A polypeptide can also be linked to a moiety (i.e., a functional group that can be a polypeptide or other compound) that enhances an immune response (e.g., cytokines such as IL-2), a moiety that facilitates purification (e.g., affinity tags such as a six-histidine tag, trpE, glutathione, maltose binding protein), or a moiety that facilitates polypeptide stability (e.g., polyethylene glycol; amino