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© 2000 American Society for Clinical Oncology
Dendritic Cells as Immunologic Adjuvants for the Treatment of CancerMemorial Sloan-Kettering Cancer Center New York, NY HARNESSING THE IMMUNE system to combat disease has long represented an important goal for immunologists. Cancer has proven especially challenging for several reasons. Most tumor antigens are self or differentiation antigens toward which the immune system is usually unresponsive, tumor cells are poor initiators of immune reactions, and the tumor microenvironment can directly inhibit immunologic reactivity. This contrasts with examples of effective immunity, like those that occur in response to many viral infections. Two articles in this issue of the Journal of Clinical Oncology1,2 address potential applications of dendritic cells in the immunotherapeutic management of cancer, raising a number of issues that merit review of this topic. Immunity Does Not Live by Antigen Alone To stimulate a cellular immune response against a tumor, one needs more than just antigen and lymphocytes. Another cell is required that can bind and present tumor antigen(s) to T lymphocytes in a way that is immunogenic. If successful, this means that the immune system reacts against, rather than ignores, all other cells bearing the same antigen. The exact cell was obscure, however, until the early 1970s, when Steinman et al3 first purified a novel leukocyte from mouse lymphoid tissue. They termed this cell a "dendritic cell," on the basis of its unusual morphology of circumferential veiled dendrites. Dendritic cells were also distinguished from other antigen-presenting cells, such as macrophages and B cells, on the basis of their potent capacity to stimulate T-cell responses, even when the other antigen-presenting cells had been removed. Unlike B lymphocytes that can respond to soluble or native antigen by transforming into antibody-secreting plasma cells, T lymphocytes can only respond to peptide fragments of protein antigens bound to surface major histocompatibility complex (MHC) molecules. MHC molecules are of two types, class I and class II. For the most part, class I MHC molecules bind intracellular or endogenous antigens that have been cut into smaller peptides in the cytosol. Class II MHC molecules bind extracellular or exogenous antigens endocytosed by the antigen-presenting cell. Recently, a novel means by which antigens from dying cells are taken up by dendritic cells and shuttled into the class I MHC pathway was reported.4 Although MHC-bound antigen may be sufficient for recognition as a target of an immune reaction, it is not sufficient to initiate the reaction. This is the basis for the unique specialization of dendritic cells, which bear not only abundant MHC molecules of both classes but also enormous amounts of costimulatory and adhesive molecules. These collectively termed accessory molecules support antigen-specific binding between T cells and MHC-complexed antigen. They also transmit all the other requisite signals to resting or naïve T cells to undergo blast transformation into activated, cytokine-secreting, and/or cytolytic T-lymphocyte effectors. The inducible increase in the constitutive expression of these accessory molecules parallels dendritic cell capture of antigen in the periphery, followed by maturation and migration to secondary lymphoid organs where dendritic cells select and stimulate antigen-specific, resting T cells. Activated T cells then exit into the periphery to execute their appointed functions. This then is the crux of the difference between tumor cells and dendritic cells as antigen-presenting cells. It is the coupling of antigen/MHC presentation by dendritic cells with potent accessory function, which tumor cells lack, together with the migratory capacity of dendritic cells from peripheral sites of antigen capture to lymphoid sites of antigen presentation, that supports dendritic cell stimulation of T-cell immunity. What’s in a Name? Dendritic cells are a trace leukocyte subpopulation in blood and tissues in the steady-state. Dendritic cells are also not identifiable in fresh blood (eg, peripheral-blood smears) without cytofluorographic detection of surface markers, as freshly isolated dendritic cells do not exhibit their unique morphology unless activated in some way. The capacity to generate dendritic cells ex vivo, however, has led to a greater appreciation of the various cells that can be called dendritic cells. Defining the development of dendritic cells from clonogenic CD34+ hematopoietic progenitor cells (HPCs) has been especially useful in this regard. For example, it is now recognized that there are at least two broad types of dendritic cells, one myeloid and the other lymphoid-related.5-10 Among the myeloid dendritic cells are Langerhans cells in epidermal or epithelial surfaces and dermal or interstitial dendritic cells in the dermis of skin or interstitium of solid organs. The latter are considered counterparts of the blood monocyte-derived dendritic cells from peripheral blood. The myeloid-related dendritic cells are the focus of applications to stimulate immunity against cancer. Consistent and reliable characteristics exist that distinguish dendritic cells from less differentiated and more immature precursors, as well as from other leukocytes. The circumferential