Toll-like Receptor 4 Engagement on Dendritic Cells Restrains Phago-Lysosome Fusion and Promotes Cross-Presentation of Antigens

Abstract

The initiation of cytotoxic immune responses by dendritic cells (DCs) requires the presentation of antigenic peptides derived from phagocytosed microbes and infected or dead cells to CD8(+) T cells, a process called cross-presentation. Antigen cross-presentation by non-activated DCs, however, is not sufficient for the effective induction of immune responses. Additionally, DCs need to be activated through innate receptors, like Toll-like receptors (TLRs). During DC maturation, cross-presentation efficiency is first upregulated and then turned off. Here we show that during this transient phase of enhanced cross-presentation, phago-lysosome fusion was blocked by the topological re-organization of lysosomes into perinuclear clusters. LPS-induced lysosomal clustering, inhibition of phago-lysosome fusion and enhanced cross-presentation, all required expression of the GTPase Rab34. We conclude that TLR4 engagement induces a Rab34-dependent re-organization of lysosomal distribution that delays antigen degradation to transiently enhance cross-presentation, thereby optimizing the priming of CD8(+) T cell responses against pathogens.

Patterns, Receptors, and Signals: Regulation of Phagosome Maturation

“…One example is the TLR4 agonist LPS, whose effect on phagosome maturation kinetics is time dependent: short stimulation of DCs with LPS (up to 6 h) does not alter phagosomal antigen degradation, while intermediate (8–18 h) and long (20–40 h) stimulations induce a delay in phagosomal antigen degradation [33]. This is achieved by the perinuclear clustering of lysosomes, which is controlled by Rab34, leading to reduced phagolysosomal fusion.…”

Cationic liposomes promote antigen cross-presentation in dendritic cells by alkalizing the lysosomal pH and limiting the degradation of antigens

“…Throughout our studies, we used endotoxinfree chicken OVA as a model protein antigen since OVA has been widely used in initial vaccine development studies as well as in studies examining the mechanisms of crosspresentation. 12,[19][20][21]36,37 BMDCs were pulsed with soluble OVA in the presence or absence of CLs. After overnight + T-cells isolated from OT-I transgenic mice that only have CD8…”

“…[9][10][11] Importantly, CLs can also enhance antigen cross-presentation 12,13 -the process by which APCs phagocytose extracellular protein antigens, process them intracellularly into peptide epitopes, and present them in the context of major histocompatibility complex-I (MHC-I) on the The specialized role of DCs in cross-presentation is supported by their unique characteristics, including their mildly degradative phagosomes compared to those of other phagocytic cells, such as macrophages; shuttling of endosomal protein antigens to the cytosol for subsequent processing via proteasomes for antigen degradation; and efficient loading of the processed antigen peptides onto MHC-I molecules via either endoplasmic reticulum (ER) or vacuolar endosomal pathways. [16][17][18][19][20][21] Prior studies have also suggested that DCs exhibit a delayed process of acidification and maturation of endolysosomes, which inhibit the activities of lysosomal proteases and deter antigen degradation. [21][22][23][24] This would allow antigens to be "rescued" from excessive degradation and shuttled to intracellular compartments for complexation with MHC-I molecules.…”

Microbiota, Immune Subversion, and Chronic Inflammation

“…The induction of a CD4 T cell response is largely determined by pathogen detection by DCs that present antigens to naïve T cells to initiate an adaptive response. DCs can also be activated to efficiently cross-present extracellular antigens on MHC-I, leading to activation of CD8 T cells (144, 145). …”