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Infection and Immunity, February 2007, p. 1055-1058, Vol. 75, No. 2
0019-9567/07/$08.00+0 doi:10.1128/IAI.00835-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Hypervirulence of a Rough Variant of the Mycobacterium abscessus Type Strain
E. Catherinot,1
J. Clarissou,2
G. Etienne,3
F. Ripoll,1
J.-F. Emile,4
M. Daffé,3
C. Perronne,2
C. Soudais,5
J.-L. Gaillard,1 and
M. Rottman1*
Laboratoire de Microbiologie,1
Département de Médecine Aiguë Spécialisée, Hôpital R. Poincaré (AP-HP) and Université de Versailles-Saint QuentinFaculté de Médecine Paris-Ile de France-Ouest, 92380 Garches,2
Institut de Pharmacologie et de Biologie Structurale du CNRSUMR 5089, Département "Mécanismes Moléculaires des Infections Mycobactériennes," 205 route de Narbonne, 31077 Toulouse Cedex,3
Laboratoire d'Anatomo-Pathologie, Hôpital A. Paré (AP-HP), 92100 Boulogne,4
Unité d'Immunologie et d'Hématologie, Hôpital Necker-Enfants Malades (AP-HP) and Laboratory of Human Genetics of Infectious Diseases, Université René DescartesINSERM U550, Faculté de Médecine Necker-Enfants Malades, 75730 Paris Cedex 15, France5
Received 24 May 2005/
Returned for modification 16 August 2006/
Accepted 30 November 2006

ABSTRACT
We isolated a rough variant of
Mycobacterium abscessus CIP 104536T
during experimental infection of mice. We show that this variant
has lost the ability to produce glycopeptidolipids, is hyperlethal
for C57BL/6 mice infected intravenously, and induces a strong
tumor necrosis factor-alpha response by murine monocyte-derived
macrophages.

