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EDITORIAL
Cardiovascular Research (2015) 106, 345–347
doi:10.1093/cvr/cvv133
Deadly tricks to combat atherosclerosis
L. Temmerman and E.A.L. Biessen*
Experimental Vascular Pathology, Cardiovascular Research Institute Maastricht (CARIM), University of Maastricht, Debyelaan 25, Maastricht 6229 HX, The Netherlands
Online publish-ahead-of-print 24 April 2015
This editorial refers to ‘Phosphatidylserine liposomesmimic
apoptotic cells to attenuate atherosclerosis by expanding polyreactive IgM producing B1a lymphocytes’ by H. Hosseini et al.,
pp. 443 –452.
The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.
* Corresponding author. Tel: +31 43 3874635, Email: erik.biessen@maastrichtuniversity.nl
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2015. For permissions please email: journals.permissions@oup.com.
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Cardiovascular disease is still the largest morbidity and mortality issue in
Western society. Although current medication has proved effective in
reducing cardiovascular disease risk, the need for additional therapeutic
options remains. In this issue, Hosseini et al. present an elegant new
concept with real bench-to-bedside potential: administration of phosphatidylserine (PS) liposomes to dampen atherosclerosis-related
immune responses by mobilizing and expanding atheroprotective B1a
cells (Figure 1).1
B cells were long believed to act anti-atherogenic. This notion was
recently refined (as reviewed by Perry et al. 2), when the groups of
Mallat and Kyaw independently demonstrated clear detrimental effects
of the classical IgG producing B2 cell subset on plaque progression,3,4
whereas Binder et al. showed B1a cells to inhibit atherosclerosis.5 The
latter finding was further expanded by Kyaw et al., imputing B1a cellmediated protective effects to natural IgM antibodies produced by
these cells.6 Expanding on their previous work the same group has, in
this study, explored new strategies to harness B1a cell’s therapeutic potential to the treatment of atherosclerosis. They argued that apoptotic
cells expressing double-stranded (ds) DNA and phosphatidyl serine
(PS) can activate TLR9 and PS-receptors TIM1/4, respectively, on B1a
cells and unleash their protective functions. To this end, they used apoptotic thymocytes and by extension, synthetic PS liposomes (previously
shown to mimick apoptotic cell-induced anti-inflammatory effects7),
to mobilize (peritoneal) B1a cells in atherosclerotic ApoE2/2 mice.
Hosseini et al. make a strong case: their work sets PS liposomes on the
map as promising therapeutic candidates in the treatment of atherosclerosis.1 Moreover, they are the first to unveal B1a cell stimulatory capacity of PS liposomes. For proof-of-concept they demonstrate that
biweekly intraperitoneal injections with apoptotic thymocytes result
in substantial reductions in atherosclerotic plaque size, necrotic core,
plaque macrophage, and T-cell content and cytokine expression
pattern. In addition, only B1a cells were significantly increased in the peritoneum of treated mice, while plasma levels of natural IgM antibodies reactive against oxidized LDL, leucocytes, and T cells were almost
doubled. The authors thus succeed in their primary goal to recruit
atheroprotective B1a cells and increase IgM antibody levels. The
beauty of the study lies however in the results obtained with PS liposomes, which can be produced in a more controlled manner and at
much larger scale and do not impart the health risks associated with
human primary or immortalized cells. Phosphatidylserine liposomes
gave a near-perfect phenocopy of apoptotic cell associated effects.
Neither apoptotic cells nor PS liposomes were able to exert their protective roles in splenectomized atherosclerotic mice, which show depletion of the peritoneal B1a cell pool,6 illustrating the critical involvement
of B1a cells in PS-mediated protection.
