Non-activated guinea-pig T cells and thymocytes express Ia antigens: FACS analysis with alloantibodies and monoclonal antibodies.

Conventional alloantisera and monoclonal antibodies to guinea-pig Ia antigens were used for analysis of Ia expression by guinea-pig T cells and thymocytes. Indirect immunofluorescent staining was performed with alloantisera or with ascitic fluid as a source of monoclonal antibody followed by flow microfluorometry analysis on the fluorescence activated cell sorter. About 80% of normal, non-activated peritoneal exudate T cells, lymph node T cells and thymocytes expressed Ia antigens. These data are therefore in contrast to studies with human or murine T cells where Ia antigens were shown to be expressed predominantly on activated but not on non-activated T cells. All the reactivity of the anti-Ia alloantisera for strain 2 T cells could be removed by absorption with an Ia-bearing B cell leukaemia, EN-L2C, but not by its Ia-negative variant, BZ-L2C. Thus, the Ia determinants identified on T and B cells are probably identical. One monoclonal antibody, 25E3, which had previously been shown by serologic analysis to react exclusively with an alloantigenic determinant of strain 2 Ia antigens displayed an unusual pattern of reactivity in that it clearly stained strain 13 thymocytes, but not mature strain 13 T or B lymphocytes. The significance of this possible expression of inappropriate Ia determinants by thymocytes remains unclear. This phenomenon might be associated with differentiation processes in the thymus.