irst observed in 1868 as Langerhans’ cells of the epidermis and then brought to attention again 25 years ago by Steinman and colleagues, dendritic cells (DC) remained enigmatic due to their scarcity and difficulties in isolation. Owing to the development of methods allowing in vitro DC generation, a wealth of information regarding their biology recently has been accumulated. Dendritic cells are now recognized as an integral part of the lymphohematopoietic system. Distributed as sentinels throughout the body, DC are poised to capture antigen, migrate to draining lymphoid organs, and, after a process of maturation, select antigen-specific lymphocytes to which they present the processed antigen, thereby initiating immune responses (Figure 1).

The 2 key DC functions, i.e., to capture and to present antigen, segregate in time and space. The soluble or particulate antigen/pathogen that invades tissues is efficiently captured by tissue DC. This triggers their migration into the proximal secondary lymphoid organ, where they mature into a developmental state that allows the selection and activation of antigen-specific T cells. Dendritic cells support the generation of not only lymphokine-secreting helper T cells, but also effector cytotoxic T lymphocytes that subsequently migrate to the site of initial injury to eliminate virally infected cells or tumor cells. The capacity of activating naive T cells is not shared by other antigen-presenting cells. In addition to being involved in the initiation of immunity, DC also appear to play an important role in the induction of immunological tolerance. In particular, thymic DC present endogenous self-peptides to newly generated thymocytes, thereby allowing the deletion of self-reactive T cells. These thymic DC may originate from a precursor cell that also gives rise to lymphocytes and natural killer cells and have thus been called lymphoid DC. There is also evidence for a DC role in the development of peripheral tolerance. Moreover, the immunoregulatory function of DC seems to also include B cells and natural killer cells.

The system of cells called dendritic cells is enormously complex. First, there is no single molecule that they uniquely express. Rather, the DC subsets, as well as maturation stages, are defined by a combination of markers as illustrated in Figure 2. Second, DC comprise 3 distinct subsets, including 2 within the myeloid lineage, Langerhans’ cells and interstitial DC, and 1 within the lymphoid lineage. Moreover, as shown in Figure 3, 3 stages of development have been delineated: 1) precursor DC patrolling through blood and lymphatics, 2) immature DC residing within virtually every tissue, and 3) mature DC residing temporarily within secondary lymphoid organs. In situ, as in the skin and lymphoid organs, DC can be identified as stellate-shaped cells with many dendrites that are long and thin, either fine or sheetlike. The shape and motility of DC suit their functions, initially the efficient capture of antigen and subsequently the selection of rare antigen-specific lymphocytes.

IMMUNOREGULATORY FUNCTION OF DENDRITIC CELLS

Antigen capture and migration

Immature DC efficiently internalize a diverse array of antigens for processing and loading onto major histocompatibility complex (MHC) molecules, as a consequence of high endocytic activity. Antigen uptake by immature DC can occur via 4 distinct mechanisms: 1) macropinocytosis, a process that allows sampling of large amounts of extracellular fluid; 2) receptor-mediated endocytosis through Fc receptors; 3) receptor-mediated endocytosis through the mannose receptor and C-type lectin receptor DEC205; and 4) engulfment of apoptotic bodies.

Cells undergoing programmed cell death (apoptosis) are rapidly cleared in vivo by phagocytes without inducing inflammation. Recently, it has been discovered that apoptotic bodies may represent a way to deliver information to the immune system. Monocyte-derived DC can engulf and process apoptotic bodies. In particular, DC loaded with apoptotic bodies derived from either macrophages or HeLa cells infected with influenza virus can stimulate the proliferation of influenza-specific T cells and the generation of class I–restricted influenza-specific CD8⁺ cytotoxic T lymphocytes. The capture of apoptotic bodies by DC is likely to account for the in vivo phenomenon of “cross-priming,” whereby antigens from tumors or transplanted organs are presented by host antigen-presenting cells. Tolerance to tissue-restricted self-antigen (peripheral tolerance) may actually be initiated by the tissue DC that capture the cells undergoing apoptosis, as occurs during normal cell turnover.

