Effects of heat-killed Lactococcus lactis subsp. lactis JCM 5805 on mucosal and systemic immune parameters, and antiviral reactions to influenza virus in healthy adults; a randomized controlled double-blind study☆

•A randomized, double-blind controlled trial in 111 healthy subjects was performed.•Lactococcus lactis JCM 5805 significantly up-regulated secreted IgA levels in saliva.•L. lactis JCM 5805 up-regulated the phagocytotic activity of neutrophils.•L. lactis JCM 5805 enhanced antiviral gene expression in PBMCs.