To address this question, we performed a comprehensive investigation in matched SG histology, transcriptomic and lesional/peripheral T-cell immunophenotyping validated in samples from four different centres which identified a subset of Tfh and Tph-cells expressing high levels of IL-21 and/or IL-21/interferon-γ (IFN-γ) under the control of ICOS as a uniquely expanded population able to identify patients with SS with ELS and MALT-L. Here, ICOS is linked to synovial sarcoma.