Physical subpopulations of mouse thymocytes: changes during regeneration subsequent to cortisone treatment

Abstract

Thymocytes from adult C3H mice were fractionated on the basis of electrophoretic mobility (EPM) differences by free-flow electrophoresis and the fractions obtained were analysed for size distribution with a Coulter Counter. The data were combined in the form of contour maps (fingerprints) representative of the various physical types of thymocytes. Four thymocyte subpopulations with distinct physical properties were thus characterized and were further shown to differ in their sensitivity to immunosuppressive drugs and in their proliferation rate. Th₁ cells possess the slowest EPM and smallest volume (95 μm³), are sensitive to cortisone (C) but resistant to cyclophosphamide (Cy) and do not incorporate ³[H]-thymidine (TdR) in vitro. Th₂ cells possess higher EPM and slightly larger volume (103 μm³) and are sensitive to both C and Cy. Th₃ cells are of still higher EPM, exhibit the largest volume (150–250 μm³) of all thymocytes, are sensitive to C and Cy and rapidly incorporate [³H]-TdR in vitro. Th₄ cells are endowed with fastest EPM and a modal volume of 130 μm³ and are resistant to both C and Cy.

The regeneration and fate of these subpopulations were investigated during the period subsequent to cortisone injection. Th₃ cells were the first to reappear on day 4–6 following treatment. Thereafter, Th₂ and Th₁ cells began to rise again and eventually reached levels higher than control by day 12–14 post-treatment. By the same time, Th₄ cells, which escaped cortisone lympholytic action became less and less visible on the fingerprint. In fact, administration of a second dose of cortisone by day 8–10 after the first treatment revealed a depletion as well as a physical modification of the C-resistant cell pool. Typical Th₄ cells were found again on day 15 after the first cortisone injection. It was only around day 20 that thymus became normally repopulated.

Taken together, these observations indicate that Th₃ cells may act as precursors for all 3 other thymocyte subpopulations and that during thymus reconstitution Th₂ and Th₁ cells are produced at a faster rate than are Th₄ cells.