Thymocytes, in contrast to peripheral T cells, were unable to generate significant numbers of anti-self-TNBS cytotoxic T cells in primary in vitro cultures. The addition to such cultures of syngeneic 1000 rad irradiated lymph node or spleen cells from mice sensitized 6 or 7 days previously with trinitrochlorobenzene (TNCB) permitted the development of an anti-self-TNBS cytotoxic T cell response. This effect was shown to be T-dependent since it was largely abolished by anti-Thy 1 plus complement and was present in nylon-wool column purified T cells.

The lymph node cells act as specific helper cells in that a) they must be restimulated in vitro by the priming antigen and b) they do not take part in the lytic phase of the response. The specificity of the helper effect is further indicated a) by the lack of help from nonsensitized lymph node cells and b) by absence of an enhanced helper effect for thymic anti-allogeneic or anti-allogeneic-TNBS responses in the presence of specific T helper cells for the anti-self-TNBS response.

Analysis of the thymocyte subpopulation able to provide precursor cells was approached by separating thymocytes into a “medullary-like” minor, peanut-lectin-agglutinable (PNA-) fraction and a “cortical-like” major, peanut-lectin-agglutinable (PNA+) fraction. It was found that the PNA- fraction was enriched in thymocytes able to mount an autonomous anti-self-TNBS cytotoxic response, whereas the PNA+ population was totally unable to generate anti-self-TNBS effector cells in the absence of helper cells. The presence of helper cells from TNCB-painted mice permitted differentiation of anti-self-TNBS cytotoxic precursor cells from the PNA+ thymocyte cultures and enhanced the level of cytotoxic cells generated in PNA- thymocyte cultures.

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This work was supported by C.N.R.S. (ATP No 3950).

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