<i>Salmonella </i>Typhimurium <i>StiP</i>-mediated upregulation of membrane protein Alx drives complement evasion via CFI-dependent C3b degradation

Invasive Salmonella Typhimurium (<i>S</i>. Typhimurium) causes lethal bloodstream infections (BSI), yet its molecular mechanisms remain unclear. We compared serum resistance and cellular invasive capabilities between invasive and non-invasive <i>S</i>. Typhimurium. Invasive strains exhibited significantly enhanced serum resistance (&gt;17-fold survival in 75% isolates) and HeLa cell invasive ability (minimum bacterial loads 3.93 × 10⁵ CFU/mL) compared to non-invasive strains (&lt; 9.8-fold survival; maximum bacterial loads 1.47 × 10⁵ CFU/mL). We next performed pan-genomic analysis and virulence gene database comparisons to elucidate the genetic mechanisms underlying robust serum resistance in invasive <i>S</i>. Typhimurium. We identified 15 differential genes unique to invasive <i>S</i>. Typhimurium, among which the <i>StiP</i> deletion strain (263<i>ΔStiP</i>) showed the greatest serum resistance reduction (2.49-fold). We further explored the role of <i>StiP</i> in host blood environment adaptation and found that 263<i>ΔStiP</i> displayed 3.12-fold reduced HeLa cell adhesion, 3.74-fold lower HeLa cell invasion, 1.92-fold decreased intra-macrophage survival, and 50% reduced serum resistance versus wild-type 263 (WT263), collectively indicating that <i>StiP</i> is critical for host blood environment adaptation in invasive <i>S</i>. Typhimurium. Mechanistically, <i>StiP</i> upregulates the membrane protein Alx, which recruits complement factor I (CFI) to accelerate C3b degradation, thereby inhibiting classical complement pathway activation and enhancing invasive <i>S</i>. Typhimurium complement evasion. In vivo, 263<i>ΔStiP</i>-infected mice exhibited 5-, 7-, and 4-fold lower bacterial loads in blood, liver, and spleen (P &lt; 0.001), respectively, with reduced pathological damage versus WT263. Thus, this study elucidates the <i>StiP</i>-Alx axis mediating complement evasion in invasive <i>S</i>. Typhimurium.侵袭性鼠伤寒沙门氏菌（*Salmonella Typhimurium*，下称*S.* Typhimurium）可引发致死性血流感染（bloodstream infections, BSI），但其致病分子机制尚未阐明。本研究对比了侵袭性与非侵袭性*S.* Typhimurium的血清抗性（serum resistance）与细胞侵袭能力。结果显示，相较于非侵袭性菌株（存活率不足9.8倍；最高菌载量为1.47 × 10⁵ CFU/mL），侵袭性菌株的血清抗性显著增强（75%的临床分离株存活率提升超17倍），且对HeLa细胞的侵袭能力更强（最低菌载量达3.93 × 10⁵ CFU/mL）。随后，本研究通过泛基因组分析（pan-genomic analysis）与毒力基因数据库比对，解析了侵袭性*S.* Typhimurium具备强大血清抗性的遗传基础。研究鉴定出15个侵袭性*S.* Typhimurium特有的差异基因，其中*StiP*缺失菌株（263ΔStiP）的血清抗性降幅最为显著，达2.49倍。本研究进一步探究了*StiP*在宿主血液环境适应中的作用，发现相较于野生型菌株263（WT263），263ΔStiP对HeLa细胞的黏附能力降低3.12倍、侵袭能力降低3.74倍、巨噬细胞内存活率降低1.92倍，且血清抗性下降50%，综合表明*StiP*对于侵袭性*S.* Typhimurium适应宿主血液环境至关重要。从分子机制层面而言，*StiP*可上调膜蛋白Alx的表达，Alx能够招募补体因子I（complement factor I, CFI）以加速C3b的降解，从而抑制经典补体通路的激活，增强侵袭性*S.* Typhimurium的补体逃逸能力。体内实验结果显示，相较于WT263感染组，263ΔStiP感染小鼠的血液、肝脏与脾脏中的菌载量分别降低5倍、7倍与4倍（*P* < 0.001），且病理损伤程度显著轻于WT263感染组。综上，本研究阐明了*StiP*-Alx轴介导侵袭性*S.* Typhimurium补体逃逸的分子机制。