<scp>l</scp> ‐Theanine Mediates Hsf1 Modulation of Intestinal Immunity Through <i>Bifidobacterium longum</i> to Alleviate Heat Stress

ABSTRACT Heat stress (HS) induces intestinal inflammation and dysbiosis, and Bifidobacterium exhibits anti‐HS roles in the intestinal tract. l ‐theanine (LTA) increases intestinal Bifidobacterium abundance in HS‐exposed mice and may modulate Hsf1 to alleviate HS‐mediated intestinal immune injury; however, the underlying mechanism remains unclear. This study found that LTA alleviated HS‐induced MODE‐K cell inflammatory injury by modulating Hsf1, and this effect was absent when Hsf1 expression was inhibited. In heat‐stressed Hsf1 knockout (KO) mice, LTA failed to regulate colon immunity and the expression of Hsf1 and its target proteins Hsp70 and Hsph1. Bifidobacterium longum (BL) ameliorated HS‐induced intestinal injury in pseudo‐germ‐free mice and inhibited the expression of Hsf1 and its target proteins, indicating its potential against HS; LTA in combination with BL was more effective than BL or LTA alone in modulating the above indices. Compared to the BL group, 200 µM LTA intervention using an in vitro co‐culture system increased BL abundance. Moreover, the co‐culture supernatant increased MODE‐K cell viability and IL‐10 and sIgA secretion under HS conditions, and this effect was not conspicuous following the targeted inhibition of Hsf1. These findings suggest that LTA mediates Hsf1 modulation of intestinal immunity by increasing the abundance of B.longum to alleviate HS. These findings provide scientific evidence for the development of anti‐HS functional foods and the extensive application of LTA.