**3. Gut's role in immune function**

The contributions of the gut microbiota to the development of the immune system have been extensively characterized. One of these characterizations suggests that the host is able to tolerate the large amount of antigens present in the gut [27], owing to the coordinated cross talk between the gut microbiota and the immune system.

Progress in the current knowledge on biodiversity of the intestinal microbiota allows us to understand the mechanisms of how different microorganisms affect the function of the body and the impact of these mechanisms. Altered microflora (dysbiosis) is generally associated with gastrointestinal disorders, but rather a microbial imbalance is associated with common diseases, which are not limited to the gastrointestinal tract [28, 29]. Studies in germ-free (GF) mice give much of the evidence about how microflora forms immune system as GF mice are completely lacking microflora. These mice show profound immune defects, such as fewer and smaller Peyer plaques and mesenteric lymph nodes and also reduced B and T cell immune response [30-32]. Therefore, the serum immunoglobulin (Ig) G and IgA levels in the gut GF mice were reduced [33, 34]. Moreover, many studies in mice and humans show that certain inflammatory diseases are associated with an altered microflora [9, 35, 36].

Inflammatory diseases are often caused by microbial dysbiosis. It was found in a prospective study of children with a high risk of developing asthma that changes in the microbiota occur before the development of the disease [37]. Regulation of immune responses requires certain species of gut commensal microbiota and perturbations in the microbiota could lead to a lack of immune regulation, the outgrowth of more pathogenic microbes, and the promotion of inflammation. The microbial composition of the microbiota in the adult human gut is mainly determined by the microorganism to which the newborn child is exposed during the first years of life. Strategies to manipulate the microbiota during infancy have been shown to prevent development of allergic and atopic diseases later in adult life [38-40]. Thus, the use of probiotics and prebiotics during the early postnatal period has been proposed for the intentional modulation of the microbiota composition. In addition, diet and exposure to microbes during pregnancy may affect the metabolic and immunological profiles of the pregnant uterus and the risk of developing the disease in the offspring [41]. The application of probiotics and prebiotics during pregnancy has been also proposed [42-44]. Differences in this composition are related lo colonization; host factors, such as sex and age; genetic factors; and health state. The dynamic state of the microbe's ecology is increasingly being associated with an expanding number of disorders. New high throughput methodologies such as metagenomics, transcrip‐ tomics, proteomics, and metabolomics in the post-genomic era have greatly helped the understanding of the mechanisms, by which the microbiota contributes to host physiology in healthy and ill individuals. Metagenomic studies of the human gut microbiota, for example, have suggested that host metabolism is affected by low bacterial diversity, which is also related to obesity and other diseases [45, 46J.

*Enterococcus* species and certain yeast strains. Numerous other LAB have shown probiotic potential in animal studies. For the treatment of IBD, several probiotics have been shown to be efficacious: *Lactobacillus casei, Lactobacillus plantarum, Lactobacillus bulgaricus,* and *Lactoba‐ cillus acidophilus*; three strains of *Bifidobacteria* and *S. thermophilus*. In recent years, evidence has accumulated that probiotic strains can exhibit the same activities as commensal bacteria,

Lactic acid bacteria are present in many feeds such as yogurt and are frequently used as probiotics to improve some biological functions of the host. Beneficial effects of the lactobacilli on the body have been identified in the treatment or prevention of acute viral gastroenteritis, after antibiotic-associated diarrhea, certain pediatric allergic diseases, necrotizing enterocoli‐ tis, and inflammatory bowel disease such as Crohn's disease and postoperative hernias. Probiotics have been long reported to aid in the treatment of many dysfunctions of the GI tract, and the mechanisms by which probiotics work have recently been elucidated. There are experimental and clinical data [18-24]. Probiotics are described as useful also in combating oxidative stress, improvement in mucosal immunity [25], and general immunity [26]. The desirable changes of the intestinal microbiota were achieved as yogurt was able to attenuate the symptoms of acute inflammation by reducing inflammatory cytokines and increasing regulatory cytokine IL-10-producing cells. The use of murine models demonstrated that the consumption of fermented milks can modulate the immune system and can maintain it in a state of surveillance, which could affront different pathologies such as cancer and intestinal

The contributions of the gut microbiota to the development of the immune system have been extensively characterized. One of these characterizations suggests that the host is able to tolerate the large amount of antigens present in the gut [27], owing to the coordinated cross

Progress in the current knowledge on biodiversity of the intestinal microbiota allows us to understand the mechanisms of how different microorganisms affect the function of the body and the impact of these mechanisms. Altered microflora (dysbiosis) is generally associated with gastrointestinal disorders, but rather a microbial imbalance is associated with common diseases, which are not limited to the gastrointestinal tract [28, 29]. Studies in germ-free (GF) mice give much of the evidence about how microflora forms immune system as GF mice are completely lacking microflora. These mice show profound immune defects, such as fewer and smaller Peyer plaques and mesenteric lymph nodes and also reduced B and T cell immune response [30-32]. Therefore, the serum immunoglobulin (Ig) G and IgA levels in the gut GF mice were reduced [33, 34]. Moreover, many studies in mice and humans show that certain

Inflammatory diseases are often caused by microbial dysbiosis. It was found in a prospective study of children with a high risk of developing asthma that changes in the microbiota occur

inflammatory diseases are associated with an altered microflora [9, 35, 36].

including immunomodulation [5,11].

