SDH loss creates an immunosuppressive tumor microenvironment through two distinct succinate-dependent mechanisms: (1) extracellular succinate is directly taken up by tumor-infiltrating T cells via MCT1 (SLC16A1), impairing TCA-cycle glucose oxidation in T cells and suppressing IFN-γ secretion and degranulation — demonstrated in human CD4+/CD8+ T cells at tumor-associated succinate concentrations (Gudgeon et al., Cell Rep 2022, PMID 35977513), with RNA-seq of SDH-deficient pheochromocytoma/paraganglioma confirming profound in-vivo IFN-γ signaling suppression; and (2) the pseudohypoxic HIF-1α program drives upregulation of IDO1 (indoleamine 2,3-dioxygenase 1), the rate-limiting enzyme in the tryptophan→kynurenine degradation pathway, with aberrant kynurenine pathway activity confirmed in metastatic SDHB-driven PPGL by multi-omics (PMID 42230482). Together these mechanisms create a profoundly T-cell-hostile TME in SDH-deficient tumors.

SDH applicability is hypothetical. PMID 42230482 is Zhou et al.'s 2026 review of metastatic PPGL multi-omics, including aberrant kynurenine metabolism and immunosuppression, not primary confirmation of HIF-driven IDO1 overexpression or a CTLA-4/Treg dependency in SDHB tumors. The proposed SDH -> kynurenine -> Treg -> CTLA-4 chain needs direct validation. Ipilimumab blocks CTLA-4 interactions with CD80/CD86, augmenting T-cell activation; reduced Treg function may contribute (Yervoy prescribing information). Fc-dependent Treg depletion is supported in human-Fc-receptor mouse models (Arce Vargas et al., PMID 29576375), but human tumor studies found no significant FOXP3+ Treg depletion (Sharma et al., PMID 30054281). It must not be described as a proven dominant human mechanism or demonstrated in SDH-deficient tumors. DART (NCT02834013) includes GIST and pheochromocytoma/paraganglioma cohorts, but histology-based eligibility is not a dedicated SDH-deficient cohort or proof of benefit in that molecular subgroup. Final overall DART results are Patel et al., 2026, PMID 41997295; combination outcomes do not establish ipilimumab monotherapy efficacy.

SDH applicability is hypothetical. PMID 42230482 is Zhou et al.'s 2026 review of metastatic PPGL multi-omics, including aberrant kynurenine metabolism and immunosuppression, not primary confirmation of HIF-driven IDO1 overexpression or a CTLA-4/Treg dependency in SDHB tumors. The proposed SDH -> kynurenine -> Treg -> CTLA-4 chain needs direct validation. Ipilimumab blocks CTLA-4 interactions with CD80/CD86, augmenting T-cell activation; reduced Treg function may contribute (Yervoy prescribing information). Fc-dependent Treg depletion is supported in human-Fc-receptor mouse models (Arce Vargas et al., PMID 29576375), but human tumor studies found no significant FOXP3+ Treg depletion (Sharma et al., PMID 30054281). It must not be described as a proven dominant human mechanism or demonstrated in SDH-deficient tumors. DART (NCT02834013) includes GIST and pheochromocytoma/paraganglioma cohorts, but histology-based eligibility is not a dedicated SDH-deficient cohort or proof of benefit in that molecular subgroup. Final overall DART results are Patel et al., 2026, PMID 41997295; combination outcomes do not establish ipilimumab monotherapy efficacy.

UniProt: P16410