ICOS and angioimmunoblastic T-cell lymphoma: In 2007, immunophenotyping and gene expression profiling analyses identified the TFH cell as the cell of origin (COO) of angioimmunoblastic T-cell lymphoma (AITL) [2–4], and this specific cellular derivation has become a cardinal defining feature of the disease [5] Over time, other peripheral T-cell lymphomas (PTCLs) have been recognized to similarly express immunophenotypic markers (CD10, BCL6, CXCL13, PD1, and ICOS) and/or gene expression signatures characteristic of TFH cells [2–4, 6–10].