Macrophage requirement for induction of in vitro anti-2,4,6-trinitrophenyl response in low-affinity receptor...

The possibility that the requirement for macrophages in the induction of an immune response is related to the antigen (Ag)-binding capacity of the lymphocyte populations that undergo activation has been examined. Spleen cells from mice immunized with 2,4,6-trinitrophenyl (TNP)-conjugated sheep red blood cells (TNP-SRBC) were filtered on a TNP-substituted Bio Gel column. The excluded cells were shown to be mainly low-affinity anti-TNP cells by using an anti-TNP plaque-forming assay and assessing the avidity of the plaque forming cells (PFC). In order to obtain a low-affinity anti-TNP precursor cell population, lymphocytes from nonimmunized mice were filtered as above. Unfiltered and filtered excluded nonimmune cells were cultured in the presence of Ag-pulsed macrophages or with the supernatants of the pulsed macrophage cultures. The filtered cells produced a specific antibody response only in the presence of the Ag-pulsed macrophages, while the total (nonfiltered) lymphocyte population contained PFC when cultured with free antigen. Determination of the avidity of the PFC confirmed the presence of low-affinity cells in the filtered population and the high affinity of the antibody-producing cells cultured in the absence of macrophages. The induction of a response in low-affinity lymphocytes appeared to require the presence of macrophages rather than that of a soluble factor present in the supernatant. It is suggested that the Ag-presenting role of macrophages are essential for the induction of the immune response in low-affinity cells, while high-affinity lymphocytes could be directly activated by free Ag.