cytoplasmic extensions that are so unique and eponymous to dendritic cells become more pronounced with maturation. Mature myeloid-related dendritic cells also lack significant expression of any epitopes specific to lymphocytes or macrophages. Mature dendritic cells increase expression of MHC, especially class II, and CD83,11 the function of which is unknown but which nevertheless remains the best available maturation marker. CD83 is also unique to the dendritic cell differentiation pathway among myeloid cells.12 Although some activated lymphocytes can express low levels of CD83, macrophages never do.12 Expression of all costimulatory and adhesive molecules (eg, CD40, CD50, CD54, CD58, CD80, and CD86, among others) increases with dendritic cell maturation, but these epitopes are in no way restricted to dendritic cells and cannot be used alone for their identification. Terminally matured dendritic cells are also consistently the most potent stimulators of T cells, by one to two logs, compared with any other candidate physiologic antigen-presenting cell, such as B lymphocytes or macrophages. A Bull Market on Dendritic Cell Expansion Previous methods to isolate these specialized leukocytes in the steady-state were laborious, time-consuming, and of low yield.13,14 When the properties and requirements for growth and differentiation of dendritic cells were elucidated, however, large numbers of dendritic cells became available. This has in turn spawned enormous zeal for the large-scale experimental use and clinical application of dendritic cells in the immunologic control of human disease. Most investigators use peripheral-blood monocytes to generate dendritic cells in vitro. These nondividing CD14+ precursors are more accessible than CD34+ HPCs. Owing to their more limited differentiation potential, blood monocytes can also yield more highly enriched populations of dendritic cells with fewer enrichment steps than required when starting with multipotent CD34+ HPCs. However, the use of CD34+ HPCs leads to substantial expansion from cycling precursors, and methods have evolved to enhance purity of the resulting dendritic cell progeny. One also more readily obtains both myeloid dendritic cell types from CD34+ HPCs, and investigations are underway to compare these in vivo as immunogens. Granulocyte-macrophage colony-stimulating factor has consistently proven to be the most pivotal cytokine for dendritic cell growth, differentiation, and survival, from all precursors in both animal and human systems. Other cytokines line up primarily on the side of supporting expansion (eg, stem-cell factor [c-kit ligand] and FLT-3 ligand), differentiation (eg, tumor necrosis factor alpha, transforming growth factor beta, and interleukin [IL]-4), or terminal maturation (eg, IL-1, IL-6, tumor necrosis factor alpha, prostaglandin E2, and type I interferon alpha/beta) or activation (eg, CD40 ligand). FLT-3 ligand is one cytokine, however, that can by itself increase circulating dendritic cells in vivo, an effect not seen when FLT-3 ligand is used alone in vitro.15,16 Other factors not yet identified must therefore contribute to its effect in vivo. Morse et al1 have extended these findings in a study of patients with metastatic colon carcinoma published in this issue. FLT3 ligand administration in vivo increased the percentage and absolute number of circulating dendritic cells as well as increased dendritic cell infiltration into metastatic lesions, when compared with untreated controls. Delayed-type hypersensitivity also increased nonspecifically to several recall antigens. The results remain descriptive, however, and data regarding the mechanism and biologic effect of dendritic cell mobilization and tumor infiltration must still be pursued. Whether expansion and differentiation of dendritic cells by cytokine administration in vivo will translate into enhanced immune responses also merits active investigation. Dendritic Cells as Trojan Horses in the War on Cancer Tumors have antigens, and dendritic cells have everything else needed to stimulate T-cell immunity. The challenge then is how best to combine the two, now that sufficient numbers of dendritic cells are obtainable. Various methods for loading tumor antigen(s) onto dendritic cells are under study, including peptide pulsing, gene transfer,17-20 tumor cell–dendritic cell fusions as heterokaryons,21 and the uptake of dying tumor cells.22 Approaches that allow dendritic cells to tailor antigenic peptides for presentation on their own MHC alleles, rather than depending on defined peptides and MHC restrictions, may ultimately prove more widely applicable. Among the many variables to test in clinical trials, there is an emerging consensus that mature dendritic cells are more effective immunogens than immature precursors and are less likely to revert to immature, less active forms. Otherwise, it is open season on the selection of types of antigen, means for moving antigen into the class I and/or II MHC presentation pathways, routes of dendritic cell vaccine administration, and even the optimal dendritic cell subset for immunization. Careful documentation of immune responses to dendritic cell tumor vaccines is also essential23-25 and does not rely solely on assessment of clinical outcomes. Sensitive biologic assays of T-cell reactivity are increasingly available to quantify changes before and after