Determining the effects of ageing on the innate mucosal immune system

The mucosal immune system in the intestine plays an important role in protection against food-borne and inhaled pathogens and their toxins. Over 26% of the UK population is >65 years old and this is expected to rise significantly in future decades. The immune response in the intestine and respiratory tract is significantly affected by ageing, a process termed 'immunosenescence'. Although many studies have addressed the age-related changes to systemic immune responses such as those in the spleen and the thymus, the mucosal immune system has received little attention. As a consequence, the mechanisms underlying the decline in immune function in the intestines of the elderly are poorly understood. The significant age-related increases in the incidence and severity of intestinal infections, cancer, inflammatory diseases, coupled with decreases in the efficacy of vaccinations, illustrate the importance of studies aimed at understanding the factors involved in this immunosenescence. Indeed, mortality from intestinal pathogens is substantially increased in the elderly. Similarly the respiratory pathogens influenza virus and Streptococcus pneumoniae also cause significant morbidity and mortality in the elderly, and vaccinations against them are much less effective in the elderly. Improving protective measures in the elderly is also a major public health priority for the World Health Organisation. A specific type of antibody molecules are secreted in the intestine (secretory IgA) and help to protect the intestine against bacterial toxins and infection by pathogenic microorganisms. The ageing-associated decline in the intestinal immune response impairs the IgA response in the intestine. However, the precise stages, cells and molecules within the mucosal immune system that are affected by ageing are not known. The research described in this proposal aims to study in detail the effects of host age on the status and function of the mucosal immune system in both the small and large intestines.
A special type of cells in the lining of the intestine (the epithelium) known as M cells, are specialised to sample the contents of intestine to allow the mucosal immune system to generate an appropriate immune response to the pathogens or toxins within in. In the absence of M cells, mucosal immune responses are less effective. Despite the important role of M cells in mucosal immunity, nothing was known about the effects of aging on their function. Our novel findings show that the function of M cells in the intestines of aged animals is dramatically reduced, significantly impeding their ability to sample the contents of the intestine. We also have evidence that these cells are adversely affected in the mucosal immune system of the upper respiratory tract. These data reveal an important, previously unrecognised, aging-related defect in the mucosal immune system's ability to monitor for ingested and inhaled pathogens. The ageing-associated decline in immune function means that vaccines are less effective in the elderly. The identification of the precise cellular and molecular factors affected in the ageing mucosal immune system is therefore crucial for the development of mucosal vaccines and effective strategies to improve intestinal immunity in aged animals and humans. Therefore, in this study we aim to fully characterise the effects of aging on M cells, identify the molecular and cellular factors which underlie such effects, and explore potential approaches to enhance M-cell maturation and mucosal immune responses in elderly animals and humans. Identification of the factors which cause immunosenescence is crucial for the development of mucosal vaccines to improve immunity in elderly animals and humans, which will ultimately reduce mortality, morbidity and the reliance on antibiotics to treat certain important bacterial infections.

Antigen (Ag) transcytosis across the follicle-associated epithelium (FAE) of Peyer's patches by cells (M cells) is an important first step in the induction of an efficient mucosal immune response. Furthermore, the targeted delivery of vaccine Ag to M cells is an effective way of inducing a specific immune response. Despite the important role of M cells in mucosal immunity, it was not known of how aging affected their function. Our data show that M-cell density in the gut-associated and nasopharyngeal-associated lymphoid tissues of aged mice is dramatically reduced, and impedes the sampling of gut lumenal Ag. Our data reveal an important, previously unrecognised, aging-related defect in the mucosal immune system's ability to sample ingested and inhaled Ag. A thorough analysis of the cellular and molecular mechanisms that underpin the dramatic aging-related decline in the functional maturation of M cells is crucial to aid our understanding of the factors that influence susceptibility to mucosally-acquired pathogens and identify novel approaches to stimulate M-cell differentiation and improve mucosal immunity in the elderly. Using a mouse model, an integrative approach will be used to fully characterise the effects of aging on M cells, identify the molecular and cellular factors which underlie such effects, and explore potential approaches to enhance M-cell maturation in the elderly. Furthermore, we will test the hypothesis that the effects of aging on M cells impair the induction of Ag-specific mucosal immune responses. To do so, the following objectives will be studied:
Objective 1: Determine the effects of aging on the GALT and NALT
Objective 2: Determine the effects of aging on Ag transcytosis across mucosal epithelia
Objective 3: Determine the effects of aging on the induction of Ag-specific IgA responses
Objective 4: Determine the stroma/lymphocyte-derived factors responsible
Objective 5: Determine the effects of aging on the epithelium-microbiota cross-talk

