Quah Ben J C, O'Neill Helen C
School of Biochemistry and Molecular Biology, Building 41, Linnaeus Way, Australian National University, Canberra, ACT 0200, Australia.
Blood Cells Mol Dis. 2005 Sep-Oct;35(2):94-110. doi: 10.1016/j.bcmd.2005.05.002.
Exosome production represents an alternate endocytic pathway for secretion. Multivesicular endosomes (MVE) fuse with the plasma membrane expelling internal vesicles or exosomes from cells. Exosome production has been recently described for immune cells including B cells, dendritic cells (DC), mast cells, macrophages and T cells. Exosomes derived from some DC populations stimulate T lymphocyte proliferation in vitro and have potent capacity to generate anti-tumour immune responses in vivo. These reported studies have involved in vitro grown mature DC expanded from precursors with cytokines. However, immature DC produce higher numbers of exosomes than mature DC and this is thought to be due to a reduction in endocytosis as DC mature, associated with reduced reformation of MVE and reduced exosome formation. This lab pioneered a method to generate immature DC in spleen long-term cultures (LTC). DC produced in cultures represent immature myeloid DC, highly endocytic but with weak capacity to stimulate T cells. LTC-DC produce exosomes and contain many MVE. This prompted a study of immunogenic potential with a view to the potential use of exosomes in vaccination and immunotherapy. DC produced in cultures represent immature myeloid DC, highly endocytic but with weak capacity to stimulate T cells. Exosomes were isolated by differential centrifugation from LTC-DC and shown by marker expression to arise by budding from the LAMP-1+ limiting endosomal membrane of MVE. These LTC-derived exosomes appear however to lack immunostimulatory markers like CD86, CD40, MHC-I and MHC-II. While LTC-DC can stimulate antigen-specific proliferation of CD4+ T cells, exosome preparations derived from antigen-pulsed DC were unable to stimulate purified naïve T cells in vitro. They were however found to weakly activate allogeneic CD8+ T cells in vitro. Tumour antigen-pulsed LTC-DC or their exosomes could induce a protective response in mice against growth of a transplanted tumour but could not induce a response to clear an existing tumour. Exosomes derived from immature DC can modulate immune responses, but do not function in direct T cell activation in vitro. Modulation of immune responses by exosomes produced by immature DC may be dependent on the presence of other antigen presenting DC subsets in the animal. The possible function of immature DC and their exosomes in maintenance of tolerance and in the induction of immunity is discussed.
外泌体产生代表了一种替代性的分泌内吞途径。多泡小体（MVE）与质膜融合，将内部小泡或外泌体排出细胞。最近已报道免疫细胞包括B细胞、树突状细胞（DC）、肥大细胞、巨噬细胞和T细胞可产生外泌体。来自某些DC群体的外泌体在体外刺激T淋巴细胞增殖，并在体内具有产生抗肿瘤免疫反应的强大能力。这些已报道的研究涉及用细胞因子从前体体外培养扩增成熟DC。然而，未成熟DC比成熟DC产生更多数量的外泌体，这被认为是由于DC成熟时内吞作用减少，与MVE的重新形成减少和外泌体形成减少有关。本实验室开创了一种在脾脏长期培养（LTC）中产生未成熟DC的方法。培养中产生的DC代表未成熟髓样DC，具有高度内吞作用但刺激T细胞的能力较弱。LTC-DC产生外泌体并含有许多MVE。这促使开展一项关于免疫原性潜力的研究，以期在外泌体用于疫苗接种和免疫治疗方面的潜在应用。培养中产生的DC代表未成熟髓样DC，具有高度内吞作用但刺激T细胞的能力较弱。通过差速离心从LTC-DC中分离出外泌体，并通过标志物表达显示其通过从MVE的LAMP-1+限制性内体膜出芽产生。然而，这些源自LTC的外泌体似乎缺乏如CD86、CD40、MHC-I和MHC-II等免疫刺激标志物。虽然LTC-DC可刺激CD4+ T细胞的抗原特异性增殖，但源自抗原脉冲DC的外泌体制剂在体外无法刺激纯化的初始T细胞。然而，发现它们在体外可微弱激活同种异体CD8+ T细胞。肿瘤抗原脉冲的LTC-DC或其外泌体可在小鼠中诱导针对移植肿瘤生长的保护性反应，但不能诱导清除现有肿瘤的反应。源自未成熟DC的外泌体可调节免疫反应，但在体外不能直接激活T细胞。未成熟DC产生的外泌体对免疫反应的调节可能取决于动物体内其他抗原呈递DC亚群的存在。讨论了未成熟DC及其外泌体在维持耐受性和诱导免疫方面的可能功能。
