Nestle Purina, Checkerboard Square, St. Louis, MO
The following article was condensed from an article by the same title, presented at the 2008 Nestle Purina Nutrition Forum, Oct 4, 2008 in St. Louis, MO
Nutritional immunology is an emerging field of study that deals with the impact of dietary components on immune function. Beyond providing essential nutrients, diet can actively influence the immune system. This is because over 65% of the immune cells in the body are located in the gut, making the gut the largest immune organ. The immune receptors of the innate immune system present in the gut are the primary targets of strategies for immunomodulation via diet.
Unlike immunodeficiency caused by malnutrition, immunodeficiencies that are age related [life-stage] or caused because of stress or dietary overindulgence need a more comprehensive strategy and cannot be addressed simply by correcting nutritional deficiencies. While macro- & micro-nutrient deficiencies due to changes in nutrient absorption & reduced food intake do contribute, immunodeficiencies that are related to age, stress and or dietary overindulgence can be more difficult to evaluate, understand and manage. More importantly, a practicing clinician has a greater likelihood of encountering immunodeficiency of the latter kind [immunodeficiency not related to malnutrition]. Immunodeficiency, irrespective of its etiology can severely undermine the health of an animal, triggering debilitating diseases such as infections, malignancies as well as autoimmune diseases.
Why enhance immune status in a healthy animal?
Age and stress are two very important factors that significantly impact the immune status of an animal. The immune response of a neonate and or an older animal tends to be less vigorous than in young adults, making them more susceptible to infection [Morein, B, 2002]. Aging and obesity are characterized by low level chronic inflammation and oxidative stress that contribute to the declining ability of the immune system to respond and self-regulate [Ungvari Z et al, 2008; Neels J G and Olefsky J M, 2006]. This chronic inflammation is believed to be one of the underlying causes for a variety of diseases such as type II diabetes, cancers, autoimmunity and declining neurological health along with a greater susceptibility to infection [Ungvari Z et al, 2008].
With age the immune system looses its plasticity, leading the organism to be a 'low responder'. Immune plasticity is the ability of the immune system to remodel itself to promptly respond to harmful stimuli and return to a quiescent state once the danger passes. The reduced plasticity associated with old age also results in reduced immune response to oxidative stress and a lower capacity in DNA-repair, leading to a condition described as 'immunosenescence', which increases the risk of age-related diseases i.e. cancer and infection [Pawelec G, et al, 1997; Makinodan, T, 1995].
Stress, especially chronic stress, can have a significant negative impact on the immune system. Both major and minor stressful events have been shown to have a profound influence on immune responses in animal and human studies. From a mechanistic stand point, stress appears to delay inflammation by reducing efficiency of immune surveillance by phagocytes [Kehrli ME, et al, 1996], decrease antigen presentation efficiency by down-regulating MHC class II molecule expression on antigen presenting cells (APCs), and delay or impair immune responses to vaccination. [Glaser, R. et al 1992; Glaser, R. et al 2000, Vedhara, K. et al 1999; Jabaaij, L. et al 1996] Among medical students taking examinations, level of stress had a negative correlation with response to hepatitis B vaccine, while the degree of social support had a positive correlation to vaccine response [Glaser, R. et al 1992]. Vaccine responses demonstrate clinically relevant alterations in an immunological response to challenge under well controlled conditions and, therefore, can be used as a surrogate for responses to an infectious challenge. Adults who show poorer responses to vaccines also experience higher rates of clinical illness, as well as longer-lasting infectious episodes [Burns, E.A. and Goodwin, J.S, 1990; Patriarca, P.A, 1994].
Age and stress can undermine the immune status in an otherwise healthy animal, triggering debilitating diseases such as infections, malignancies or autoimmune diseases. Hence optimizing immune status can significantly enhance the quality of life.
How Can Diet Influence the Immune System?
Gut is the Largest Immune Organ
Besides being the gateway for nutrient intake, the gut is the largest immune organ with over 65% of all the immune cells in the body and with over 90% of all IgG producing cells [Bengmark S, 1999; Brandtzaeg P, et al, 1989]. Immune stimuli from environmental antigens, especially microbiota in the gut, are crucial for development of a healthy immune system [Cebra JJ, 1999]. Germ free animals tend to have a very underdeveloped immune system, underscoring the role played by symbiotic microflora and associated environmental antigens. The GALT [Gut Associated Lymphoid Tissue] is unique in its ability to be exposed to a diverse array of antigens from the foods and from the over 1000 species of commensal microorganisms and yet remain quiescent until it encounters a threat, such as a pathogen. This response is initiated by molecules called PAMPs (Pathogen Associated Molecular Patterns) expressed by microbial pathogens and recognized by cells within the GALT system. PAMPs are highly conserved motifs present in microorganism, and include such triggers as lipopolysaccharides (LPS) from the gram-negative cell wall, lipoteichoic acids from the gram-positive cell wall, the sugar mannose, N-formylmethionine found in bacterial proteins, double-stranded RNA from viruses, and glucans from fungal cell walls.
