Regulatory T Cells, a Potent Immunoregulatory Target for CAM Researchers: Modulating Allergic and Infectious Disease Pathology (II)

Regulatory T (Treg) cells maintain dominant control of immune responses to foreign materials and microbes. Appropriate Treg cell suppression of immune responses is essential for the maintenance of efficacious defensive responses and the limitation of collateral tissue damage due to excess inflammation. Allergy and infection are well studied and frequent afflictions in which Treg cells play an essential role. As such, they provide excellent models to communicate the significance and relevance of Treg cells to complementary and alternative medicine (CAM).


Regulatory T Cells, a Potent Immunoregulatory
Regulatory T (T reg ) cells maintain dominant control of immune responses to foreign materials and microbes. Appropriate T reg cell suppression of immune responses is essential for the maintenance of efficacious defensive responses and the limitation of collateral tissue damage due to excess inflammation. Allergy and infection are well studied and frequent afflictions in which T reg cells play an essential role. As such, they provide excellent models to communicate the significance and relevance of T reg cells to complementary and alternative medicine (CAM).

T reg Ubiquity and Universality: Relations to Foreign Bodies
The major principles underlying the dynamic immune system, including constituents, interrelationships and feedback mechanisms have been established, with emphasis on regulatory T (T reg ) cells (1). The goal of this article is (i) to detail T reg cells, (ii) to delineate T reg cell function in allergy and infection and (iii) to examine the pathological consequences of aberrant T reg cell activity. Allergy and infection are wellstudied and frequent afflictions, and as such, they provide excellent models to communicate the significance and relevance of T reg cells to complementary and alternative medicine (CAM).
Considerable inflammation, resulting from allergic hypersensitivity or immune response to infection, has the potential to induce deleterious effects on an individual's tissues and overall well-being. Recent evidence has served to elucidate the mechanism of action and substantiate the usage of a veritable array of traditional herbs, folk medicines and other compounds found in nature, which have been employed to attenuate inflammatory complications. Of interest to practitioners, researchers and patients of CAM modalities are those compounds that maintain powerful immunomodulatory capacity via direct or indirect action on T reg cells (Table 1).
A cornucopia of herbal medicines has shown clinical effectiveness in the attenuation of allergic and infection-induced inflammatory pathology. Traditional therapeutics for allergic complications includes immunotherapy, antihistamines and glucocorticoids. A newly researched compound, termed antiasthma herbal medicine intervention (ASHMI), which is an extract of three herbs, has shown effectiveness and benefit over traditional treatment options for asthma. ASHMI mainly downregulates T H 2 cell responses, increases lung function through direct modulation of smooth muscle contraction and decreases peripheral blood eosinophils and serum IgE. ASHMI does not induce a state of general immunosuppression like steroids, such as prednisone, which suppress T H 1 and T H 2 cell responses.
Treatment of infection with antimicrobial drugs poses an array of complications, including resistance and physical ailments, e.g. diarrhea. Allium sativum, extracted from garlic, has the potential to bolster T H 1 cell-mediated responses to pathogens, such as Leishmania major. The proposed mechanism includes enhancement of T H 1 cytokine response, T lymphocyte proliferation and NK cell activity. This method of antimicrobial treatment offers benefits over pharmaceutical drugs; however, its efficacy and mechanism of action needs further research and elucidation. CAM benefits greatly from research done to determine a scientific basis for confident treatment decisions, as it lends credibility to CAM modalities and, most importantly, offers efficacious treatment options to a large segment of the population afflicted with allergic and infectious complications. Knowledge of the dynamic relationship between T reg cells and immune system responses to foreign antigens is essential in order to approach T reg cells as a clinical target for the alleviation of complications arising from allergy, asthma, dermatitis and infection.

T reg Subsets: Three of a Kind
The family greatly responsible for immunomodulation consists of two key subsets: naturally arising and peripherally induced T reg cells. Although the ontogenic relationship between the two is not well understood, both subsets have been characterized by distinct differentiation patterns and functions (19).

Naturally Occurring T reg Cells
Naturally occurring T reg (nT reg ) cells compose 5-10% of peripheral T cells, maintain a distinct lineage and develop in the thymus. nT reg cells are directed to largely aid in tolerance to self-antigen in the periphery via suppressive actions on both T H 1 and T H 2 cell-mediated immune responses, with greater specificity for the former (20)(21)(22)(23)(24).
nT reg cells inhibitory action is contact dependent. Interaction of CTLA-4 and TGF-b1, expressed on the cell surface, with respective ligands and receptors on target cells, triggers the downregulation of effector cell IL-2Ra receptors and a subsequent decrease in fitness (20,23,25). Alternate mechanisms proposed include the perforin-dependent induction of T cell apoptosis and the reduction of dendritic cell's ability to prime T cells, through direct suppression of cytokine secretion or tryptophan metabolism (2,15,(26)(27)(28).

