Distinct subpopulations of thymus-dependent lymphocytes. Tracing of the differentiation pathway of T cells by use of preparatively electrophoretically separated mouse lymphocytes.

Thymus-dependent cells from thymus and peripheral lymphoid organs were preparatively separated by means of free flow electrophoresis into various subpopulations which were defined in terms of θ (theta) antigen content, negative surface charge, graft-versus-host (GvH) reactivity, hydrocortisone sensitivity, cell volume and morphological details. Most thymocytes in the cortex have a low negative surface charge, high θ antigen content, are hydrocortisone-sensitive and immuno-incompetent. On the basis of electronic cell sizing this group consists of a large population of 90 μm cells (T) and a small population of 175 μm cells (T), the latter being less hydrocortisone-sensitive than the former. A minority of thymocytes resides in and around the medulla and has high negative surface charge, a medium θ antigen content, is hydrocortisone-resistant and reveals low GvH reactivity. These cells are medium sized (125 μm), electrophoretically bimodal (T had a medium and T a high negative surface charge) and on the basis of morphological criteria are metabolically more active than the thymocytes of low negative surface charge. In the peripheral lymphoid organs, all thymus-dependent cells show high negative surface charge and have the lowest observed θ antigen content and the highest observed GvH reactivity. These cells fall into two populations of which one is 125 μm with lower negative surface charge and the other is 90 μm with a somewhat higher negative surface charge. These 125 μm cells (T), which morphologically resemble the 125 μm thymocytes, are less GvH-reactive than the 90 μm (T) cells, which seem to be resting cells. On the basis of these data, a possible sequence of steps in the maturation of T cells was constructed as follows: in the cortex of the thymus T thymocytes are transformed into T and these develop into T and T thymocytes which have higher negative surface charge, lower θ antigen content and are in an advanced stage of maturity. After further loss of θ antigen these cells, which are in the medulla, emigrate into the periphery and are finally transformed into highly immunocompetent T cells possessing the highest observed negative surface charge.