Micronized and heat-treated lactobacillus plantarum LM1004 stimulates host immune responses via the TLR-2/MAPK/NF-κB signalling pathway in vitro and in vivo

Although nanometric dead Lactobacillus plantarum has emerged as a potentially important modulator of immune responses, its underlying mechanism of action has not been fully understood. This study aimed to identify the detailed biochemical mechanism of immune modulation by micronized and heat-treated L. plantarum LM1004 (MHT-LM1004, <1 μm in size). MHT-LM1004 was prepared from L. plantarum LM1004 via culture in a specifically designed membrane bioreactor and heat treatment. MHT-LM1004 was shown to effectively induce the secretion of TNF-α and IL-6 and the mRNA expression of inducible nitric oxide synthase (iNOS). MHT-LM1004 enhanced the expression of TLR-2, phosphorylation of MAPKs (ERK), and nuclear translocation of NF-κB in a dose-dependent manner. Oral administration of MHT-LM1004 (4 × 10⁹ or 4×10¹¹ cells/kg mouse body weight) increased the splenocyte proliferation and serum cytokine levels. These results suggested that MHT-LM1004 effectively enhances early innate immunity by activating macrophages via the TLR-2/MAPK/NF-κB signalling pathway and that this pathway is one of the major routes in immune modulation by the Lactobacillus species.