Department of Comparative Medicine, Yale School of Medicine, New Haven, CT 06520, USA; Department of Immunobiology, Yale School of Medicine, New Haven, CT 06520, USA. Department of Comparative Medicine, Yale School of Medicine, New Haven, CT 06520, USA; Department of Immunobiology, Yale School of Medicine, New Haven, CT 06520, USA; Yale Center for Research on Aging, Yale School of Medicine, New Haven, CT 06520, USA. Electronic address: vishwa.dixit@yale.edu.