EXPRESSION OF BOTH B7-1 AND CD28 CONTRIBUTES TO THE IL-2 RESPONSIVENESS OF CTLL-2 CELLS

The CTLL-2 bioassay is used frequently to determine interleukin-2 (IL- 2) concentrations in experimental samples, including samples that cont ain reagents which affect the CD28-B7 interaction. We therefore evalua ted whether the CD28-B7 pathway plays a role in the growth of CTLL-2 c ells. Flow cytometry demonstrated that CTLL-2 cells express both CD28 and B7-1. CTLA4-immunoglobulin (CTLA4-Ig) inhibited the growth of CTLL -2 cells over a range of IL-2 concentrations, suggesting that the CD28 -B7 interaction plays an important role in the growth of CTLL-2 cells. Anti-B7-1 antibody also inhibited CTLL-2 proliferation at all concent rations of IL-2. These results indicate that the CTLL-2 bioassay may n ot be a reliable means of determining IL-2 levels in experimental samp les containing reagents that affect the CD28-B7 interaction. They also suggest that co-expression of CD28 and B7 may contribute to the growt h of malignant T cells.