Peripherally Induced T reg Cells
The adaptive T reg cell subset includes type 1 T reg (T r 1) cells and T helper 3 (T H 3) cells. T r 1 and T H 3 cells are implicated in immune responses to foreign antigens in the periphery, alongside nT reg cells (22).
While extrathymic generation of T reg cells is not as well understood as nT reg cell generation, T r 1 and T H 3 cells are derived from naïve T cells in an environment supporting suitable antigenic and cytokine stimulation (24,29). Located in peripheral lymphoid tissue, these cells produce a distinct cytokine profile upon TCR-mediated activation (22). T r 1 and T H 3 cells mediate immunosuppression through the secretion of IL-10 and TGF-b1 (21,23,30,31). T r 1 and T H 3 cells become significant in peripheral self-tolerance when the pool of selfantigen-specific nT reg cells is deficient (31).

T reg Cell Singularity
For the purposes of this review and owing to the fact that definitive comparative research on the subsets is insufficient for precise differentiation, nT reg , T r 1 and T H 3 cells will be collectively referred to as T reg cells.

At One with Nature: Modulating Responses of Allergenic Challenge
Aberrant immune responses to environmental allergens are relatively common with wide variations in severity and manifestation. Depending on the allergen size and mode of exposure, an atopic individual may experience afflictions ranging from asthma to seasonal allergic rhinitis to atopic dermatitis. Atopic allergic sensitization involves the overproduction of IgE against environmental allergens, e.g. grass, house dust mites, pollen and animal proteins (30,32). Allergen-specific IgE, on the surface of mast cells and basophils, upon binding to allergen, triggers the release of histamine and mediators resulting in immediate symptomology (33). Often, patients with allergic diseases have a deficient ability to suppress T cell responses to allergen by T reg cells (32,34). T reg cell's role in the prevention of sensitization to allergens has recently been explored (32). Evidence of significant T reg cell suppressive action on T H 2 cell-mediated immune responses supports the notion that their depletion or functional dysregulation may be responsible for atopic pathology (32).

Allergy
Seasonal allergic rhinitis, more commonly known as Hay Fever, involves the deposition of allergens on the nasal mucosa followed by an immediate hypersensitivity reaction. Allergens involved, like grass pollen, are typically too large to enter into the lower airways, rendering asthmatic complications unusual. T reg cells have the capability to reduce or prevent T H 2 cell-mediated allergic rhinitis and atopic sensitization disorders (32,34).
Allergen-specific T reg cells are also modulators of immune response to dietary allergens and intimately involved in the development of oral tolerance. Karlsson et al. illustrates T reg cell functionality in an experiment utilizing children with allergy to cow's milk (35). A majority of the allergic children developed oral tolerance to milk following a period of milk restriction. Development of b-lactalbumin-specific T reg cells, in those children who developed tolerance to cow's milk, was found to be responsible for the newfound tolerance. Induction of tolerance to allergen, via antigen-specific T reg cell generation, is noted in individuals encountering pollen, dust mites and other environmental antigens (2,32). This signifies the malleability of immune system response and balance afforded by antigen-specific T reg cells (35,36).
Taken altogether, the response of an individual to allergen encounter is dependent on many factors, including the quantity and activity of antigen-specific T reg and the T H 2 effector populations (36).

Asthma
Asthma is a chronic airway inflammatory disease triggered by allergic exposure and hallmarked by airway inflammation, bronchial hyperreactivity, lung eosinophilia and excessive T H 2 cytokine production (28,34).
Inappropriate T H 2 cell response to inhaled allergens elicits airway inflammation. The complex biosignaling cascade resulting in asthma manifestation is mediated by the over production of IL-4, IL-5, IL-9 and IL-13, which serves to regulate IgE production and to sequester effector cells to the airway. Effector cells that are directed towards airway tissues enhance airway inflammation and hyperresponsiveness through the generation of additional proinflammatory cytokines and autocoids (32,37). In many cases, asthma pathology is due to an excess of T H 2 cell quantity or activity, leading to a skewing towards a proinflammatory cytokine profile (28).
Mouse models of airway inflammation allow for the examination of allergen-specific T reg cells activity in vivo. The transfer of ovalbumin (OVA) peptide-specific T reg cells to OVA sensitized mice reduced T H 2 type cytokine expression in the lungs, airway hyperreactivity and effector cell recruitment. The ameliorative effects were dependent upon IL-10; however, T H 2 cells were the source of IL-10 secretion, rather than T reg cells (34). T reg cells may reduce inflammatory response through a contact-dependent manner, e.g. enhancing the secretion of IL-10 by T H 2 cells.
T reg cells aid in the suppression of inflammatory responses to inhaled antigen and are essential for the induction of allergenic tolerance. Administration of inhaled allergen, with prior induction of allergen-specific T reg cells, prevents allergen sensitization and airway inflammation upon later exposure. Thus, immunotherapy offers a powerful function based on the modulation of allergen-specific T reg cell suppression of T H 2 responses (32). (38). While various treatment options exist, including the use of steroids, antihistamines and aggregative factor elimination, an increasing number of patients are finding little relief and are requiring other therapeutic modalities, some of which may be derived from CAM (39).

