Introduction

T cells from young or old donors can be cloned and cultured in vitro, provided that they are supplied with growth factors and intermittently stimulated via their surface receptors for antigen and costimuli. The critical advantage of using monoclonal populations is that the changes observed over time are not due to age-associated alterations in the proportions of different T cell subsets, a confounding factor plaguing innumerable studies. Mostly, peripheral blood lymphocytes are the starting material for cloning, but other sources of T cells, such as lymph node, behave in the same manner. On initial stimulation, T cells upregulate growth factor receptors, which are soon down-regulated again regardless of the presence of growth factors unless restimulation via the antigen receptor (or under certain circumstances, other surface receptors) takes place. Restimulation is required for re-upregulation of growth factor receptors and continued growth. Restimulation too soon after antigen exposure, on the other hand, may lead to activation-induced cell death by apoptosis (activation-induced cell death, or AICD), whereby the definition of ''too soon'' changes according to circumstances, importantly, including the age of the clone, and is also different from clone to clone. Thus, T cell clones (TCC) must be maintained in careful balance between too much antigen (apoptosis) and too little (growth termination). Under appropriate culture conditions, all major subsets of T cells can be cloned (i.e., CD4+ TCR2, CD8+ TCR2, CD4, 8-negative TCR1, and NKT cells). However, the latter two types are more difficult to maintain. Clonal longevity studies are rare because of the time-consuming nature of the experiments, but some data are available on the behavior of human T cell clones from different sources.

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