An important attribute of DC at various stages of their maturation is their mobility, which enables them to move from the blood to peripheral tissues and from peripheral tissues to lymphoid organs. The selective migration of DC, their residence in a given tissue, and their migratory capacity are tightly regulated events. Chemotactic factors released by the target tissue and modulation of surface adhesion molecules are involved in these processes. For example, interleukin (IL)-1 and tumor necrosis factor (TNF) activate and mobilize Langerhans’ cells by down-regulating the surface expression of E-cadherin, thereby loosening their interactions with keratinocytes. Interstitial DC likewise migrate from the kidney and heart in response to IL-1 and TNF. Dendritic cells express several chemokine receptors such as CCR1 (receptor for RANTES), CCR2 (receptor shared by MCP-1 and MCP-3), CCR3 (receptor for eotaxin), CCR5 (receptor for MIP-1 and ?, and RANTES), CCR6 (receptor for MIP-3), and CCR7 (receptor for MIP-3?). CCR1, CCR5, and CCR6 are expressed on immature DC and are down-regulated during maturation. Conversely, CCR7 is lacking on immature DC but is induced upon activation. Importantly, MIP-3 is preferentially produced at sites enriched with immature DC, while MIP-3? is preferentially expressed within the paracortex of secondary lymphoid organs where mature DC migrate. Thus, the coordinated expression of distinct chemokine receptors may play a critical role in the migration of DC at various stages of maturation.

Antigen presentation

The mixed lymphocyte reaction, in which T cells proliferate in response to allogeneic antigen-presenting cells, remains the most reliable functional assessment of histocompatibility. Dendritic cells are at least 30- to 100-fold more efficient than other antigen-presenting cells in inducing the mixed lymphocyte reaction. Numerous cytokines, including IL-2, IL-4, and interferon-, are released when DC stimulate T cells. While CD4⁺ cells account for much of the T-cell proliferation during the mixed lymphocyte reaction, DC also can stimulate CD8⁺ T cells without CD4⁺ help, although higher DC numbers are needed. The ability to prime naive T cells to proteins that require processing into peptides constitutes a unique and most critical function of DC.

The interaction between DC and T cells is coordinated by several molecules (Figure 4). “Signal 1” results from recognition of MHC-peptide complexes on DC by antigen-specific T-cell receptor. High levels of several adhesion molecules expression, like integrins ?1 and ?2 and members of immunoglobulin superfamily (CD2, CD50, CD54, CD58), enhance cell-cell interaction and signaling. A variety of accessory molecules, coexpressed on DC (B7.1/CD80, B7.2/CD86, CD40) and interacting with ligands and counterreceptors on T cells, constitute together “signal 2,” which is required to sustain T-cell activation. CD86 on DC is so far the most critical molecule for amplification of T-cell responses both in humans and mice. The interaction between CTLA-4/CD28 on T cells and CD80/CD86 on DC may play a role in the regulation of type 1 vs. type 2 T-cell development (Th1 vs. Th2). In particular, B7.1/CD80 may promote Th1 responses, whereas B7.2/CD86 ligation may skew toward Th2 responses.

While the DC–T cell interaction has been traditionally viewed as a 1-way interaction with DC activating T cells, there is now evidence that T cells play an important role in activating DC, in particular via CD40 ligand (CD40L)–CD40 signaling. Ligation of CD40 increases DC viability, induces their maturation (manifested by increased expression of CD80, CD83, and CD86), and leads to cytokine release, including IL-1, TNF, chemokines, and importantly IL-12, a key cytokine for the generation of Th1 responses. This results in enhanced T-cell stimulatory capacity. The CD40-activated DC can trigger T-killer responses in the absence of helper T cells and thus act as a temporal bridge between a CD4⁺ T helper and a T killer cell. However, CD40 activation of DC can be bypassed by inflammatory agents, as provided by an adjuvant, or by viral infection.

