MONOCLONAL-ANTIBODY 2D10 RECOGNIZES A NOVEL T-CELL COSTIMULATORY MOLECULE ON ACTIVATED MURINE B-LYMPHOCYTES

We have developed a panel of rat mAbs against dibutyryl cAMP-activated 5C2 cells. In this panel, one mAb, 1G10, recognized murine B7. Anothe r mAb designated 2D10 did not bind to murine B7 but could recognize a surface molecule expressed only on dibutyryl cAMP-activated 5C2 mouse B lymphoma cells or on LPS-stimulated splenic B cells. This new molecu le is referred to as early T cell costimulatory molecule-1 (ETC-1). Fr om both activated 5C2 cells and splenic B cells, mAb 2D10 immunoprecip itated a 59- to 60-kDa protein, which was different from the 47- to 55 -kDa murine 87 protein precipitated from the same cell populations. FA CS analysis showed that, in contrast to B7, the expression of ETC-1 on 5C2 cells was induced by lower concentrations of dibutyryl cAMP and d isplayed a faster kinetics. LPS-stimulated splenic B cells expressed r elatively low levels of 87 and much higher levels of ETC-1. Importantl y, in an Ag presentation assay using activated 5C2 cells as APC, the s ecretion of IL-2 by C8A3 T hybrids was partially inhibited by mAb 2D10 alone and completely blocked by combination use of mAbs 2D10 and 1G10 in a dose-dependent and synergistic fashion. In a one-way primary MLR , mAb 2D10 alone at 0.1 to 1 mu g/ml inhibited T cell proliferation by 19 to 56%. However, an additive blocking effect (up to 76%) was obser ved when two mAbs were added in combination. Thus, our data have demon strated that a novel T cell costimulatory molecule is present on activ ated murine B cells, which, in cooperation with B7, may play a critica l role in optimal T cell activation.