Gnotobiological models and methods in immunology

Immune activation by microbiome shapes the colon mucosa: Comparison between healthy rat mucosa under conventional and germ-free conditions

Germ-free animals (GF) are those without a microbiome since birth. This particular biological model has become one of special interest with the growing evidence of importance of the microbiome in the life, development, adaptation, and immunity of humans and animals in the environments in which they live. Anatomical differences observed in GF compared with conventionally-reared animals (CV) has given rise to the question of the influence of commensal microflora on the development of structure and function (even immunological) of the bowel. Only recently, thanks to achievements in microscopy and associated methods, structural differences can be better evaluated and put in perspective with the immunological characteristics of GF vs. CV animals. This study, using a GF rat model, describes for the first time the possible influence that the presence of commensal microflora, continuously stimulating mucosal immunity, has on the collagen scaffold organization of the colon mucosa. Significant differences were found between CV and GF mucosa structure with higher complexity in the CV rats associated to a more activated immune environment. The immunological data suggest that, in response to the presence of a microbiome, an effective homeostatic regulation in developed by the CV rats in healthy conditions to avoid inflammation and maintain cytokine levels near the spontaneous production found in the GF animals. The results indicated that collagen scaffold adapted to the immune microenvironment; therefore, it is apparent that the microbiome was able to condition the structure of the colon mucosa.

Early development of immune system in pigs

Prenatal and early postnatal immune system development has been studied in minipigs. First leukocytes were observed in the yolk sac and fetal liver (FL) on the 17th day of gestation, the majority of them being SWC3(+). The colonization of the thymus (TH) with leukocytes was observed 21 days later. Two waves of fetal TH colonization with pro-T cells were deduced from the frequency of thymocyte subsets. Thymic B cells and immunoglobulin-secreting cells (Ig-SC) were studied by flow cytometry and ELISPOT, respectively. When the total numbers of fetal Ig-SC were compared, the TH was identified as the main source of natural antibodies and the only site of IgA and IgG synthesis. In germ-free animals, the TH also represented the major site of IgG and IgA production and the number of Ig-SC was not influenced by colonization with microflora. FL and bone marrow were identified as primary B lymphopoietic sites. The phenotype of B precursors was characterized and pre-B II cells were shown to be the dominant mononuclear fraction between DG50 and DG105. In the periphery, relative proportions of lymphocyte subsets were determined. Studies in gnotobiotic piglets have revealed that the appearance of CD4(+)CD8(+) T cells and CD2(-) B cells is absolutely dependent on the contact of immune system with live viruses and bacteria, respectively.

Autoimmunity, immunodeficiency and mucosal infections: chronic intestinal inflammation as a sensitive indicator of immunoregulatory defects in response to normal luminal microflora

Despite the fact that target antigens and the genetic basis of several autoimmune diseases are now better understood, the initial events leading to a loss of tolerance towards self-components remain unknown. One of the most attractive explanations for autoimmune phenomena involves various infections as possible natural events capable of initiating the process in genetically predisposed individuals. The most accepted explanation of how infection causes autoimmunity is based on the concept of "molecular mimicry" (similarity between the epitopes of an autoantigen and the epitopes in the environmental antigen). Infectious stimuli may also participate in the development of autoimmunity by inducing an increased expression of stress proteins (hsp), chaperones and transplantation antigens, which leads to abnormal processing and presentation of self antigens. Superantigens are considered to be one of the most effective bacterial components to induce inflammatory reactions and to take part in the development and course of autoimmune mechanisms. It has long been known that defects in the host defense mechanism render the individual susceptible to infections caused by certain microorganisms. Impaired exclusion of microbial antigens can lead to chronic immunological activation which can affect the tolerance to self components. Defects in certain components of the immune system are associated with a higher risk of a development of autoimmune disease. The use of animal models for the studies of human diseases with immunological pathogenesis has provided new insights into the influence of immunoregulatory factors and the lymphocyte subsets involved in the development of disease. One of the most striking conclusion arising from work with genetically engineered immunodeficient mouse models is the existence of a high level of redundancy of the components of the immune system. However, when genes encoding molecules involved in T cell immunoregulatory functions are deleted, spontaneous chronic inflammation of the gut mucosa (similar to human inflammatory bowel disease) develops. Surprisingly, when such immunocompromised animals were placed into germfree environment, intestinal inflammation did not develop. Impairment of the mucosal immune response to the normal bacterial flora has been proposed to play a crucial role in the pathogenesis of chronic intestinal inflammation. The use of immunodeficient models colonized with defined microflora for the analysis of immune reactivity will shed light on the mode of action of different immunologically important molecules responsible for the delicate balance between luminal commensals, nonspecific and specific components of the mucosal immune system.

