Vaccine has been part of human fight against infectious diseases for more than two centuries. An adjuvant is often required for a vaccine or antigen to induce a strong immune response, making it a crucial component in vaccines. Aluminum salts, such as aluminum hydroxide, are widely used in various human vaccines. Before the recent approval of AS04, for decades, aluminum-containing adjuvants were the only approved vaccine adjuvant for human use in the United States. Even AS04 contains aluminum hydroxide. Despite their demonstrated favorable safety profile, aluminum salts can only weakly or moderately potentiate antigen-specific antibody responses, and is generally considered incapable of enhancing cellular immune responses. Therefore, there continues to be a need to search for safe and more potent vaccine adjuvants. Our long-term goal is to develop a safe vaccine adjuvant that is more potent than the traditional aluminum adjuvants. Recently we discovered that the adjuvant activity of the traditional aluminum hydroxide can be significantly enhanced by modifying one of the physical properties of the aluminum hydroxide suspension. These exciting findings point to the potential of this novel aluminum hydroxide formulation as a novel vaccine adjuvant. However, to fully appreciate the feasibility using this novel aluminum hydroxide-containing adjuvant in future human vaccine development, there is a critical need to further characterize its adjuvant activity and to assess its safety in animal models, and the present application is designed to address this critical need. Specifically, we will (i) characterie the immune responses induced by a protein antigen adsorbed on our novel aluminum hydroxide-containg adjuvant. Bacillus anthracis protective antigen protein will be used as a functional model antigen in this study to characterize the specific B cell and T cell responses induced; (ii) elucidate the mechanisms underlying the potent adjuvant activity of our novel aluminum hydroxide-containing adjuvant; and (iii) assess the safety/toxicity of our aluminum hydroxide-containing adjuvant. The most innovative aspect of our proposed research is to enhance the potency of the traditional aluminum hydroxide adjuvant and to overcome its limitations by an innovative physical modification. Our expected outcomes from this project are to thoroughly characterize the immune responses induced by antigens adsorbed on our new aluminum hydroxide-containing adjuvant and to preliminarily assess its safety profile in a rodent model. Moreover, we expect to understand why the adjuvant activity of traditional aluminum hydroxide can be favorably modulated by modifying one of its physical properties. Collectively, these outcomes will provide a sound scientific foundation for future development of this novel aluminum hydroxide-containing adjuvant into a safe and more potent human vaccine adjuvant, which can be used in developing new vaccines as well as re-formulating existing vaccines.