Antimicrobial peptides are major effectors of the innate immunity of multicellular organisms including human and play a critical role in host defense, and their importance has been widely recognized. The epithelium of the intestine is the largest surface area exposed to the outer environment including pathogens, commensal microbiota and foods. The Paneth cell lineage of intestinal epithelial cells produces and secretes α-defensin, an antimicrobial peptide, and functions in innate enteric immunity by removing pathogens. It also contributes to intestinal homeostasis through symbiosis with commensal microbiota. α-Defensin has selective activities against bacteria, eliciting potent microbicidal activities against pathogenic bacteria; however, it elicits minimal or no bactericidal activity against commensal bacteria. Therefore, α-defensins regulate the composition of the intestinal microbiota in vivo and play a role in homeostasis of the entire intestine. Recently, relationships between dysbiosis, abnormal composition of the intestinal microbiota, and diseases such as inflammatory bowel disease, and lifestyle-related diseases including obesity and atherosclerosis have been reported. However, the details of these mechanisms are still unknown. Because α-defensins regulate the composition of the intestinal microbiota, Paneth cells and their α-defensins may play a key role in linking microbiota and disease.

Human beings are higher organisms which evolved in 1G. When humans are exposed to the 0G space environment, they experience a large physiological change. Here, I introduce the changes, how to deal with them, and the findings accumulated so far. Physiological changes in the human body that occur in the space environment are characterized by the rapid progress of phenomena (orthostatic hypotension, bone loss, muscle atrophy, cardiopulmonary function decline, immune decline, etc.) similar to aging. The elucidation of these mechanisms contributes to the development of new treatments for the medical treatment of elderly people on the ground. Furthermore, I also give an outline of space life, and introduce various space technologies contributing greatly to life on the earth.