Review Generation of Cellular Immune Responses to Antigenic Tumor
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CMLS, Cell. Mol. Life Sci. 57 (2000) 290–310 1420-682X/00/020290-21 $ 1.50+0.20/0 © Birkha¨user Verlag, Basel, 2000 Review Generation of cellular immune responses to antigenic tumor peptides G. A. Pietersza,*, V. Apostolopoulosb and I. F. C. McKenziea aThe Austin Research Institute, Studley Rd, Heidelberg, Victoria, 3084 (Australia) bDepartment of Molecular Biology, Scripps Research Institute, 10550 Nth Torrey Pines Rd (BCC-206), La Jolla (California 92037, USA), Fax +1 613 9287 0600, e-mail: [email protected] Received 28 June 1999; received after revision 13 October 1999; accepted 26 October 1999 Abstract. Tumor immunotherapy is currently receiving generated endogenously or given exogenously can enter close scrutiny. However, with the identification of tu- the class I pathway, but a number of other methods of mor antigens and their production by recombinant entering this pathway are also known and are discussed means, the use of cytokines and knowledge of major in detail herein. While the review concentrates on induc- histocompatibility complex (MHC) class I and class II ing cytotoxic T cells (CTLs), it is becoming increasingly presentation has provided ample reagents for use and apparent that other modes of immunotherapy would clear indications of how they should be used. At this be desirable, such as class II presentation to in- time, much attention is focused on using peptides to be duce increased helper activity (for CTL), but also presented by MHC class I molecules to both induce and activating macrophages to be effective against tumor be targets for CD8+ cytolytic T cells. Many peptides cells. Key words. Tumor antigens; vaccines; class I presentation; class II presentation; peptides; toxins; adjuvants; antigen targeting; clinical trials; CTL epitopes. Introduction 1. Detection of tumor antigens Tumor antigens have been detected either as unique Every 20 years there is a reemergence of interest in antigens found on tumors (especially melanoma) and on tumor immunotherapy: in the 1970s BCG mixed with few normal tissues; or they are present in larger tumor cell lysates was used, and while some initial amounts in tumors, so that the relative amount of successes were found in melanoma, the procedure was antigen in a tumor is much greater than in the normal ultimately abandoned. Tumor immunotherapy has tissue, e.g. mucins are present in 40–100-fold higher again reemerged, but this time there are many different concentrations in breast cancer than normal resting ways of attacking tumors, and optimism is high that breast epithelium [1]. The antigens—either tumor-spe- these will work. The reason for the recent emergence of cific or tumor-associated—can be oncogenic products, tumor immunotherapy is based predominantly on the expressed uniquely in the tumor after gene mutation or use of genetic engineering techniques, which have made rearrangement, or be overexpressed in cancers to give a difficult techniques a reality. The major advances are in differential expression in tumors. It should be noted several areas (table 1): that particularly in melanoma, the starting point for detecting tumor antigens was the description of cyto- * Corresponding author. toxic lymphocytes (CTLs) against tumor antigens such CMLS, Cell. Mol. Life Sci. Vol. 57, 2000 Review Article 291 as MAGE, gp100 and tyrosinase [2], whereas other 3. Cytokines antigens, such as mucins, could be suitable targets sim- Cytokines have been described in the last 20 years, and ply because there is a lot present in the tumor and the now there are 25 cytokines [IL-1-18, TNF-h,-i, description of CTLs followed the identification of interferon (IFN) and others] and 50 chemokines and mucins as a potential tumor target [3, 4]. In most cases their receptors [5, 6]. There is almost an embarrassment the identification of the tumor antigens was accompa- of riches here, for to understand the mode of action of nied by the isolation of the complementary DNA each cytokine and their interactions in a clinical setting (cDNA) or genes encoding the antigens. is an enormous undertaking. These agents will be useful in manipulating immune responses to tumor antigens, particularly in patients whose immune system is sup- 2. Production of tumor antigens pressed [7, 8]. The antigen can be produced in large amounts by recombinant techniques, either as fusion proteins in 4. Function of the immune system bacterial or other systems, or as soluble molecules in There is now more knowledge of how the immune eukaryotic systems. Furthermore, synthetic peptides can system functions, and how cellular immune responses be made from the tumor antigen; this must be one of are generated [9]. Major histocompatibility complex the major technical breakthroughs in tumor im- (MHC) restriction is now understood in molecular munotherapy—previously whole tumors were irradi- terms and refers to the restricted repertoire of peptides ated or used as lysates, and such preparations contained presented by either class I or class II MHC