Regulation of B lymphocytes

Dendritic cells, well established as the most capable of T-cell stimulators, are now known to have major effects on B-cell growth and immunoglobulin secretion. B cells and DC are both antigen-presenting cells and both are essential for antibody responses, but for entirely different reasons. Dendritic cells activate and expand T helper cells, which in turn induce B-cell growth and antibody production. But there is a more direct DC–B cell dialogue as well. Naive B cells respond uniquely to the interstitial, non-Langerhans’ cell type of DC. By secretion of soluble factors (including IL-12), DC directly stimulate the production of antibodies and the proliferation of B cells that have been activated by T cell–associated CD40L. Dendritic cells also orchestrate immunoglobulin class-switching of T cell–activated B cells. Soluble factors like IL-10 and transforming growth factor (TGF)-? can induce secretion of IgA1, while expression of IgA2 appears to be strictly dependent on a direct DC–B cell interaction. This indicates that DC are in control of mucosal immunity, and, in fact, DC can be found in mucosal lymphoid tissues beneath antigen-transporting M cells and in T-cell areas.

GENERATION OF HUMAN DENDRITIC CELLS IN VITRO

Dendritic cell development from CD34⁺ progenitors

The DC progenitors represent a small fraction of CD34⁺ hematopoietic progenitor cells derived from bone marrow or peripheral blood. These progenitors grow and differentiate into precursor DC in response to granulocyte-macrophage colony–stimulating factor (GM-CSF) and TNF, and this process can be enhanced and/or modulated by cytokines such as c-Kit ligand, Flt-3-ligand, IL-3, TGF-?, and IL-4 (IL-13). Tumor necrosis factor is critical in DC development, and the addition of neutralizing antibody in the early days of culture blocks DC differentiation in favor of granulocytic pathway.

CD34⁺ hematopoietic progenitor cells contain progenitors for 2 discrete myeloid DC populations: the epidermal Langerhans’ cells and the interstitial DC. The mature DC derived from these 2 in vitro–generated precursor DC subsets are equally potent in stimulating the proliferation of naive, allogeneic CD45RA⁺ T cells or of autologous T cells in the presence of staphylococcal enterotoxin A. However, interstitial DC demonstrate a potent and long-lasting antigen uptake capacity (FITC-dextran or peroxidase) that is about 10-fold higher than that of Langerhans’ cells. The high efficiency of antigen capture of interstitial DC correlates with the expression of nonspecific esterase activity, a marker of the lysosomal compartment. In contrast, Langerhans’ cells do not express nonspecific esterase activity. The most striking functional difference between these subsets is the ability of interstitial DC, but not Langerhans’ cells, to induce naive B-cell proliferation.

Dendritic cell development from blood precursors

Three subsets of precursor DC circulate in the blood: the CD14⁺ monocytes, the CD11c⁺ precursor DC, and the CD11c^- precursor DC. Monocytes can differentiate into macrophages or into cells displaying features of immature DC in response to macrophage colony–stimulating factor or GM-CSF and IL-4 (IL-13), respectively. These monocyte-derived immature DC become mature DC upon CD40L and/or lipopolysaccharide signaling or when cultured with TNF or monocyte-conditioned medium.

Considerable advances have been made in the characterization of the non-monocyte precursor DC in blood. After depletion of T cells, B cells, natural killer cells, and monocytes from blood mononuclear cells, precursor DC can be identified as HLA-DR^hi cells that are CD11c⁺ or CD11c^- with reciprocal expression of CD45RA. Precursor DC of similar phenotype also can be isolated from tonsils with CD11c⁺ precursor DC, which carry immune complexes, principally found within follicles. CD11c^- precursor DC are dependent for survival and differentiation of IL-3 and/or CD40L. These cells may belong to the lymphoid DC subset.

DENDRITIC CELLS IN CLINICAL DISEASE STATES

Being such important regulators of immune responses, DC are implicated in the pathophysiology of several diseases. As a consequence, DC can also become a target or a tool for various immunotherapy protocols.

