Abstract
Exosomes are lipid bound nanovesicles that are formed via the inward budding of the endosomal membrane, then released upon fusion of the endosomal limiting membrane with the plasma membrane. The majority of exosome studies involve the use of exosomes from bone marrow-derived dendritic cells or immortalised cell lines. This research project has focused on exosomes derived from primary B cells in response to T cell signalling, in particular via the CD40 and the interleukin-4 receptors. The fate of exosomes following their release is largely unknown. However as we have previously identified that B cell-derived exosomes are enriched in the antigen presenting molecules major histocompatibility complex class I (MHC-I), MHC-II and immunoglobulin, this may implicate B cell-derived exosomes in the transfer of antigen. We have sought to address the physiological role of primary B cell-derived exosomes and their interactions with other cells within secondary lymphoid tissue in vivo.
We identified sialoadhesin (CD169; Siglec-1) as a novel mechanism for the specific capture of B cell-derived exosomes via surface expressed α2,3-linked sialic acids. In particular, exosomes were captured by marginal metallophilic and subcapsular sinus macrophages within the spleen and lymph node respectively. In vitro binding assays performed on naïve spleen and lymph node tissue sections confirmed that exosome binding to CD169 was not solely due to preferential fluid flow to these areas. Despite the presence of this specific capture mechanism, the circulation half-life of exosomes in the blood of wild-type and CD169-deficient mice was similar, with a short half-life of approximately two minutes. This likely indicated the presence of CD169-independent capture and uptake mechanisms. However immunofluorescent microscopy revealed intravenously or subcutaneously administered exosomes displayed altered distribution within CD169-deficient compared to wild-type mice. In vivo, exosomes freely accessed the outer marginal zone rim of SIGN-R1+ marginal zone macrophages and to a lesser degree also the F4/80+ macrophages in the spleen. In the lymph node, exosomes appeared less well contained to the subcapsular sinus and had migrated into the medullary sinus. In vivo exosomes were efficiently captured in a CD169-independent manner by Kupffer cells within the liver of both wild-type and CD169-deficient mice, but this capture was not observed in vitro indicating this capture to be purely a physiological response. Interestingly, although the human splenic marginal zone contains relatively few CD169+ macrophages, a pilot experiment involving one human spleen sample revealed that B cell-derived exosomes were also specifically captured in a marginal zone binding pattern using an in vitro binding assay. While this capture within the human spleen sample was likely CD169-independent, the striking similarity between exosome binding within murine and human splenic tissue, implicates the splenic marginal zone as a conserved mediator of exosome capture across species.
We next established a role for B cell-derived exosomes in the promotion of in vivo T cell proliferative and cytotoxic responses. In vivo endogenous cytotoxic T lymphocyte (CTL) responses to exosomal antigen showed an absolute dependence on the presence of CD4+ T cells, CD8+ T cells and natural killer (NK) cells, where the loss of any one of these subsets led to the complete loss of CTL response. Furthermore, NK cell involvement appears independent of interferon-gamma. Interestingly, in the absence of CD169, exosomal antigen induced an enhanced CTL response. This implicates CD169+ macrophages in suppression of the immune response to exosomal antigen, possibly via limiting exosome access to potent antigen presenting cells. While the precise pathway for exosome induced in vivo CTL responses still remains to be determined, we have identified that CD4+ T cells, CD8+ T cells and NK cells each play a critical role in the response to exosomal antigen.