ICOS and neoplasm: In this context, ICOS-L is expressed either by TA-plasmacytoid DC (pDC) in breast [152], ovarian [153] and gastric [162] cancers or by tumor cells in follicular lymphoma [155], acute myeloid leukemia [163] or melanoma [156] and plays a major role in the expansion of ICOS+ TA-Tregs (Figure 4).