Cross-Presentation of Tumor Antigens Is Ruled by Synaptic Transfer of Vesicles among Dendritic Cell Subsets

Migratory dendritic cells (DCs) in tumors transport antigens and share them with lymph node resident DCs through cross-presentation. In this issue of Cancer Cell, Ruhland et al. demonstrate that transport and transfer of tumor antigens in vesicles is a dominant pathway to load resident DCs for presentation to T cells. Migratory dendritic cells (DCs) in tumors transport antigens and share them with lymph node resident DCs through cross-presentation. In this issue of Cancer Cell, Ruhland et al. demonstrate that transport and transfer of tumor antigens in vesicles is a dominant pathway to load resident DCs for presentation to T cells. Dendritic cells (DCs) are professional antigen presenting cells (APCs) specialized for the uptake, processing, and presentation of antigens to effector cells to elicit immune responses or tolerance. They originate from hematopoietic stem cells and are sparse in frequency but widely distributed in tissues and lymphoid organs. DCs are classified as conventional DCs (consisting of two subsets, cDC1 and cDC2) and plasmacytoid DCs, and they are further classified based on their location as migratory DCs (mDCs) or lymphoid resident DCs (rDCs). mDCs are located in peripheral tissues and comprise mDC1 (CD103+XCR1+CLEC9A+) and mDC2 (CD11b+SIRPα+) subsets. They migrate to draining lymph nodes in a CCR7-dependent manner after antigen encounter to induce tolerance in the steady state or immunity toward pathogens or tumor cells (Roberts et al., 2016Roberts E.W. Broz M.L. Binnewies M. Headley M.B. Nelson A.E. Wolf D.M. Kaisho T. Bogunovic D. Bhardwaj N. Krummel M.F. Critical Role for CD103(+)/CD141(+) Dendritic Cells Bearing CCR7 for Tumor Antigen Trafficking and Priming of T Cell Immunity in Melanoma.Cancer Cell. 2016; 30: 324-336Abstract Full Text Full Text PDF PubMed Scopus (320) Google Scholar, Salmon et al., 2016Salmon H. Idoyaga J. Rahman A. Leboeuf M. Remark R. Jordan S. Casanova-Acebes M. Khudoynazarova M. Agudo J. Tung N. et al.Expansion and Activation of CD103(+) Dendritic Cell Progenitors at the Tumor Site Enhances Tumor Responses to Therapeutic PD-L1 and BRAF Inhibition.Immunity. 2016; 44: 924-938Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar). Previous studies established hierarchical functions for these subsets: mDC1s prime CD8+ T cells, while mDC2s prime CD4+ T cells (Pooley et al., 2001Pooley J.L. Heath W.R. Shortman K. Cutting edge: intravenous soluble antigen is presented to CD4 T cells by CD8- dendritic cells, but cross-presented to CD8 T cells by CD8+ dendritic cells.J. Immunol. 2001; 166: 5327-5330Crossref PubMed Scopus (429) Google Scholar). Moreover, the prevailing paradigm has been that mDCs constantly gather antigens from their local environments, whereas rDCs acquire antigens from sources such as lymphatic drains (rich in apoptotic bodies and peptides) in addition to cellular sources, e.g., DC vaccines or migrating cells that release antigens (Yewdall et al., 2010Yewdall A.W. Drutman S.B. Jinwala F. Bahjat K.S. Bhardwaj N. CD8+ T cell priming by dendritic cell vaccines requires antigen transfer to endogenous antigen presenting cells.PLoS One. 2010; 5: e11144Crossref PubMed Scopus (74) Google Scholar). Cytotoxic CD8+ T cells are critical for the clearance of tumor cells through the recognition of peptides presented on major histocompatibility complex-I (MHC-I). DCs can acquire antigens exogenously for MHC-I presentation, a specialized process known as cross-presentation, and cDC1s have been shown to have the highest antigen presentation capacity among DC subsets (Roberts et al., 2016Roberts E.W. Broz M.L. Binnewies M. Headley M.B. Nelson A.E. Wolf D.M. Kaisho T. Bogunovic D. Bhardwaj N. Krummel M.F. Critical Role for CD103(+)/CD141(+) Dendritic Cells Bearing CCR7 for Tumor Antigen Trafficking and Priming of T Cell Immunity in Melanoma.Cancer Cell. 2016; 30: 324-336Abstract Full Text Full Text PDF PubMed Scopus (320) Google Scholar). cDC1s utilize an array of mechanisms to acquire exogenous antigens including endocytosis, phagocytosis of dying apoptotic or necrotic cells and their debris, receptor mediated uptake (e.g., immune complexes), intercellular antigen transfer via exosomes, and cross-dressing (transfer of MHC-peptide complexes between cells through trogocytosis) (Albert et al., 1998Albert M.L. Sauter B. Bhardwaj N. Dendritic cells acquire antigen from apoptotic cells and induce class I-restricted CTLs.Nature. 1998; 392: 86-89Crossref PubMed Scopus (1978) Google Scholar, Wakim and Bevan, 2011Wakim L.M. Bevan M.J. Cross-dressed dendritic cells drive memory CD8+ T-cell activation after viral infection.Nature. 