Identification of a T helper cell-derived lymphokine that activates resting T lymphocytes

A novel lymphokine with apparent molecular size of 10 to 12 kilodaltons is secreted from helper T cell clones within hours after cross-linking their T cell antigen-MHC (major histocompatibility complex) receptors (T3-Ti). This lymphokine, termed interleukin-4A (IL-4A), stimulates resting lymphocytes by binding to a surface component (or components) of the alternative T11 pathway and subsequently by inducing interleukin-2 (IL-2) receptors. The activation process is neither dependent on antigen specificities of the recruited population or the presence of macrophages. It appears, therefore, that IL-4A is a mediator involved in amplifying the T cell immune response.