Haematopoietic growth factors are released in cultures of H-2-restricted helper T cells, accessory cells and specific antigen.

Colony formation by mammalian haematopoietic cells in culture depends on specific glycoprotein growth factors. 'Colony-stimulating factors' for granulocytic and macrophage precursors (G- and M-CSFs), as well as for pluripotential and early committed erythroid cells ('burst promoting activity, or BPA) are released in cultures of stimulated spleen, lymph node or peripheral blood cells. In such systems the induction stimulus has been either allogenic cell antigens, or lectins which stimulate T cells, such as pokeweed mitogen, concanavalin A (Con A) or phytohaemagglutin and both T cells and adherent cells are implicated in the process. The complexity of the spleen cell populations used in earlier in vitro studies has made it difficult to establish the cellular source of the factors and the mechanisms leading to their release. Pure populations of continuously growing cell lines of either monocytic/macrophage or T-cell character have already been shown to release haematopoietic activities constitutively. However, such evidence cannot prove that these functions are also expressed by the normal counterparts of these lines. We have reduced the complexity of the spleen cell system by supplying helper T cells in the form of pure clonal populations of known physiological function and antigenic specificity, and demonstrate here that activity is released when helper T cells are cultured with specific antigen, and that release depends on H-2 (I-A region) restricted interaction with accesory cells present in spleen or normal peritoneal cavity.