Immune System–Tumor Crosstalk Under Microgravity: Mechanistic Insights, Challenges, and Translational Perspectives

Despite notable progress in cancer therapy, immune evasion remains a major obstacle to effective treatment outcomes. In the context of spaceflight, astronauts are exposed to unique environmental stressors—particularly microgravity and radiation—that profoundly affect cellular and immune homeostasis. Emerging evidence suggests that microgravity alters key cellular processes, including proliferation, apoptosis, adhesion, and oncogenic signaling pathways such as NF-κB and ERK1/2. Concurrently, microgravity (µg) disrupts immune regulation, potentially facilitating both tumor progression and treatment resistance. Of particular concern is the upregulation of human endogenous retroviruses (HERVs), especially HERV-K and HERV-W, under µg conditions, which may exacerbate inflammatory responses and immune system dysregulation. While some studies indicate that µg may impair tumor growth, others reveal enhanced immune evasion and reduced antitumor immunity. Importantly, insights from µg research extend beyond space medicine and provide translational opportunities for terrestrial oncology, including the development of physiologically relevant 3D tumor models for drug screening, the identification of mechano-sensitive pathways (FAK/RhoA, YAP/TAZ) as therapeutic targets, and novel immunotherapeutic strategies involving epigenetic modulation and checkpoint inhibition. This review critically examines the dual role of µg in modulating cancer progression and immune function. We synthesize findings on how µg shapes immune responses, alters tumor–immune system interactions, and impacts the efficacy of immunotherapeutic approaches. Finally, we highlight translational opportunities and challenges for optimizing cancer immunotherapy and precision oncology in both spaceflight and Earth-based environments.