T-cell maturation in the human thymus and tonsil: peanut agglutinin binding T lymphocytes in thymus and tonsil differ in maturation stage.

The finding of peanut agglutinin (PNA) binding capacity, supposed to be a marker of immature lymphocytes, within the T-cell population of the human thymus (58%) and tonsil (10%) prompted the comparison of maturation stages of PNA binding (PNA+) and nonbinding (PNA-) T cells in both organs. The proliferative response after mitogenic stimulation of purified PNA+ fractions was significantly less than that of purified PNA- fractions. The results of mitogen dose-response experiments, of variation in time of culture harvest, and of addition of irradiated allogeneic peripheral blood non-T cells indicated the intrinsic mitogen unresponsiveness of cells in the PNA+ fractions. The mitogen response of tonsil fractions was higher than that of thymocyte fractions. Cells with an immature immunologic phenotype were enriched in the thymocyte PNA+ fraction, and almost absent in the tonsil fractions. Both tonsil fractions contained cells with the immunologic phenotype of mature T cells, and showed a purine interconversion enzyme makeup comparable to mature T lymphocytes. It is concluded that the tonsil PNA+ T cell is a functionally immature lymphocyte which is in a further maturation stage than PNA+ or PNA- thymocytes. The presence of PNA+ T cells outside the thymus is of relevance for the clinical evaluation of PNA binding assays and suggests the occurrence of T-cell maturation within the tonsil environment.