Transplantation immunity

Interstitial DC originally were suspected to be the passenger leukocytes that led to primary allograft reactions. Indeed, DC migrate from cardiac or liver allografts to the T-cell areas of recipient spleens, where they effectively prime antigen-specific immune responses. The depletion of DC from solid organ grafts (kidney, heart, islets of Langerhans, and thyroid) could prolong graft survival. Clinical trials aimed at depleting donor kidney DC showed beneficial effects. Furthermore, MHC-incompatible tissue devoid of DC could only provoke responses comparable to those induced by minor histocompatibility differences. Very little is known about the role of DC in graft-versus-host disease, but they are likely to be involved, as all the involved sites are populated by DC. Dendritic cells, which are radioresistant, theoretically contribute to direct donor T-lymphocyte allosensitization and prime for the donor immune reactivity that results in the clinical syndrome of graft-versus-host disease.

Transplantation tolerance

The spontaneous acceptance of transplanted livers in mice despite MHC mismatch suggests the existence of tolerance-induction pathways that can be exploited especially by this organ. Inasmuch as liver represents an early site of hematopoiesis, it has been hypothesized that precursor DC are seeded from the liver graft to recipient lymphoid tissue after transplantation. Supporting evidence comes from the identification of donor-derived cells in recipient bone marrow or spleen, whereas such cells are not observed in marrows of mice that are rejecting heart allografts. Microchimerism has also been detected in the tissues or blood of human kidney or liver transplants studied 2 to 30 years posttransplant. Some of the donor cells appear to have been candidate DC. While it can be argued that this microchimerism is merely a consequence of long-term allografting, it is equally plausible that microchimerism actively supports induction of transplantation tolerance. For example, costimulatory molecule–deficient DC progenitors fail to stimulate a primary mixed lymphocyte reaction and induce donor-specific T-cell anergy. Administering costimulatory molecule–deficient precursor DC to normal mice also allows the subsequent engraftment of vascularized cardiac allografts. Thus, in addition to having a role in central tolerance, DC are now regarded as potential modulators of peripheral immune responses, offering a new approach to the immunosuppressive therapy of allograft rejection or autoimmunity.

Contact allergy

Contact sensitivity is a T cell–mediated immune reaction occurring after cutaneous immunization and challenge with low-molecular-weight chemicals (haptens) that covalently bind to self or exogenous proteins. Hapten-modified proteins are then processed by Langerhans’ cells that subsequently migrate to draining lymph nodes to initiate immune responses. Unlike classical delayed-type hypersensitivity to proteins or cellular antigens, mediated primarily by class II MHC-restricted CD4⁺ T cells, the T-cell response to haptens appears more complex and may involve CD4⁺ T cells and/or CD8⁺ T cells, depending on the hapten and the mouse strain. Responses to dinitrofluorobenzene in C57 BL/6 mice are mediated by class I MHC-restricted CD8⁺ effector T cells that can be primed by class I MHC⁺, class II MHC– DC. The response is down-regulated by CD4 regulatory T cells that are primed by class II MHC⁺, class I MHC⁺ DC. In vivo application of IL-10, a potent inhibitor of T cell and DC function, before allergen exposure induces antigen-specific tolerance in mice.

Asthma

In the lung, the network of airway DC is particularly well developed to capture inhaled antigen. Upon encounter of inhaled antigen, airway DC migrate to the draining lymph nodes of the lung and induce primary immune responses. Dendritic cells also are important for presenting inhaled antigen to previously primed Th2 cells, leading to chronic eosinophilic airway inflammation. The number of DC is significantly higher in the airways of asthmatics compared with control subjects, as is the proportion of DC expressing FcE R I-a. Thus, DC may play a significant role in the onset and perpetuation of allergic asthma, and targeting DC may represent an important new approach to the treatment of asthma.

Autoimmunity

Finally, in rheumatoid arthritis, the synovial fluid contains cells that are comparable to blood DC, although the frequency (1% to 5%) is 10-fold higher. Psoriatic skin–derived DC are more active stimulators of autologous T-cell proliferation than are either psoriatic blood–derived or normal skin–derived DC. These psoriatic DC are not more potent in supporting superantigen induced T-cell proliferation, however, which suggests that the autostimulatory potency of psoriatic skin DC may be a critical alteration leading to the skin lesion.

