Initiation of an in vitro response to sheep erythrocytes (SRBC) requires the production of a nonspecific soluble mediator which is present in the culture supernatants of antigen-stimulated thymus-derived (T) cells. This factor was shown to act early in the immune response and probably directly affects bone marrow derived (B) cells. However, the stimulation of the PFC response caused by the factor depends upon the presence of some intact T cells. These T cells can still function after 1300 rads of ionizing radiation. The factor does not simply cause proliferation of small numbers of surviving T cells, and is exhausted from the medium when cultures of B cells with the active supernatant are stimulated with antigen.

Further evidence for two functions for T cells during the antibody response in culture has been obtained by studying the ability of cultures of B and A cells to respond when the two T cell functions are provided sequentially. For example, if B cells are preincubated in vitro for 24 hr with an active supernatant, they can give subsequent PFC responses either in vivo or in vitro when mixed with irradiated normal (unprimed) T cells. Investigation of the need for intact T cells has shown that this requirement, unlike the nonspecific factor, represents an antigen-specific function for T cells. This specificity has been demonstrated with irradiated T cells from mice tolerant to SRBC. In the presence of the active supernatant and a suitable B cell source these irradiated T cells can reconstitute the response to horse erythrocytes but not SRBC.

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