Provided herein are engineered non-catalytic, non-toxic tetanus toxin variants and methods of using such engineered tetanus toxin variants as low dose, protective vaccines that are non-toxic and more potent than their respective chemically inactivated toxoids. In addition, provided herein are conjugate vaccine carriers comprising engineered tetanus toxin variants and methods of using such conjugate vaccines to elicit T-cell dependent immune memory responses which can target a broad spectrum of microbial pathogens as a single vaccine.