A cytofluorimetric study of T lymphocyte subsets in rat lymphoid tissues (thymus, lymph nodes) and peripheral blood: a continuous remodelling during the first year of life

We previously demonstrated that the rat thymus undergoes a progressive remo delling long before the appearance of typical signs of involution [Quaglino , D., Capri, M., Bergamini, G., Euclidi, E., Zecca, L., Franceschi, C., Pas quali Ronchetti, I., 1998. Age-dependent remodelling of rat thymus. Morphol ogical and cytofluorimetric analysis from birth up to one year of age. Eur. J. Cell. Biol. 76, 156-166]. To focus better on the complex remodelling th at occurs in the rat immune system during the first year of life, we analys ed the phenotype profile of thymocytes, and T lymphocytes from mesenteric l ymph nodes and peripheral blood of the same animals by flow cytometry. Two experimental sets were performed simultaneously using the same animal strai n, but starting and ending the study at different ages (15 days up to 300 d ays in the first experimental set, and 90 days up to 360 days of life in th e second). In the rat these ages appear to be crucial not only for developm ental, maturative and early involutional processes of the thymus, but also of the entire immune system. The main findings were the following: (1) in t he thymus, CD8(-)CD3(-) cells increased, CD5(+) alpha beta TCR- and CD8(+)C D4(+) thymocytes decreased, while the most mature cell subset appeared well preserved with ageing; (2) in the lymph nodes, T helper and T cytotoxic ly mphocytes decreased in the most aged animals. Memory/activated CD4(+)CD45RC (-) T cells decreased, while naive/resting CD4(+)CD45RC(+) cells increased in the youngest animals and decreased in the oldest. CD8(+)CD45RC(-) and CD 8(+)CD45RC(+) lymphocytes showed a complex age-dependent trend, and (3) in peripheral blood, minor modifications were evident, such as an age-dependen t increase in the alpha beta TCR(+)CD25(+) cell subset. Some of these chang es were related to the developmental process, while others could likely be interpreted as early signs of immunosenescence. The role of these modificat ions in immune system is discussed within the framework of the remodelling hypothesis of immunosenescence. The age-dependent changes in these three ly mphoid compartments, however, appear to be different and only partially ove rlapping, thus suggesting that the maturational, developmental and ageing p rocesses have distinct characteristics in the central and peripheral lympho id organs. (C) 2000 Elsevier Science Inc. All rights reserved.