FcγRIIB交联依赖性激动性抗体LVGN6051(抗CD137)和LVGN7409(抗CD40)的开发机制[1]
1.TNFSF-TNFRSF表达谱和生物学功能
肿瘤坏死因子超家族(TNFSF)和肿瘤坏死因子受体超家族(TNFRSF)分别由19种配体和29种受体组成，包括CD137 (TNFRSF9，4-1BB) 和CD40 (TNFRSF5)[2]。TNFSF和TNFRSF与适应性免疫系统由共同的祖先进化而来[3-5]。进化史表明，TNFSF和TNFRSF成员结构和功能具有高度保守性。在结构上，他们都具有相似的激活下游信号的配体受体三聚体结构。功能上，TNFRSF成员具有某种程度上共性的表达谱和生物学功能，例如，促进T细胞和B细胞的增殖或树突细胞和吞噬细胞的成熟。同时，每一个TNFSF或者TNFRSF家族成员的表达谱，信号网络和对于免疫系统功能的影响都应该有其个性化的区别。例如CD27，CD37和OX40在刺激T细胞对抗肿瘤时分别发挥不同作用。CD27和HVEM表达在激活早期的休眠的T细胞上，而OX40 和CD137信号则出现在激活晚期的T细胞，OX40 和 CD137分别主要影响CD4 T和CD8 T细胞[6]。另一项关于CD27，CD137，OX40 和 GITR对CD8 T细胞细胞因子分泌的影响研究表明，尽管CD137和CD27增加了免疫系统对刺激的敏感性，但是只有CD137能够延长应答反应持续时间，增加细胞因子分泌量，相反，GITR和OX40几乎没有影响[7]。在用自体肿瘤细胞进行的体外肿瘤浸润淋巴细胞(TILs)刺激实验中，与OX40+、PD-1+、CD25+和CD69+ TILs相比，CD137+ TILs上IFN-γ、TNF-α、颗粒酶B、穿孔素和IL-2等效应因子表达量最高，提示CD137+ TILs可能具有最强的抗肿瘤细胞毒性。
2. TNFRSF信号的激活依赖于受体聚集反应
TNFRSF可以被其相应的配体TNFSF严格调控激活。如上所述，TNFRSF和TNFSF成员在TNFRSF-TNFSF相互作用和下游信号激活方面都很保守。TNFRSF受体是1型跨膜蛋白，细长的结构，主要包含3-4个高度保守的“半胱氨酸富集结构域”(CRDs)。TNFSF三聚体主要结合到TNFRSF的第2和第3 CRDs 上，促进有活性的TNFSF3-TNFRSF3复合体的形成（图1）[8-10]。该复合体是TNFRSF信号激活的最小单位。事实上，对于大部分TNFRSF成员来说，仅依靠这种三聚体本身并不足以激活下游的信号通路，需要进一步在TNFSF3-TNFRSF3三聚体的基础上形成多聚体才能完全激活TNFRSFs信号[11]。
图1.TNFSF3-TNFRSF3复合体的聚集过程[1]。
三聚体膜结合TNFSF通过TNF同源区（TDH）结合到TNFRSF富含半胱氨酸区（CDR），招募三个TNFRSF受体分子，形成TNFSF3-TNFRSF3复合体。然而，对于大部分TNFRSF成员来说，仅依靠这种三聚体本身并不足以激活下游的信号通路（包括CD40、CD137、OX40和GITR），需要进一步在TNFSF3-TNFRSF3三聚体的基础上形成多聚体才能完全激活TNFRSFs信号。
以下是TNFRSF配体与受体之间的激活机制，激动性抗体需要能够促进TNFRSF受体低聚体的形成(图2)。
图2 xLinkAb工作模型：FcγRIIB在TNFSFR-IgG-FcγRIIB复合物聚集中的作用[1]。
3. TNFRSF激动性抗体的治疗潜力
许多TNFRSF成员被评估为潜在的新型免疫治疗靶点，特别是考虑到它们在细胞毒性T细胞的激活、克隆扩增和生存方面的重要的直接或间接作用，类似PD-1的调节机制。事实上，20世纪90年代末，靶向TNFRSF的研究在就引起了医学界的兴趣，甚至早于PD-1拮抗剂的开发。除了T细胞共刺激因子CD137外，CD40作为一种罕见的髓系细胞共刺激因子也是肿瘤免疫治疗的重要靶点，有望改善肿瘤微环境，挑战冷肿瘤。如今，有数十种以单克隆或多特异性抗体形式的CD137和CD40激动剂候选药物正在进行临床试验开发、临床扩展策略包括单药或者与免疫检查点抑制剂(如抗CTLA4或抗PD-1/PD-L1抗体)或化疗等标准治疗方案联合治疗[12-15]。表1和表2分别列出了具有代表性的CD137 和 CD40激动性抗体的临床开发情况。
表1. 用于癌症治疗的CD137激动行抗体的临床研究，单药或联合用药
表2. 用于癌症治疗的CD40激动性抗体的临床研究，单药或联合用药
4. 总结
TNFRSF信号通路刺激多种免疫细胞的激活和增殖，包括髓系细胞和T细胞，为癌症免疫治疗的开发提供了潜在靶点。然而，TNFRSF激动性抗体在临床中成功的案例很少，可能是由于早期研发对于TNFRSF下游信号通路依赖受体多聚体的机制不清晰，或者由于剂量限制毒性，临床开发受限于过低的剂量。激活性抗体的激动活性可受到Fab、铰链或Fc区域的影响。通过多年临床前及临床研究积累，Fc-Fc𝛾RIIB相互作用已被确定为激动活性(疗效和毒性)的主要决定因素。FcγRIIB可通过抗体介导TNFRSF受体聚集，激活其下游信号通路，发挥体内抗肿瘤活性的重要作用。体外研究表明，无Fc交联依赖性的强激动性抗体在与靶标结合时表现出激动性，在有FcγRs的情况下成为超级激动剂，导致临床很难克服的毒性问题。相反，在没有Fc交联的情况下，另一类抗TNFRSF抗体可能呈现很弱或无检测到的激动活性，却可在Fc-FcγRIIB交联时表现出强激动活性，这类Fc-FcγRIIB介导的条件性激动剂更有临床开发空间。
经Fc工程化改造的， 与FcγRIIB选择性结合，FcγRIIB交联依赖的TNFRSF激活抗体，掀起了治疗药物临床开发的新浪潮。Fc工程化改造技术应适用于TNFRSF的大部分成员，包括CD40、CD137、OX40、GITR和CD27。此外，交联依赖性可用于开发抗TNFRSF的双特异性或多特异性抗体，其中FcγRIIB可被肿瘤抗原或免疫靶点替代或增加。与这些肿瘤选择性靶点的结合可导致多价肿瘤靶向-抗体-TNFRSF复合物的形成，进而在肿瘤微环境中促进TNFRSF的聚集和下游信号选择性激活。
综上所述，免疫共刺激靶点，特别是TNFRSF成员如CD137和CD40，使得肿瘤免疫疗法仍存在巨大发展空间。从临床疗效、安全性、生产工艺和分子稳定性等方面考虑，IgG结构的单克隆抗体激动剂相比于早研阶段的双抗有其独特的优势。包括礼进生物公司开发的xLinkAb模型在内，进一步探索FcγRIIB交联依赖的Fc工程改造方法成为下一步研究重点[1]。
本文仅作信息分享，不代表礼进生物公司立场和观点，也不作治疗方案推荐和介绍。如有需求，请咨询和联系正规医疗机构。
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