Alternate splicing converts human CD137 from costimulatory to immunosuppressive function

Human membrane-bound CD137 (mCD137) is a well-known costimulatory molecule; however, it is alternatively spliced into two transcripts (sCD137-1, 2) whose function is not yet known. Here, we show that sCD137 isoforms lack the CRD4 region and form unique structures compared to mCD137. Human activated Tregs produce both CD137 splice isoforms, which are rapidly upregulated after cell activation, and identify an activated Treg phenotype along with FOXP3, CTLA4, and sCTLA4. We engineered recombinant Fc-Hu-sCD137 variants, which are immunosuppressive, inhibiting IFN-γ secretion and proliferation in purified CD4⁺ and CD8⁺ T cells in an APC-independent manner. These effects are mediated by the downregulation of S6 and 4EBP1 of the mTOR complex 1 pathway. Human sCD137 variants, in contrast to the membrane-bound form, are immunosuppressive and may be a novel treatment for inflammation and autoimmunity.