Co-expression of CD4 and CD8 molecules and de novo expression of MHC class II antigens on activated rat T cells.

Rat T-cell populations were analysed for their surface expression of CD4, CD8 and class II MHC antigens using monoclonal antibodies (McAbs) W3/25, Ox8 and Ox6 in immunofluorescence and immunoenzyme cytochemistry. Resting T cells expressed either CD4 (W3/25) or CD8 (Ox8) molecules, but not both, and did not express class II MHC antigens. In contrast, when purified T cells were stimulated by Con A or by alloantigens in a mixed lymphocyte reaction (MLR), over 60% of the cells expressed class II MHC antigens after 48-72 hr. The expression of class II MHC antigens by the T-cell blasts was transient and no longer evident after 96 hr of activation. In addition, T cells activated by either alloantigen or lectin expressed CD4 (W3/25) and CD8 (Ox8) molecules simultaneously: 30% of cells activated in MLR and 50% of Con A-activated cells expressed both markers within 48 hr of activation. The overlapping of the CD4- and CD8-positive subpopulations within the T-cell population was inferred from the results of staining for each marker separately and confirmed using a double-immunoenzyme staining method.