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Under the Microscope: Dendritic Cells (Feb 2022)

  
Under the Microscope: Dendritic Cells


by:

Alejandro Barquero, MPharm, MSc
Leitat Medical
Barcelona, Spain

Daniel Benítez Ribas, PhD
Hospital Clínic Barcelona
Barcelona, Spain

Raquel Cabezón, PhD
Hospital Clínic Barcelona
Barcelona, Spain

Declarations of interest: none.


Phase contrast image of isolated human monocyte-derived dendritic cells in culture 7 days with rhIL-4 and GM-CSF and exposed 24 hours with heat-inactivated Salmonella typhimurium (10).

What?

Dendritic cells (DCs) are a diverse group of specialized antigen-presenting cells (APCs) derived from bone marrow precursors and form a widely distributed cellular system throughout the body. DCs have a key role in the initiation and regulation of innate and adaptive immune responses (1, 2). They are considered “sentinels” of the immune system for their prominent role in immune-surveillance and capturing pathogens. Via their cell surface pattern recognition receptors (PRRs), they act as “sensors” for activation in response to encountered infections, and as “conductors” of the immune response thereafter. In other words, DCs specialize in capturing and processing antigens, converting proteins to peptides that are presented on major histocompatibility complex (MHC) molecules that are subsequently recognized by T lymphocytes, giving rise to various immunological responses.

Distinct markers and receptors identify subtypes of DCs, which also differ in location, migratory pathways, detailed immunological function and dependence on infections or inflammatory stimuli for their generation. In general, DCs are classified into conventional DCs (cDCs), plasmacytoid (pDCs), and DCs derived from monocytes (mDCs) (2-5).

 

How?

DCs can recognize pathogens, tissue damage signals, and tumour antigens by specific receptors and then migrate to the secondary lymphoid organs where they present antigens and activate naïve and memory T lymphocytes (both CD4 and CD8). DCs may induce the development of diverse immunological responses, either Th1, Th2, Treg, or Th17. DCs are involved in many diseases, for example, contact hypersensitivity, infectious and  autoimmune diseases, and cancer (2), so they can be applied in a variety of therapeutic approaches:

  • In antitumor immunotherapy, by induction of effective immunological responses against cancer cells, through DCs potent ability to initiate and regulate adaptive immune responses (1, 4-6).
  • In hypersensitivity conditions and autoimmune diseases, applying strategies to increase the tolerogenic activity of DCs (1, 2).

A deeper understanding of DC biology and harnessing the therapeutic potential of these subsets, highly specialized in driving specific immune responses, may increase the therapeutic scope of DCs and improve the outcomes of their clinical application, and in addition, a combination of DC application and other strategies would potentiate immune responses.

Who and When?

In 1966, Robert Mishell and Richard Dutton added sheep red blood cells to a suspension of mouse spleen cells, generating the first primary antibody responses in vitro. Experiments mixing antigens with lymphocytes to measure antibody responses brought an unexpected finding: lymphocytes alone were not sufficient to produce immune responses, and accessory cells were required to initiate immunity (7).

In 1970, Ralf Steinman began a postdoctoral fellowship at Rockefeller University in the laboratory of Zanvil Cohn, who had spent his career studying macrophages. In the context of his first assignment at Cohn’s laboratory, Steinman was studying if antigens degraded to amino acids by macrophages were displayed to lymphocytes inducing immunity. However, no evidence of antigen uptake or retention by macrophages was found (7,8).

Next, Steinman and Cohn decided to further study the adherent cell compartment of mouse spleen cells as the potential source of different accessory cells. In 1973, using phase-contrast light microscopy, mixed in with the expected macrophages, Steinman identified a separate small population of extensively branched, motile and mitochondria rich cells that did not display the prominent phagocytic vacuoles typical of macrophages (9). Based on their distinct appearance, these cells were named Dendritic Cells (DC).

Steinman hypothesized that DCs were key immune response initiators, an idea that did not initially gain much support. There was a debate by that time if these proposed new cells were an artefact, or just too rare to be relevant. By using electron microscopy and the best ultra-structural techniques available, Steinman and Cohn ultimately confirmed the existence of DCs as a novel cell type, with features distinct from typical macrophages and monocytes. The first hints of DC function were provided by patterns of proteins expressed on their cell surface. Via the mixed leukocyte reaction, a technique used to mimic T cell-mediated rejection of donor tissue during transplantation, Steinman showed that DCs could indeed initiate T cell activation responses and doing so with 100 to 1,000 times more potency than bulk spleen cells. These findings suggested that DCs presented antigens to naive lymphocytes and were equally potent at stimulating both T cell cytotoxicity and antibody responses. Steinman´s notable discovery of dendritic cells and its role in adaptive immunity was honoured with the 2011 Nobel Prize in Physiology or Medicine (7,8).

 

How to?

DCs are most usually generated in vitro by differentiating peripheral blood mononuclear cells (PBMCs) or from CD34+ hematopoietic stem cells (HSCs). Alternative procedures are based on isolation of monocytes (CD14+ cells) as DC precursors, including apheresis, elutriation, filtration, adherence to plastic, positive selection of CD14+ monocytes and negative selection via lymphocyte depletion (16). In vitro differentiation of isolated CD14+ monocytes into immature DCs typically involves supplementation with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 or -13, which lead to the formation of immature DCs. Activation to immunogenic or tolerogenic mature DCs is achieved after adding inflammatory cytokines, TLR-ligands or immunosuppressive agents, respectively (16,17).

