**2. Modulation of innate immunity by probiotics**

Innate immunity is one of the major arms in our immune system and consists of a complex complement cascade that acts as a physical and chemical barrier. It works to protect against infectious agents by recognizing conserved features of pathogens that become quickly activated to help destroy microbial invaders and to produce factors such as cytokines to activate adaptive immune response.

The most recognized innate mechanism comes from the concept of a barrier. This intrinsic wall helps evade foreign microbe penetration and prevents all the deleterious effects of colonization. The three major components that have been studied in barrier protection are mucin production, reinforcement of tight junctions in the epithelial layer, and enzyme regulation. Mucin production made by epithelial cells helps deter pathogen attachment. The permeable gel-like layer offers innate immunity by helping release secretory IgA, which prevents invading pathogen adherence. Additionally, the mucin layer helps identify self with nonself and can activate the immune system against invaders. Pathogen-associated molecular patterns are embedded in commensal microbiome organisms and these are recognized by Toll-like receptors to be noninvasive microbiota. The mechanisms of mucin for barrier enforcement are well known but studies now are starting to show how probiotics may help with boosting this barrier production [4]. *In vivo* rat studies have shown that administration of a specific probiotic mixture named VSL#3, which included strains of *Lactobacilli*, *Bifidobacteria*, and *Streptococci*, was associated with increased mucin gene expression and secretion. Increased mucin production theoretically leads to a more robust barrier [5]. The third barrier mechanism includes enzyme activity modification. Foreign microbes can also invade by activating destructive enzymatic processes. Pathogens secrete enzymes like Β-glucuronidase that result in toxic metabolites and can be pre-carcinogenic in the intestines. *Bifidobacterium longum* is a probiotic that was used in animal studies and showed a decrease in fecal B-glucuronidase activity and abnormal intestinal crypt structure by about 53% [6].

Tight junctions between epithelial cells help create a firm seal and prevent invasion. *Lactiplantibacillus plantarum* WCFS1, a probiotic, was noted to increase proteins like zonula occludens-1 (ZO-1) and transmembrane occludins near tight junctions, which help promote a good seal and ensure integrity of the epithelial barrier [7]. Another probiotic, *Lactobacillus rhamnosus*, was used to pretreat intestinal epithelial cells of pigs and was later exposed to enterotoxigenic *Escherichia coli* (ETEC). Pretreated cells have less TNF-α inflammatory response, higher ZO-1/occludin levels, and helped deter pathogenic adhesion to the epithelium. TNF-α activity correlates with an inflammatory cytokine response, which can lead to cell injury, so a subdued response helps taper these detrimental events [8]. In addition to the formation of a strong barrier, the components of the epithelial layer also add to its fortitude. Basolateral cells in the intestine have B cells that secrete sIgA, a secretory IgA transporter, which helps build a robust innate immunity. This secretory immunoglobulin works through a process called immune exclusion, which is when sIgA recognizes surface molecules on pathogens and prevents adherence. Probiotics like *Bifidobacterium* 

### *Translation of Immunomodulatory Effects of Probiotics into Clinical Practice DOI: http://dx.doi.org/10.5772/intechopen.109864*

*breve, L. rhamnosus*, and *Lactobacillus casei* have shown to increase the sIgA production and thus prevent colonization [9, 10].

Competitive exclusion is another important innate mechanism used to prevent pathogenic growth. The general concept here is that one microbe outcompetes, through various mechanisms, another and dominates the microbiome. Probiotics take advantage of this principle by creating toxic environments, competitively taking over resources, and producing antimicrobial bacteriocins to overtake pathogens [9].

Another key area of probiotic function comes from the cytokine cascade that leads to immune activity. Several examples exist but to understand their function, a brief review of immune cells and cytokines will help showcase the various mechanisms. Natural killer (NK) cells are lymphocytes that work to kill foreign pathogens with their cytotoxic proteases. Monocytes include macrophages and dendritic cells, which work by phagocytosis and present antigens to adaptive immune cells, respectively. IL-10 and IL-4 are anti-inflammatory interleukins that can prevent cell damage [11]. A plethora of studies exists to showcase how particular strains evoke a complex cytokine pathway. Daily consumption of *Lactobacillus salivarius*, a probiotic from breast milk, increased production of natural killer cells, monocytes, immunoglobulins, and IL-10 [12]. The SETOPROB study showed that probiotics like *L. rhamnosus*, *L. casei*, and *B. breve* increased IL-4, IL-10 and fecal secretory IgA. These cytokine and cell activations lead to downregulation of inflammation and prompt the activation of the adaptive immune system [9].

Finally, probiotics help maintain homeostasis by way of pathogen recognition and T cell regulation. Pattern recognition receptors (PRRs) bind to pathogen-associated molecular patterns (PAMPs), or damage-associated molecular patterns (DAMPs), which are expressed on most pathogens. PRRs are made up of toll-like receptors (TLRs) and NOD-like receptors (NODLRs), which function to activate immune activation and protect the cytoplasm. Additionally, TLR activation by PAMPs or DAMPs on monocytes triggers T cell activation and naïve T cells are prompted to differentiate. Activation of TLRs and NODLRs prompts cytokine cascade activation and the resulting inflammation could facilitate cell damage. Probiotics, however, regulate nuclear factor-κB (NFκB) and dampen the inflammatory response [9, 13–15].

The innate immunity is the body's initial defense mechanism and is made up of a variety of pathways to fortify the barriers and activate immune cascades. Probiotics assist in this pathway in many ways as outlined above. This initial response lends itself to initiate the acquired immunity discussed below, which goes on to form a more longlasting immune response.
