Regulation of the Cell Biology of Cross-presentation
Watch the October 28, 2020 Global Immunotalk on the topic by Professor Blander.
Major histocompatibility complex class I (MHC-I) molecules, present on all nucleated cells, present peptides at the cell surface to CD8 T cells. These peptides provide a snapshot of the protein expression profile of a cell, and can derive from either cellular proteins or viruses and bacteria that infect the cells. Professional antigen presenting dendritic cells also conduct cross-presentation to present peptides derived from extracellular sources, such as virus-infected cells or tumor cells, by their MHC-I molecules. Cross-presentation plays an important role in anti-viral and anti-cancer immunity. There has been a lot of interest and controversy surrounding the mechanisms of cross-presentation. Much of it has centered on pathways and sites of peptide processing. We have taken a fundamentally different approach investigating how cross-presentation is regulated reasoning that signals from Toll-like receptors (TLR), which detect microbial structures and alert the immune system, would be prime candidates for exerting regulation.
There is an urgent need for developing T cell vaccines. Current vaccine technologies have yielded limited success for protection against infections like tuberculosis, malaria, HIV and hepatitis C where CD8 T cells prevent and control infection. Licensed vaccines generate mostly neutralizing or opsonizing antibodies effective against extracellular pathogens. Understanding the regulation of cross-presentation will inform novel combinations of adjuvants and antigen targeting methods that achieve the most robust subcellular mobilization of MHC-I for cross-priming CD8 T cells.
Toll-like receptor signals mobilize ERC stores of MHC-I for cross-presentation
Dendritic cells (DC) present antigens to naive T cells and mediate their priming and activation into effector T cells. We found a large number of MHC-I molecules contained within the endosomal recycling compartment (ERC) of DC, where many surface molecules are internalized to undergo slow recycling to the plasma membrane. We discovered a TLR-regulated vesicular trafficking pathway that delivers ERC-resident MHC-I molecules specifically to phagosomes carrying microbial antigens. When internalized cargo carries microbial ligands that engage TLR signaling, synaptosomal-associated protein 23 (SNAP-23) on phagosomes is phosphorylated by IKK2 stabilizing interactions between ERC cellubrevin and endobrevin and a phagosomal syntaxin to mediate fusion of ERC-derived vesicles with the microbe-containing phagosome. ERC-phagosome fusion delivers critical numbers of MHC-I for cross-presentation precisely where they are needed during infection. Our work has shown that the control of subcellular MHC-I trafficking lies at the core of the regulation of cross-presentation (See Nair-Gupta P. et al. Cell 2014; Blander J.M. Annu Rev Immunol 2018).
How to create a cross-presentation compartment in dendritic cells?
Two pathways of vesicular traffic converge on the cross-presentation compartment in dendritic cells. The schematic illustrates these pathways after phagocytosis of microbial cargo by dendritic cells. The ERGIC pathway traffics vesicles from the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) and is dependent on pairing of the ER SNARE Sec22b with syntaxin-4 on the phagosomal membrane (Cebrian I et al. Amigorena S and Savina A Cell 2011). This pathway delivers the transporter associated with antigen processing (TAP), other components of the peptide-loading complex (PLC), and possibly also the ER retrotranslocon Sec61. The ERGIC pathway is mobilized independently of TLR signals during phagocytosis, regardless of the nature of the phagocytosed cargo. The ERC pathway is regulated by TLR and it traffics vesicles from the ERC to phagosomes to deliver MHC-I molecules specifically to phagosomes carrying microbial antigens (Nair-Gupta P et al. Cell 2014). The small GTPase Rab11a is important for formation of the MHC-I-rich ERC. The arrow from the phagosome to the plasma membrane represents traffic of microbial peptide (green)-loaded MHC-I to the cell surface for recognition by CD8 T cells.
Traffic of MHC-I from the ERC to phagosomes is dependent on TLR signals, while traffic of components of the peptide-loading complex (PLC) from ERGIC to phagosomes is independent of TLR signals and dependent on the ER-SNARE Sec22b.
Non-canonical cross-presentation that escapes Toll-like receptor control
Many clinically important human viruses such as Herpesviridae and Poxviridae have devised strategies to block the classical pathway of MHC-I presentation to evade cytotoxic CD8 T cells. Viruses can do so by blocking the transporter associated with antigen processing (TAP), a heterodimeric ATP-binding-cassette transporter on the endoplasmic reticulum membrane that transports cytosolic peptides generated by the proteasome into the ER for loading onto MHC-I molecules. The current paradigm holds that TAP blockade in dendritic cells (DC) critically compromises their ability to prime an anti-viral CD8 T cell response. It is widely understood that cross-presentation by uninfected TAP-sufficient DC that have phagocytosed infected dying cells is primarily responsible for priming an anti-viral CD8 T cell response. DC with compromised TAP are considered to be no longer functional.
We found that TAP blockade in DC mobilizes a counter cell-intrinsic strategy to rescue MHC-I presentation and ensure the activation of a CD8 T cell response (Barbet, G, Nair-Gupta P et al. Nat Immunol 2021). Upon blockade of TAP, the ERC, which harbors MHC-I molecules in DC, becomes depleted and can no longer deliver MHC-I to phagosomes, negatively impacting TLR-regulated cross-presentation (Schematic). Under these conditions, DC traffic MHC-I to phagosomes from the ERGIC instead to restore MHC-I cross-presentation. We have named this pathway non-canonical cross-presentation.Our findings suggest there is a previously unrecognized pathway for priming a population of CD8 T cells which recognize epitopes that are TAP-independent, and kill cells infected with viruses seeking to hide from the conventional TAP-dependent CD8 T cells. We call this pathway non-canonical cross-presentation because it relies on a different sub-cellular source of MHC-I molecules and is not subject to TLR control. Research on this novel pathway will be critically important to understand the full spectrum of CD8 T cells mobilized against viral infection. We are working to understand the consequences of losing TLR control on cross-presentation during infection.
Read the Accelerating Cancer Immunotherapy Research (ACIR) April 14, 2021 Weekly Digest highlighting our recent work.
Ongoing work: Regulation of endocytic antigen cross-presentation
Studying the regulation of cross-presentation uncovered the critical component that dictates its outcome: the trafficking of MHC-I molecules (See Nair-Gupta P. et al. Cell 2014; Blander J.M. Annu Rev Immunol 2018). TLR signals do so by controlling the routing of MHC-I specifically to phagosomes carrying bacteria or infected apoptotic cells. Whether this general principle applies to the cross-presentation of antigens derived from endocytosed proteins or viruses remains elusive. We have found that the cross-presentation of endocytic antigen is also regulated by TLR signals, but the regulatory mechanisms are different from those controlling phagocytic antigen cross-presentation. Although the molecular basis for this regulation appears to be different, we hypothesize that the regulation of endocytic antigen cross-presentation nevertheless relies on controlling the traffic of MHC-I to endosomes containing internalized antigens. We are working to define the cellular source of MHC-I important for endocytic antigen cross-presentation. Using genetic, cell biological, and biochemical approaches, this work will determine the distinct receptor systems that orchestrate the regulation of endocytic antigen cross-presentation at different steps.
Schematics created with BioRender.com
When TAP is blocked, MHC-I molecules can still traffic to phagosomes, but do so from their new location in the ERGIC by coopting Sec22b-dependent vesicular traffic. While this rescues CD8 T cell priming, it bypasses control by TLR signals.