An appropriate innate immune response sets the stage that directs the course of the adaptive immune response, in part via antigen presenting cells (APCs) and cytokines released from APCs. APC function is central to the altered immune response that is characteristic of the neonatal immune system, decreased immune response of an aging immune system and immune response during stress. In all three cases, due to the lack of inflammatory cytokines such as IL-1, IL-12, APCs responding to an immune challenge are not able to efficiently upregulate MHC Class II molecules or co-stimulatory molecules such as CD86 [Murtaugh MP, Foss DL 2002]. Lack of these cytokine signals also modifies the adaptive immune response reducing its efficiency and giving it a Th2 bias [Murtaugh MP, Foss DL 2002]. The resulting immune response tends to be not as efficient.
The immune receptors of the innate immune system present in the gut serve as the primary targets of strategies for immunomodulation via diet. For example, toll-receptor agonists such as yeast β-glucans [Decuypere J, et al 1998], yeast mannans [Pietrella D et al, 2002], nucleic acids [Holen E, 2006] and probiotics [Savendra JM, 2007] are great examples of use of PAMPs as Immune Response Modifiers [IRMs]. These IRMs initiate cytokine secretion which activates local APCs to upregulate MHC Class II and co-stimulatory molecules, enabling them to present antigen efficiently to T lymphocytes and initiate an effective immune response.
The enhanced immune function induced by dietary IRMs can spread to the entire immune system. The concept of a common mucosal immune system is being proposed because of the trafficking of activated lymphocytes from induction to effector sites, with significant overlap with the non-mucosal immune system [Hannant D., 2002]. Thus, immune cells carry the messages from the GALT to the rest of the immune system.
Nutrition Interacts with the Immune System at Multiple Levels
Interaction between nutrition and the immune system takes place at multiple levels and for simplicity can be considered in a framework of four stages. Stages I and II are passive because they involve providing the immune system with essential nutrients to allow the immune system to function well. Stages III and IV are active, targeted approaches to immune modification. These stages focus on modifying the immune response using specific agents such as IRMs that primarily target the PAMP receptors in the gut.
Stage I: Complete Nutrition: At the primary level the focus revolves around dietary energy, protein, vitamins (Vitamin A, C and E) and trace minerals such as zinc and iron. Macrominerals, such as calcium and magnesium drive signaling mechanisms in the immune system and are therefore also important for enhanced immune response.
Stage II: Optimizing macro & micro nutrients: The second stage involves optimizing key nutrients that are critical for the immune cells. The immune system has a need for certain nutrients and providing greater amounts of these key nutrients may target the immune cells better. A temporary deficiency of a key nutrient can negatively impact the immune system. For example, during strenuous exercise, muscle cells preferentially use glutamine as their energy source and as a result there is a reduction of glutamine levels in circulation. Glutamine is also the preferred energy source for immune cells and following strenuous exercise because of low levels of glutamine in circulation, immune cells cannot function efficiently if challenged, making these athletes vulnerable to infections immediately after vigorous bouts of exercise [Gleeson M, et al, 2004].
Another key nutrient needed for a healthy immune system is protein. At a molecular level, proteins make up the structural components and mediate key process of the immune system. Receptors, cytokines, immunoglobulin's, complement components, etc. are all proteins. Dietary protein at a level that maximizes protein turnover optimizes immune function [Scrimshaw N S and SanGiovanni J P, 1997].
Providing nutrients to address oxidative stress and subsequent damage to cellular DNA is another example of the optimizing nutrients for immune function. Aging, along with other environmental stressors, tends to increase the levels of oxidative damage to cellular DNA, including immune cells. Oxidative DNA damage due to free radicals produced during cellular metabolism is one of the primary causes of cell death (Cooke MS et al 2003). Increased apoptosis can break immune tolerance to self-antigens resulting in autoimmunity (Cline AM & Radic MZ, 2004). Immunosenescence is characterized by decreased response to mitogens, decreased cytokine production and changes in signal transduction. (reviewed in Diet and Human Immune Function; Ed Hughes D A, Karlington G L and Benidich, A; 2003, Humana Press; Greeley, E. H. et al 2001). Various dietary strategies can help address senescence, tissue damage and apoptosis associated with aging, including:
1. Caloric restriction (CR): Apart from increasing the life span (Barger J et al 2003; Kealy 2002), data from laboratory animals have demonstrated that CR reduces immunosenescence (Pahlavani MA, 2004). Recent data from a CR study conducted in Labrador retriever dogs clearly shows that CR can help retard immunosenescence (Greeley E. et al 2006). Feeding pets to maintain a lean body condition is likely to help aging animals maintain a healthier immune system.
2. Antioxidants: Increased levels of antioxidants such as Vitamin C (Anderson R et al 1990), Vitamin E (Diet and Human Immune Function; Ed Hughes D A, Karlington G L and Benidich, A; 2003, Humana Press) and carotenoids (β-carotene, α-carotene, lycopene, astaxanthin, etc) may help prevent damage mediated by these free radicals. There are a number of reports documenting the benefits of carotenoids in dogs, particularly in older animals (Massimino S et al 2003; Kim HW et 2000).