DENDRITIC CELLS AND VIRUSES

Viruses use several mechanisms to escape the immune system and to survive and replicate. Because of the pivotal role in the initiation of immune response, DC represent a target of choice for viruses. Sequestration within the DC themselves may provide a very efficient strategy for viruses not to be identified by the immune system. Moreover, due to the distribution of DC throughout body surfaces like skin and mucosa, DC provide a means for virus to access other cells like T cells.

Influenza virus

Dendritic cells are infected upon exposure to influenza virus, but they remain viable and produce little infectious virus. This contrasts with monocytes/macrophages that produce infectious virus while undergoing apoptosis. Infected DC, but not infected monocytes/macrophages or B cells, can induce recall cytotoxic T-lymphocyte responses by CD8⁺ T cells. Perhaps most relevant to in vivo biology, DC can acquire influenza antigens from virus-infected apoptotic cells and subsequently stimulate class I MHC-restricted CD8⁺ cytotoxic T lymphocytes.

Measles virus

Measles virus causes a profound immunosuppression that is responsible for the high morbidity and mortality induced by secondary infections. While the mechanism of immune suppression is poorly understood, it might be the consequence of virus replication within leukocytes, especially within the lymphoid system. Infected T cells and monocytes die by apoptosis, particularly within syncytia identifiable in vivo in the submucosal areas of tonsils and pharynx after virus has started replicating.

The wild type measles virus as well as the vaccine strains can infect human DC. This infection results in the surface expression of hemagglutinin on a large proportion of DC and the generation of giant syncytia. Infectious virions are produced, and DC eventually undergo apoptosis. Infected DC are unable to stimulate proliferation of alloreactive T cells, which may be explained by a major cytopathic effect of the virus on DC. It is therefore unclear how immunity against measles is ever established. One possibility is that noninfected DC may capture measles virus–induced apoptotic bodies, as occurs with influenza virus, and subsequently initiate cytotoxic T-lymphocyte responses. Alternatively, measles virus may differentially affect the various DC subsets or maturational stages, as evidenced by the fact that measles virus–infected, immature DC induce T-cell death, whereas T-cell viability is not altered by infected mature DC.

Human immunodeficiency virus

Because DC express CD4, the receptor for human immunodeficiency virus (HIV), early studies analyzed whether DC would act as transporters of the virus, initially deposited on the mucosa, to activated T cells in secondary lymphoid organs; or as permissive sites for virus replication. These studies eventually led to the finding that explosive HIV replication occurs when DC and resting T cells are cocultured. Most of the viral production from DC–T cell cocultures occurs within syncytia that are heterokaryons of DC and T cells. Each cell type brings a specific transcription factor, allowing viral genome expression. In accordance with these in vitro studies, HIV-expressing syncytia have been found in vivo at the surfaces of mucosal lymphoid tissues like tonsils and adenoids. Moreover, it has been demonstrated that HIV-1 can infect DC in vitro through interactions with chemokine receptors.

A deficit of circulating DC may explain the early loss of CD4⁺ memory T cells. It is hoped that an improved understanding of the pathogenic role of HIV in the DC system will facilitate the use of DC to establish long-term immunity against HIV.

DENDRITIC CELLS AND TUMOR IMMUNOLOGY

The immune system has the potential to reject tumors, as evidenced by occasional spontaneous remissions in renal cell carcinomas and melanomas. Tumor regression occurs through various pathways, one of which is mediated by cytotoxic T lymphocytes recognizing class I MHC peptide complexes on the tumor cell surface. For this to happen, antigen-presenting cells (and more specifically DC) should first home into the tumor, capture tumor antigens, and then migrate to secondary lymphoid organs to initiate T-lymphocyte responses against the tumor-associated antigens. The final or efferent step of the antitumor immune response occurs when the primed tumor-associated antigen–specific cytotoxic T lymphocytes leave the secondary lymphoid organs and return to the tumor to kill the malignant cells.

