The role of CD4 in antigen-independent activation of isolated single T lymphocytes

The membrane molecule CD4 (L3T4) is thought to facilitate activation of Class II H-2-restricted T cells by binding to Ia determinants on antigen-presenting cells. Recent reports suggest that CD4 can also contribute to antigen-independent activation by anti-T cell receptor (TCR) antibodies. An assay which measures the secretion of two lymphokines, granulocyte-macrophage colony-stimulating factor and interleukin 3 (IL-3), by single T cells activated with an anti-TCR antibody, F23.1, was used to analyze the effects of anti-CD4 antibodies on antigen-independent T cell activation. Single cells of a CD4+F23.1+ clone were micromanipulated into wells to which F23.1 had been immobilized, and their lymphokine secretion was measured 24 hr later. The frequency of lymphokine-secreting cells was consistently reduced up to 10-fold in the presence of soluble anti-CD4 antibody (GK1.5) but only up to 2.5-fold by an antibody to the cell adhesion molecule, LFA-1. In both bulk and single-cell cultures, responses to suboptimal concentrations of F23.1 were more susceptible to inhibition by GK1.5 than responses to optimal F23.1. The failure of GK1.5 to inhibit IL-2-stimulated lymphokine synthesis in bulk cultures suggested that CD4 ligation did not deliver a negative signal to the clone. By contrast, when either anti-CD4 or anti-LFA-1 was immobilized on the same surface as F23.1, the frequency of lymphokine-secreting cells could be increased up to 10-fold. It is concluded that anti-CD4 antibodies can act directly on the responding T cell to affect TCR-dependent activation, in the absence of interaction with antigen-presenting cells or any other cell type.