Helper activity of T cells stimulated in long-term culture.

Thymus-derived (T) cells obtained from three categories of in vitro antigen-induced proliferative responses were assayed as sources of helper T cells. These categories are exemplified by (a) direct stimulation of keyhole limpet hemocyanin (KLH)-primed cell with KLH, which results in high indexes of proliferation; (b) direct stimulation of apoferritin-primed cells with apoferritin, which does not result in indexes of proliferation above background levels; (c) trans-stimulation of unprimed cells with X-irradiated KLH-primed cells which results in indexes of proliferation comparable to category (a). Our results indicate that levels of [3H]thymidine incorporation by proliferating populations are not an accurate reflection of helper T cell generation. Directly stimulated KLH and apoferritin-primed cells give rise to highly enriched populations of antigen-specific helper T cells which support both IgM and IgG antibody responses in vitro. Moreover, these specific helper T cells are functionally restricted by products encoded by the I region of the major histocompatibility complex (MHC). Helper T cells generated in KLH-trans-stimulated cultures are not KLH-specific in that comparable levels of helper activity are expressed using either KLH or apoferritin as carriers. These non-KLH-specific helper T cells only support the production of IgM antibody in vitro and they are not functionally restricted by MHC products.