Biogerontology 1: 247–254, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 247 Research article Longevity in vitro of human CD4+ T helper cell clones derived from young donors and elderly donors, or from progenitor cells: age-associated differences in cell surface molecule expression and cytokine secretion Graham Pawelec 1,* , Erminia Mariani 2 , Ben Bradley 3 & Rafael Solana 4 1 Tübingen Ageing and Tumour Immunology Group, Section for Transplantation Immunology and Immunohaematology, Zentrum für Medizinische Forschung, Waldhörnlestr. 22, D-72072 Tübingen, Germany; 2 University of Bologna, Italy; 3 University of Bristol, England; 4 University of Cord´ oba, Spain; * Author for correspondence (e-mail: graham.pawelec@uni-tuebingen.de; fax: +49-7071-294464) Received 22 December 1999; accepted in revised form 5 January 2000 Key words: immunosenescence, replicative senescence, T-cell clones, T-cell longevity, T cells Abstract The effectiveness of the adaptive immune system relies upon extensive proliferation of an initially small number of antigen-specific T cells. At the end of a successful response, the majority die by apoptosis and a small minority joins the memory cell pool. Upon re-challenge with antigen, these memory cells must again undergo clonal expansion in order to mediate an effective response. Thus, T cells are subjected to marked proliferative stress which may result in clonal exhaustion due to replicative senescence. In other systems made up of rapidly proliferating cells (e.g. in the gut) individual clones are identical and are replaced at the end of their lifespan by differentiation from a stem cell reservoir. However, because of the unique clonal distribution of antigen receptors on T cells, mere replacement with other T cells is not sufficient to maintain the integrity of the system. Moreover, the very source of new T cells decreases with age (due to thymic involution). Therefore, the adaptive immune system may be uniquely susceptible to the deleterious effects of replicative senescence. Particularly in humans, in vivo studies of the behaviour of individual T-cell clones in the body is difficult. However, T-cell longevity, measured as proliferative capacity in terms of population doublings, can be usefully modelled at the clonal level in vitro. This paper discusses the surprisingly little that is known about the average longevity, variation between clones, and the maximal longevity of human T cells under clonal culture conditions in vitro. From our own studies, we show that average lifespan of human T cells is as little as 17 PD; however, established clones reach 35 PD on average, with maximum longevity generally in the region of 60–80 PD, regardless of the source of the cloned cells. Expression of surface molecules in general did not differ strikingly between young and old donors, but the frequency of clones secreting IL-10, and the amount secreted per clone was higher in the elderly than in the young. Conversely, the frequency of clones secreting IL-6 and the amount secreted per clone was higher in the young. Abbreviations: CE – cloning efficiency; CEA – clonal expansion ability; PD – population doubling; TCC – T-cell clones Introduction Progenitor cells rearrange genes for the T-cell antigen receptor in the thymus and following successful posi- tive and negative thymic selection, they are released to the periphery. There they may remain for long periods as resting ‘naïve’ cells until stimulated by spe- cific antigen. Because of space constraints, and the large potential antigenic universe to which they may be required to respond, only a small number of T cells with each antigenic specificity exists in the body. Therefore, the T-cell response requires waves of clonal expansion to generate sufficient cells for successful