Serovars of Mycobacterium avium complex isolated from AIDS and non-AIDS patients in Spain

The mycobacterial glycopeptidolipids: structure, function, and their role in pathogenesis

Glycopeptidolipids (GPLs) are a class of glycolipids produced by several nontuberculosis-causing members of the Mycobacterium genus including pathogenic and nonpathogenic species. GPLs are expressed in different 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 AIDS patients and pulmonary infections in non-AIDS patients. Previous studies have defined the gene cluster responsible for GPL biosynthesis and more recent work has characterized the function of the individual genes. Current research has also focused on the GPL's role in colony morphology, sliding motility, biofilm formation, immune modulation and virulence. These topics, along with new information on the enzymes involved in GPL biosynthesis, are the subject of this review.

Mycobacterium avium 104 deleted of the methyltransferase D gene by allelic replacement lacks serotype-specific glycopeptidolipids and shows attenuated virulence in mice

Mycobacterium avium is a major opportunistic pathogen of AIDS patients in the United States. The understanding of M. avium pathogenesis has been hampered by the inability to create gene knockouts by homologous recombination, an important mechanism for defining and characterizing virulence factors. In this study a functional methyltransferase D (mtfD) gene was deleted by allelic replacement in the M. avium strain 104. Methyltransferase D is involved in the methylation of glycopeptidolipids (GPLs); highly antigenic glycolipids found in copious amounts on the M. avium cell surface. Interestingly, the loss of mtfD resulted in M. avium 104 containing only the non-serotype specific GPL. Results also suggest that the mtfD encodes for a 3-O-methyltransferase. The absence of significant amounts of any serotype-specific GPLs as a consequence of mtfD deletion indicates that the synthesis of the core 3,4-di-O-methyl rhamnose is a prerequisite for synthesis of the serotype-specific GPLs. Macrophages infected with the mtfD mutant show elevated production of tumour necrosis factor-alpha (TNF-alpha) and RANTES compared to control infections. In addition, the M. avium 104 mtfD mutant exhibits decreased ability to survive/proliferate in mouse liver and lung compared to wild-type 104, as assessed by bacterial counts. Importantly, the mtfD mutant complemented with a wild-type mtfD gene maintained an infection level similar to wild-type. These experiments demonstrate that the loss of mtfD results in a M. avium 104 strain, which preferentially activates macrophages in vitro and shows attenuated virulence in mice. Together our data support a role for GPLs in M. avium pathogenesis.

Serotype-specific modulation of human monocyte functions by glycopeptidolipid (GPL) isolated from Mycobacterium avium complex

Immunomodulating activity of glycopeptidolipids (GPL), separated from different serovars of Mycobacterium avium complex (MAC), on macrophage functions was compared. When peripheral blood mononuclear cells (PBMC), from healthy donors showing strongly positive reactions to mycobacterial purified protein derivatives (PPD), were incubated with heat-killed Staphylococcus aureus coated with GPL serovar 4, phagocytosis of monocytes increased in dose-dependent manner. However, coating with GPL serovar 9 did not show any effects. After phagocytosis of heat-killed S. aureus, the phagosome-lysosome (P-L) fusion in monocytes was inhibited dose-dependently by coating of S. aureus with GPL serovar 4, but not serovar 9. These results indicate that GPL serovar 4 facilitates invasion of MAC into monocytes and renders resistance to bactericidal reactions due to the inhibition of P-L fusion. Regarding accessory function of macrophages in proliferative responses of T cells, the addition of GPL serovar 4 to cultures resulted in significant inhibition of anti-CD3 monoclonal antibody (mAb)-induced proliferation, whereas both serovar GPLs did not cause reduction of cell viability. Furthermore, the PPD-specific T cell proliferative response was downregulated markedly by GPL serovar 4, but weakly suppressed by GPL serovar 9. These results indicated that the immunomodulating activity of GPL on macrophage functions is serovar-dependent.

