INDIRECT DEMONSTRATION OF THE LIFETIME FUNCTION OF HUMAN THYMUS

The aim of this study was to test the hypothesis that human thymus mai ntains its function as the site of early T cell development throughout life, but to a progressively diminishing extent. Mononuclear cell sus pensions prepared from the samples of 39 human thymuses were analysed for the total number of cells per gram of thymus tissue, percentage of single marker-positive CD2, CD4 and CD8 cells, percentages of double- positive CD4 CDS and CD2 CD8 cells, double-negative CD4 CD8 cells, abs olute numbers of these cells per gram of tissue, and extent of the in vitro proliferation upon stimulation with concanavalin A (Con A), phyt ohaemagglutinin (PHA) and pokeweed mitogen (PWM) mitogens. The main ou tcome measures were How cytometric data on thymus lymphoid cell compos ition (according to CD classification), expressed as percentages and n umbers of cells per gram of thymus tissue. The total number of mononuc lear cells expressed per gram of thymus tissue exponentially decreased with age, The slope of none of the analysed cell subpopulations diffe red from the slope of the line constructed for age-related decline of the total number of mononuclear cells (-0.024 on a semilogarithmic sca le). The thymuses of all ages contained all analysed cell subpopulatio ns in approximately the same proportions: percentages of these cell su bpopulations did not change with age, except for all CD4(+) (P = 0.017 ) and double-positive CD4(+) CD8(+) (P=0.016) cells, which tended to d ecrease with age. The extent of proliferation of thymus cells upon sti mulation with T and B cell mitogens was unrelated to age. We conclude that the thymus retains its function as the site of differentiation of T lymphocytes throughout life, With respect to the number of involved lymphoid cells, the function exponentially decreases with age.