Abstract 1599: mTOR enables alveolar macrophage survival and GM-CSF-STAT5-PPARγ signaling: Genetic and temsirolimus evidence

Abstract Mechanistic target of rapamycin (mTOR) inhibitors, widely used in transplant and oncology settings, can induce a noninfectious pneumonitis linked to disrupted alveolar macrophage (AM) function. AMs uniquely require granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling through CSF2RA, STAT5, and PPARγ to maintain identity and survival, yet the extent to which mTOR participates in this pathway in AMs remains undefined. Given that alveolar and interstitial macrophages (IMs) occupy distinct microenvironments with potentially divergent dependencies, the purpose of this study was to determine whether AMs exhibit a unique, non-redundant requirement for mTOR downstream of GM-CSF and how this requirement preserves AM survival and alveolar niche homeostasis. To investigate mTOR dependence in AMs, we generated conditional myeloid mTOR knockout mice (LysMCremTORf/f) and performed lung histopathology, flow cytometry, and signaling analyses across AMs and IMs. GM-CSF stimulation assays evaluated STAT5 phosphorylation in wild-type versus knockout AMs. Bone-marrow cultures were used to generate AM-like cells with GM-CSF to test mTOR-dependent differentiation and survival. WT AM-like cells were treated with rapamycin and GM-CSF. Intraperitoneal temsirolimus was administered to WT mice three times per week at 30 mg/kg/wk for four weeks. Human Protein Atlas datasets were interrogated for expression patterns of key signaling pathways in human AMs. LysMCremTORf/f mice were generated and developed pulmonary alveolar proteinosis with AM loss, lipid-laden foamy AMs, and surfactant accumulation, while IMs and other tissue macrophages remained intact, indicating niche-specific mTOR dependence. Temsirolimus produced a similar AM depletion in vivo. AMs displayed higher baseline mTOR activity, CSF2RA, phosphorylated STAT5, and PPARγ than IMs. GM-CSF induced robust STAT5 phosphorylation in wild-type but not knockout AMs, whereas IMs showed neither basal nor inducible STAT5 activation. Human datasets demonstrated concordant AM-selective enrichment of CSF2RA, JAK2, STAT5, mTOR, phosphorylated S6, PPARγ, C/EBPβ, and downstream effectors (CD36, MerTK, Bcl-2 family proteins). In vivo, mTOR-deficient AMs underwent progressive apoptosis with loss of PPARγ despite preserved CSF2RA. In vitro, GM-CSF drove AM-like differentiation marked by Siglec F, CD206, and CD11c regardless of genotype. In contrast, mTOR-deficient AM-like cells failed to upregulate p70 S6 kinase, C/EBPβ, PPARγ, CD36, MerTK, Bcl-2, and Bcl-xL. Rapamycin impaired survival of wild-type AM-like cells in a dose-dependent manner, partially rescued by increasing GM-CSF. Together, these findings identify mTOR as a non-redundant regulator of GM-CSF survival signaling in AMs—a dependency absent in IMs—supporting an AM mechanism for mTOR inhibitor-associated alveolar injury. Citation Format: Samson Hennessy-Strahs, Xiaojun Su, Si Sun, Ge Deng, Wen Muo, Tasha Miller, Xiang Xiao, Xian C. Li. mTOR enables alveolar macrophage survival and GM-CSF-STAT5-PPARγ signaling: Genetic and temsirolimus evidence [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1599.