molecules— only small amounts of antigen which was not repro- the repertoire being different for each MHC allele. This ducible in production, consistency or stability. These knowledge has led to the peptide approach to tumor problems have been largely overcome by making syn- immunotherapy. Peptides (8–9mers) can be presented thetic peptides or recombinant proteins. In addition, by MHC class I molecules, leading to the generation of monoclonal antibodies to the tumor antigens can be CD8+-mediated cellular responses consisting of CTLs used in the identification and purification of tumor and cytokine secretion (predominantly IFN-k and TNF- antigens. Thus, tumor specific and tumor associated h). Class II molecules present longer peptides (10– antigens have been identified and are available in virtu- 15mer) to CD4+ cells which can make different ally unlimited amounts. cytokine responses—the so-called T1 or T2 responses. Thus far in tumor immunotherapy, most workhas concentrated on the presentation of peptides by MHC class I molecules to CD8+ cells, and attempts to gener- Table 1. Recent advances in tumor immunotherapy. ate CD8+ CTLs are currently at the forefront of tumor immunotherapy. Unfortunately, many tumors survive Reference and evade the immune system by having major alter- Identification of tumor antigens (see table 3) ations or deletions of their MHC class I molecules Production of tumor antigens 157, 158, 159 which would decrease CTL activity [10]. Thus far, little prokaryotic expression has been done on class II presentation of tumor anti- eukaryotic expression synthetic peptides gens, and as most solid tumors do not express class II Identification and production of cytokines molecules, tumor cells cannot be directly attacked by and chemokines cytokine-secreting CD4 cells. However, infiltrating class IL-1-18 160 II+ cells, e.g. macrophages, could lead to the genera- TNFh, i 160 + IFNh, i, k 160 tion of CD4 helper cytokine secretion to improve the + chemokines 5, 6 induction of CD8 CTLs and have direct cytolytic or Knowledge of the immune system destructive effects. In addition, there is much knowledge presentation of peptides by MHC class 9 of the intracellular trafficking of peptides—either into I and II the class I or class II pathways, and much of this review peptide trafficking into cells and into 161 the pathway will concentrate on ‘diverting’ peptides from intracellu- affinity vs. tolerance 162 lar degradation pathways in lysosomes and late endo- / DC in vitro in vivo 12 somes into the cytoplasm where they can be processed Assays of cellular immunity and ultimately presented by MHC class I molecules to CTL/CTLp (LDA) 13 + proliferation 14 induce CD8 effector cells [11]. DTH biopsy 130 cytokine production 5. Antigen-presenting cells (APCs) ELISPOT; intracytoplasmic cytokines 15, 16 tetramer binding 17, 21 The dendritic cell (DC) has emerged as a central player in generating either antibodies after class II presenta- 292 G. A. Pietersz, V. Apostolopoulos and I. F. C. McKenzie Generation of cellular immune responses tion or CD8 CTLs after class I presentation. A recent Antigen-processing pathways innovation has been to obtain DCs from patients, iso- MHC class I presentation pathways late and expand in vitro, feeding them peptides and An essential first step in CD8+ T cell generation is transferring such ‘loaded’ DCs into the patient [12]. the uptake and presentation of peptides by MHC Others are using such loaded DCs to generate specifi- molecules by antigen-presenting cells (APCs). MHC cally activated T cells in vitro prior to transfer. class I proteins consist of three subunits, the heavy chain made up of h1, h2 and h3 domains of M =45 6. Measuring cellular immunity r kDa and a light chain i2-microglobulin (12 kDa); in Over the last 5 years much time has been expanded close association with the h1/2 domain is an 8–9mer on measuring CTLs or their precursors, using tedious peptide in the MHC groove (fig. 1). All three compo- limiting dilution (LDA) assays, often after multiple nents are crucial for the formation of a stable com- rounds of restimulation in vitro taking up to 5–6 plex. MHC class I molecules can present endogenous weeks to obtain results [13]. Lymphocyte proliferation nuclear and cytosolic peptides, and exogenous anti- of either CD4 or CD8 cells has also been performed, gens. For MHC class I presentation, exogeneous anti- but it is difficult to interpret the meaning of this in gens are taken up by APC into endosomes and escape patients immunized with complex structures like into the cytosol (either directly or after degradation) whole tumors [14]. In vivo delayed type hypersensitiv- where they enter the proteasome, which is responsible ity (DTH) responses have been measured—often un- for their further degradation (fig. 2). As will be dis- rewarding in immunosuppressed patients, but more cussed, the presence of a peptide (of either endoge- recently, the biopsy and histological examination of nous or exogenous origin) in the cytosol is required DTH reaction has been of value; still, it is difficult to for processing to the class I pathway.