ANTIGEN-PULSED, INTERLEUKIN-4-TREATED B-CELLS ACTIVATE PRIMED T-CELLSIN-VITRO BUT NOT NAIVE T-CELLS IN-VIVO

The ability of B cells to act as effective antigen-presenting cells is a source of debate which centres on the degree of activation of eithe r B cells or T cells. We have investigated whether B cells treated wit h interleukin 4 (IL-4) can express the two signals required to activat e T cells: MHC Class 2/antigenic peptide complexes(signal 1) and the c ostimulatory molecules B7-1 and B7-2 (signal 2). We have also determin ed whether these cells could activate antigen-experienced T cells in v itro and whether they could prime naive T cells in vivo. We found that B cells expressed abundant MHC Class 2 molecules and moderate levels of B7-2 after 24 h culture in IL-4 with or without purified protein de rivative (PPD) but B7-1 was not detectable. PPD-pulsed, IL-4 treated B cells induced antigen-experienced T cells to proliferate in vitro but these cells failed to prime naive T cells in vivo when injected into mice. We conclude that signals, in addition to those induced with IL-4 , are required for B cells to initiate an immune response to antigen.