Major Histocompatibility Complex-Restricted and Unrestricted Interactions in the T Cell-Dependent Activation of Hapten-Binding B Cells

The requirements for linked recognition and major histocompatibility complex-restricted interactions in helper T cell-dependent activation of purified unprimed and primed haptenbinding B cells have been investigated. The activation of unprimed hapten-binding B cells required specific antigen and restricted interactions between helper T cells and B cells. As expected, specific hapten-carrier conjugates were essential for the activation of B cells at low antigen doses. Interestingly, however, supraoptimal concentrations (125 ltg/ml) of carrier protein alone could substitute for specific conjugates in the activation of unprimed B cells. The requirement for restricted helper T cell-B cell interactions was unchanged under these conditions. These results are discussed in terms of the signalling requirements for B cell activation.