The Molecular Mechanism of Age & Vitamin E-induced Change in the Immune Response
Aging is associated with decline in T cell mediated function, which pre-disposes the elderly to higher incidence of infectious diseases and cancer.
We have demonstrated that CD4+ T cells from old mice are less likely to form effective immune synapses than those from young mice as observed with lower redistribution of key signaling molecules Zap70, LAT, Vav, and PLCγ to the immune synapse and this age-related reduction in the formation of effective immune synapse is more dramatic in naïve than in memory populations ( Marko et al, 2007 ). The age-associated impairment in effective immune synapse formation and distribution of above mentioned signaling molecules in CD4+ T cells can be reversed by vitamin E. We further showed that the defect in immune synapse formation was related to impaired phosphorylation of membrane associated LAT (Marko et al, 2009 ).
Figure 1. This illustration depicts pathways in CD4+ T cell activation that are affected in older mice
Our current studies are focused on determining molecular events which lead to age-related decline in effective immune synapse formation and its reversal by vitamin E. Furthermore, we are utilizing lipidomic and proteomic approaches to determine key membrane related changes associated with aging in T cells.
Our microarray analysis of gene expression profiles in T cells has provided additional information in delineating the underlying mechanism of age-related decrease in T cell function. We showed that T cells of old mice express significantly higher levels of suppressors of cytokine signaling 3 (SOCS3) and lower levels of growth factor independence-1 (Gfi-1) compared to those of young mice (Han et al, 2006 ).
These results suggest that relative to young T cells, less increase in activation-induced Gfi-1 expression in old T cells may result in higher levels of SOCS3 expression and suppressed T cell proliferation. We also showed that vitamin E supplementation in old mice increased expression of the genes for cell cycle related proteins including cyclin B, Cdc2 (Cdk1), and Cdc6, which suggests that altered cell cycle related proteins might underlie the effect of vitamin E in promoting cell division cycle and IL-2 production.
Nutrition, Genetic Background & Respiratory Infection
Respiratory infections are among the leading causes of death in the elderly. Influenza virus, a common respiratory pathogen, accounts for over 10,000 deaths annually and over 40,000 deaths during epidemic years. Individuals 65 years and older accounted for 89% of all influenza associated deaths in 1992. These deaths were not typically due to influenza infection alone, but from complications that developed due to influenza infection. Previously, our laboratory showed that E supplementation reduces influenza viral infection in aged mice. Recently, we have shown that 1 year of supplementation with 200 IU per day of vitamin E has a protective effect on upper respiratory tract infections in elderly nursing home residents. Our current studies are investigating the role of cytokine polymorphism and its interaction with vitamin E on cytokine production and resistance to respiratory infection. We have found that the impact of vitamin E on TNFα production varies depending on the type of single nucleotide polymorphism (SNP) in TNFα gene (Belisle et al, 2009 ). Furthermore, we found that protective effect of vitamin E in reducing pneumonia was dependent on sex and SNP in IL-10 gene (Belisle et al, 2010 ).
Zinc plays an important role in immune function. In a recently published study, we investigated the association between serum zinc and pneumonia in nursing home elderly. Results showed that subjects with low serum zinc concentrations had a lower incidence of pneumonia, fewer new antibiotic prescriptions, a shorter duration of pneumonia, and fewer days of antibiotic use compared to those with normal serum zinc level. Normal baseline serum zinc concentrations were associated with a reduction in all-cause mortality (Meydani SN et al, 2007 ).
These results indicate that zinc supplementation to maintain normal serum zinc concentrations in the elderly may help reduce the incidence of pneumonia and associated morbidity. We are currently conducting a study to determine the effect of zinc and immune function in the nursing home elderly.
Increase of population over 60 years of age continues to rise in developed and developing countries like Latin America. In so doing, health care costs are expected to rise as well; therefore, it is important to develop and test low-cost, simple nutritional and behavioral changes to improve nutrition, strengthen immune function, and minimize the risk of infectious disease in the elderly. We performed a cross-sectional study of elderly Ecuadorians in a low-income community of Quito, Ecuador to define the occurrence of infectious diseases and nutritional deficiencies. We found that micronutrient deficiencies were prevalent in elderly Ecuadorians and they predicted low immune response and higher incidence of respiratory infections. These data suggest that interventions to correct nutritional deficiencies will improve the immune response and reduce incidence of infection in elderly Ecuadorians (Hamer et al, 2009 ).
