Roles of Sectm1a in macrophages and cardiac function during sepsis

宿主免疫系统失调会削弱心脏功能，极大地增加了患 危重病人败血症导致的死亡。尽管进行了数十年的密集研究，但基本机制仍然存在 难以捉摸。特别是，尸检结果表明，大多数脓毒症患者都有未解决的感染灶， 这表明，巨噬细胞功能受损无法根除入侵的病原体。因此，任何 增强巨噬细胞功能的策略有望提高患者的总体存活率。我们最近 新发现，分泌和跨膜蛋白1(Sectm1)的水平通常很高 在髓系免疫细胞和上皮细胞中的表达在脓毒症患者的血液中显著降低 患者多于健康的捐赠者。我们的最新数据也一致地表明，Sectm1a， 人Sectm1的小鼠同源物，在外周血单核细胞和腹膜中显著减少 从脓毒症小鼠收集的巨噬细胞，与对照小鼠相比。因此，Sectm1a-淘汰赛(KO) 小鼠表现出：1)血液和腹膜灌洗液中的细菌负荷更高，2)全身 炎症反应，以及3)心功能恶化以及存活率较低 败血症条件下野生型(WT)小鼠。机制上，Sectm1a-KO巨噬细胞显示 吞噬和杀菌能力减弱。相比之下，Sectm1a在 巨噬细胞的吞噬能力和杀菌活性增强。因此，测试将非常重要 巨噬细胞特异性过表达Sectm1a是否可通过促进 细菌清除。我们的实验数据还显示，巨噬细胞Sectm1a可以引发自分泌作用。 通过与TNFRSF18(肿瘤坏死因子受体超家族膜18)的膜受体强烈结合 称为GITR或CD357，以下简称GITR)。后续实验显示，治疗巨噬细胞 重组Sectm1a蛋白激活GITR信号转导，进而增强吞噬能力。 因此，测试在脓毒症小鼠体内注射重组Sectm1a蛋白是否不仅促进 细菌清除，还可减轻全身炎症反应，改善心肌功能 以及动物的生存。这些翻译思想将通过追求三个具体目标来检验：1)定义 利用巨噬细胞特异性的Sectm1a在多菌败血症时巨噬细胞中的确切作用 Sectm1a-过度表达的小鼠模型；2)确定Sectm1a-诱导的抗- 脓毒症依赖GITR，使用GITR基因敲除的小鼠模型；3)研究治疗方法 利用重组Sectm1a蛋白治疗脓毒症的可能性。拟议的研究预计将 鉴定Sectm1a是一种有效和新颖的宿主免疫调节因子，是预防败血症的主要保护者。如果 经过验证，这项提案的发现应该会为增强巨噬细胞提供新的治疗选择 在脓毒症期间清除细菌的功能，并有望将脓毒症导致的死亡降至最低。

Dysregulation of the host immune system that weakens cardiac function greatly increases the odds of sepsis-induced death in critically ill patients. Despite decades of intensive study, basic mechanisms remain elusive. In particular, autopsy results indicate that most patients with sepsis had unresolved infectious foci, suggesting that impaired macrophage function fails to eradicate the invading pathogens. Therefore, any strategies that boost macrophage function would be expected to improve overall patient survival. We recently made the novel findings that the levels of secreted and transmembrane 1 (Sectm1), a protein normally highly expressed in immune cells of myeloid lineage and epithelial cells, was significantly lower in the blood of septic patients than healthy donors. Consistently, our latest data also showed that the expression levels of Sectm1a, a mouse homolog of human Sectm1, were greatly reduced in peripheral blood monocytes and peritoneal macrophages collected from septic mice, compared to control mice. Accordingly, Sectm1a-knockout (KO) mice exhibited: 1) a higher bacterial load in the blood and peritoneal lavage fluid, 2) an increased systemic inflammatory response, and 3) a deteriorated cardiac function as well as a lower survival rate, compared to wild-type (WT) mice under sepsis conditions. Mechanistically, Sectm1a-KO macrophages showed the impaired capacity of phagocytosis and bacterial killing. By contrast, forced expression of Sectm1a in macrophages augmented phagocytic capacity and bactericidal activity. Hence, it will be important to test whether macrophage-specific overexpression of Sectm1a can protect against sepsis through enhancing bacterial clearance. Our pilot data have also revealed that macrophage Sectm1a can elicit autocrine action by binding strongly to the membrane receptor of TNFRSF18 (TNF receptor superfamily membrane 18, also known as GITR or CD357, hereafter GITR). Follow-up experiments showed that treatment of macrophages with recombinant Sectm1a protein activated GITR signaling, which in turn enhanced phagocytic capacity. Thus, it will be important to test if injection of recombinant Sectm1a protein into septic mice not only promotes bacterial clearance, but also reduces the systemic inflammatory response, and improves myocardial function as well as animal survival. These translational ideas will be tested by pursuing three specific aims: 1) Define the exact role of Sectm1a in macrophages during polymicrobial sepsis, using a macrophage-specific Sectm1a-overexpressing mouse model; 2) Identify the mechanism by which Sectm1a-elicited anti- sepsis is dependent on GITR, using a GITR-knockout mouse model; and 3) Investigate the therapeutic potential using recombinant Sectm1a protein to treat sepsis. The proposed studies are expected to identify Sectm1a as a potent and novel regulator of host immunity and a major protector against sepsis. If verified, the findings from this proposal should provide new therapeutic options for boosting macrophage function in the clearance of bacteria during sepsis and hopefully, to minimize sepsis-induced death.