Tumors are infiltrated with dendritic cells

Immunohistological analysis of carcinomas using S100 staining as a marker for DC demonstrated that an increased number of DC located within tumors was associated with better prognosis. In situ evaluation of carcinomas reveals frequent infiltration with macrophages and with class II MHC^high CD80^low CD86^low DC with low allostimulatory capacity. A functional analysis of infiltrating DC in treatment-responding vs. progressing metastatic melanoma of the same patient showed that DC infiltrating the responding metastasis have the characteristics of mature DC, with potent allostimulatory properties. In contrast, DC within progressing melanoma metastases are poor allostimulators. The alteration of DC functions in cancer appears to extend beyond the tumor site, as blood DC from patients suffering from stage III and IV breast cancer show decreased allostimulatory capacity. Tumor cells may actively inhibit DC development and function, for example by release of IL-10 and vascular endothelial cell growth factor.

Dendritic cells presenting tumor antigens can cure cancer in mice

Experiments over the past few years have demonstrated the feasibility of eradicating tumors in mice with tumor-associated antigen–loaded DC. Several antigen-delivery systems have been employed, including pulsing with peptides of known sequence, isolating undefined peptides by acid elution from tumor cell lines, or using retroviral or adenoviral vectors. In all instances, the induction of MHC-restricted cytotoxic T-lymphocyte responses and considerable antitumor effects have been observed. Most recently, tumor peptide–pulsed, DC-derived exosomes (subcellular structures containing high levels of MHC molecules and peptides) have been used successfully to prime specific cytotoxic T lymphocytes in vivo and to eradicate or suppress growth of established murine tumors. In the context of effective antigen delivery, the capture and processing of tumor-derived apoptotic bodies represent a new and promising opportunity.

Dendritic cells can be safely injected into humans

Significant clinical responses have been observed in pilot trials using blood-derived DC loaded with lymphoma idiotype. Some clinical responses also have been observed in prostate cancer using monocyte-derived DC pulsed with prostate-specific membrane antigen peptide. Furthermore, melanoma peptide–pulsed, DC-induced clinical regression was reported in 5 of 16 patients treated, 2 of which showed a complete response of all evaluable disease.

Transposing to human cancer the encouraging results observed in mice after DC immunotherapy will require significant efforts for multiple reasons. First, cancer in humans is in no way comparable to experimental tumors in animal models. Second, the complexity of the DC lineage, with its diverse subsets, stages of maturation, and methods of generation, necessitates that each step be tested independently. Furthermore, the nature of the tumor antigens and the optimal method for loading DC with those tumor antigens represent additional parameters for careful analyses. Strategies that introduce antigen into DC, but allow the DC to select and tailor peptides for presentation on available MHC molecules, would circumvent the need to identify tumor-specific peptides with known MHC restrictions a priori. Such approaches would also offer the theoretical advantage of introducing both helper and cytolytic antigenic epitopes for the generation of effective cytotoxic T lymphocytes. Route of administration (intravenous vs. intracutaneous vs. intranodal), the dose of DC, and the frequency of injections also need to be established.

Assuming successful induction of strong antitumor cytotoxic T-lymphocyte activity in patients after DC immunization, there are still caveats to the long-term success of DC-based immunotherapy of cancer. Cytotoxic T lymphocytes may not readily migrate to the tumor site. Tumor variants may lose the class I MHC expression required for cytotoxic T-lymphocyte recognition. Tumor variants also may lose expression of critical tumor antigens, or express surface molecules like Fas ligand, or secrete cytokines like IL-10 that inactivate cytotoxic T lymphocytes. Patients may experience either tumor-related or drug-induced immune suppression that would render cytotoxic T-lymphocyte priming inefficient in vivo. In this case, the priming process may best be accomplished in vitro, followed by adoptive transfer to the diseased host. Despite these potential pitfalls, the prospects are bright for immunotherapy of human cancer and probably for other diseases, using in vitro–generated DC. An alternative approach may be to directly increase the levels of DC in vivo that are capable of capturing tumor antigens and turning in specific immune responses. Accordingly, administration of Flt-3-ligand to mice challenged with fibrosarcoma has been shown to induce complete tumor regression in a significant proportion of mice and decreased tumor growth in the remaining mice. There is, however, some evidence that this effect may not be due to the generation of specific cytotoxic T lymphocytes, but rather to the activation of natural killer cells by the Flt-3-ligand-elicited DC.

