Thymus-derived cell (T cell) activation by heterologous antigens as a replacement of specific immune T cells in the transfer of the secondary response to sheep erythrocytes.

Spleen cells from LAF(1) mice hyperimmune to sheep erythrocytes (SE) lost their ability to transfer a secondary response to irradiated recipients after incubation with anti-theta and rabbit complement in vitro. Small numbers of specific immune cells even when taken 3 days after a primary SE injection reconstituted the direct and indirect plaque-forming cell responses. Larger numbers of cells sensitized to B. abortus (or keyhole limpet hemocyanin), and given together with the corresponding antigen, also partially reconstituted the ability to respond to SE. This property was mediated by theta-bearing cells and was interpreted as due to a nonspecific humoral factor liberated by specifically activated T cells and acting on B cell proliferation or maturation.