CELLULAR VACCINES AND MODULATIONS THERE OF
Dendritic Cell Vaccines
Recently, much attention has focused on the area of dendritic cell (DC) vaccines in the treatment of cancers. The immunological basis of current approaches to therapeu-tic cancer vaccination (often called “vacci-treatment”) has been established over the past decade or longer. These new devel-opments are mainly based on the lessons learned from the clinical testing of these approaches. In particular, three lessons are worthy of note: First, recent randomized phase III trials suggest that vacci-treatment with autologous DCs expressing pros-tatic acid phosphatase or with autologous tumor-derived heat shock protein (HSP gp96) peptide complexes are showing progress in cancer patient survivals. Sec-ond, immunological monitoring of many clinical trials has failed to identify a sur-rogate marker for clinical outcomes. Third, many articles and reviews suggest that protective immunity to human cancer is elicited by the mutated antigenic repertoire unique to each cancer.
Focusing more closely on the type of DC needed to achieve its vaccine poten-tial, the subsets of DCs could prove criti-cal. Much of the current research is being carried out with monocyte-derived DCs, which are potent and homogeneous stimu-lators of immunity. Monocyte-derived DCs can be readily generated within a few days in large numbers (300 million–500 million mature DCs per apheresis) from precursors in the blood without the need for pretreat-ing patients with various cytokines such GM-CSF or FLT 3-L. Rather, one obtains populations of immature DCs by exposing monocytes to GM-CSF and IL-4, and then they are differentiated into mature DCs by various stimuli such as toll-like recep-tor (TLR) ligands (LPS or poly I:C), inflam-matory cytokines (IL-1β, TNF-α, IL-6, and PGE2), or CD40L. The use of DCs that have received a maturation stimulus is likely to be important to induce strong immunity. It has become clear that antigen delivered on immature or incompletely matured DCs can even induce tolerance. However, the type and the duration of the maturation stimulus remain to be determined and may influence efficacy. At this time, monocyte-derived DCs are the most accessible and homogeneous populations of DCs.
Monocyte-derived DCs were the first to be used for treating melanoma patients, and several pilot studies have been published. Most used defined antigens in the form of peptides, but in some studies, tumor lysates or autologous DC tumor hybrids were also employed. The first trial, published in 1998 by Nestle and colleagues, aroused great interest given an overall response rate of 30 percent in stage IV patients (i.e., distant metastases), including complete responses. An important point in the first study by Nestle and colleagues was that they use fetal calf serum (FCS) during DC genera-tion, and this might have contributed to the observed effects by providing nonspe-cific helper epitopes and by promoting the maturation of DCs.
Jonuleit and colleagues directly com-pared within each of eight patients the immunogenicity of immature DCs (gen-erated according to Nestle and colleagues in FCS-containing media using GM-CSF and IL-4) to mature DCs generated in the absence of FCS and matured by a cocktail consisting of TNF-α, IL-1β, IL-6, and PGE2 mimicking the composition of monocyte-conditioned medium. These two differ-ent DC populations were administered intranodally into opposite inguinal lymph nodes. FCS-free mature DCs induced stron-ger T-cell responses, both to the two recall antigens used (tetanus toxoid and PPD/ tuberculin) and to tumor peptides. Interest-ingly, however, both immature and mature DCs showed an expansion of peptide-spe-cific T cells by tetramer staining; yet, only mature DCs induced IFN-γ-producing and lytic CD8+ T cells. These findings suggest the interesting possibility that the imma-ture DCs might have induced regulatory T cells rather than effector T cells, an obser-vation previously noted in studies with normal volunteers.
Cancer patients and healthy subjects often harbor a repertoire of self-reactive T cells and antibodies. This led to the idea that if one could break immunological tolerance to these self-antigens in a con-trollable manner one would find a “thera-peutic window” in which an autoimmune response might damage cancers more than normal tissues. This approach has worked reasonably well with chemotherapies, which, although not cancer specific, can confer clinical benefit with acceptable mor-bidities.
The efforts to create cancer vaccines using allogeneic cell lines, differentiation antigens (such as gp100 and MART1), cancer testes (CT) antigens (such as MAGE, NY-ESO-1), or other common molecules (such as carcinoembryonic anti-gen, mucins, prostate-specific antigen, and prostatic acid phosphatase) represent this approach. Within this approach lie several subthemes. Thus, CT antigens, which are not expressed on normal somatic tissues but only on cancers or gonads, might be a better target for breaking tolerance than are differentiation antigens expressed on somatic tissues. Another subtheme is the idea that artificially mutated differentiation or CT antigens as vaccines might be better at breaking tolerance than their wild-type counterparts. Finally, there is a multiplic-ity of choices of delivery agents for these antigens – whole proteins, peptides, RNA, DNA, viral vectors, DCs, and so on.
The importance of innate immunity and the emergence of the toll-like receptors have led to the recognition that immune modifiers must be an essential compo-nent of any cancer vaccine. The inclusion of QS21, ISCOMS, Montanide, heat shock proteins, CpG, BCG, and granulocyte-macrophage colony-stimulating factor with cancer vaccines reflects this. Of inter-est in this respect are the anecdotal success of Coley’s toxin at the turn of the nineteenth century (a heat-killed mixture of strep-tococcal cells and probably superantigen broths), which may now be reinterpreted in the light of the newly identified immune response modifiers.
Finally, a better understanding of the controls that act on T cells to stimulate or inhibit them has led to the use of reagents to enhance antitumor T-cell activity. For exam-ple, blocking antibodies to the inhibitory T-lymphocyte antigen CTLA-4, manipula-tion of regulatory T cells, antibodies to PDL or its ligand PDLI, and enhancement of co-stimulating molecules like B7 on antigen-presenting cells are a few examples of reagents that are being used in this manner in conjunction with vacci-treatments.