Synthesis and Immunological Comparison of Differently Linked Lipoarabinomannan Oligosaccharide–Monophosphoryl Lipid A Conjugates as Antituberculosis Vaccines

A monophosphoryl lipid A (MPLA) derivative having the 6′-OH group substituted with an NH2 group was synthesized and coupled with the upstream terminal tetrasaccharide of mycobacterial lipoarabinomannan (LAM) via an amide bond to create a novel type of MPLA-based fully synthetic glycoconjugate vaccine. The same tetrasaccharide was also coupled with MPLA at the 1-O-position. Immunological activities of the two synthetic conjugates were evaluated in mice and compared. Both afforded robust overall and IgG antibody responses, but intraperitoneal injection elicited responses significantly stronger than those from subcutaneous injection. It was thus speculated that MPLA conjugates might act via stimulating B1 lymphocytes present in the intrapleural and peritoneal cavities. Moreover, the 6′-N-conjugate afforded antibody titers much higher than those of the 1-O-conjugate. These results revealed not only the self-adjuvant property of MPLA conjugates to elicit robust IgG antibody responses but also the impact of MPLA structure on the immunological activity of its conjugates. It was concluded that LAM oligosaccharide-MPLA conjugates, especially 6′-N-linked, are promising candidates as antituberculosis vaccines worthy of further investigation. Additionally, the 6′-amino derivative of MPLA was proved to be a useful carrier for the development of fully synthetic carbohydrate-based conjugate vaccines.