Japanese Journal of Clinical Oncology

There has been increasing development in technology for cell processing (`graft engineering'), moving from the experimental to the clinical settings. PBSC can be collected from the body's entire pool of hematopoietic stem cells to provide more stem cells than by bone marrow aspiration performed at localized iliac bones; this makes `cell component therapy' far more effective with PBSC. A new strategy in the use of PBSC is the positive selection of CD34⁺ hematopoietic progenitor (CD34⁺) cells in autologous or allogeneic settings. In the autologous setting, the primary advantage of using selected CD34⁺ PBSC is reduced tumor cell contamination during PBSC preparation. On the other hand, in the allogeneic setting, CD34⁺ selection methods are used to reduce the incidence and severity of graft-versus-host disease (GVHD) (2,3). CD34⁺ selection may also be used as a target of gene therapy, for the induction of immune tolerance in solid organ transplantation, treatment of patients with autoimmune disease, and very recently, as a source of dendritic cells (DC) for cancer immunotherapy.

The rationale of PBSCT has been to provide maximum antitumor effect by high-dose chemo/radiation therapy. However, recent observations in patients undergoing allogeneic stem cell transplantation support that total leukemia eradication by high-dose therapy is not mandatory. Cure of the disease is achievable if reduction of the tumor burden can reach a level low enough at a stage of minimal residual disease to be controlled by patient's own immune function or by adoptive immunotherapy (4,5).Nevertheless, lack of effective tumor antigen presentation and immuno-inhibitory tumor environment have been the primary obstacles for conventional immunotherapy using lymphokine-activated killer cells or cytotoxic T-cells (CTL). Hence, new dimension of anticancer immunotherapy incorporating DCs, long-ignored cells in immunology, has been evolving.

DCs hold an important place in the immune system communication web as the most potent antigen-presenting cells that initiate the antigen-specific immune response (6). They are the only known natural cells that can present antigen to naive T cells, i.e. T cells that have not been previously exposed to antigen, in the context of human leukocyte antigen (HLA) molecules and other co-stimulatory factors such as CD86. They prime CD8+ T-cells for cytotoxic responses and do CD4+ cells for helper function. They are derived from bone marrow and migrate through the blood, as a scarcely presenting cell population (<0.5%), into nonlymphoid tissues, where they develop to a stage of immature DCs which are characterized by a high capability for antigen capture and processing, but carry low T-cell stimulatory capability and lack of typical dendritic cell morphology. Inflammatory mediators, such as tumor necrosis factor (TNF)-, Interleukin-1 or lipopolysaccharide (LPS), then promote DC maturation and migration out of nonlymphoid tissues into the afferent lymph to seed into T cell areas of secondary lymphoid tissues (7). Mature DCs loose the ability to capture antigen, but acquire a capability to stimulate T-cells.

The basic mechanisms by which immature DCs process antigens for MHC class II loading are now rather well understood (8). Following education by antigen-loaded DC, naive CD4+ T-cells differentiate into antigen-specific class II restricted `helper' T-cells which are important both as effector cells and as providers of help for CD8+ CTL precursors and for B-cells. CTL are capable of eradicating tumors or virus infections. In addition to this pathway, DC can load their major histocompatibility complex (MHC) class I peptides which are derived from both exogenous antigens and proteins synthesized in the cytosol via the classical pathway to initiate class I MHC-restricted CTL immunity, which can be primed and expanded directly by DCs either ex vivo or in vivo. Thus, DC is a potent vehicle for the effective initiation of T- and B-cell-mediated immune responses. A paucity of markers for DC and resultant lack of a clear definition of a DC used to be a major setback, but there are now useful DC-specific monoclonal antibodies available in human which include HLA-DR, CD80, CD83, CD86 and CD123. DCs have been studied extensively in mouse model systems of active immunotherapy of cancer. Most of such studies found DC to be effective in treating or preventing tumors in an antigen-specific fashion and are strongly supportive of the notion that active immunotherapy of cancer with DC is worthwhile pursuing in clinical trials in humans.

In the past several years, cell component therapy has considerably been evolved by the development of graft engineering techniques. Accordingly, the hematotherapy society has been clearly moving into a new era. Together with a growing list of human tumor associated antigens, consequent availability of a large number of DC has opened up new possibilities for generation of CTL responses to tumor specific antigens. Inhibition of anti-cancer immune function in tumor-bearing environment might be bypassed by direct delivery of tumor antigens on isolated autologous DCs; autologous DCs from patients can be collected by apheresis, expanded ex vivo, pulsed with tumor-specific peptide and then, reinfused to the patients to induce tumor-specific CTL. Several cellular sources including bone marrow cells, cord blood, CD34+ peripheral blood cells and CD14+ monocytes will be used to generate DC (9). This process is a homologous to an autologous blood transfusion. As an alternative to peptides, DC can be transfected with the relevant genes using expressing vectors (10).

First reports on the safety and the immuno-stimulatory activity of peptide-loaded DC in clinical phase I studies disclosed that following intravenous infusion, DCs are first trapped by the lung, and then home to the T-cell areas in liver and spleen 24 h later. Although this induces dose-responsive immune reaction of T-cells in most cases, it remains to be confirmed whether this will be translated into clinically recognized regression of tumor. How rapidly this field would expand and how relevant this strategy would become totally depend on efficient dissemination of standardized training, quality control for the new procedures and development of well-organized clinical trial.