Liquid chromatography/mass spectrometry analysis of small-scale glycopeptidolipid preparations to identify serovars of Mycobacterium avium-intracellulare complex

AIMS

The antigenic glycopeptidolipids (GPLs) from Mycobacterium avium-intracellulare complex (MAC) are grouped into 28 serovars on the basis of the variable oligosaccharide sequences and the core structures. To facilitate the identification of MAC serovars by employing liquid chromatography/mass spectrometry (LC/MS), the diversity in fatty acyl moieties and the number of acetyl groups of GPLs should be characterized.

METHODS AND RESULTS

Employing a small-scale preparation method, sufficient quantities of intact GPLs could be obtained from several colonies of MAC within 4 h. Tandem mass spectrometry of GPLs showed the presence of common fragment ion at m/z 1048 in the main molecular species of all reference strains. It revealed that the acyl moieties had similar diversity among all serovars. Furthermore, intact GPLs had mainly one or two acetyl groups. This allowed us to determine the masses of each serovar based on intact GPLs and to classify 16 isolates from patients by LC/MS.

CONCLUSIONS

The present serotyping method using LC/MS analysis improved the precision of measurements and shortened the procedure time compared with conventional thin-layer chromatography or the seroagglutination test method.

SIGNIFICANCE AND IMPACT OF THE STUDY

This proposed method proves useful for identifying serovars of MAC for epidemiological and pathogenic research purposes.

Characterization of virulence, colony morphotype and the glycopeptidolipid of Mycobacterium avium strain 104

SETTING

Members of the Mycobacterium avium complex (MAC) are responsible for mycobacterial disease in children, the aged and in immunocompromised individuals. The complex consists of different species, serovars and morphologic forms that vary in virulence. One isolate of the MAC is currently being sequenced (MAC 104) and was chosen based on its derivation from an AIDS patient and the fact that it could be genetically manipulated.

OBJECTIVE

MAC 104 was therefore analyzed for virulence, colony morphotype and expression of the glycopeptidolipid (GPL) responsible for serotying differences and the rough to smooth morphological switch.

RESULTS

The isolate was found to be virulent in the murine model of low-dose aerosol infection in that it could colonize the lung, proliferate within the tissue and disseminate to other organs. MAC 104 expressed a variety of colony morphotypes, the most prevalent of which were smooth opaque, smooth transparent and rough. All three morphotypes could persist in the lung; however, the transparent and rough morphotypes grew more rapidlyinvivo. The rough morphotype was unusual in that it expressed an atypical form of the GPL usually absent from rough morphotypes.

CONCLUSION

This characterization complements the genome data and confirms that MAC 104 behaves similarly to other MAC isolates.

Characterization of immune responses during infection with Mycobacterium avium strains 100, 101 and the recently sequenced 104

Mycobacterium avium strain 104 was chosen as the M. avium isolate to sequence, as it is virulent to humans, stable and readily transfectable. As this strain has not been widely studied we sought to investigate the pattern of 104 infection in mice. Bacterial growth and the immune response generated were compared with infection with the low virulence M. avium strain 100, and the high virulence common laboratory strain, 101. Mycobacterium avium strains 104 and 101 grew progressively within mice, while strain 100 was gradually cleared. Strains 104 and 101 induced strong T cell activation and spleen cell cultures produced similar levels of IFN-gamma. In mice infected with strain 100 no significant T cell activation or IFN-gamma production was measured. Further, mice infected with strain 104 or 101 also displayed comparable inflammatory responses and similar granuloma formation, while only minimal inflammation was seen in mice infected with strain 100. Strains 101 and 104 also grew in a similar fashion in bone-marrow-derived macrophages and induced significant levels of TNF and nitric oxide. Thus infection with M. avium strain 104 induced an immunological response comparable to M. avium strain 101 and, with the availability of its sequence, should be a useful tool for designing new vaccines or drugs therapies to treat the increasing incidence of M. avium infection in humans.