Where to?

The first immunotherapeutic drug based on autologous DCs to obtain marketing authorisation status by FDA was Sipuleucel-T (Provenge) in 2010. Sipuleucel-T is a somatic cell-based medicine (sCTMP), based on autologous peripheral blood mononuclear cells activated with PAP-GM-CSF (11). In 2013, EMA followed FDA, granting authorisation to Sipuleucel-T as well. However, it was withdrawn due to poor commercial performance driven by cost of manufacture in the European Union in 2015.

Currently, Sipuleucel-T is being tested in clinical trials in the US in combination with other anticancer therapies (NCT01804465, NCT02463799, NCT01881867).

Although DC vaccines have shown low efficacy in the treatment of solid and haematological tumours as a monotherapy, combination with other anticancer therapies are anticipated to provide an effective tool for shaping a targeted antitumor T-cell response (11-13). Examples of such promising combinational therapies currently under evaluation for cancer treatment include:

Although the initial clinical applications referred above focus on cancer treatment, the scope of potential DC-based treatments is vast, and varied. For instance, other clinical initiatives are exploring DCs to fight infectious diseases, i.e. HIV; or autoimmune diseases, by using tolerogenic DCs, i.e. Type 1 Diabetes Mellitus, Rheumatoid Arthritis, Crohn's disease, or Multiple Sclerosis (14,15).

Did you know that… Dendritic cells are named for their probing, ‘tree-like’ or dendritic shapes.

References

  1. Wculek SK, Cueto FJ, Mujal AM, Melero I, Krummel MF, Sancho D. Dendritic cells in cancer immunology and immunotherapy. Nat Rev Immunol. 2020;20(1):7-24.
  2. Castell-Rodríguez A, Piñón-Zárate G, Herrera-Enríquez M, Jarquín-Yáñez K, Medina-Solares I. Dendritic Cells: Location, Function, and Clinical Implications. Biol Myelomonocytic Cells. 2017.
  3. Ali Roghanian. Dendritic Cells [Internet]. Bitesized Immunology. 2021 [cited 18 January of 2022]. Available at: https://www.immunology.org/public-information/bitesized-immunology/cells/dendritic-cells
  4. Gardner A, de Mingo Pulido Á, Ruffell B. Dendritic Cells and Their Role in Immunotherapy. Front Immunol. 2020;11(May):1-14.
  5. Cabeza-Cabrerizo M, Cardoso A, Minutti CM, Pereira Da Costa M, Reis E Sousa C. Dendritic Cells Revisited. Annu Rev Immunol. 2021;39:131-66.
  6. Wang Y, Xiang Y, Xin VW, Wang XW, Peng XC, Liu XQ, et al. Dendritic cell biology and its role in tumor immunotherapy. J Hematol Oncol. 2020;13(1).
  7. Nussenzweig MC. Ralph Steinman and the discovery of dendritic cells. Nobel Lect. 2011;1–31.
  8. Katsnelson A. Kicking off adaptive immunity: The discovery of dendritic cells. J Exp Med. 2006;203(7):1622.
  9. Rowley DA, Fitch FW. The road to the discovery of dendritic cells, a tribute to Ralph Steinman. Cell Immunol. 2012;273(2):95–8.
  10. Cabezón R, Ricart E, España C, Panés J, Benitez-Ribas D. Gram-Negative Enterobacteria Induce Tolerogenic Maturation in Dexamethasone Conditioned Dendritic Cells. PLoS One. 2012;7(12).
  11. Filin IY, Kitaeva K V., Rutland CS, Rizvanov AA, Solovyeva V V. Recent Advances in Experimental Dendritic Cell Vaccines for Cancer. Front Oncol. 2021;11(September):1–7.
  12. Mastelic-Gavillet B, Balint K, Boudousquie C, Gannon PO, Kandalaft LE. Personalized dendritic cell vaccines-recent breakthroughs and encouraging clinical results. Front Immunol. 2019;10(APR).
  1. Sadeghzadeh M, Bornehdeli S, Mohahammadrezakhani H, Abolghasemi M, Poursaei E, Asadi M, et al. Dendritic cell therapy in cancer treatment; the state-of-the-art. Life Sci. 2020;254.
  2. Phillips BE, Garciafigueroa Y, Trucco M, Giannoukakis N. Clinical tolerogenic dendritic cells: Exploring therapeutic impact on human autoimmune disease. Front Immunol. 2017;8(OCT):1–9.
  3. Landau NR. Honing the T cell response to HIV: Turning off the noise. EBioMedicine. 2021;64:2020–1.
  4. Sallusto F, Lanzavecchi A. Efficient presentation of soluble antigen by cultured human dendritic cells is maintained by granulocyte/macrophage colony-stimulating factor plus iuterleukin 4 and downregulated by tumor necrosis factor α. J Exp Med. 1994;179(4):1109–18.
  5. Chometon TQ, Da Silva Siqueira M, Sant´anna JC, Almeida MR, Gandini M, De Almeida Nogueira ACM, et al. A protocol for rapid monocyte isolation and generation of singular human monocyte derived dendritic cells. PLoS One. 2020;15(4):1–16.

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