Many scientists will significantly benefit from the novel data generated from this proposal which addresses important gaps in our understanding of the effects of aging on the mucosal immune system. Scientist working in the aging field will benefit from having access to the extremely useful resource of a range of tissues, tissue fluid, cells, DNA/RNA samples from a large number of aged mice (2 thru to 18 mo. old) generated from this project. In order to faciltate this, samples and data sets will be made available to interested scientists, for example, via the BBSRC-supported Shared Ageing Research Models portal and bio-bank (www.sharmuk.org).

Researchers in industry and in academia will also benefit from data derived from this project describing the effects of aging on the uptake of mucosal antigens. The targeting of vaccine antigens to M cells is an efficient way of inducing antigen-specific IgA responses. Data describing the influence of aging on the maturation and function of M cells in both the GALT and NALT will aid the design of novel therapeutics, especially those which improve the efficacy of vaccines in the elderly.

UK policy makers will have interest in the project's data on an important gap in our knowledge of the effects of aging on the mucosal immune system. This may influence their assessments on the efficacy of vaccines in the elderly, or susceptibility to important pathogens (eg: Salmonella, influenza). Indeed, the applicant's recent study describing the effects of aging on TSE susceptibility (Brown et al. 2009 J. Immunol. 183, 5199) was discussed by the UK Spongiform Encephalopathies Advisory Committee. Improved vaccine efficacy in the elderly will ultimately lead to a reduction in the use of antibiotics in this population.

Pharmaceutical/vaccine companies will have interest in outputs from this study. During the project the applicant will consult with the Institute's Business Development and Commercialisation Department to seek potential Industry partners to exploit the project's data.

This study will enable the scientists working on the project to acquire many transferable skills in two major disciplines: in vivo immunology and aging. The scientist will also develop import skills in in vivo biology (a currently recognised research priority) and high resolution bioimaging, transcriptomics and bioinformatics. This is a collaborative. Thus, the scientist will gain opportunities to develop their team-working, networking and collaboration skills. The scientist will have many opportunities to present data at external meetings.

Data from this study will be disseminated to the international scientific community using a combination of publication in quality peer-reviewed journals and presentation at scientific meetings eg: international and national scientific conferences, seminars at other research institutions and lectures to undergraduate students. Dr. Mabbott is regularly invited to present data at these events. Eg: he was invited to present his data at last year's BBSRC-sponsored Symposium on 'Ageing: intestine, immunity, microbiota and diet', and chaired The Immunology of Ageing 2014 meeting. Dr. Mabbott is keen to explore opportunities to communicate data from this project to the public through events such as the "Doors Open" day held annually at the Institute, and also his activities as a BBSRC-funded Schools Regional Champion. The release of potential news-worthy publications will be discussed with the Institute and BBSRC press officers and press releases issued when appropriate, as recent examples show: Dr Mabbott's recent study on the effects of aging on BSE transmission (Brown J Gen Virol 2014) received media interest; Data from BBSRC grant (BB/526471/1; McCulloch 2011 PLoS Pathogens) were also featured in the media, on the BBSRC website and in BBSRC Business; Our study on the effects of Salmonella on M cells (Tahoun 2012 Cell Host & Microbe) was also featured by the media and BBSRC.