Immunology and Cell Biology (2004) 82, 644–650 doi:10.1111/j.1440-1711.2004.01279.x © 2004 Australasian Society for Immunology Inc. Special Feature Challenges facing adjuvants for cancer immunotherapy CIRCE MESA and LUIS E FERNÁNDEZ Vaccine’s Department, Centre of Molecular Immunology, Havana, Cuba Summary An adjuvant is defined as a product that increases or modulates the immune response against an antigen (Ag). Based on this general definition many authors have postulated that the ideal adjuvant should increase the potency of the immune response, while being non-toxic and safe. Although dozens of different adjuvants have been shown to be effective in preclinical and clinical studies, only aluminium-based salts (Alum) and squalene–oil–water emulsion (MF59) have been approved for human use. However, for the development of therapeutic vaccines to treat cancer patients, the prerequisites for an ideal cancer adjuvant differ from conventional adjuvants for many reasons. First, the patients that will receive the vaccines are immuno-compromised because of, for example, impaired mechanisms of antigen presentation, non-responsiveness of activated T cells and enhanced inhibition of self- reactivity by regulatory T cells. Second, the tumour Ag are usually self-derived and are, therefore, poorly immunogenic. Third, tumours develop escape mechanisms to avoid the immune system, such as tumour editing, low or non-expression of MHC class I molecules or secretion of suppressive cytokines. Thus, adjuvants for cancer vaccines need to be more potent than for prophylactic vaccines and consequently may be more toxic and may even induce autoimmune reactions. In summary, the ideal cancer adjuvant should rescue and increase the immune response against tumours in immuno-compromised patients, with acceptable profiles of toxicity and safety. The present review discusses the role of cancer adjuvants at the different phases of the generation of antitumour immunity following vaccination. Key words: immune adjuvant, impaired dendritic cell, tumour immunotherapy. Antigen uptake and antigen-presenting cells conditioning phase The first phase of vaccination occurs at the site of injection and includes not only Ag delivery into dendritic cells (DC), but also DC activation. In many cases, a vehicle facilitates Ag delivery and the adjuvant provides the context to promote DC activation. For the purpose of this review the vehicle or Ag delivery system and the adjuvant will be discussed as a single agent that can facilitate both processes. Dendritic cell activation It is well known that an infectious context promotes DC activation and the development of immunity while absence of infection fails to do so. 1 Janeway first explained this phenom- enon by proposing that APC possess germline-encoded pattern recognition receptors (PRR) that recognize and are triggered by evolutionarily conserved molecules essential to pathogen function (the pathogen associated molecular pattern [PAMP]), which are absent from the host cells. 2 In addition, APC could be activated by signals made by other cells in response to PAMP. 3 These signals are able to activate DC and are called exogenous and endogenous ‘danger signals’, respec- tively. These findings opened new opportunities for vaccine design, and the concept of danger signals has been successfully used to develop more powerful and successful adjuvants. Among the exogenous danger signals that have demon- strated excellent adjuvant activity are Bacillus Cal- mette–Guerin (BCG) and bacterial non-methylated DNA containing CpG-ODN sequences. BCG has been successfully used as an antitumour agent in the treatment of both superfi- cial bladder cancer and melanoma 4,5 and CpG-ODN has shown the ability to stimulate a potent, tumour-specific immune response in mice. 6 These preclinical investigations have led to the ongoing development of both these agents as components of clinical cancer immunotherapy. BCG and CpG-ODN are known to activate DC in human and murine systems, as indicated by induction of cytokine production, including IL-12, and upregulation of MHC I, MHC II, CD80 and CD86 7–10 (Table 1). Another exogenous danger signal that warns of bacterial infection is the outer membrane vesicles derived from Neisseria meningitidis. Using this knowledge, a new adjuvant approach in which gangliosides are incorpo- rated into the outer membrane complex to form very small size proteoliposomes (VSSP) has been described. 11 VSSP has been demonstrated to have the ability to activate, in vitro and in vivo, mouse and human DC, with corresponding IL-12p40/ p70, TNF-α and IL-6 production 12 (Table 1). This adjuvant has the ability to render highly tolerated gangliosides immu- nogenic. 13 Phase I trials of VSSP in melanoma and breast cancer patients have demonstrated the safety and immuno- genicity of these preparations. 14 Basu et al. have shown that necrotic cells, but not apop- totic cells, can release heat shock proteins (HSP) 15 and several research groups have shown that different HSP can activate DC and macrophages making the HSP the first large family of endogenous danger signals 16 (Table 1). Tumour–peptide–HSP Correspondence: Circe Mesa, Vaccine’s Department, Centre of Molecular Immunology, 216 esq 15, Atabey, Playa, CP 16040, C. Habana, Cuba. Email: circe@ict.cim.sld.cu Received 26 May 2004; accepted 6 July 2004.