Plasticity of T cell function. Cloned EL-4 lymphoma cells may help or suppress a primary antibody response depending on their own concentration and the assay system.

Cloned EL-4 lymphoma T cells were tested in limiting dilution experiments for their capacity to suppress or to help the primary humoral immune response of spleen cells (or T cell-depleted spleen cells) to the Ag SRBC and 4-hydroxy-3-iodo-5-nitro-phenyl-keyhole limpet hemocyanin. EL-4 clones are able to suppress up to 80% of the total IgM responses in both systems, as well as to help. Suppression and help fluctuate between high and low levels with the numbers of EL-4 cells placed into tissue cultures. In Poisson plots, this is reflected as a "typical curve": usually one or two frequencies are estimated (e.g., integral of 1/4 and approximately 1/1000 for suppression and approximately 1/6 and approximately 1/200 for help), which appear regulated with increasing numbers of cells seeded. Control experiments showed that EL-4 cells need to be alive to exert the effects. EL-4 cells do not serve as additional antigen, do not induce an isotype switch and are not cytotoxic. Help and suppression are not restricted by the MHC. Help requires the presence of a small number of normal T cells in the assay system, indicating that EL-4 cells do not replace specific helper T cells. When a number of control cell lines were analyzed under identical circumstances, similar effects were observed with most long term T cell lines or clones expressing a T cell receptor, whereas cells of non-T lineages and T cells not expressing a T cell receptor did not show the phenomena. The results suggest a functional plasticity of T cells, dependent on cell numbers and the assay system used, and expressed via T cell communication.