The aging immune system and nutritional interventions

The increased numbers of elderly people pose a major burden to public health care and society. DNA damage is considered to be the major origin of age-related changes in the body. With aging, the immune system becomes deregulated and is characterized by a low-grade inflammation (inflammaging). In this thesis, we investigate the effects of nutritional and microbial interventions on the aging immune system. In <strong>chapter 2</strong>, we elaborate on the role of basophils in the immune system, particularly in the initiation and perpetuation of allergic immune responses. We found that basophils and dendritic cells interact <em>in vitro</em>, which reciprocally affects their surface markers and cytokine production. Thus, by modulating cytokine production and surface marker expression on dendritic cells, basophils may act as accessory cells in immune responses. Because little is known about the effects of aging on basophils, we investigated in <strong>chapter 3</strong> whether basophils are affected with aging. We found that frequencies of basophils in the spleen of aging mice are increasing, while their phenotype in bone marrow and spleen changes. Moreover, to investigate the role of microbiota in the aging process, we studied the effects of microbiota transfer from young or aged mice into germfree mice. Aging, and microbiota from aged mice, in particular affect differentiation and function of basophil precursors. These findings warrant further studies on the role of basophils in T helper-2 immune responses with aging. The contribution of macrophages to inflammaging is described in <strong>chapter 4. </strong>Important aspects for macrophage polarization and function, like autophagy and cellular metabolism, are discussed. Targeting of aged macrophages by (nutritional) interventions may open up new therapeutic opportunities for elderly. In <strong>chapter 5, </strong>we studied the <em>in vitro</em> interaction between bacterial supplementations and immune cells (whole spleen cells and macrophages). We noticed that aged immune cells mount a different response to bacterial strains than young immune cells. Based on these outcomes, we selected three bacterial strains (<em>Lactobacillus plantarum</em> WCFS1, <em>Lactobacillus casei</em> BL23, <em>Bifidobacterium breve</em> DSM20213) for <em>in vivo </em>application in <strong>chapter 6. </strong>We used <em>Ercc1-/Δ7</em> mice, which lack fully functional ERCC1 protein. As a consequence, DNA repair is compromised, which results in accelerated aging features in all organs, including the immune system. We supplemented <em>Ercc1-/Δ7</em> mice, as well as control <em>Ercc1+/+</em> mice with the three selected bacterial strains. We observed that <em>L. plantarum</em> prevented the age-related decline in mucus barrier function of <em>Ercc1-/Δ7</em> mice, whereas <em>B. breve</em> exacerbated the age-related decline in mucus barrier. <em>L. casei</em> supplementation elevated multiple systemic inflammatory markers in <em>Ercc1-/Δ7</em> mice, including Ly6Chi monocytes, neutrophils, and Th17 cells in spleen. Strikingly, we found major changes in the mucus barrier and immune system after supplementation of <em>Ercc1-/Δ7</em> mice with <em>L. plantarum</em> and <em>L. casei</em>, but not after supplementation of <em>Ercc1+/+</em> mice. Therefore, we conclude that caution is needed in the selection of candidate probiotic strains for supplementation of aging individuals. In <strong>chapter 7</strong>, we took a different approach to modulate the aging immune system by applying dietary tryptophan restriction in <em>Ercc1+/+</em> and <em>Ercc1-/Δ7</em> mice. We observed that in both mouse models dietary tryptophan restriction modulated B cell development and microbiota composition. In particular, we found a near-complete absence of B cell precursors in the bone marrow after dietary tryptophan restriction. The decline in B cell precursors was correlated with decreased abundance of the <em>Akkermansia</em> and <em>Alistipes </em>bacterial strains in the intestine. Thus, our results show that dietary tryptophan restriction is a powerful intervention to shape immunity and gut microbiota, also in aging. In <strong>chapter 8</strong>, we assessed the role of microbiota in the aging gut and immune system. Microbiota from young and aged mice were transferred to germfree mice. Aged microbiota induced higher T helper-1 cell and regulatory T cell frequencies in the spleen. In the ileum, the expression of inflammatory markers was increased after transferring aged microbiota, accompanied by differences in the abundance of microbial species. We conclude that senescent microbiota contribute to the inflammaging observed in aging mice. In <strong>chapter 9</strong>, we discuss the findings presented in this thesis, concluding with directions for future research. In summary, our studies show that the aging gut and immune system of mice can be modulated by nutritional and/or microbial interventions. Interestingly, our mouse models clearly provide evidence that age-related effects could be reverted or prevented by these interventions. Nevertheless, our studies at the same time show the need for translational research in order to apply the presented dietary and microbial interventions in elderly.