TEXT
Mycobacterium abscessus (formerly
Mycobacterium chelonae subsp.
abscessus) is an emerging, rapidly growing mycobacterium that
causes a wide spectrum of human infections, including skin and
soft tissue infections (
3,
26), lung infections (
11,
24), and
disseminated infections in patients either under immunosuppressive
therapy (
3,
23) or with a Mendelian syndrome conferring susceptibility
to mycobacteria (
5). Extended lung infections and disseminated
infections raise serious therapeutic issues because
M. abscessus strains are resistant to most antibiotics and are associated
with a particular high fatality rate (
3,
23).
M. abscessus organisms may be isolated with a smooth (S) or a rough (R) morphotype from clinical samples (23). Recently, Byrd and Lyons described an R strain of M. abscessus from an ileal granuloma in a patient with Crohn's disease, and they reported the spontaneous in vitro dissociation of this isolate into an S variant (4). Interestingly, these authors showed that the S variant was severely attenuated in its ability to infect the murine host and to persist in human monocytes. M. abscessus may thus undergo an S/R phase variation linked to pathogenicity. The molecular basis for the S or R appearance of M. abscessus, as well as the reason for the difference in pathogenicity of S and R variants, is presently unknown (12).
We have recently used M. abscessus CIP 104536T (=ATCC 19977T) (19) to study the immune control of M. abscessus infection in the murine host. This strain, which was provided by the Laboratoire de Référence des Mycobactéries (Institut Pasteur, Paris, France) has an S morphotype on ordinary solid media such as Trypticase soy agar (BioMérieux, Marcy l'Etoile, France) (Fig. 1A). Following the intravenous (i.v.) injection of 107 CFU of CIP 104536T into immunoglobulin µ chain knockout mice (15), we obtained mycobacterial colonies of the R morphotype from deep organs of several animals on day 90 following infection. We verified that these colonies were truly M. abscessus by partial hsp65 sequencing (21). One isolated colony (CIP 104536T-R) was selected, reisolated, and cryopreserved at 80°C using cryobeads (Mast Diagnostics, Reinfeld, Germany). The R morphotype of this variant was confirmed to be stable after 10 subcultures on solid media and three iterative in vivo passages. Thus, in contrast with previous studies reporting the switch of an R M. abscessus strain into an S morphotype (4), we show here that R variants may be selected in vivo from an M. abscessus strain with an S morphotype.
Glycopeptidolipids (GPLs), also called C-mycosides or J-substances,
represent up to 85% of the surface-exposed lipids in
M. abscessus and a number of other nontuberculous mycobacterial species,
such as
M. chelonae,
M. smegmatis, and
M. avium (
8). Studies
have shown that the lack of GPLs at the mycobacterial surface
is often associated with rough colony morphology in
M. smegmatis and
M. avium mutants obtained spontaneously or by insertional
mutagenesis (
1,
2,
8). We thus studied whether CIP 104536T-R
was affected in its ability to express GPLs. We also tested
the 390S and 390R
M. abscessus variants described by Byrd and
Lyons (
4), which were kindly provided by Blaine Beaman (University
of California Davis, Davis, CA). Mycobacteria were grown in
Middlebrook 7H9-0.05% Tween 80 medium (Difco-Becton Dickinson,
Le Pont-de-Claix, France) at 37°C. Lipids were extracted
from mycobacterial cells with a mixture of chloroform and methanol
and further partitioned by methanol precipitation as previously
described (
27). The GPLs (250 µg lipid for each deposit)
were identified, by comparison with standard GPLs purified from
M. smegmatis (
27), by thin-layer chromatography on silica gel
Durasil 25-precoated plates (Macherey-Nagel, Düren, Germany)
run in chloroform-methanol-water (60:16:2 [vol/vol/vol]). The
sugar-containing GPL-like compounds were identified by spraying
plates with 0.2% anthrone in concentrated sulfuric acid, followed
by heating at 110°C (
8). As shown in Fig.
1B, the S strains
CIP 104536T and 390S both produced GPL-like compounds of comparable
thin-layer chromatography mobilities and in similar quantities.
In contrast, no GPL-like compounds were detectable at any growth
phase in cell extracts of CIP 104536T-R and 390R. The precise
nature of these GPL-like compounds was resolved by matrix-assisted
laser desorption ionization-time-of-flight mass spectrometry.
All of the GPL-associated pseudomolecular ions ([M + Na]
+) detected
in the smooth strains perfectly matched the GPLs previously
described for
M. abscessus in calculated molecular weight (
16).
In contrast, these GPL-associated pseudomolecular ions were
nondetectable in the rough strains (data not shown). Thus, both
CIP 104536T-R and 390R appear to have lost the ability to produce
GPLs.
The virulence of M. abscessus CIP 104536T and CIP 104536T-R was evaluated in C57BL/6 mice injected intravenously. C57BL/6 mice were purchased from Elevage Janvier (Le-Genest-Saint-Ile, France). All animals were 8 to 9 weeks old when infected and were hosted in a confinement class II facility under filter covers, placed in single-use cages with irradiated litter, and fed irradiated chow and autoclaved water ad libitum. Experiments were conducted in accordance with the guidelines of the animal welfare committee of the Université de Versailles-Saint Quentin. Mycobacterial strains were grown at 37°C in Middlebrook 7H9-0.05% Tween 80 medium, harvested by centrifugation, and frozen with 10% glycerol. Aliquots of 1 ml were stored at 80°C until required and then titrated. Challenge inocula were prepared from rapidly thawed frozen aliquots. Bacterial clumps were eliminated by iterative passages through a 29.5-gauge needle, and suspensions were then diluted appropriately in phosphate-buffered saline (PBS). Mice were inoculated by i.v. injection with 107 CFU via the lateral tail vein in a volume of 0.2 ml. This challenge was lethal to all 12 mice injected with CIP 104536T-R as early as 5 days postinfection, whereas no mortality was observed for 90 days in the group of mice injected with CIP 104536T (Fig. 2), consistent with the results of our previous experiments. Similar experiments were conducted with the 390S and 390R variants. All 12 mice injected with 107 CFU of 390S were still alive 90 days postinfection, whereas 11 out of 12 mice injected with the 390R variant died within 15 days of infection (Fig. 2). Thus, the R-morphotype variants CIP 104536T-R and 390R were clearly hyperlethal in the i.v. murine model. The spleens and livers had similar bacterial counts for both the S and R morphotypes of CIP 104536T from days 1 to 4 (data not shown). Mice challenged with 107 CFU and 108 CFU of heat-inactivated CIP 104536T-R (30 min at 95°C; yielding >99.99% inactivation) were asymptomatic, with 100% survival on day 90. Thus, the hypervirulence of the R variants requires live mycobacterial cells or, alternatively, may depend on the expression of thermolabile determinants.
GPLs have recently been shown to interfere with the production
of tumor necrosis factor-alpha (TNF-

) by murine macrophages
(
13). We thus studied the production of TNF-

by murine macrophages
infected with S and R variants. Bone-marrow-derived macrophages
(BMMs) were prepared from 6- to 8-week-old C57BL/6 mice (Elevage
Janvier, Le-Genest-Saint-Ile, France). Mice were killed by carbon
dioxide narcosis, and their femurs and tibias were aseptically
removed. Bone marrow was flushed with PBS. Cells were washed
once in PBS and resuspended in Dulbecco modified Eagle medium
(DMEM)-Glutamax (Invitrogen GIBCO, Cergy-Pontoise, France) supplemented
with 20% L-cell conditioned medium (LCCM) and 10% fetal calf
serum (FCS), and 5
x 10
4 cells per well were inoculated onto
24-well plates (TPP, Trasadingen, Switzerland) in a volume of
1 ml. After 6 days of incubation (37°C, 5% CO
2 atmosphere),
nonadherent cells were eliminated with two washes, and fresh
DMEM containing 10% FCS-10% LCCM was added. BMMs were used for
infection 24 h later. Mycobacteria were prepared as described
above for virulence studies. BMMs were infected for 1 h at 37°C
at a multiplicity of infection of 5:1. BMMs were then washed
twice in PBS to remove extracellular bacteria, and fresh DMEM
containing 10% FCS-10% LCCM and 40 mg/liter amikacin was added.
Macrophages were lysed with 1% Triton at 12 and 24 h postinfection
to assess bacterial survival. TNF-

