T cells stimulate catabolic gene expression by the stromal cells from giant cell tumor of bone

► Two T cell lines stimulate PTHrP, RANKL, MMP13 gene expression in GCT cell cultures. ► CD40 expressed by stromal cells; CD40L detected in whole tumor but not cultures. ► Effect of CD40L treatment on GCT cells increased PTHrP and MMP13 gene expression. ► PTHrP treatment increased MMP13 expression, while inhibition decreased expression. ► T cells may stimulate GCT stromal cells and promote the osteolysis of the tumor.