Gnotobiotic mouse model's contribution to understanding host-pathogen interactions

What we need to know about the germ-free animal models

The gut microbiota (GM), as a forgotten organ, refers to the microbial community that resides in the gastrointestinal tract and plays a critical role in a variety of physiological activities in different body organs. The GM affects its targets through neurological, metabolic, immune, and endocrine pathways. The GM is a dynamic system for which exogenous and endogenous factors have negative or positive effects on its density and composition. Since the mid-twentieth century, laboratory animals are known as the major tools for preclinical research; however, each model has its own limitations. So far, two main models have been used to explore the effects of the GM under normal and abnormal conditions: the isolated germ-free and antibiotic-treated models. Both methods have strengths and weaknesses. In many fields of host-microbe interactions, research on these animal models are known as appropriate experimental subjects that enable investigators to directly assess the role of the microbiota on all features of physiology. These animal models present biological model systems to either study outcomes of the absence of microbes, or to verify the effects of colonization with specific and known microbial species. This paper reviews these current approaches and gives advantages and disadvantages of both models.

Elucidating the role of microbes in celiac disease through gnotobiotic modeling

Celiac disease (CeD) is a common immune-mediated disease triggered by the ingestion of gluten in genetically predisposed individuals. CeD is unique in that the trigger (gluten), necessary genes (HLA-DQ2 and DQ8), and the autoantigen (tissue transglutaminase) have been identified, allowing additional environmental co-factors, like the intestinal microbiota, to be studied through relevant in vivo models. Murine models for CeD have come a long way in the past decade and there are now in vitro and in vivo tools available that mimic certain aspects of clinical disease. These models, many of which express the CeD risk genes, have recently been used to study the mechanisms through which the microbiota play a role in CeD pathogenesis through a gnotobiotic approach. Historically, the generation of gnotobiology technology in mid-20th century allowed for the study of immunity and physiology under a complete absence of microbes (axenic) or known colonized status (gnotobiotic). This enabled understanding of mechanisms by which certain bacteria contribute to health and disease. With this perspective, here, we will discuss the various murine models currently being used to study CeD. We will then describe how utilizing axenic and gnotobiotic CeD models has increased our understanding of how microbes influence relevant steps of CeD pathogenesis, and explain key methodology involved in axenic and gnotobiotic modeling.

Francisella and Antibodies

Immune responses to intracellular pathogens depend largely upon the activation of T helper type 1-dependent mechanisms. The contribution of B cells to establishing protective immunity has long been underestimated. Francisella tularensis, including a number of subspecies, provides a suitable model for the study of immune responses against intracellular bacterial pathogens. We previously demonstrated that Francisella infects B cells and activates B-cell subtypes to produce a number of cytokines and express the activation markers. Recently, we documented the early production of natural antibodies as a consequence of Francisella infection in mice. Here, we summarize current knowledge on the innate and acquired humoral immune responses initiated by Francisella infection and their relationships with the immune defense systems.

Effects of Pediococcus pentosaceus LI05 on immunity and metabolism in germ-free rats

Many Pediococcus spp. have health-promoting benefits, and Pediococcus pentosaceus LI05 is one such species that was proved to be beneficial in previous studies. Our research aimed to determine the immune and metabolic effects of P. pentosaceus LI05 on germ-free rats. Germ-free rats were gavaged with P. pentosaceus LI05 suspensions (1 × 10⁹ CFU) for 2 weeks, and 3 weeks later, blood, spleen, intestine and liver samples were gathered for metabolome, intestine morphology, immunity, and transcriptomics analyses. Oral gavage of P. pentosaceus LI05 reduced the bodyweight of rats, which manifested as increased fecal carbohydrate concentrations, decreased intestinal fat intake and the hepatic fat synthesis gene expression, and accelerated fat-to-glycogen conversion. In addition, P. pentosaceus LI05 exhibited an anti-inflammatory ability, reducing serum proinflammatory cytokine levels and increasing intestinal subepidermal CD4⁺ cell levels. Furthermore, administration of P. pentosaceus LI05 increased the antimicrobial ability and enhanced the liver detoxification function. These results indicate that as a probiotic, P. pentosaceus LI05 ameliorates the hampered immune response of GF animals and improves the metabolism of fat and toxic substances.

The role of the gut microbiome in graft fibrosis after pediatric liver transplantation

Liver transplantation (LT) is a life-saving option for children with end-stage liver disease. However, about 50% of patients develop graft fibrosis in 1 year after LT, with normal liver function. Graft fibrosis may progress to cirrhosis, resulting in graft dysfunction and ultimately the need for re-transplantation. Previous studies have identified various risk factors for the post-LT fibrogenesis, however, to date, neither of the factors seems to fully explain the cause of graft fibrosis. Recently, evidence has accumulated on the important role of the gut microbiome in outcomes after solid organ transplantation. As an altered microbiome is present in pediatric patients with end-stage liver diseases, we hypothesize that the persisting alterations in microbial composition or function contribute to the development of graft fibrosis, for example by bacteria translocation due to increased intestinal permeability, imbalanced bile acids metabolism, and/or decreased production of short-chain fatty acids (SCFAs). Subsequently, an immune response can be activated in the graft, together with the stimulation of fibrogenesis. Here we review current knowledge about the potential mechanisms by which alterations in microbial composition or function may lead to graft fibrosis in pediatric LT and we provide prospective views on the efficacy of gut microbiome manipulation as a therapeutic target to alleviate the graft fibrosis and to improve long-term survival after LT.