2011; 471: 629-632Crossref PubMed Scopus (174) Google Scholar). However, the relative role of each pathway in the cross-presentation of tumor antigens by mDC1s and mDC2s and the contribution of lymph node resident DC subsets has remained unclear. Ruhland et al. determine that tumor antigens are located within vesicles in intratumoral mDC1s and mDC2s in a murine melanoma model (Ruhland et al., 2020Ruhland M.K. Roberts E.W. Cai E. Mujal A.M. Marchuk K. Beppler C. Nam D. Serwas N.K. Binnewies M. Krummel M.F. Visualizing Synaptic Transfer of Tumor Antigens Amongst Dendritic Cells.Cancer Cell. 2020; 37 (this issue): 786-799Abstract Full Text Full Text PDF Scopus (21) Google Scholar). When the mDCs migrate to the lymph node, these vesicles are transferred to resident DCs. Vesicles consist of membrane-bound structures containing tumor antigens that are transferred across an undefined synapse requiring close and sustained contact between the donor and recipient DCs, with the former supplying vesicles to multiple recipient DCs. Both mDC1 and mDC2 subsets are efficient donors and able to share vesicles with rDCs as well as retain sufficient vesicles for efficient antigen presentation. T cell activation is strictly restricted to only those DCs that have acquired vesicles; the mDC1, mDC2, and rDC1 subsets are capable of activating CD8+ T cells, whereas CD4+ T cell activation is specifically restricted to mDC2s (Figure 1). The figure illustrate the summary of the process described. rDC1s exhibit a superior ability to acquire the vesicles from donor DCs compared to rDC2s, a process actively regulated by a phosphoinositide 3-kinase (PI3K)-dependent mechanism. Even though both rDC subsets were capable of acquiring antigen-containing vesicles, only rDC1s could cross-present the antigens to CD8+ T cells, confirming a previously established order between the two subsets (Pooley et al., 2001Pooley J.L. Heath W.R. Shortman K. Cutting edge: intravenous soluble antigen is presented to CD4 T cells by CD8- dendritic cells, but cross-presented to CD8 T cells by CD8+ dendritic cells.J. Immunol. 2001; 166: 5327-5330Crossref PubMed Scopus (429) Google Scholar). Other mechanisms of cross-presentation (acquisition of exosomes or dying cells; cross-dressing) appear to be less relevant in this model, at least in vitro, and further experiments will be necessary to address their contributions in vivo, especially as tumors grow in size. Why would both mDC1s and rDC1s be required for generating anti-tumor CD8+ T cells? Ruhland et al. show that rDC1s generate T cells with a short-term effector phenotype (higher KLRG1/IL-7R alpha ratio, lower CD69 intensity) compared to mDC1s. rDC1s are known to boost memory CD8+ T cells in the context of viral infection, and the data presented here confirm the inherent ability of these DC subsets for generating effector T cell responses efficiently (Wakim and Bevan, 2011Wakim L.M. Bevan M.J. Cross-dressed dendritic cells drive memory CD8+ T-cell activation after viral infection.Nature. 2011; 471: 629-632Crossref PubMed Scopus (174) Google Scholar). The authors observe increased KLRG1+ effector T cells following stimulation with rDC1s than mDC1s and conclude that tumor loading of these cells might skew away from memory phenotypes. However, KLRG1+ effector CD8+ T cells can differentiate into memory cell lineages that retain proliferative and cytotoxic activity necessary for effective long-term anti-tumor immunity (Herndler-Brandstetter et al., 2018Herndler-Brandstetter D. Ishigame H. Shinnakasu R. Plajer V. Stecher C. Zhao J. Lietzenmayer M. Kroehling L. Takumi A. Kometani K. et al.KLRG1+ Effector CD8+ T Cells Lose KLRG1, Differentiate into All Memory T Cell Lineages, and Convey Enhanced Protective Immunity.Immunity. 2018; 48: 716-729.e8Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar). Functional ablation from dLN or nodal activation with suitable adjuvants for inducing strong T cell responses will determine how effectively these DCs modulate anti-tumor immunity. The study sheds light on a coordinated sequence of DC involvement in antigen transfer from the tumor microenvironment (TME) to draining lymph nodes (dLN). mDCs ferry antigens to the dLN at very early stages of tumor development, and vesicles are progressively acquired by rDCs. The induction of an effector-type T cell response from rDC1s may be necessary to mount a rapid potent antigen-specific T cell response for tumor clearance, while the mDC1s sustain this through the development of more long-term memory. Despite the best efforts of both DCs in the TME and the lymph nodes to deliver antigens to T cells, tumors continue to grow. Optimal intratumoral mDC numbers are required to generate effective immunity (Salmon et al., 2016Salmon H. Idoyaga J. Rahman A. Leboeuf M. Remark R. Jordan S. Casanova-Acebes M. Khudoynazarova M. Agudo J. Tung N. et al.Expansion and Activation of CD103(+) Dendritic Cell Progenitors at the Tumor Site Enhances Tumor Responses to Therapeutic PD-L1 and BRAF Inhibition.Immunity. 