The present invention concerns methods and compositions involving modified LcrV proteins from Yersinia bacteria. These methods and compositions can be employed to invoke an immune response in a subject against the bacteria, while not suppressing the immune system as much as the native LcrV protein. In certain embodiments, the present invention relates to vaccines, as well as methods to protect a subject against Yersinia pestis and plague. Moreover, the present invention concerns methods and compositions for suppressing a subject's immune system using non-native LcrV polypeptides.