3. Pre-biotics support or help maintain normal gut flora. Intestinal microflora play an important role in keeping the immune system primed to prevent colonization by pathogenic microbes. However under certain conditions, such as following antibiotic therapy, GI infections, various stressors, the normal flora in the GI is perturbed and may lead to an overgrowth of harmful bacteria. Prebiotics such as inulin help the animal maintain a healthy commensal population in the gut (Flickinger EA & Fahey GC Jr. 2002).
Stage III Active modulation of the immune system: In stage three, the emphasis is to actively interact with the immune system attempting to modulate its function towards a desired goal. Some examples include:
1. Induction of a Th1 bias & enabling efficient antigen presentation: A Th1 (pro-inflammatory) response is important for protection against microbial infections. Th1 component of the immune system is boosted by stimulating the immune system with probiotic bacteria or PAMP expressing moieties (e. g yeast β-glucans). Probiotics (Enterococcus faceum SF68, Lactobacilli sp., Bifidobacteria sp. etc) in diet have been shown to enhance markers of immune status in dogs (Benyacoub J et al 2003). Milk bioactives from bovine colostrum have immune enhancing effects in both human and murine studies and is an interesting IRM. Colostrum [and whey protein which has a similar composition] contains immunoglobulins, cytokines, lactoferrin, and lactoperoxidase, each of which can influence the immune system [Artym J, et al, 2003]. In multiple studies, mice fed milk bioactives produced significantly higher serum and intestinal antibodies to several antigens and had enhanced resistance to pneumococcal infection [Lowe PPL et al, 2003; Bounous G & Kongshavn PK, 1989]. In a human study conducted in highly trained cyclist, low-dose bovine colostral protein concentrate supplementation favorably modulated immune parameters during normal training and after an acute period of intense exercise, which contributed to lowering of the incidence of upper respiratory illness [Cecilia MS et al, 2007] We evaluated the immune enhancing effect of bovine colostrum in adult dogs during exercise. [Satyaraj R & Reynolds A, unpublished data 2008] Our results demonstrate that adding bovine colostrum significantly enhanced their immune status as measured by the response to canine distemper vaccine as well as increased level of GALT activity.
2. Managing inflammation to prevent further damage: Chronic inflammation is central to the pathophysiology of a number of diseases, including cardiovascular diseases, diabetes mellitus, arthritis, and neurological diseases (Alzheimer's, impaired cognition) (Casserly I & Topol E 2004). Physiologically the effects of inflammation are mediated by prostaglandins and leukotrienes, both end products of the arachidonic acid metabolism. A diet rich in DHA and EPA, both omega-3-fatty acids, can control the damaging effects of inflammation, because of the reduction in the levels of active prostaglandins and leukotrienes.
3. Disease specific modifications: Induction of a local Th2 bias in animals with inflammatory bowel disease using dietary means is an example of a targeted approach to immunomodulation. Probiotic microbes have been characterized based on the cytokines responses they induce. Certain bacteria induce secretion of anti-inflammatory cytokines such as IL-10, TGF-β and IL-13 (Ma D et al 2004). These probiotic agents provide an opportunity to help animals suffering with inflammatory bowel diseases. Similarly, TGF-β rich ingredients such as colostrum and whey proteins are being increasingly used to effectively address localized inflammatory conditions in the gut.
Stage IV "Personalized nutrition: predictive, preventive and personalized nutrition: Interaction between diet, environment and genome ultimately defines health status and can be critical in influencing chronic disease (Ames, BN &. Gold LS, 1998; Ames BN. 2001; Kaput, J 1994; Milner JA 2004). Over the last few decades the science of pharmacogenomics, which deals with the genetic basis underlying disease susceptibility and variable drug response in individuals, has brought about a paradigm shift in the pharmaceutical industry by moving it from a "one drug fits all" towards personalized therapy. This process has been greatly accelerated by advances in the 'omics fields: single nucleotide polymorphisms analysis, transcriptomics (cDNA analysis), proteomics and metabolomics.
The concept of "personalized medicine" has been expanded into nutrition. Although "personalized nutrition" is still in its infancy, it is practiced in principle in the dietary management of diabetes or maintaining a healthy lipid profile to manage risk of cardiovascular disease. For a practical personalized diet strategy, there are two basic requirements: A clear understanding of the disease pathogenesis and the availability of reliable disease biomarkers to identify either susceptibility or diagnose disease. Biomarkers are an objectively measured characteristic that is an indicator of normal biological processes, pathological processes, or pharmacological responses to a therapeutic intervention. The ultimate goal is to modify physiology through this "personalized" dietary regimen before the animal enters into the disease continuum, preventing, delaying or managing disease and thereby enhancing quality of life.
In summary, as research advances in understanding complex physiological networks in health and disease, the role played by the immune system and its interaction with diet takes a whole new meaning. As our understanding of the relationship between nutrition and the immune system deepens, a vast array of diet based options to address immune needs will become available. The food we eat and feed our pets can clearly deliver several other benefits beyond basic nutrition and therein lays the promise of immunonutrition.
Antigen Presenting Cell
Gut Associated Lymphoid Tissue
Immune Response Modifiers
Pathogen Associated Molecular Pattern
T Helper Type 1
T Helper Type 2
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