CD28 mAbs with distinct binding properties differ in their ability to induce T cell activation: analysis of early and late activation events

A panel of eight different CD28 mAbs was used to analyse the structure-function relationships of the CD28 molecule. The results of binding inhibition experiments show a complex and heterogeneous pattern of inhibition; however a subgroup of mAbs was identified, namely CD28.1, CD28.3, and CD28.5, which exhibited almost identical inhibition profiles. To test the hypothesis that the different binding specificities are related to functionally distinct subregions of the CD28 molecule, the ability of each mAb to (i) induce IL-2 release and (ii) increase intracellular calcium [(Ca2+)i] in Jurkat T cells was analysed. The results show that the mAbs CD28.1, CD28.3, and CD28.5 are almost totally unable to induce IL-2 release, and their ability to increase (Ca2+)i is relatively low. All other mAbs are able to induce a marked (Ca2+)i rise, however they strongly differ in their ability to induce IL-2 release. Such differences cannot be explained by differences in the isotypes or binding kinetics of the mAbs. These results imply the existence of functionally distinct subregions on the CD28 molecule. In addition, the (Ca2+)i rise may be associated with either high or low IL-2 secretion following CD28 triggering.