dendritic cell–tumor vaccination. These include, for example, enzyme-linked immunosorbent assay–based tests for detecting cytokines secreted by activated T cells (ELISpot assays) and tetramer staining, whereby a fluorochrome-tagged tetrameric complex of antigen and MHC molecules binds specific antigen-reactive T cells. Although less sensitive, the usual assay of 51Cr release from tumor targets killed in vitro by T cells, after being activated in vivo by dendritic cell–tumor immunization, remains a standard assay of cytotoxic T cell development. Initial animal studies and clinical trials of human dendritic cell vaccines are generating encouraging preliminary results, both in patients with cancer and in healthy volunteers.23-26 Many investigators are focussing on melanoma to validate dendritic cell immunotherapy,23,25 because immunologic responses associated with disease activity have been better documented in melanoma than in other tumors. Immunodominant peptide antigens and MHC restrictions are also far better defined for melanoma. Immunotherapeutic interventions are also being evaluated in prostate cancer, as exemplified by the study of patients with hormone-refractory disease by Small et al2 published in this issue. These investigators used a modification of early techniques for isolating autologous human dendritic cells, not from defined precursors differentiated in recombinant cytokines but from the high-density fraction of fresh blood after a brief period of culture.13,26,27 During this culture, cells were exposed to a recombinant fusion protein, linking full-length human prostatic acid phosphatase and granulocyte-macrophage colony-stimulating factor, and then infused intravenously. Those postvaccination proliferative responses that did develop were specific for the immunizing antigen. Delayed time to disease progression also correlated with development of a proliferative response to prostatic acid phosphatase and a higher dose of infused cells. T-cell cytotoxic activity was not measured. Unfortunately, the proportion of mature dendritic cells infused also cannot be ascertained, as a specific dendritic cell epitope or group of epitopes was not used for their identification. In fact, 60% to 70% of the infused cells were CD3+ T cells that neither presented antigen nor provided costimulatory signals. Control assays, such as the allogeneic mixed leukocyte reaction, used to confirm the stimulatory capacity of the infused cells, also demonstrated much lower activity than would be exerted by a properly validated population of dendritic cells. All That Glitters May or May Not Be Gold There is great promise in the use of dendritic cells as immune adjuvants for treating human disease, especially cancer. Nevertheless, many unknowns remain that will challenge the successful and routine application of these cells as therapeutic agents. For example, tumors are adept at suppressing immune responses and evading immune recognition. Activated T cells require adequate mobilization and access to tumor sites. Coordination with other therapies that limit systemic tumor spread will also be important. In fact, most tumor immunologists anticipate an adjuvant role for dendritic cell vaccines, emphasizing the continued need for therapy that reduces tumor burden beforehand. Nevertheless, the increased access that basic and clinical investigators now have to these unique cells offers enormous possibilities. These should be exploited in logical, stepwise increments and evaluated in a careful and critical manner. The dendritic cell system is far more complex than originally suspected, but with such complexity come opportunities to develop effective approaches to initiating and controlling T cell–mediated immunity for treating cancer. REFERENCES
1.
Morse MA, Nair S, Fernandez-Castal M, et al: Preoperative mobilization of circulating dendritic cells by Flt3 ligand administration to patients with metastatic colon cancer. J Clin Oncol 18: 3883-3893, 2000
2.
Small EJ, Fratesi P, Reese DM, et al: Immunotherapy of hormone-refractory prostate cancer with antigen-loaded dendritic cells. J Clin Oncol 18: 3894-3903, 2000 3. Steinman RM, Lustig DS, Cohn ZA: Identification of a novel cell type in peripheral lymphoid organs of mice: III. Functional properties in vivo. J Exp Med 139: 1431-1445, 1974[Abstract] 4. Albert ML, Sauter B, Bhardwaj N: Dendritic cells acquire antigen from apoptotic cells and induce class I-restricted CTLs. Nature 392: 86-89, 1998[Medline] 5. Caux C, Dezutter-Dambuyant C, Schmitt D, et al: GM-CSF and TNF-a cooperate in the generation of dendritic Langerhans cells. Nature 360: 258-261, 1992[Medline]
6.
Young JW, Szabolcs P, Moore MAS: Identification of dendritic cell colony-forming units among normal CD34+ bone marrow progenitors that are expanded by c-kit-ligand and yield pure dendritic cell colonies in the presence of granulocyte/macrophage colony-stimulating factor and tumor necrosis factor a. J Exp Med 182: 1111-1120, 1995
7.
Szabolcs P, Avigan D, Gezelter S, et al: Dendritic cells and macrophages can mature independently from a human bone marrow-derived, post-CFU intermediate. Blood 87: 4520-4530, 1996
8.
Caux C, Vanbervliet B, Massacrier C, et al: CD34+ hematopoietic progenitors from human cord blood differentiate along two independent dendritic cell pathways in response to GM-CSF+TNF alpha. J Exp Med 184: 695-706, 1996
9.
