Some strains of lactic acid bacteria can regulate the host's intestinal immune system. Bacterial cells and membrane vesicles (MVs) of Limosilactobacillus antri JCM 15950T promote immunoglobulin A (IgA) production in murine Peyer's patch cells via toll-like receptor (TLR) 2. This study aimed to investigate the role of lipoteichoic acid (LTA), a ligand of TLR2, in the immunostimulatory activity of these bacterial cells and their MVs. LTA extracted from bacterial cells was purified through hydrophobic interaction chromatography and then divided into fractions LTA1 and LTA2 through anion-exchange chromatography. LTA1 induced greater interleukin (IL)-6 production from macrophage-like RAW264 cells than LTA2, and the induced IL-6 production was suppressed by TLR2 neutralization using an anti-TLR2 antibody. The LTAs in both fractions contained two hexose residues in the glycolipid anchor; however, LTA1 was particularly rich in triacyl LTA. The free hydroxy groups in the glycerol phosphate (GroP) repeating units were substituted by d-alanine (d-Ala) and α-glucose in LTA1, but only by α-glucose in LTA2. The dealanylation of LTA1 slightly suppressed IL-6 production in RAW264 cells, whereas deacylation almost completely suppressed IL-6 production. Furthermore, IL-6 production induced by dealanylated LTA1 was markedly higher than that induced by dealanylated LTA2. These results indicated that the critical moieties for the immunostimulatory activity of L. antri-derived LTA were the three fatty acid residues rather than the substitution with d-Ala in GroP. LTA was also detected in MVs, suggesting that the triacyl LTA, but not the diacyl LTA, translocated to the MVs and conferred immunostimulatory activity. IMPORTANCE Some lactic acid bacteria activate the host intestinal immune system via toll-like receptor (TLR) 2. Lipoteichoic acid (LTA) is a TLR2 ligand; however, the moieties of LTA that determine its immunostimulatory activity remain unclear because of the wide diversity of LTA partial structures. We found that Limosilactobacillus antri JCM 15950T has three types of LTAs (triacyl, diacyl, and monoacyl LTAs). Specifically, structural analysis of the LTAs revealed that triacyl LTA plays a crucial role in immunostimulation and that the fatty acid residues are essential for the activity. The three acyl residues are characteristic of LTAs from many lactic acid bacteria, and our findings can explain the immunostimulatory mechanisms widely exhibited by lactic acid bacteria. Furthermore, the immunostimulatory activity of membrane vesicles released by L. antri JCM 15950T is due to the transferred LTA, demonstrating a novel mechanism of membrane vesicle-mediated immunostimulation.