of HPV 16 E7-specific IgA [20]. α4β7 integrin -mediated homing to the intestine has already been established [35,36], and a similar mechanism has been suggested for the vagina [21]. These studies have indicated that the migration (homing) of Lactobacillus-reactive memory/effector cells occurs from the gut to the vagina (gut-vagina axis), and it is not surprising that antibodies recognizing the usual components of Lactobacillus (polyreactive or species-specific) are produced. To the best of my knowledge, no studies have reported that the same Lactobacillus strains are shared between the intestine and vagina. Instead, phylogenetic studies of L. crispatus and L. gasseri in the gut and vagina suggest that different strains, adapted to their respective environments, may be established in the two organs, respectively [37,38]. Therefore, inducing Lactobacillus-specific rather than strain-specific antibodies would be important.Overall, IgA induction for Lactobacillus can occur in the small intestine (especially in the ileum). Then Lactobacillus-reactive and integrin αEβ7+ memory/effector cells migrate from the intestine to the vagina and produce Lactobacillus-specific IgA, resulting in the increased number of IgA-coated Lactobacillus in the vagina. Another possibility to increase the number of IgA-coated Lactobacillus in the vagina is through direct migration from the gut. IgA-coated L. jensenii has been confirmed in fecal samples from healthy women [39]. Additionally, probiotic Lactobacillus strains administered orally were confirmed in the vagina [40][41][42][43]. Therefore, these observations suggest that Lactobacillus already coated with IgA in the gastrointestinal tract may also migrate directly to the vagina.Finally, these IgA coatings may promote stable colonization of Lactobacillus in the vagina (Figure 1). In vitro study suggested that IgA can enhance the mucosal binding of Lactobacillus in the gut [14]. Therefore, IgA-coated Lactobacillus adhering to vaginal mucus may facilitate their vaginal colonization; however, further studies are needed to determine the effects of differences in the composition of mucus between the intestinal tract and vagina. Notably, a lower quantity of IgA attachment per bacterium was observed when women with L. crispatus-dominant microbiota exhibited a higher level of IgA coating on vaginal bacteria than those with other microbiota compositions [18]. Another investigation revealed a notable preference for IgA coating of taxa linked to vaginal dysbioses (bacterial vaginosis), such as Sneathia and Prevotella species [22]. Hence, an unknown mechanism that distinguishes the degree of IgA coating may balance pathogen clearance and host-microbial symbiosis in the vagina. Interestingly, Lactobacillus obtained from undernourished diet-fed mice demonstrated a significantly reduced capacity to bind IgA, indicating that Lactobacillus may have adapted mechanisms for evading IgA [14]. Therefore, the nutritional environment may also regulate the IgA coating of Lactobacillus in the vagina.Mutualistic symbiosis has developed as a result of millions of years of coevolution between the host and microorganisms, wherein the microbiota supports host metabolic processes and the host gives bacteria nourishment and a preferable environment [23]. The dominance of the human vagina by Lactobacillus can also be regarded as a result of its coevolution with humans [11], which contribute to women's health [1][2][3]. Nonetheless, the process by which Lactobacillus establishes dominance in the vaginal microbiota is still not fully understood.Notably, among Lactobacillus species, only L. iners is regarded as an undesirable bacteria because it is associated with recurrent bacterial vaginosis (dysbiosis of the vaginal microbiota) [44]. The significance of L. iners during the coevolution between humans and Lactobacillus remains unknown. However, the metabolic capacity and small genome size of L. iners compared to other vaginal Lactobacillus species indicate the potential for targeting metabolic differences (such as cysteine and oleic acid) to either inhibit L. iners or even enhance other species of Lactobacillus [45,46]. These metabolic differences may provide clues to coevolution.In addition to the present hypothesis involving IgA, another mechanism of the gut-vagina axis is the estrobolome [47] which was not the focus of the present study. Briefly, some bacteria in the gastrointestinal tract can deconjugate estrogens that were previously conjugated in the liver. Subsequently, the reabsorption of deconjugated estrogen into the systemic circulation occurs. Circulating estrogen affects the distal epithelium of the vagina by modifying the physiological characteristics of vaginal epithelial cells, including glycogen and mucus production. Elevated glycogen levels promote the dominance of Lactobacillus in the vaginal environment because glycogen acts as a crucial energy source for vaginal Lactobacillus [48][49][50].In conclusion, IgA induction for Lactobacillus in the small intestine may promote colonization of this bacterium in the vagina via IgA regulation. If the present hypothesis is valid, prior or simultaneous oral administration of probiotics could enhance the colonization of the same bacteria administered vaginally. A further understanding of the relationship between the female reproductive tract and other organs is required to establish effective treatments.