Gone with the Antibody
Authors: Silvia Guglietta, Maria Rescigno
Journal: Immunity (2018)

Bacterial metabolites can reach distant organs, and in this issue of Immunity, Uchimura et al., 2018Uchimura Y. Fuhrer T. Li H. Lawson M.A. Zimmermann M. Yilmaz B. Zindel J. Ronchi F. Sorribas M. Hapfelmeier S. et al.Antibodies set boundaries limiting microbial metabolite penetration and the resultant mammalian host response.Immunity. 2018; 49 (this issue): 545-559Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar show a fast systemic distribution of microbiota metabolites. This distribution is controlled by antibodies that accelerate bacterial transit through the small intestine, resulting in reduced local and systemic metabolite penetration and attenuation of immune responses. Bacterial metabolites can reach distant organs, and in this issue of Immunity, Uchimura et al., 2018Uchimura Y. Fuhrer T. Li H. Lawson M.A. Zimmermann M. Yilmaz B. Zindel J. Ronchi F. Sorribas M. Hapfelmeier S. et al.Antibodies set boundaries limiting microbial metabolite penetration and the resultant mammalian host response.Immunity. 2018; 49 (this issue): 545-559Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar show a fast systemic distribution of microbiota metabolites. This distribution is controlled by antibodies that accelerate bacterial transit through the small intestine, resulting in reduced local and systemic metabolite penetration and attenuation of immune responses. The gut microbiota harbors trillions of bacteria that have co-evolved with the host and are essential for maintaining its health. The host has developed a series of physical barriers that allow the containment of the gut microbiota in the intestinal tract, therefore preventing bacterial spread to systemic circulation and distant organs. Altered composition of the gut microbiota has been associated with several pathologies in the gastrointestinal tract, such as inflammatory bowel diseases and colorectal cancer (Tilg et al., 2018Tilg H. Adolph T.E. Gerner R.R. Moschen A.R. The intestinal microbiota in colorectal cancer.Cancer Cell. 2018; 33: 954-964Abstract Full Text Full Text PDF PubMed Scopus (375) Google Scholar). It is becoming obvious that besides the intestine, the gut microbiota modulates homeostasis and inflammation in virtually all host organs through a series of metabolic and immune regulatory functions (Martinez et al., 2017Martinez K.B. Leone V. Chang E.B. Microbial metabolites in health and disease: Navigating the unknown in search of function.J. Biol. Chem. 2017; 292: 8553-8559Crossref PubMed Scopus (81) Google Scholar). Given the tight containment of the gut microbiota within the intestine, how can a host-wide reach be explained? The answer lies in the ability of the intestinal bacteria to produce a large spectrum of metabolites that act throughout the host and complement the host metabolism at the same time. In this issue of Immunity, Uchimura et al., 2018Uchimura Y. Fuhrer T. Li H. Lawson M.A. Zimmermann M. Yilmaz B. Zindel J. Ronchi F. Sorribas M. Hapfelmeier S. et al.Antibodies set boundaries limiting microbial metabolite penetration and the resultant mammalian host response.Immunity. 2018; 49 (this issue): 545-559Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar propose that absorption of bacterial metabolites in the small intestine results in a fast systemic distribution that can cause inflammatory responses. The production of immunoglobulin A (IgA) can reduce this phenomenon by dragging bacteria in the colon, where the penetration of bacterial metabolites is minimal. The idea that microbial metabolites can penetrate distant host organs emerges from several studies that have attempted to provide a deeper understanding of the dynamic interactions between host and gut microbiota. For instance, during conventionalization of germ-free mice, the establishment of a microbial flora has been shown to correlate with profound alterations in the transcriptomic and metabolomic profile of intestinal tissues, and significant changes have also been found in the urine metabolome, therefore suggesting a systemic impact of bacterial colonization (El Aidy et al., 2013El Aidy S. Derrien M. Merrifield C.A. Levenez F. Doré J. Boekschoten M.V. Dekker J. Holmes E. Zoetendal E.G. van Baarlen P. et al.Gut bacteria-host metabolic interplay during conventionalisation of the mouse germfree colon.ISME J. 2013; 7: 743-755Crossref PubMed Scopus (72) Google Scholar). However, the above-mentioned study did not discriminate between microbial- and host-derived metabolites in response to conventionalization. In addition, it was not known how and whether the host immune responses could modulate the effect of bacterial metabolites. In this issue of Immunity, by using a unique set of experimental tools, Uchimura et al. provide deeper insights into this matter. In order to allow an unequivocal discrimination of the metabolite source, the authors