Grouard G, Rissoan M-C, Filgueira L, et al: The enigmatic plasmacytoid T cells develop into dendritic cells with IL-3 and CD40-ligand. J Exp Med 185: 1101-1111, 1997
10.
Kadowaki N, Antonenko S, Lau JY, et al: Natural interferon alpha/beta-producing cells link innate and adaptive immunity. J Exp Med 192: 219-226, 2000 11. Zhou L-J, Tedder TF: Human blood dendritic cells selectively express CD83, a member of the immunoglobulin superfamily. J Immunol 154: 3821-3835, 1995[Abstract] 12. Zhou L-J, Tedder TF: HB15, a specific marker for human blood dendritic cells, in Schlossman SF, Boumsell L, Gilks W, et al (eds): Leukocyte Typing V, White Cell Differentiation Antigens. Oxford, United Kingdom, Oxford University Press, 1995, pp 695-697 13. Young JW, Steinman RM: Accessory cell requirements for the mixed leukocyte reaction and polyclonal mitogens, as studied with a new technique for enriching blood dendritic cells. Cell Immunol 111: 167-182, 1988[Medline]
14.
Freudenthal PS, Steinman RM: The distinct surface of human blood dendritic cells, as observed after an improved isolation method. Proc Natl Acad Sci U S A 87: 7698-7702, 1990
15.
Pulendran B, Banchereau J, Burkeholder S, et al: Flt3-ligand and granulocyte colony-stimulating factor mobilize distinct human dendritic cell subsets in vivo. J Immunol 165: 566-572, 2000
16.
Maraskovsky E, Daro E, Roux E, et al: In vivo generation of human dendritic cell subsets by Flt3 ligand. Blood 96: 878-884, 2000
17.
Song W, Kong H-L, Carpenter H, et al: Dendritic cells genetically modified with an adenovirus vector encoding the cDNA for a model tumor antigen induce protective and therapeutic antitumor immunity. J Exp Med 186: 1247-1256, 1997
18.
Specht JM, Wang G, Do MT, et al: Dendritic cells retrovirally transduced with a model tumor antigen gene are therapeutically effective against established pulmonary metastases. J Exp Med 186: 1213-1221, 1997 19. Nair SK, Heiser A, Boczkowski D, et al: Induction of cytotoxic T cell responses and tumor immunity against unrelated tumors using telomerase reverse transcriptase RNA transfected dendritic cells. Nat Med 6: 1011-1017, 2000[Medline]
20.
Boczkowski D, Nair SK, Nam JH, et al: Induction of tumor immunity and cytotoxic T lymphocyte responses using dendritic cells transfected with messenger RNA amplified from tumor cells. Cancer Res 60: 1028-1034, 2000
21.
Gong J, Avigan D, Chen D, et al: Activation of antitumor cytotoxic T lymphocytes by fusions of human dendritic cells and breast carcinoma cells. Proc Natl Acad Sci U S A 97: 2715-2718, 2000 (published erratum appears in Proc Natl Acad Sci U S A 97:5011, 2000) 22. Albert ML, Darnell JC, Bender A, et al: Tumor-specific and anti-neuronal killer cells in paraneoplastic cerebellar degeneration. Nat Med 4: 1321-1324, 1998[Medline]
23.
Thurner B, Haendle I, Roder C, et al: Vaccination with mage-3A1 peptide-pulsed mature, monocyte-derived dendritic cells expands specific cytotoxic T cells and induces regression of some metastases in advanced stage IV melanoma. J Exp Med 190: 1669-1678, 1999 24. Dhodapkar MV, Krasovsky J, Steinman RM, et al: Mature dendritic cells boost functionally superior CD8(+) T-cell in humans without foreign helper epitopes. J Clin Invest 105: R9-R14, 2000[Medline]
25.
Schuler-Thurner B, Dieckmann D, Keikavoussi P, et al: Mage-3 and influenza-matrix peptide-specific cytotoxic T cells are inducible in terminal stage HLA-A2.1+ melanoma patients by mature monocyte-derived dendritic cells. J Immunol 165: 3492-3496, 2000 26. Hsu FJ, Benike C, Fagnoni F, et al: Vaccination of patients with B-cell lymphoma using autologous antigen-pulsed dendritic cells. Nat Med 2: 52-58, 1996[Medline] 27. Markowicz S, Engleman EG: Granulocyte-macrophage colony-stimulating factor promotes differentiation and survival of human peripheral blood dendritic cells in vitro. J Clin Invest 85: 955-961, 1990
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Copyright © 2000 by the American Society of Clinical Oncology, Online ISSN: 1527-7755. Print ISSN: 0732-183X
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