GOALS OF DENDRITIC CELL RESEARCH

Dendritic cells have the potential to engage in functions as diverse as the induction of immunity vs. the induction of tolerance, and much remains to be learned about them. In particular, the mechanisms regulating the balance between immunizing and tolerance-inducing DC must be investigated. While the cellular and molecular events involved in T-cell activation by DC are becoming better established, there are enormous deficits in the knowledge of how DC could induce tolerance, especially in the periphery. Answers to these questions will permit the therapeutic manipulation of the DC system. Initially, defined DC populations generated in vitro will be administered to patients to induce either immunity (as required in cancer and infectious diseases) or tolerance (as required in allergy, autoimmunity, and transplantation). Finally, one may directly target DC in vivo using specific pharmacological agents. While single agents like steroids or Flt-3-ligand exert effects on DC in experimental models, more sophisticated strategies targeting various DC subpopulations and various stages of maturation probably will be necessary to enhance or inhibit specific immune responses with precise control. Although the tasks are immense, considerable means from academic, government, private, and industrial sources are now being devoted to DC research. It should not be long before DC-targeted therapy becomes part of numerous medical interventions.

THE STUDY OF DENDRITIC CELLS AT BAYLOR INSTITUTE FOR IMMUNOLOGY RESEARCH

Scientists of the Baylor Institute for Immunology Research are studying DC pathophysiology and are developing immunotherapy protocols targeting or employing DC. Several projects have been initiated in collaboration with various clinical investigators at BUMC. We are studying the biological consequences of Flt-3-ligand administration in lymphoma patients and in healthy volunteers. This project is a collaboration with the bone marrow transplantation unit and Dr. Joseph Fay in particular, as well as with Immunex (Seattle, Wash.), which is providing Flt-3-ligand. Early results show mobilization of various subsets of circulating DC and their progenitors/precursors. Furthermore, we have designed a clinical trial that uses peptide-loaded DC in patients with metastatic melanoma. Our goals are to assess 1) the safety and tolerability of antigen-pulsed DC therapy for patients with metastatic melanoma, 2) the induction of antigen-specific T-cell and B-cell functions, and 3) the eventual clinical response to these vaccination protocols. We hope to initiate the trial before the end of 1998. To generate patients’ DC from their CD34⁺ hematopoietic progenitors, we have designed a special laboratory in the Lieberman Research Building. Dendritic cells will be generated ex vivo using GM-CSF, Flt-3-ligand, and TNF; subsequently loaded with synthetic peptides from 4 melanoma-associated antigens; and then injected into the patient both intravenously and intradermally.

Several studies also are under way to determine the DC status in various diseases. Thus, we are exploring the various populations of blood DC in patients undergoing liver transplantation in collaboration with liver transplant surgeons Drs. G?ran Klintmalm and Marlon Levy. We are studying the status of DC in breast carcinomas in collaboration with Dr. George Netto in the Department of Pathology and with breast cancer surgeons Drs. Sally Knox and Michael Grant. Preliminary data show tumor infiltration with immature DC, while mature DC are found in the peritumoral areas. Finally, DC function in patients suffering from prostate cancer is being evaluated in collaboration with Dr. Michael Goldstein at the Center for Urology Research, and clinical trials are being designed. Another actively pursued research aim is to evaluate DC status in infectious diseases. To this end, we are studying interactions of DC with Toxoplasma gondii, a project of Dr. Tyler Curiel, a Baylor Institute for Immunology Research investigator. In collaboration with Dr. Chris Cutler, assistant professor at the Baylor College of Dentistry, we are analyzing the interactions of DC with oral pathogens.

After nearly 2 years of work in physically distant laboratories, we are excited about moving into the new state-of-the-art Lieberman Research Building. We are confident that it will attract other talented scientists. The scientific programs hosted at the Lieberman Building will eventually permit to us to provide patients with novel therapeutic procedures.