Differential contribution of thymic outputs and peripheral expansion in the development of peripheral T cell pools.

The number of peripheral T cells in mice increases up to 100-fold in the first few weeks of life. We have followed the fate of Thy-1 congenic T cells transferred into newborn recipients, to evaluate the relative contribution of thymic output versus peripheral expansion in the constitution of peripheral T cell pools during post-natal development. The results show that in normal animals there is essentially no peripheral expansion of T cells, which show slow turnover rates (1 to 2 months) along that time period. The rates of cell accumulation in the periphery require, therefore, an average of 1 x 10(6)-2 x 10(6) mature thymic emigrants/day for the first 3 weeks of life.