T cell-replacing factor in specific antibody responses to influenza virus by human blood B cells

Abstract

In man, B cell maturation factors obtained from T cells or T cell lines have been shown to induce antibody formation in mitogen or anti-immunoglobulin activated B cells, and in some continuous B cell lines, but the relationships between these factors and B cell differentiation factors in antigen-specific antibody responses is unclear. We have now shown that supernatants from phytohemagglutinin-activated tonsil cells, or from the Gibbon Ape T cell line MLA-144, can substitute for T cells in the specific antibody response by human blood B cells to influenza virus. Thus, T cell-depleted non-rosette-forming (E-) cells prepared from peripheral blood mononuclear cells made antibody when cultured with antigen and factor together, whereas control cultures of E- cells with either antigen or factor alone did not. Moreover, E- cells cultured with factor and influenza virus strain A/X31 made antibody to A/X31, but not the non-cross-reacting strain, B/HK (and vice versa) showing that the response was antigen specific. The activity in these supernatants, therefore, fulfilled the functional definition of T cell-replacing factor (TRF). The possibility that interleukin 2 (IL 2) present in the TRF-containing supernatants was expanding residual T cells in the E- preparations to provide normal T cell help was excluded in three different ways. First, E- cells depleted of T (Leu4+) cells to undetectable levels made normal amounts of antibody when cultured with antigen and TRF. Secondly, a limiting dilution technique was employed to show that help in cultures of E- cells and TRF was not mediated by antigen-specific T helper cells. Thirdly, TRF-containing supernatants depleted of IL2 retained activity, whereas purified IL2 was inactive. Preliminary purification of TRF by gel filtration on Ultrogel AcA54 columns showed that all the activity eluted in a single peak between 35 000 and 43 000 molecular weight. This distinguishes human TRF from IL 2 and from other B cell maturation factors with a molecular weight range of 15 000-20 000 which act on continuous B cell lines. In addition to TRF, supernatants from phytohemagglutinin-activated tonsils also contained a factor which could induce polyspecific IgM production, but only in cultures containing significant numbers of T cells. This additional activity may have been due to IL 2, and provides an explanation for the apparent T cell-dependent effects sometimes observed in experiments designed to test B cell differentiation factors on T cell-depleted normal B cells.