Arabino-mycolates derived from cell-wall skeleton of Mycobacterium bovis BCG as a prominent structure for recognition by host immunity.

Arabino-mycolates are components of the cell-wall skeleton of Mycobacterium bovis BCG (BCG-CWS). It is known that synthesized arabinomycolates induce the production of tumor necrosis factor alpha (TNF-α) in murine macrophage cell lines at an intensity similar to that of BCG-CWS. However the immunological activity of natural arabino-mycolates isolated from BCG has not been investigated, probably due to the complexity of the molecule. In this paper, we investigated the immunostimulatory activity of arabino-mycolates isolated from BCG-CWS by acid hydrolysis. Arabino-mycolates obtained by acid hydrolysis from the originally prepared CWS (SMP-105) of M. bovis BCG Tokyo 172 strain consisted mainly of mono-arabinose mono-mycolate, penta-arabinose tetra-mycolate and hexa-arabinose tetramycolate fractions. Arabino-mycolates significantly induced TNF-α production with an intensity comparable to that of CWS and enhanced delayed type hypersensitivity (DTH) reactions against inactivated tumor cells. Arabino-mycolates-induced TNF-α production was completely dependent on TLR2 and MyD88 pathways. These findings indicate that isolated natural arabino-mycolates possess potent adjuvant immunostimulatory activity.

Phagocytosis plays a dual role in activating dendritic cells; digestive production of active Toll-like receptor ligands and cooperation with Toll-like receptor signaling.

Phagocytosis is an initial step in innate immunity, which is also stimulated by signals via Toll-like receptors (TLRs); however, the cooperation of phagocytosis with signals through TLRs to establish acquired immunity is unknown. We found that phagocytosis is an essential process to induce an immune reaction against an insoluble TLR ligand. Cell-wall skeleton prepared from Mycobacterium bovis BCG (BCG-CWS), an insoluble TLR2 ligand, activated and matured murine splenic dendritic cell (DC) line BC-1 as well as a soluble TLR2 ligand, Pam3CSK4. Surprisingly, BC-1 maturation with BCG-CWS was completely suppressed by inhibiting phagocytosis, while that with Pam3CSK4 was not affected. Moreover, BCGCWS induced intense delayed-type hypersensitivity (DTH) reactions against mitomycin C-inactivated Lewis lung carcinoma cells but Pam3CSK4 did not. These results suggested that the phagocytosis process enables the insoluble TLR2 ligand to activate DCs via TLR2 comparable to a soluble TLR2 ligand in vitro, and stimulating TLR2 alone is not sufficient to establish T cell-mediated immunity in vivo. It is therefore conceivable that the process of phagocytosis induces additional effects on TLR2-stimulated DCs to activate cellmediated immunity in vivo.