Acceptor site(s) for T cell-replacing factor (TRF) on B lymphocytes. II. Activation of B cells by cross-linkage or aggregation of the TRF acceptor molecule.

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Abstract
An in vitro experimental system was established to demonstrate the TRF-substituting activity of an alloantiserum raised in TRF low-responder (DBA2/2Ha x BALB/c)(DC)F, male mice against TRF high-responder parental BALB/c B cells. The TRF substituting activity of the antiserum was apparent in that anti-Thy-1 plus C-treated, DNP-primed B cells from TRF high-responder mice were effectively stimulated, whereas B cells from TRF low-responder DBA/2Ha mice were not, as evidenced by the induction of secondary anti-DNP IgG PFC responses. The specificity of the reaction of the antibody with a component present on the TRF high-responder B cells was also substantiated by the fact that antibody activity was virtually eliminated by absorption with B cells from various TRF high-responder mice but not by TRF low-responder DBA/2Ha B cells. The IgG fraction of antiserum and the F(ab')2 and Fab' fragments of the antibody, which possess a comparable reactivity in regard to the TRF acceptor site(s), were prepared, and analysis of the B cell-triggering mechanism by the antibody was carried out. The results revealed that both the IgG fraction and F(ab')2 fragment, but not the monovalent Fab' fragment, demonstrated effective TRF-substituting activity, indicating that cross-linkage or aggregation of the TRF acceptor site(s) may give rise to differentiation signals to the B cells.