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

Abstract

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 μm³ cells (T₁) and a small population of 175 μm³ cells (T₂), 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 μm³), electrophoretically bimodal (T₃ had a medium and T₄ 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 μm³ with lower negative surface charge and the other is 90 μm³ with a somewhat higher negative surface charge. These 125 μm³ cells (T₄), which morphologically resemble the 125 μm³ thymocytes, are less GvH-reactive than the 90 μm³ (T₅) 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 T₁ thymocytes are transformed into T₂ and these develop into T₃ and T₄ 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 T₅ cells possessing the highest observed negative surface charge.