Promotes apoptosis, possibly via a pathway that involves the activation of NF-kappa-B. Can also promote apoptosis mediated by BAX and by the release of cytochrome c from the mitochondria into the cytoplasm. Trophic-factor deprivation triggers the cleavage of surface APP by beta-secretase to release sAPP-beta which is further cleaved to release an N-terminal fragment of APP (N-APP). Negatively regulates oligodendrocyte survival, maturation and myelination. Plays a role in signaling cascades triggered by stimulation of T-cell receptors, in the adaptive immune response and in the regulation of T-cell differentiation and proliferation. Negatively regulates T-cell responses and the release of cytokines such as IL4, IL5, IL10, IL13 and IFNG by Th2 cells. Negatively regulates the production of IgG, IgM and IgM in response to antigens. May inhibit the activation of JNK in response to T-cell stimulation. Also acts as a regulator of pyroptosis: recruits CASP8 in response to reactive oxygen species (ROS) and subsequent oxidation, leading to activation of GSDMC (PubMed:34012073). {ECO:0000269|PubMed:21725297, ECO:0000269|PubMed:22761420, ECO:0000269|PubMed:34012073, ECO:0000269|PubMed:9714541}. This is the function of TNFRSF21 (TNF receptor superfamily member 21, Ensembl gene identifier ENSG00000146072).