| Glycopeptidolipids (GPLs) are a class of glycolipids produced by several non-tuberculosis-causing members of the Mycobacterium genus. GPLs are surface-exposed and expressed in different structural forms, with production of highly antigenic, typeable serovar-specific GPLs in members of the Mycobacterium avium complex (MAC). M. avium and M. intracellulare, which comprise this complex, are slow-growing mycobacteria noted for producing disseminated infections in persons with an advanced stage of AIDS, and pulmonary infections in non-AIDS patients. Studies from our laboratory, and from others, have implicated that GPLs function as virulence factors. However, little is known about how GPLs specifically interact with host cell macrophages, and thereby modulate the immune response.;We have found that certain GPLs can stimulate the NFkappaB pathway, as well as MAPK p38 activation and TNF-alpha secretion upon exposure to murine bone marrow-derived macrophages. This stimulation was dependent on TLR2 and MyD88, but not TLR4. Interestingly, the serovar 1 and 2 GPLs activated macrophages in a MyD88- and TLR2-dependent manner, while non-specific GPLs (nsGPLs) and the serovar 4 GPL were non-stimulatory. Using mass spectrometry and NMR analyses, we next characterized the molecular requirements of the GPL-TLR2 interaction. We determined that the extent of the respective acetylation and methylation of the 6-deoxytalose and rhamnose contained within the core GPL structure dictated whether the GPL signaled through TLR2.;Lastly, we showed that phagosomes containing silica beads coated with nsGPLs limited the acidification and delayed recruitment of late endosomal/lysosomal markers compared to phagosomes containing phosphatidylcholine-coated beads. The carbohydrate component of the GPL was required, as beads coated with only the de-glycosylated lipopeptide core of the GPL failed to delay phagosome-lysosome fusion. Moreover, the ability of GPLs to delay phagosome maturation was dependent on the expression of the macrophage mannose receptor.;Together, these data support that GPLs are able to modulate the macrophage response, and that they are capable of inducing distinct cellular processes based on whether or not they engage specific receptors. The results also support a strong structure-function correlation, and suggest that M. avium may alter the structure or composition of its GPLs to avoid detection by the host immune system. |
