We have found that an I-J+ I-A- antigen-presenting cell (APC) is required for Ts3 activation in vivo. Together with the I-J restriction previously reported for Ts3 induction (Takaoki, M., M.-S. Sy, A. Tominaga, A. Lowy, M. Tsurifiji, B. Benacerraf, R. Finberg, and M. I. Greene, 1982, J. Exp. Med., 156:1325), it appears that this I-J+ APC is responsible for I-J restriction in the triggering of Ts3. This restriction may be exerted via a pre-Ts3 associative recognition of antigen and I-J encoded determinants, analogous to the T helper recognition of antigen in the context of I-A and I-E determinants.

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