AbstractThe data presented in this report describe an antigen‐independent activation pathway leading to reinduction of proliferation of class II major histocompatibility complex (MHC)‐restricted murine T cell lines that after previous antigen‐specific stimulation reverted to a resting state. Antigen‐independent proliferation and interleukin 2 (IL 2)‐receptor expression occur in the presence of splenic accessory cells, exogenous IL 2 and a soluble factor(s) provisionally termed T cell‐stimulating factor(s) (TSF). Each of these components is essential for inducing growth. TSF is found in the supernatant of an autoreactive T cell line upon stimulation with syngeneic accessory cells. Neither TSF nor accessory cells can be replaced by IL1 and by some other cytokines. Monoclonal antibodies against class IIMHC molecules, the T cell receptor and L3T4 do not block this antigen‐independent stimulation. This demonstrates that the function of the accessory cell in this system is not MHC restricted and that the T cell receptor is also not involved. Furthermore, it is suggested that the blocking of L3T4 molecules by antibody will mediate a negative signal only if T cells are triggered via their anti
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