200 Immunopathology and Immunomodulation

inflammation on its part.

**3. Gut's role in immune function**

talk between the gut microbiota and the immune system.

#### **4. Probiotic modulation of the gastrointestinal mucosal immune system**

Perhaps one of the most important aspects of probiotic bacteria is the ability to modulate the host GIT mucosal immune system locally and systemically The interaction between the probiotic microbe with the resident microbiota, gastrointestinal epithelia and gut immune cells to produce an immunomodulatory response is quite complex, and has been reviewed exhaus‐ tively [34, 47- 49].

The expression of cytokines and chemokine genes was carried out by activated nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinase signalling cascades, mediated of MAMPs, PRRS (including NOD-like receptor, Toll-like receptors, and C-type lectin receptors). Lipoteichoic acids (DMA), peptidoglycan and S-layer proteins are mostly found MAMPs from probiotic microorganisms. [48, 50] [Figure 1].

Multiple studies have explored the immunomodulatory effect of these MAMPs using func‐ tional genomic techniques. Various studies have demonstrated a significant reduction in product ion of proinflammatory cytokines with a simultaneous increase in anti-inflammatory IL-10 and the down regulation of pro-inflammatory IL-12 and TJMF-H. [23, 51J. Microflora in the intestine promotes mucosal barrier function, and also improves the immunity of the host to enteric infection. During the active infection a cytokine normally produced is IL-1β, which is critical for neutrophil restoration and eradication of the pathogen. Microflora play a vital role in the production of homeostatic levels of pro-IL-1β in local intestinal macrophages. The

**Figure 1.** Probiotic modulation of the gastrointestinal mucosal immune system. While intestinal epithelial cells (IECs) exposed to pathogenic microbes or related stimuli produce proinflammatory mediators such as interleukin 8 (IL-8) and tumour necrosis factor a (TNF-a), probiotics suppress the production of these cytokines and instead induce anti-in‐ flammatory mediators such as transforming growth factor b (TGF-b) and thymic stromal lymphopoietin (TSLP), which can promote the differentiation of immature dendritic cells (iDCs) to regulatory dendritic cells (DCregs). Macrophages in the inflamed mucosa produce high amounts of IL-6, and probiotics can decrease their IL-6 production and increase IL-10 production.

gut microbiota can also enhance host immunity through MyD88-independent mechanisms (MyD88 – Myeloid differentiation primary response gene 88). Notably, colonization of GF mice by commensal bacteria induces development of Th-17 cells in the intestine, which is important for protection against *Citrobacter rodentium* infection [52].

#### **5. Immune cells**

Probiotics regulate host innate and adaptive immune responses by modulating the functions of dendritic cells, macrophages, and T and B lymphocytes. Probiotics regulate immunomo‐ dulalory functions through the activation of toll-like receptors, which is one of the mechanisms of regulation. Recent studies indicate that probiotics activate innate immunity by enhancing adaptive immune response [20, 53]. One of the proposed mechanisms is by activation of tolllike receptors.

Regulatory dendritic cells are the primary professional antigen presenting cells (APCs) modulating adaptive immune responses. Probiotics containing *L. acidophilus, L. casei, L. reuteri, E. bifidium,* and *Streptococcus thermophilus*, stimulate dendritic cells to produce IL-10, TGF-β, COX-2, and indoleamine 2,3-dioxygenase, which in turn increase the formation of CD4 Foxp3 regulatory T cells (Tregs) and the suppressor activity of naturally occurring CD4 CD25 Tregs. They also decrease responsiveness of T and B lymphocytes and the number of T helper (Th) 1, Th2, and Th17 cytokines without inducing apoptosis. This mixture suppressed 2,4,6trinitrobenzenesulfonic acid-induced intestinal inflammation, which was associated with enrichment of CD4 Foxp3 Tregs in the inflamed regions, as was found by in vivo studies. Thus, probiotics that enhance the generation of regulatory dendritic cells to induce Tregs, represent a potential therapeutic approach for inflammatory disorders [50, 54].

Nowadays, the exact mechanism of interaction between probiotic microorganisms and host cells remains elusive. Nevertheless, there is enough gathered information that microbiota in the gut could affect the immune response at both systemic and mucosal levels. Some putative mechanisms include: influence of the microflora itself, amelioration of membrane barrier function, and direct effects of probiotic microorganisms on different epithelial and immune cell types. Many patients with inflammatory bowel disease (IBD) use probiotics to manage this intestinal condition. Downregulation of production of proinflammatory cytokines and other inflammatory mediators seems to constitute important mechanisms for the partial ameliora‐ tion of colitis, seen with numerous LAB strains in various models. It must also be noted that TNF-α blocking agents are also quite successful in the treatment of patients with CD (Crohn's disease) [55]. However, it should be taken into account that different probiotic bacterial species and strains have various beneficial effects and therefore need to be selected in a more rational manner to treat human diseases.
