Numerous attempts have been made to increase the potency of nonviral genetic vaccines through genetic modifications, targeting strategies, and boosting regimens, to
name just a few. Adjuvants are defined as anything added to a vaccine that increases the immune response in terms of magnitude, duration, or time of onset (72). By this definition even micro-injury during inoculation with vaccine formulations (73-75) or the haplotype of an individual (76) can be conceivably called an adjuvant. As it relates to DCs, adjuvancy can be more tightly defined as anything that induces progression toward an optimal level of signal 1 (antigen presentation enhancements such as in delivery systems) and signal 2 (such as costimulatory molecule and cytokine up-regulations by using "immunostimulatory adjuvants"). Enhancing the presentation to signal 1 seems fairly straightforward by increasing expression of the antigen in the proper cell type. However, methods for enhancing the optimal presentation of signal 2 remain unclear, in part because of the complex dialogue between lymphocytes.
It is clear, however, that signal 2 requires the up-regulation of costimulatory molecules and the secretion of Th1 and Th2 cytokines. What causes this reaction to a stimulus is not fully understood. One theory states that the immune system is finely tuned to react to "danger signals" (77). These signals distinguish between when to mount an attack, in the case of an invading pathogen, and when to suppress immune rejection, in the case of regularly surveyed "self/nondangerous" antigens. The current dogma is that the immune system induces tolerance to some antigens in certain circumstances (e.g., without signal 2 or in the presence of some other signal), and that tumor cells may have the ability to down-regulate this signal (9). Attempts to modulate the immunostimula-tory properties of genetic vaccines have resulted in incremental increases in vaccine potency and understanding of the immune system.
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