Throwing Water on the Inflammation: T reg Cell's Delicate Relations to Infectious Pathology
To deal with microorganisms, the body has evolved intricate defense mechanisms. The process of pathogen control involves the recruitment of immune system cells to the site of infection. Necessary components include inflammatory cells, cytotoxic T cells and NK cells. The generation of antigen-specific T reg cells is a crucial regulatory element in the immune response to infection by bacteria, parasites, fungi and viruses, as well as the fostering and maintenance of tolerance to non-pathogenic microbes (21,40). Dysregulation of T reg cellmediated anti-inflammatory pathways poses potentially great risks to health (41).
Effective immune response to pathogen is often accompanied by a great deal of inflammation. In excess, this inflammation can cause collateral tissue damage and pathology that necessitates the activation and proliferation of pathogenspecific T reg cells (19,21,42-44; Fig. 1). Conversely, a decrease in defense responsiveness owing to underlying eCAM 2006;3 (2) immunosuppression renders the host susceptible to pathogenic infection and subsequent harm. Depending on a number of variables, a reconstitution of immune system responses or T reg cell populations may be necessary in lieu of appropriate antimicrobial or inflammatory therapy.

Bacterial Infections
IL-10 secreting T reg cells attenuate bacterial-induced abnormalities caused by massive inflammation. This immunosuppressive cytokine is imperative for dampening excessive inflammation, owing to T H 1 responses and increase in TNFa production, while it is potentially detrimental to pathogen attack, thus making the level of IL-10 in sites of inflammation extremely delicate and specific (44).
A variety of bacterial-induced inflammatory diseases ranging from peritonitis from Escherichia coli, to chronic gastritis from Helicobacter pylori and to chronic hepatitis from Helicobacter hepaticus, reinforce a correlation between IL-10 deficiencies and disease severity. To illustrate this, mice with a deficiency in IL-10, upon exposure to Listeria monocytogenes, have a greater severity of brain lesions, because of increased proinflammatory cytokine production in the brain (44)(45)(46). In this instance, either specialized T reg cells or their IL-10 secretions help to limit T H 1 cell-mediated inflammation and damage during infection.
Recognition of hazardous microbes, allergens and toxins as pathogenic agents activates the gastrointestinal immune system. Antigen-specific T reg cells, which mediate oral tolerance to commensal microbes, differentiate between harmless inhabitants of the gut and pathogens. A break in the development or maintenance of oral tolerance may result in an astounding array of detrimental inflammatory disorders, including inflammatory bowl disease (IBD) and colitis.
IBD and colitis are conditions in which the immune system of patients reacts excessively to indigenous intestinal bacteria. T reg cell depletion in these disorders effectively breaches tolerance and allows for massive inflammation in the gut. In vivo transfer of T reg cells suppresses disease development, through IL-10, TGF-b and CTLA-4-dependent mechanisms (21).
Colonization of gastric and duodenal mucosa by H. pylori induces strong immune responses involving innate immune system cells as well as H. pylori-specific T and B cells (27). T reg cells dampen the immune response to H. pylori, effectively limiting acute infection-induced pathology, at the cost of bacterial persistence and long-term pathology, i.e. chronic infection (27). Prevention of inflammation via IL-10 and TGF-b may prove useful in the control of H. pylori infection (21).
Interestingly, varieties of bacteria including Mycobacterium tuberculosis, Yersinia entercolitica and Bordetella pertussis induce production of IL-10 by macrophages. This cytokine manipulation efficiently triggers immunosuppression and attenuates bacterial attack, by means of inducing development of IL-10 secreting T reg cells (46,47).
The appropriate balance between inflammation and bacterial destruction so that the body accrues minimal tissue damage while putting up adequate host defense as well as development of tolerance to commensals are essential roles of IL-10 secreting T reg cells. Thus, they pose a promising target for the suppression of extreme inflammation and tissue damage during bacterial exposure or infection. It is important to note that different clinical outcomes may result from T reg cell activation status.

