ICOS is a key mediator of PI3K activation in TFH cell [56], and is part of the diagnostic criteria for AITL, with ICOS positivity significantly associated with AITL (95%) vs. other PTCL subtypes (0%–40%) [57] (Figure 5G). Here, ICOS is linked to mature T-cell and NK-cell non-Hodgkin lymphoma.