Requirement of monocytes and T-helper cells during development of tumor cell cytotoxicity in targeted T cells.

In cocultures of human placental alkaline phosphatase(PLAP)-positive MO4 tumor cells and human peripheral blood mononuclear cells (PBMC), also containing a heteroconjugate (7E8-OKT3) synthesized between the anti-PLAP monoclonal antibody 7E8 and the anti-CD3 antibody OKT3, and supplemented with low levels of recombinant interleukin-2 (rIL-2), T cells are progressively activated, resulting in tumor cell lysis. To unravel the contribution of PBMC subsets to the generation of this targetable cytotoxicity, PBMC subsets were studied after their isolation by cell sorting, either from fresh PBMC or from PBMC pre-activated with OKT3 and rIL-2. Whereas no targetable cytotoxicity was found in Fc-receptor-bearing CD3- cells, tumor cells were lysed by CD3+ T cells (mostly CD8+) isolated from pre-activated PBMC. When isolated from fresh PBMC, neither the CD8+ T cell subset, nor the total CD3+ T cell population developed significant targetable cytotoxicity, even in the presence of rIL-2. Thus, additional cell types are essential for the CD8+ T cell activation. Indeed, CD4+ T cells isolated from pre-activated but not from fresh PBMC were capable of eliciting cytotoxicity in fresh CD8+ T cells. The non-targeted monocytes were found to be the activators of the CD4+ T cells. In summary, targeting T cells to the surface of a tumor cell is not sufficient per se to achieve activation and lysis. The progressive tumor cell lysis by targeted T cells seems to be initiated by non-targeted monocytes activating CD4+ T cells, these cells in turn promoting CD8+ T cell activation, necessary for the development of cytotoxicity.