Fungal Infections
T reg cell activation limits inflammatory pathology induced by fungal infection but compromises fungal clearance. Pneumocystis carinii and Candida albicans are used in models to ascertain T reg cell function. Infection of T reg cell deficient mice with P. carinii yields fatal pulmonary inflammation, with damage owing to CD4 þ effector T cells. T reg cell infusion prevents inflammation and disease development at the cost of increased pathogen load (19,21,(48)(49)(50). Similarly, T reg cell depletion renders adequate control of C. albicans infection while allowing for large-scale gastrointestinal inflammation to ensue (21).

Parasitic Infections
IL-10 or TGF-b secreting T reg cells have great utility in the balance between parasite clearance and induced immunopathology, as seen in malarial, Plasmodium chabaudi and L. major infections.
The severity of malarial infection directly correlates to the ratio of TGF-b and IL-10 levels to TNF-a levels, i.e. a greater proportion of suppressive cytokines attenuates severity of infection-induced inflammation. IL-10-deficient mice, infected with P. chabaudi, maintain severe infection and massive inflammatory responses resulting in significant host damage (44,(51)(52)(53). In the case of L. major infection, removal of T reg cells results in the effective parasite clearance, however bad lesions and a robust T H 2 response ensue (19,21,27,54). This substantiates the importance of TGF-b and IL-10 secreting T reg cells in controlling parasitic infection and pathology.
Various parasites that have adapted to T reg cell host-defense mechanisms have the ability to modulate T reg cell cytokine production and activation (55)(56)(57). Therefore, the role of T reg cells in inflammatory pathology and pathogen clearance may vary significantly between individual parasitic infectious agents.

Viral Infections
T reg cells reduce the severity of immune-mediated inflammatory lesions in viral-induced diseases through the suppression of pathogenic CD4 þ T cell activity and the limitation of inflammatory cell sequestration.
Chronic hepatitis C virus infection results in massive hepatic inflammation and damage. In liver biopsies, there is an inverse correlation between peripheral T reg cells and histological inflammatory score (21). T reg cells, specifically those secreting IL-10, are essential for the attenuation of such organ detriment.
Theiler's virus induces murine encephalomyelitis, a T H 1 cell-mediated inflammatory disease of the central nervous system, and provides a model of human multiple sclerosis from which exploration of T reg cell functionality in autoimmune detriment is possible. B cell proliferation and autoantibody production, in some viral infections, plays a major role in the development of viral-induced autoimmunity (51,58). Infected mice may experience acute encephalomyelitis or chronic demyelinating disease depending on the strain. Virusspecific T reg cells sufficiently suppress this aberrant CD4 þ T H 1 cell-mediated response (59). T reg cell modulation of reactions to self, regardless of causation, holds promising implications for a broad spectrum of deleterious autoimmune diseases.
The Maginot Line: T reg Cell's Blockade of Allergic and Infectious Immune Responses Innovative research and subsequent elucidation of T reg cell involvement in various atopic and infectious pathologies opens up numerous avenues for ameliorative therapies while describing mechanisms of disease pathogenesis. It is apparent that the T reg cell quantity and activation state are integral and equally important factors in the development and maintenance of inflammatory immunopathology (36).
T reg cell involvement in immune response to pathogens is delicate since T reg cells actively suppress immunopathology during infection, while concomitantly supporting persistence of infection during chronic disease. Aberrant modulation of immune responses by T reg cells may be owing to inappropriate quantity or functionality of T reg cells. Excessive immune suppression results in enhanced pathogen survival, through clearance inhibition, which may lead to long-term persistence, pathogen damage and increased potential for transmission (21). On the other hand, depressed T reg cell activity allows little control of inflammatory responses resulting in collateral tissue damage (21,41). A balance between effector T and T reg cell responses in sites of chronic infection may allow parasite survival in host while maintaining host immune memory and control of the pathogen (44).
Pathogens are evolving self-serving strategies to increase survival potential, through the establishment of favorable conditions for T reg cell priming, recruitment and survival (21). Certain pathogens and their products, e.g. Staphylococcal superantigen B, HTLV-1 and HIV, directly target and modulate T reg cell function (2). Evolved pathogen mechanisms for T reg cells manipulation underscores the power and utility that T reg cells hold and presents another means for medical treatment of related diseases, e.g. implementation of means to effectively antagonize pathways in which pathogens directly modulate T reg cell functioning.
Increased research is necessary in order to determine T reg cell functioning in relation to individual allergic and pathogen induced disease states. This will afford CAM researchers insight into the appropriate means of approaching a variety of human disorders with respect to T reg cells.
T reg cell's essential role in the management of allergy and infection has been detailed using specific allergens or pathogens as examples. Harmony between regulatory and effector arms of the immune system is a necessity for good health. T reg cell intricacy and specificity to individual allergens or pathogens impels further research and highlights T reg cells overall importance to human health and CAM. The conceptual framework laid down is consistent with various disease states, including autoimmunity and tumor pathogenesis, which will be a futile subject.