NKT细胞同源辅助促进B细胞记忆应答的机制研究

Cognate help of CD4+ Tfh cell to B cell is essential for humoral immune response against infections. NKT cell can regulate humoral immunity, as it has been shown inhibiting autoreactive B cells and autoimmune diseases or promoting antibody production against infections. Although the mechanisms are not clear, several studies have reported that cognate and noncognate help providing by NKT cell to B cell can enhance antibody production. Noncognate NKT help depends on DC activation and DC primed CD4+ Tfh-B cell cognate interaction. In contrast, CD4+ Tfh cell is not required for cognate NKT help. NKT cell recognizes lipid ligand presented by CD1d molecule of antigen specific B cell and forms a direct NKT-B conjugate. Lipid ligands of NKT cell widely exist in nature. Cognate help of NKT cell could be crucial to humoral immune response against T-independent (TI) antigens, which fail to induce CD4+ Tfh cell help. Studies based on model antigens such as alpha-galactosylceramide (aGC)-nitrophenyl (NP) conjugates concluded that cognate NKT help to B cells failed to induce memory response. However, results from our lab using liposome particles carrying aGC and polysaccharide antigen elicited pure cognate NKT-B interactions in vivo. And in consequence, prolonged antibody responses with isotype switch, affinity maturation, and B memory differentiation were observed. Preliminary data from our lab using same liposome particles carrying aGC and hapten antigen NP, as the antigen used by other group in previous study, suggested NKT cognate help to B cell was competent to induce memory response, which was not limited to polysaccharide antigen. Further studies are required to explain the different outcome in our study, which may be related to several differences between the experimental systems used such as the distinct chemical nature of the B-cell antigens, antigens presenting cells, and B cell origin. Here, we proposed to illustrate the mechanisms in regulating NKT cognate helped B memory differentiation. Our work will help to understand humoral immunity and highlight the potentiality of NKT cognate helped vaccines especially against TI antigens.

B细胞记忆应答是体液免疫阻止病原微生物感染的关键。CD4+ Tfh细胞在促进抗体亲和力成熟、记忆B细胞分化和抗体再次应答中扮演重要角色。对于无法被CD4+ Tfh细胞识别的细菌多糖等TI抗原，则难以产生高亲和力抗体和记忆应答。近年的研究表明,NKT细胞同样参与对体液免疫的调控，可通过对B细胞同源和非同源辅助促进抗体应答。非同源辅助依赖CD4+ Tfh细胞对B细胞的作用，同源辅助依赖NKT细胞与B细胞的直接相互作用。现有观点认为，NKT细胞的同源辅助只能引起迅速而短暂的抗体应答，不能引起记忆应答。然而，我们近期发表的工作以及预实验结果都表明，NKT细胞的同源辅助可以促进抗体亲和力成熟和记忆应答。本项目将在前期工作的基础上，进一步揭示调控NKT-B同源作用的关键细胞和关键分子，阐明NKT细胞同源辅助促进B细胞记忆应答的细胞分子机制。我们的工作将对体液免疫的调控机制以及疫苗设计提供新的理论基础。

体液免疫是机体抵抗病源微生物的重要防线。在此过程中，TFH细胞对B细胞的辅助，是促进抗体亲和力成熟、类型转换、记忆性B细胞分化和抗体再次应答的关键。然而，对于细菌多糖等TI抗原，由于无法被TFH细胞识别，难以引起高亲和力IgG抗体和记忆应答。因此，这类抗原的疫苗效果较差，无法对机体起到保护作用。iNKT细胞也参与对B细胞功能的调控，然而iNKT细胞是否能直接辅助抗体记忆应答、如何辅助抗体记忆应答仍不清楚。我们的研究发现,iNKT细胞可以替代TFH细胞，在TFH缺陷小鼠中通过对B细胞的直接辅助,促进抗体的亲和力成熟以及记忆应答。在免疫后的小鼠中,我们发现了可长期存活的记忆性iNKTFH细胞。记忆性iNKTFH的形成依赖于DC对iNKT细胞的活化。而B细胞与iNKT细胞的相互作用对记忆性iNKTFH细胞的分化并不重要，但确是再次应答中iNKTFH细胞增殖、记忆性B细胞增殖和抗体记忆应答的关键。同时，我们的研究还说明了在缺乏外源脂类抗原的情况下，LPS促进的内源性脂类抗原不足以诱导B细胞与iNKT细胞的相互作用以及抗体记忆应答。因此，细菌自身能合成脂类抗原是iNKT细胞通过直接辅助促进抗感染的抗体记忆应答的关键。我们的研究结果为发展基于iNKT细胞的疫苗设计提供了理论基础，说明了其适用的范围。此外，iNKT细胞作为组织驻留细胞参与对组织免疫稳态的维持，与疾病的发生密切相关。我们的工作还阐释了组织微环境中各因素对iNKT细胞的功能调控机制以及与肿瘤、组织炎症发生的关系。