Early infection-induced natural antibody response

There remains to this day a great gap in understanding as to the role of B cells and their products-antibodies and cytokines-in mediating the protective response to Francisella tularensis, a Gram-negative coccobacillus belonging to the group of facultative intracellular bacterial pathogens. We previously have demonstrated that Francisella interacts directly with peritoneal B-1a cells. Here, we demonstrate that, as early as 12 h postinfection, germ-free mice infected with Francisella tularensis produce infection-induced antibody clones reacting with Francisella tularensis proteins having orthologs or analogs in eukaryotic cells. Production of some individual clones was limited in time and was influenced by virulence of the Francisella strain used. The phylogenetically stabilized defense mechanism can utilize these early infection-induced antibodies both to recognize components of the invading pathogens and to eliminate molecular residues of infection-damaged self cells.

Methods for Establishment and Maintenance of Germ-Free Rat Models

Numerous studies have demonstrated that the gut microbiota plays a vital role in human health and disease development. Although the number of studies on host–microbiota interactions have increased in recent years, the underlying pathogenesis of dysbiosis-related diseases are still largely unknown. Germ-free (GF) rodent models, with the animals housed in sterile isolators and completely free of microbiota, are useful tools to advance our understanding of host–microbiota relationship in vivo. Although protocols concerning the establishment and maintenance of GF mouse models have previously been reported, the establishment, maintenance and monitoring of GF rodents are labor-intensive, tedious and take experience and skills. The aim of our study was to establish a GF rat model for the following microbiota-related researches and provide an easy-to-use protocol for the establishment and maintenance of GF rat model in detail, including steps to set up the isolator, sterilize the flexible isolator bubble, import food, water and other supplies, and methods to acquire newborn GF rats, hand rearing of suckling GF rats and reproduction of GF offspring. During the hand feeding period, the body weight of suckling GF rats was weighed once a day to ascertain the amount of artificial milk was given. Based on our results, the body weight of suckling GF rats decreased 1 week after birth and then began to increase. Methods for verifying the quality of the model like assessing the sterile status of the rat colony are also described. Moreover, possible difficulties and challenges, especially during gavage, and suggestions to avoid contamination will be discussed. The protocol presented will facilitate the establishment of GF rat models and downstream microbiota-related researches.

Early cellular responses of germ-free and specific-pathogen-free mice to Francisella tularensis infection

Bacteria that are highly virulent, expressing high infectivity, and able to survive nebulization, pose great risk to the human population. One of these is Francisella tularensis, the etiological agent of tularemia. F. tularensis is a subject of intense scientific interest due to the fact that vaccines for its immunoprophylaxis in humans are not yet routinely available. One of the substantial obstacles in developing such vaccines is our insufficient knowledge of processes that initiate and regulate the expression of effective protective immunity against intracellular bacteria. Here, we present data documenting the different pattern of cellular behavior occurring in an environment unaffected by microbiota using the model of germ-free mice mono-associated with F. tularensis subsp. holarctica strain LVS in comparison with a classic specific-pathogen-free murine model during early stages of infection.

'Cyclical Bias' in Microbiome Research Revealed by A Portable Germ-Free Housing System Using Nested Isolation

Germ-Free (GF) research has required highly technical pressurized HEPA-ventilation anchored systems for decades. Herein, we validated a GF system that can be easily implemented and portable using Nested Isolation (NesTiso). GF-standards can be achieved housing mice in non-HEPA-static cages, which only need to be nested 'one-cage-inside-another' resembling 'Russian dolls'. After 2 years of monitoring ~100,000 GF-mouse-days, NesTiso showed mice can be maintained GF for life (>1.3 years), with low animal daily-contamination-probability risk (1 every 867 days), allowing the expansion of GF research with unprecedented freedom and mobility. At the cage level, with 23,360 GF cage-days, the probability of having a cage contamination in NesTiso cages opened in biosafety hoods was statistically identical to that of opening cages inside (the 'gold standard') multi-cage pressurized GF isolators. When validating the benefits of using NesTiso in mouse microbiome research, our experiments unexpectedly revealed that the mouse fecal microbiota composition within the 'bedding material' of conventional SPF-cages suffers cyclical selection bias as moist/feces/diet/organic content ('soiledness') increases over time (e.g., favoring microbiome abundances of Bacillales, Burkholderiales, Pseudomonadales; and cultivable Enterococcus faecalis over Lactobacillus murinus and Escherichia coli), which in turn cyclically influences the gut microbiome dynamics of caged mice. Culture 'co-streaking' assays showed that cohoused mice exhibiting different fecal microbiota/hemolytic profiles in clean bedding (high-within-cage individual diversity) 'cyclically and transiently appear identical' (less diverse) as bedding soiledness increases, and recurs. Strategies are proposed to minimize this novel functional form of cyclical bedding-dependent microbiome selection bias.

Innate Immune Recognition: Implications for the Interaction of Francisella tularensis with the Host Immune System

The intracellular bacterial pathogen Francisella tularensis causes serious infectious disease in humans and animals. Moreover, F. tularensis, a highly infectious pathogen, poses a major concern for the public as a bacterium classified under Category A of bioterrorism agents. Unfortunately, research has so far failed to develop effective vaccines, due in part to the fact that the pathogenesis of intracellular bacteria is not fully understood and in part to gaps in our understanding of innate immune recognition processes leading to the induction of adaptive immune response. Recent evidence supports the concept that immune response to external stimuli in the form of bacteria is guided by the primary interaction of the bacterium with the host cell. Based on data from different Francisella models, we present here the basic paradigms of the emerging innate immune recognition concept. According to this concept, the type of cell and its receptor(s) that initially interact with the target constitute the first signaling window; the signals produced in the course of primary interaction of the target with a reacting cell act in a paracrine manner; and the innate immune recognition process as a whole consists in a series of signaling windows modulating adaptive immune response. Finally, the host, in the strict sense, is the interacting cell.