Role of Accessory Cells in the Activation of Pure T Cells via the T Cell Receptor‐CD3 Complex or with Phytohaemagglutinin

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Abstract
The ability of different subpopulations of blood mononuclear cells to serve as accessory cells in the activation of CD4+ and CD8+ T cells via Ti‐CD3 or with phytohaemagglutinin (PHA) was studied. Pure CD4+ or CD8+ T cells did not respond to particle‐bound anii‐CD3 monoclonal antibodies (MoAb) or PHA, whereas responses were seen when non‐T cells served as accessory cells. Removal of class II‐positive cells from peripheral blood mononuclear cells (PBMC) or from non‐T cells diminished, but did not completely abolish, the responses in both T cell subsets, indicating that the accessory cells are mainly found among the class II‐positive cells. However, the class II molecules themselves were not involved, as demonstrated in antibody‐blocking experiments. Removal of monocytes decreased the ability of non‐T cells to serve as accessory cells for both CD4+ and CD8+ cells in PHA activation. In contrast, the removal of monocytes resulted in an enhanced activation by anti‐CD3 MoAb in CD4+ T cells, while the activation of CD8+ T cells was less affected. Positively selected B cells were effective accessory cells in anti‐CD3 and PHA activation. Furthermore, Epstein‐Barr virus (EBV)‐transformed B cell lines were very potent accessory cells both in anti‐CD3 and PHA activation of T cells, and showed the strongest accessory cell function observed in this system on a per cell basis.

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