2016; 44: 924-938Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar, Spranger et al., 2017Spranger S. Dai D. Horton B. Gajewski T.F. Tumor-Residing Batf3 Dendritic Cells Are Required for Effector T Cell Trafficking and Adoptive T Cell Therapy.Cancer Cell. 2017; 31: 711-723.e4Abstract Full Text Full Text PDF PubMed Scopus (477) Google Scholar), and cDC maturation seems to be necessary to drive T cell memory formation. Ruhland et al. show that in the steady-state setting, antigen capture is mostly restricted to mDC1s, and antigen is not shared with rDCs (Ruhland et al., 2020Ruhland M.K. Roberts E.W. Cai E. Mujal A.M. Marchuk K. Beppler C. Nam D. Serwas N.K. Binnewies M. Krummel M.F. Visualizing Synaptic Transfer of Tumor Antigens Amongst Dendritic Cells.Cancer Cell. 2020; 37 (this issue): 786-799Abstract Full Text Full Text PDF Scopus (21) Google Scholar). Delivery of an inflammatory stimulus, however, induces the mDC1s to transfer antigens to rDCs, suggesting context-specific regulation of which DC subsets have access to antigens and can induce a T cell response. Detailed characterization of the vesicles and their biogenesis during steady state or tumor development, as well as deciphering the precise mechanism and requirements of vesicle transfer between the subsets, will be key for developing novel approaches for cancer therapy that promote antigen presentation and T cell activation. Indeed, efforts to expand tumor-associated APCs with Flt3L (Salmon et al., 2016Salmon H. Idoyaga J. Rahman A. Leboeuf M. Remark R. Jordan S. Casanova-Acebes M. Khudoynazarova M. Agudo J. Tung N. et al.Expansion and Activation of CD103(+) Dendritic Cell Progenitors at the Tumor Site Enhances Tumor Responses to Therapeutic PD-L1 and BRAF Inhibition.Immunity. 2016; 44: 924-938Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar, Hammerich et al., 2019Hammerich L. Marron T.U. Upadhyay R. Svensson-Arvelund J. Dhainaut M. Hussein S. Zhan Y. Ostrowski D. Yellin M. Marsh H. et al.Systemic clinical tumor regressions and potentiation of PD1 blockade with in situ vaccination.Nat. Med. 2019; 25: 814-824Crossref PubMed Scopus (132) Google Scholar) and delivery of intratumoral immune modulatory stimuli to promote DC maturation and migration (Kyi et al., 2018Kyi C. Roudko V. Sabado R. Saenger Y. Loging W. Mandeli J. Thin T.H. Lehrer D. Donovan M. Posner M. et al.Therapeutic Immune Modulation against Solid Cancers with Intratumoral Poly-ICLC: A Pilot Trial.Clin. Cancer Res. 2018; 24: 4937-4948Crossref PubMed Scopus (37) Google Scholar, Hammerich et al., 2019Hammerich L. Marron T.U. Upadhyay R. Svensson-Arvelund J. Dhainaut M. Hussein S. Zhan Y. Ostrowski D. Yellin M. Marsh H. et al.Systemic clinical tumor regressions and potentiation of PD1 blockade with in situ vaccination.Nat. Med. 2019; 25: 814-824Crossref PubMed Scopus (132) Google Scholar) are newer approaches to enhance anti-tumor immunity. The elegant experiments of Ruhland et al. highlight the immunological relevance of antigen transfer between DC subsets. They also indicate that immune responses to tumor antigens may depend more heavily upon intercellular vesicle transfer than in other circumstances (Wakim and Bevan, 2011Wakim L.M. Bevan M.J. Cross-dressed dendritic cells drive memory CD8+ T-cell activation after viral infection.Nature. 2011; 471: 629-632Crossref PubMed Scopus (174) Google Scholar). A better understanding of the relative role of DC subsets in modulating the CD8+ T cell response will also be critical for optimizing therapeutic immune-based approaches to treat cancers. Nina Bhardwaj serves on the advisory board for Neon Therapeutics, Tempest, Checkpoint Sciences, Curevac, PrimeVax, Novartis, Array Biopharma, Roche, Avidea, Boehringer Ingelheim, Rome Therapeutics Carisma, BreakBio, and Roswell Park Comprehensive Cancer Center. Visualizing Synaptic Transfer of Tumor Antigens among Dendritic CellsRuhland et al.Cancer CellJune 08, 2020In BriefRuhland et al. capture a novel vesicle-encapsulated and contact-dependent transfer of tumor-derived material and antigens between dendritic cells, leading to effective and varied T cell priming in the lymph node. Full-Text PDF Open Archive