was measured in culture supernatants
at 12 and 24 h postinfection using an enzyme-linked immunosorbent
assay duoset kit (R&D Systems, Lille, France). The mean
result for duplicate wells was determined by following the manufacturer's
recommendations. Student's
t test was used for statistical analysis
of data. As shown in Fig.
3, CIP 104536T-R induced a significantly
higher level of TNF-

production than CIP 104536T at both 12
and 24 h postinfection (
P = 0.014 and 0.017, respectively).
Similar results were observed with 390R and 390S (Fig.
3). A
study of bacterial survival in BMMs done in parallel showed
no significant differences between S and R isogenic strains
(not shown).
The lack of activation of BMMs by the smooth variant could result
from an inhibitory effect of the GPLs that it synthesizes. In
order to test this hypothesis, we incubated CIP 104536T and
CIP 104536T-R in PBS with purified GPLs for 60 min prior to
infection of BMMs and measurement of TNF-

release. Two GPL extracts
were used: "native GPLs" (80 to 90% purity) were obtained by
methanol precipitation of the lipid extract (
23) and "deacetylated
GPLs" (>98% purity) were obtained by mild alkaline hydrolysis
of the methanol supernatant (
18). GPLs were used at 1- and 100-fold
concentrations by incubating 10
8 CFU with GPLs extracted from
10
8 and 10
10 CFU, respectively. Infecting BMMs in the presence
of either amount of GPLs did not significantly alter the TNF-
response to either the rough or the smooth variant (Fig.
4),
suggesting that GPLs were not inhibitory components per se.
One of the trademarks of mycobacteria is the presence of a complex
array of lipidic molecules exposed at the cell surface (
6).
The most superficial layer is made up of glycolipids which,
depending on the species, can be either GPLs (e.g.,
M. abscessus,
M. smegmatis,
M. avium) or phenolglycolipids (e.g.,
M. tuberculosis).
There is an increasing number of studies showing that mycobacterial
glycolipids play a major role in pathogenicity by interfering
with the host immune system. GPLs have been shown to inhibit
the phagocytic properties of human macrophages (
27) and to modulate
the induction of TNF-

synthesis in murine macrophages (
13).
Recently, the hypervirulent phenotype of
M. tuberculosis strains
belonging to the W-Beijing family was shown to be associated
with the production of a particular glycolipid that inhibits
the release of TNF-

and other key proinflammatory cytokines
(
20). The results of our study are consistent with these data
because we showed that surface-exposed GPLs interfered with
the release of inflammatory effector molecules from murine macrophages
after interaction with
M. abscessus.
We cannot exclude the possibility that the hyperlethality observed with R variants of M. abscessus is related to mediators of the host innate response other than TNF-
. Indeed, production of TNF-
in response to mycobacterial infection orchestrates the early induction of chemokines and is required for the organization of the leukocytes recruited into granulomas (22). As such, it is essential to the control of pathogenic mycobacteria in the murine model (7, 10, 14). However, hyperproduction of TNF-
could directly participate in a lethal "toxic-shock"-like syndrome (9) or be reflective of a dysregulated immune response. The mechanisms by which mycobacterial glycolipids interfere with host cell responsiveness also remain to be elucidated. Mycobacterial GPLs can insert into phospholipid monolayers and alter their biological properties (17, 25). However, our experiments using purified GPLs favor the hypothesis that some proinflammatory components, possibly underlying lipids, are overexpressed or unmasked in rough variants. This hypothesis is currently under investigation by our group.

ACKNOWLEDGMENTS
We thank Veronique Vincent and Cristina Gutierrez (Institut
Pasteur, Paris, France) and Blaine Beaman, (UC-Davis, Davis,
CA) for kindly providing the strains used in this study.
This work received financial support from the association "Vaincre la Mucoviscidose." E. Catherinot was supported by the Legs Poix (Chancellerie des Universités de Paris).

FOOTNOTES
* Corresponding author. Mailing address: Laboratoire de Microbiologie, Hôpital Raymond Poincaré, 104 Bd Raymond Poincaré, 92380 Garches, France. Phone: 33.1.47.10.46.22. Fax: 33.1.47.10.79.49. E-mail:
martin.rottman{at}rpc.aphp.fr.

Published ahead of print on 4 December 2006. 
Editor: J. L. Flynn

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Infection and Immunity, February 2007, p. 1055-1058, Vol. 75, No. 2
0019-9567/07/$08.00+0 doi:10.1128/IAI.00835-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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