Age-Associated Adipose Tissue, Inflammation & the Role of Nutrition
Aging is associated with increase in type 2 diabetes (T2D) incidence; a low-grade inflammation in adipose tissue is implicated in development of insulin resistance; age-associated inflammation has been observed in certain tissues. In a recent study, we investigated the effect of aging on inflammatory status of adipose tissue. Our results showed that visceral fat from old mice had significantly higher mRNA expression of pro-inflammatory cytokines IL-1β, IL-6, TNF-α, and COX-2 and lower expression of anti-inflammatory PPAR-γ than those of young. We further showed that adipocytes are not only major contributor to this inflammatory state but they also increase production of inflammatory cytokines in adipose tissue macrophages, thus further propagating inflammation. We further showed that sphingolipid ceramide, through activation of NF-κB, was involved in this age-associated upregulation of inflammation in adipocytes. We are currently determining whether functional foods such as wolfberry supplementation could affect age-associated inflammatory state in adipose tissue.
Obesity & Iron Deficiency
The obesity epidemic continues to expand at a global scale, iron deficiency is the most prevalent micronutrient deficiency in the world, and the world’s population is becoming older, with elderly also being affected by the obesity epidemic. Concurrently, obese people of all ages show a particularly high prevalence of iron deficiency, where chronic inflammation and the upregulation of a small hormone peptide, hepcidin, may be playing a central role. Our research is focused on determining how the inflammatory environment inherent in obesity, both in young and old individuals, affects the regulation of hepcidin in different tissues and cell types, including immune cells.
Caloric Restriction Study
In an ongoing NIA supported multi-center randomized controlled clinical trial for the health benefit of caloric restriction (CR), we are determining the effect of 25% CR for 24 months on T cell-mediated functions of adult subjects, as well as to exploring its underlying mechanisms. The results from this study will be the first documentation of the impact of CR on immune response in humans, a biologically meaningful and clinically relevant biological marker shown to be sensitive to CR in various animal models.
Olive Oil Study
Olive oil consumption is suggested to have health benefit particularly in reducing inflammation and maintaining a healthy immune response. We are starting a prospective, randomized controlled nutrition intervention trial to evaluate immunological impact of high olive oil consumption for 3 months in the obese elderly people.
Modulation of the Immune Response by Functional Foods
We are pursuing the role that several different foods may play in modulating the immune response.
We have recently conducted animal studies and shown that dietary supplementation with white button mushroom enhanced natural killer activity, which is associated with higher levels of IFN-γ and TNF-α (Wu et al, 2007 ). In another study we showed that in vitro supplementation promoted maturation of bone marrow derived dendritic cells and increased their antigen presenting function (Ren et al, 2008 ). We are currently using an influenza infection mouse model to determine whether the enhanced immune response by consumption of mushrooms could result in improved resistance to viral infection to determine its clinical relevance.
We have reported that EGCG, a major active component in green tea, inhibits T cell proliferation and cell cycle progression and this effect is more profound in CD4+ T cells than CD8+ T cells (Wu et al, 2009 ; Pae et al, 2010 ). This effect suggests a potential application in mitigating T cell-mediated autoimmune disorders. We are currently testing this hypothesis using an autoimmune experimental encephalomyelitis (EAE) animal model. The EGCG fed animals showed a significant improvement in both symptom and pathology. The mechanism for this effect is currently under investigation.
We found enhance lymphocyte proliferation in mice fed palm oil-derived tocotrienols--together with tocopherols they form the vitamin E family (Ren et al, 2010 ). We further showed that different subtypes of tocotrienols exert their effect with different efficacy. This is the first time that an immuno-modulating effect of tocotrienols has been identified. Further studies will determine the underlying mechanisms as well as their efficacy in improving the immune response in humans.
Several ongoing animal studies are designed to determine the impact of other dietary components and functional food on immune response and resistance to infection. These projects include effect of wolfberry and probiotics on resistance to influenza infection and immune response, and the effect of combination of multi-nutrients and whey-based protein on immune function.
The Nutritional Immunology Laboratory is involved in collaborative projects with several investigators within and outside Tufts University. Some of these are highlighted below.
Effect of calorie restriction on immune response of human, in collaboration with Dr Susan Roberts of JMUSDA-HNRCA as part of her project “Dietary Energy Restrictions and Metabolic Aging in Humans”.
Vaccination: Role of vitamin E and zinc status, in collaboration with Drs Laura Coleman and Edward Belongia of Marshfield Clinic, Marshfield, WI.
Maternal obesity and offspring immune dysregulation, in collaboration with Dr Sarbattama Sen of Tufts Medical Center.