T & B lymphocytes in thymus of SJL/J mice

Abstract

THE thymus reaches relative maximal development early in life followed by apparent programmed involution, which is eventually expressed in reduced functions of the T cell system. In the SJL/J mice we observed an interesting deviation of the normal thymus pattern, namely a secondary increase in thymus weight from the age of 7–9 months onwards (average weight of 2-month-old thymus being 60 mg compared with 130 mg in a 14-month-old normal mouse). The timing of this secondary thymus weight increase seems to coincide with the comparatively early, age-related, progressive decrease in the immunere-sponsiveness of SJL/J mice described recently¹. We therefore investigated the thymus population pattern in SJL/J mice in relation to age increase, using membranal antigenic markers as well as functional criteria. Based on antigenic properties of the cell surface of mouse thymocytes, the thymus population can be divided into two defined subpopulations. The major population (about 85%), which resides mainly in the cortex, possesses high levels of θ antigen and low levels of surface H-2 alloantigens. The remaining minor population has antigenic properties similar to peripheral T cells, namely low levels of θ and high levels of H-2 (refs 2 and 3). The immunological reactivity is usually attributed to the small, high H-2 subpopulation⁴. Thymocytes belonging to the B cell compartment are rarely found⁵.