ICOS and angioimmunoblastic T-cell lymphoma: In the 4th edition of the WHO classification of lymphoid tumors in the 2017 revision, AITL resides under the umbrella of PTCLs with a follicular helper T-cell (TFH) phenotype, which is defined as positive immunostaining for at least two of the following TFH markers: CD10, Bcl-6, PD-1, CXCL13, and CXCR5, ICOS, and SAP (3).