The authors propose TIM-1, a PS recognizing molecule, as a mediator
of apoptotic cell or PS liposome-induced effects on B1a cells.1 However,
the actual involvement of TIM-1, and more in general the molecular
mechanism of the profound effects remains somewhat obscure. The
moderate increase in IL-5 plasma levels, a known B1a cell mitogen, in
treated mice suggests that indirect B1a cell induction by PS-activated
phagocytes may also takes place. Moreover, it is still unclear whether
PS liposomes act by activating B1a cells in peritoneum and how such
interaction implicates the spleen. B1a cells are rather scarce in spleen,
and although Rothstein et al. have identified a splenic CD138+ B1a
subset particularly effective in elaborating IgM,8 they are generally
thought to play a modest role in IgM production. This raises the question
to what extent CD169+ marginal zone macrophages (MZM) may contribute to the PS-liposome effects as well. As shown by Ravishankar
et al. they can release tolerogenic IDO9 and regulatory T-cell recruiting
CCL22 in response to apoptotic cell-exposed dsDNA, leading to tolerance induction.10 In line, MZM depletion led to increased autoimmunity
and proinflammatory cytokine production, echoing the PS liposome
phenotype. Further studies will be required to conclusively establish
the precise mode of action of PS liposomes in atherosclerosis.
In summary, Hosseini et al. present a fresh and new therapeutic angle
to tackle atherosclerosis, and by extension many other chronic inflammatory disorders. In essence, they managed to hijack a protective mechanism the body has created to avoid excess inflammation upon normal
cell death. Their paper leaves us with a number of exciting questions.
Firstly, will B1a cells—the key players in PS liposome-induced atheroprotection—turn out to be equally potent and PS liposome targetable
in other chronic inflammatory or autoimmune diseases. Iv injected
PS liposomes have been used in other animal models of arthritis11
and myocardial infarction,12 though their beneficial actions were
in those cases entirely attributed to PS-activated phagocytic effects.
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Editorial
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Figure 1 Apoptotic cells expose phosphatidyl serine (PS) residues on the membrane. Artificial PS-coated liposomes are able to serve as
apoptotic-cell-mimics. Administration of apoptotic cells or PS liposomes will promote B1a-cell activity and expansion in the peritoneum, possibly
through interaction of PS with TIM-1 receptors. Activated B1a cells will produce increased levels of natural IgM antibodies, which are secreted into the
blood where they confer protection against atherosclerotic plaque formation by (i) sequestering oxidized LDL, (ii) inducing elimination of apoptotic
cells, and (iii) reducing autoantibody formation, in part by interaction of IgM/OxLDL immune complexes with Fc(micro) receptors on other immune
cells. Apoptotic cell and PS liposome dependent B1a cell activation implicates the spleen by a still unknown mechanism; although a role of the recently
identified splenic CD138+ B1a cells, representing a major source of IgM, cannot be excluded, it seems more likely that this involves IgM/Fc(micro)R
induced tuning of T/B cell status in spleen or the participation of (splenic) CD169+ macrophages, which are primarily responsible for PS-marked cells/liposomes engulfment and subsequent release of the B1a mitogen IL-5.
Phosphatidylserine-induced increases in IgM production were not
described, though based on the data presented here they should definitely be looked into. Secondly, the authors report a PS liposome B1a cellmediated effect on several polyreactive IgM subtypes, ensuring a broad
tolerogenic response. While this offers clear opportunities for targeting
other inflammatory diseases, given that natural IgMs are currently
attracting considerable interest as potent immune suppressive agents,13
such tolerogenic state could impact on host defense against tumourassociated (neo)epitopes or pathogens. Secondly, is this methodology
directly translatable to a human setting? In this regard it is worth
noting that IgM memory B cells have recently been proposed as the
human antipode of B1a cells. Whether this subset is equally capable of
mediating atheroprotective responses to PS liposome exposure will
be one of the outstanding questions for the near future. Another issue
relates to PS-liposome administration. Obviously ip injection is not fit
for human application, but may well be critical for B1a dependent atheroprotection. Moving forward, researchers will have to find an effective
way to keep the B1a cell targeting both specific as well as efficient
enough, also when translated into more clinical applications. Thirdly,
Hosseini et al. chose to study effects on atherosclerosis onset; studies
are awaited on PS liposome effects in more advanced stages of
disease. Based on the presented pilot work, a positive outcome is to
Editorial
be expected, and would greatly uplift the therapeutic value of their
methodology. All in all, the impressive body of data presented here
lays a thorough foundation for further studies into the therapeutic use
of PS liposomes in the prevention of cardiovascular disease.
References
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Phosphatidylserine liposomes mimic apoptotic cells to attenuate atherosclerosis by
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