colonized germ-free mice with 13C-labeled HA107 E. coli. Therefore, 13C-labeled metabolites of bacterial origin and 12C-labeled host metabolites could be distinguished and efficiently traced by high-resolution mass spectrometry. Further, given that the HA107 strain of E. coli is engineered with cell-wall auxotrophy and was previously shown to be unable to replicate in vivo, the 13C-labeled bacterial metabolites did not become diluted with 12C host nutrient sources released during in vivo bacterial growth (Hapfelmeier et al., 2010Hapfelmeier S. Lawson M.A. Slack E. Kirundi J.K. Stoel M. Heikenwalder M. Cahenzli J. Velykoredko Y. Balmer M.L. Endt K. et al.Reversible microbial colonization of germ-free mice reveals the dynamics of IgA immune responses.Science. 2010; 328: 1705-1709Crossref PubMed Scopus (526) Google Scholar). Interestingly, bacteria-derived metabolites were present in all body fluids as early as 2 hr after oral administration of HA107, and bacterial amino acids were able to penetrate all host tissues. Fatty acids were mainly detected in the intestine and in the mesenteric lymph nodes, suggesting a lymphatic rather than a hematic spreading. After 18 hr, all bacterial metabolites were absent from the small intestinal fluids and had reached the caecum and colon, which are anatomically located in the terminal part of the intestinal tract and are characterized by a surface with minimal absorptive capability. Nevertheless, bacteria-derived metabolites could still be detected in small intestinal tissues, in line with the idea that the small intestine represents the predominant absorptive surface. The penetration of the HA107-derived metabolites was comparable to that of a parental strain with the ability to replicate in vivo, ruling out the possibility that higher fragility of the replication incompetent strain could cause stronger systemic exposure to metabolites. Microbial colonization induces profound changes in the host and specifically in the intestine, where increased epithelial cell proliferation and maturation and increased infiltration by immune cells have been extensively described (Bäckhed et al., 2005Bäckhed F. Ley R.E. Sonnenburg J.L. Peterson D.A. Gordon J.I. Host-bacterial mutualism in the human intestine.Science. 2005; 307: 1915-1920Crossref PubMed Scopus (3612) Google Scholar). Given that mice that are administered the HA107 strain of E. coli return to their initial germ-free status 2 weeks after oral administration, it is a perfect system for assessing the role of “microbiota conditioning” in the uptake of its metabolites or in bacterial transit in the intestine. Consistent with their previous observation (Hapfelmeier et al., 2010Hapfelmeier S. Lawson M.A. Slack E. Kirundi J.K. Stoel M. Heikenwalder M. Cahenzli J. Velykoredko Y. Balmer M.L. Endt K. et al.Reversible microbial colonization of germ-free mice reveals the dynamics of IgA immune responses.Science. 2010; 328: 1705-1709Crossref PubMed Scopus (526) Google Scholar), the authors found that transient colonization led to a durable upregulation of Ig-related genes and of several metabolic pathways. Hence, conditioning germ-free mice with an initial 13C HA107 administration and performing a subsequent challenge after they returned to their germ-free status enabled the investigators to understand how and whether the immune responses elicited in the host by bacteria colonization could affect the penetration of bacteria-derived metabolites. In addition, the authors compared metabolite penetration between naive and previously conditioned germ-free wild-type (WT) and Igh-J−/− mice after re-challenge to assess the role of Ig in metabolite uptake. They observed substantial differences between naive and unconditioned groups. Indeed, in conditioned mice early after bacterial challenge, bacteria-derived metabolites were preferentially eliminated in the urine, whereas in unconditioned mice the metabolites mainly accumulated in the peritoneal fluids. These differences might have been due to the immune system given that important changes were found in the conditioned mice depending on their ability to produce Ig. Indeed, 2 hr after HA107 administration, conditioned WT mice showed a higher amount of bacteria-derived metabolites in the caecum and colon fluids than conditioned antibody-deficient mice and were able to clear the bacterial load in the large intestine by 18 hr after bacterial administration. These results clearly indicate that the ability to produce antibodies correlates with faster bacterial transit in the intestinal tract. IgAs have been shown to favor enchained growth of pathogens, which is effective in accelerating pathogen clearance from the gut lumen (Moor et al., 2017Moor K. Diard M. Sellin M.E. Felmy B. Wotzka S.Y. Toska A. Bakkeren E. Arnoldini M. Bansept F. Co A.D. et al.High-avidity IgA protects the intestine by enchaining growing bacteria.Nature. 