Structural Differences in Lipomannans from Pathogenic and Nonpathogenic Mycobacteria That Impact CD1b-restricted T Cell Responses

Abstract

Mannosylated molecules on the Mycobacterium tuberculosis surface are important determinants in the immunopathogenesis of tuberculosis. To date, much attention has been paid to mannose-capped lipoarabinomannan, which mediates phagocytosis and intracellular trafficking of M. tuberculosis by engaging the macrophage mannose receptor and subsequently binds to intracellular CD1b molecules for presentation to T cells. Another important mannosylated lipoglycan on the M. tuberculosis surface is lipomannan (LM). Comparative structural detail of the LMs from virulent and avirulent strains is limited as is knowledge regarding their differential capacity to be recognized by the adaptive immune response. Here, we purified LM from the avirulent M. smegmatis and the virulent M. tuberculosis H 37 R v , performed a comparative structural biochemical analysis, and addressed their ability to stimulate CD1b-restricted T cell clones. We found that M. tuberculosis H 37 R v produces a large neutral LM (TB-LM); in contrast, M. smegmatis produces a smaller linear acidic LM (SmegLM) with a high succinate content. Correspondingly, TB-LM was not as efficiently presented to CD1b-restricted T cells as SmegLM. Thus, here we correlate the structure-function relationships for LMs with CD1b-restricted T cell responses and provide evidence that the structural features of TB-LM contribute to its diminished T cell responsiveness.The Mycobacterium tuberculosis surface is particularly rich in mannose-containing biomolecules, including mannosecapped lipoarabinomannan (ManLAM), 2 the related lipoman- Mycobacterial LM is an extremely heterogeneous population of lipoglycan molecules with two well defined domains, a mannosyl-phosphatidyl-myo-inositol (MPI) anchor and a mannan polymer. The intrinsic heterogeneity of the population resides in the length and branching of the mannan polymer, the number of fatty acids present in the MPI anchor, and the presence or absence of other undetermined acyl groups. Specifically, mannan structure of LM consists of a linear ␣-(136)-linked mannopyranosyl backbone that is linked to position 6 of the inositol of its MPI anchor. LMs are considered multimannosylated forms of PIMs because both types of molecules share a common MPI anchor (6). Differences in the degree of acylation and nature of the fatty acids present in the LM MPI anchor have been studied in detail in a few mycobacterial species (reviewed in Ref. 6). Palmitic and tuberculostearic acids are always present in the C-1-and C-2-position of the glycerol. In addition, a third and fourth fatty acid of variable nature may be present in the C-6-position of the (132)-linked mannose and the C-3-position of the myo-inositol (6), respectively.Structural differences in LM populations among some pathogenic mycobacterial strains exist (6). Overall, LM molecules have been shown to regulate cytokine, oxidant, and T cell responses (1). LM populations from M. tuberculosis H 37 R v associate with DC-SIGN but not with the MR and induce apoptosis and IL-12 production (1). BCG LM popu...

Macrophages in tuberculosis: friend or foe

“…TNF-α production is robust for M. smegmatis LM-stimulated human macrophages, but not for LM from Mtb. This diversity was found to be due in part to differential microRNA regulation [373] in response to subtle variations in the structure of the molecule, such as the presence of increased succinates in M. smegmatis LM [374]. These results are consistent with the decreased TNF-α response seen in virulent mycobacteria infection compared to nonpathogenic mycobacteria [375].…”

Divergent Effects of Mycobacterial Cell Wall Glycolipids on Maturation and Function of Human Monocyte-Derived Dendritic Cells

“…Erdman ManLAM has since then been reported to induce significantly lower amounts of TNF not only compared to ManLAM from H37Ra [30], [31], [32] but also compared to ManLAM from the “virulent” H37Rv laboratory strain [39]. ManLAM from Mtb Erdman also failed to induce IL-12p40 in mouse MΦ, while AraLAM did [61].…”

Cationic Antimicrobial Peptides and Biogenic Silver Nanoparticles Kill Mycobacteria without Eliciting DNA Damage and Cytotoxicity in Mouse Macrophages

“…In the case of M. marinum, no additive activity was observed with both AgNPs and NK-2, whereas LLKKK-18 alone showed complete bacterial killing. This discrepancy could be due to differences in the properties of AMPs and NPs and the cell wall composition of both bacterial strains (42). AMPs act on cell membrane by forming stable pores after inserting into the membrane or by forming transient pores by destructing phospholipids bilayers.…”