Commensal enteric bacteria engender a self-limiting humoral mucosal immune response while permanently colonizing the gut

We have employed a germfree mouse model to study the development and persistence of a humoral mucosal immune response to a gram-negative murine commensal organism, Morganella morganii. M. morganii bacteria rapidly colonize the gut, resulting in hypertrophy of Peyer's patches (PP), including germinal center reactions (GCR), and the development of specific immunoglobulin A (IgA) responses detected in vitro in PP fragment cultures and by ELISPOT assays of lamina propria cells. The GCR peaks 14 days after infection and begins to wane thereafter. Upon colonization, the organisms successfully translocate to the mesenteric lymph node and spleen, but the number of translocating bacteria begins to drop with the onset of a specific IgA response. A clonal B-cell microculture technique was used to determine the frequency of specific IgA plasmablasts and IgA memory cells. The frequencies of preplasmablasts were seen to be higher in the earlier stages of germinal center development, whereas the frequencies of antigen-specific memory cells appeared to remain at a relatively constant level even after 193 days postmonoassociation. We suggest that a successful secretory IgA response can attenuate chronic stimulation of GCR even though the bacteria persist in the gut. The observed developing hyporesponsiveness to a chronically present commensal organism may be relevant to the use of bacterial vectors for mucosal immunization.

The minipig as a model in gnotobiology

Due to its close physiological and morphological resemblance to man, the pig occupies an important position in biomedical research. The minipig lends itself particularly well to germfree research. It is easy to deliver, maintain, and utilize in isolators. The colostrum-free, newborn germfree piglet lacking passively obtained antibodies is of an exclusive use if distinguishing the factors of natural and adaptive immunity is needed. It fits also well in studies investigating the earliest mechanisms of antibody synthesis. Under germfree conditions, the pig has been proved an excellent model for testing the action of ionizing radiation as well as of selected bacterial strains when monoassociated. Recently, conventional pigs have been used also for studies in total or selective decontamination of the intestinal tract by antibiotics and strict isolation.

The use of gnotobiological models for the studies of immune mechanisms

Germfree (GF) animals are widely used for discrimination of natural mechanisms (genetically determined) from adaptive mechanisms of immunity induced by external antigens. GF rats fed at least for first 5 days after birth with artificial diet LNa, deficient in unsaturated fatty acids, were in contrast to those fed with K 50 diet deeply suppressed in humoral and cellular immune reactivities during their life. GF piglets, deprived from maternal colostrum (CF) as a source of passively transferred maternal antibody, immunized with preformed Ag/Ab complexes (HSA anti HSA) responded according to the composition of complexes (Ab excess, equivalence, Ag excess) by low or high dose tolerance or by secondary response. Similar pictures of response were obtained in groups of presensitized GF-CF piglets by immunization with different doses of antigen. It is concluded that 3 different types of response are the result of Ag/Ab complexes acting or formed on the lymphocyte membrane and exerting stimulatory or down-regulatory signals.

Selective decontamination, induced colonization resistance and connected immunological changes in piglets

14-d-old conventional piglets were picked from normal piggery, washed with disinfectants, placed into isolators suitable for germfree work, fed a sterile diet and treated with peroral antibiotics (nalidixic acid, kanamycin, and nystatin). Beginning with day 5 or 7, Enterobacteriaceae were not found in feces. The absence of these bacteria was proved by inoculation of germfree newborn piglets with caecal content. In selectively decontaminated piglets, the white blood cell count in blood had fallen to 6 X 10(9)/L; this decrease was due to an extremely low number of granulocytes (to 0.8 X 10(9)/L). On day 35, IgG-positive cells almost disappeared from the spleen, whereas IgA cells were found in an unusually great amount. Corresponding changes in serum levels were established. The colonization resistance effect in Enterobacteriaceae-deprived piglets was confirmed; settling of introduced various E. coli strains did not occur or was delayed.

Lysosomal enzyme activities in polymorphonuclear leukocytes, macrophages, serum, and spleen of conventional, germ-free, and antigen-free Minnesota miniature swine

The activities of lysozyme, myeloperoxidase, and elastase were lower in PMNs and AMs from GF and AF Minnesota miniature piglets than in the leukocytes from their CONV counterparts. In the spleen and serum of gnotobiotic piglets only the levels of lysozyme were slightly reduced. Substantially depressed activities of these LEs were found also in PMNs from precolostral piglets in comparison with PMNs from their CONV mother. The bisassociation of GF piglets with Enterococcus liquefaciens and Escherichia coli caused an increase of LE activities in their AMs, spleens, and sera. Fewer LEs were released after phygocytic stimulation with zymosan from PMNs of GF, AF, and precolostral piglets than from PMNs of CONV animals of the same age. These data suggest that the antigenic-microbial stimulation is important for the development of normal lysosomal enzyme activities in PMNs and AMs from gnotobiotic animals.

Immunobiology of germfree mice infected with Nocardia asteroides

Nocardia asteroides GUH-2 was administered either intranasally or by intravenous inoculation into (i) conventionally grown, (ii) germfree, and (iii) lipopolysaccharide-treated germfree NIH:S mice. The number of bacteria within the lungs, brain, kidneys, adrenals, liver, spleen, and blood was quantitated at 3, 24, 72, and 168 h after infection. Further, the histological changes that occurred in each of these organs after infection were studied. The data demonstrated that germfree mice were significantly more susceptible to the acute phase of infection caused by N. asteroides than the conventionally grown animals. The brains and lungs of these mice were affected most dramatically. Pretreatment of the germfree mice with lipopolysaccharide completely reversed this enhanced susceptibility and rendered the animals more resistant to infection than the conventionally grown mice. These observations establish further the role of macrophage activation and the development of cell-mediated immunity in host resistance to nocardia. In addition, the presence of a resident microflora within the host appears to be important in the development of resistance to systemic nocardial infections.