2017; 544: 498-502Crossref PubMed Scopus (216) Google Scholar). In line with these findings, further analyses confirmed that the small intestinal fluids of conditioned WT mice contained flagellin-specific IgA. Although IgAs were also shown to bind authentic bacterial metabolites in specific ELISA binding studies, no differences were found between WT and Igh-J−/− mice. On the basis of these results, IgA controls metabolite penetration not by acting on bacterial metabolites directly but rather by reducing bacteria motility and accelerating their transit through the small intestine, maybe by favoring enchainment. Accordingly, in Igh-J−/− mice, the absence of any Ig enabled the bacteria to move against the intestinal flow so that they could persist longer in the small intestine. As a consequence, at later time points bacteria-derived metabolites were found to accumulate in intestinal tissues. Further uptake of microbial molecules was also detected in the serum of Igh-J−/− mice and might explain the presence of microbial metabolites also in other peripheral tissues, such as the adipose tissue (Figure 1). Consistent with the observation that there is a higher local and systemic exposure to bacteria and their derived metabolites in the absence of IgA, Ig-deficient mice also showed a stronger release of inflammatory and non-inflammatory cytokines. Examination of the metabolites of both microbial and host origin in intestinal tissues and distant organs allowed the authors to conclude that, in the absence of antibodies, the persistent exposure of the host to free bacteria in the small intestine correlates with a status of lipidemia. Indeed, in circulation, as well as in the pancreas of antibody-deficient mice, there were increased amounts of non-sterified fatty acids, such as arachidonic acid and the derived compounds leukotrienes, and these were most likely associated with increased innate immune responses. It remains to be established whether the absence of metabolite-directed antibodies favors metabolite hematic dissemination, given that in their “free form,” metabolites could easily cross the gut vascular barrier (Spadoni et al., 2015Spadoni I. Zagato E. Bertocchi A. Paolinelli R. Hot E. Di Sabatino A. Caprioli F. Bottiglieri L. Oldani A. Viale G. et al.A gut-vascular barrier controls the systemic dissemination of bacteria.Science. 2015; 350: 830-834Crossref PubMed Scopus (309) Google Scholar). Although the transient monocolonization of germ-free mice with 13C-labeled HA107 E. coli proved to be a powerful and elegant tool in this work, it does not provide information about the complex dynamics of metabolite penetration in the presence of a diversified microbiota. Nevertheless, the present findings set the stage for future studies aiming at unravelling the intricate metabolic interaction between host and microbiota and could be highly relevant in patients suffering from the retention of high bacterial load in the small intestine, which is a condition associated with an increasing number of pathologies. Besides environmental entheropaties in children in developing countries, in which a dysregulated IgA production has indeed been described, small intestinal bacterial overload is also found in cystic fibrosis patients and in individuals with cirrhosis, where it correlates with malabsorption, pancreatic insufficiency, and endotoxemia (Bauer et al., 2002Bauer T.M. Schwacha H. Steinbrückner B. Brinkmann F.E. Ditzen A.K. Aponte J.J. Pelz K. Berger D. Kist M. Blum H.E. Small intestinal bacterial overgrowth in human cirrhosis is associated with systemic endotoxemia.Am. J. Gastroenterol. 2002; 97: 2364-2370Crossref PubMed Scopus (171) Google Scholar, De Lisle et al., 2011De Lisle R.C. Mueller R. Boyd M. Impaired mucosal barrier function in the small intestine of the cystic fibrosis mouse.J. Pediatr. Gastroenterol. Nutr. 2011; 53: 371-379Crossref PubMed Scopus (38) Google Scholar). The etiology of these clinical manifestations still remains very elusive, and it is tempting to speculate that defects in antibody production or function could at least partially contribute to it. If it is true that “all diseases begin in the gut,” as Hippocrates said, according to Uchimura et al., a healthy immune system makes the most bacteria end up in the colon given that bacteria are gone with the antibody. The authors would like to thank Mirja Loth for support with the figure preparation. Antibodies Set Boundaries Limiting Microbial Metabolite Penetration and the Resultant Mammalian Host ResponseUchimura et al.ImmunitySeptember 4, 2018In BriefBacteria-derived metabolites pervade the mammalian host, shaping immunity and metabolism. Using stable isotope tracing, Uchimura and colleagues profile the scope and depth of host tissue penetration by bacterial metabolites. Extensive host immune and metabolic responses to microbial metabolite penetration are constrained by secretory antibodies that limit microbial small-intestinal dwell time. Full-Text PDF Open Access