Abstract summary 

The hygiene or "Old Friends" hypothesis proposes that the epidemic of inflammatory disease in modern urban societies stems at least in part from reduced exposure to microbes that normally prime mammalian immunoregulatory circuits and suppress inappropriate inflammation. Such diseases include but are not limited to allergies and asthma; we and others have proposed that the markedly reduced exposure to these Old Friends in modern urban societies may also increase vulnerability to neurodevelopmental disorders and stress-related psychiatric disorders, such as anxiety and affective disorders, where data are emerging in support of inflammation as a risk factor. Here, we review recent advances in our understanding of the potential for Old Friends, including environmental microbial inputs, to modify risk for inflammatory disease, with a focus on neurodevelopmental and psychiatric conditions. We highlight potential mechanisms, involving bacterially derived metabolites, bacterial antigens, and helminthic antigens, through which these inputs promote immunoregulation. Though findings are encouraging, significant human subjects' research is required to evaluate the potential impact of Old Friends, including environmental microbial inputs, on biological signatures and clinically meaningful mental health prevention and intervention outcomes.

Authors & Co-authors:  Lowry Christopher A CA Smith David G DG Siebler Philip H PH Schmidt Dominic D Stamper Christopher E CE Hassell James E JE Yamashita Paula S PS Fox James H JH Reber Stefan O SO Brenner Lisa A LA Hoisington Andrew J AJ Postolache Teodor T TT Kinney Kerry A KA Marciani Dante D Hernandez Mark M Hemmings Sian M J SM Malan-Muller Stefanie S Wright Kenneth P KP Knight Rob R Raison Charles L CL Rook Graham A W GA

Study Outcome 

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