Effect of age on functional plasticity of CD4+ T cells infiltrating a localized secondary immune response in humans

Aging is accompanied by a marked susceptibility to infectious diseases, which inflict heavy toll upon the rapidly aging society with regard to lost productivity, mounting health costs and loss of life. The progressive decline with age of the immune system - immunosenescence - is the primary underlying cause of the age-related increase in this susceptibility. Despite decades of research, important gaps remain in our understanding of the fundamental nature of the process, as well as in our practical ability to protect older adults against infectious diseases.
Our bodies mount immune responses when attacked by infectious organisms. The central participants in this response are our white cells (leucocytes), whose role is to protect us from the infective agents. Based on the molecules they secrete different populations (subsets) of leucocytes have been identified which play different roles during immune responses. A separate subset of leucocytes is known as regulatory cells and their main role is the control of other leucocytes. The major problems that are inherent in the study of human immunology is that generally, only leucocytes from the blood compartment are studied and that virtually all studies are performed in vitro. We have developed and validated a new model for the study of a human memory T cell response in humans in vivo. This involves the injection of a recall antigen in the skin, followed by either the biopsy of the lesion for histological analysis of the immune infiltration, or the harvest of infiltrating cells using suction blister technology. Using this model, we propose to clarify the functional profile of cells that accumulate in the skin during immune responses induced by different antigens and determine whether these leucocyte populations behave differently in the old individuals. Specifically we wish to examine the relationship between regulatory cells and other functional leucocyte populations and explore mechanisms which control their behaviour. This information may enable us to manipulate this balance artificially.

There are considerable gaps in understanding of the behaviour of both regulatory and effector cells during immune responses in vivo and crucial questions, especially in humans, remain unanswered. We previously developed a model to study a memory T cells response to antigen-injection in the skin of normal human volunteers (recall response); in this model we can perform histological investigations of infiltrating leucocytes at different times after antigen challenge, or study their function after isolation from suction blisters induced at the site of injection. This allows us to follow the course of a response from initiation to resolution and gives us access to the human tissues where immune response is taking place. We have previously observed that cutaneous recall response is significantly reduced in old individuals. In this study we want to investigate the kinetics with which different functional CD4+ T cell populations accumulate and disappear during an immune response and to clarify some of the mechanisms involved in the decreased responses in the old. We postulate that the balance between human CD4+ T cell subsets such as Th1, Th17 and Foxp3+Tregs is altered in the old individuals leading to decreased immune responsiveness. To test this hypothesis we will first determine the kinetics with which different functional T cell populations (Th1, Th17 and Treg) accumulate and proliferate during recall response in vivo in young and old. Functional populations will be identified by cytokine secretion and by the expression of lineage-specific transcription factors (T-bet, ROR-??c and Foxp3). Secondly, we will investigate whether different T cell populations preferentially undergo apoptosis or senescence in the old individuals. Thirdly, we will use class II tetramer staining to determine if responsive and regulatory T cells of the same Ag-specificity accumulate in parallel. Finally, we will manipulate the mechanisms that drive the functional inter-conversion of skin derived effector and regulatory populations and investigate whether cells at different stages of differentiation show equal plasticity. To do this we will use T cells isolated from the skin to generate antigen specific clones in vitro and test whether their functional profiles can be changed by exogenous cues and whether the stage of differentiation is a factor in this process.