Unraveling the basic biology and clinical significance of the chlamydial plasmid.

2011·1.1 MB·English

by  Rockey, Daniel D.

Minireview
Unraveling the basic biology and clinical significance
of the chlamydial plasmid
Daniel D. Rockey
Chlamydial plasmids are small, highly conserved, nonconjugative, and non­ syndromes caused by otherwise related
integrative DNA molecules that are nearly ubiquitous in many chlamydial strains and species.
species, including Chlamydia trachomatis. There has been significant recent Such variability is not found in
progress in understanding chlamydial plasmid participation in host–microbe chlamydia. Several chlamydial species
interactions, disease, and immune responses. Work in mouse model systems contain one of a homologous set of
and, very recently, in nonhuman primates demonstrates that plasmid­ 7,500 base pair plasmids with a copy
deficient chlamydial strains function as live attenuated vaccines against number that is approximately four­
genital and ocular infections. Collectively, these studies open new avenues fold greater than that of the chromo­
e some (Thomas et al., 1997; Pickett et al.,
n of research into developing vaccines against trachoma and sexually transmitted
ci chlamydial infections. 2005; Fig. 1). Within C. trachomatis
i clinical isolates, the plasmid is virtually
d
e Human pathogenic chlamydiae are These obligate intracellular bacteria ubiquitous. There are occasional stud­
M ies showing plasmid­negative clinical
members of a successful and unique develop within a membrane­bound
strains, but little is known about the
l lineage of bacteria (Collingro et al., vacuole termed the inclusion (Fig. 1),
a epidemiology and significance of these
t 2011), which infect and cause disease in and existence within the inclusion de­
n relatively rare isolates (Peterson et al.,
a wide variety of animals (Longbottom fines much about the biology of the
e
m and Coulter, 2003). Although anti­ lineage. The challenges to understanding 1990; An et al., 1992; Farencena et al.,
1997). Chlamydial plasmids are non­
i biotic chemotherapy is quite effective in and preventing chlamydial disease are
r conjugative and nonintegrative. They
e treating diagnosed primary uncompli­ amplified by the difficulties of work­
p do not encode antibiotic resistance
cated ocular and urogenital chlamydial ing with an obligate intracellular bacte­
x
genes and do not show signs of genetic
E infections, prevention of the serious rial species, in which the development
pathological sequelae, which can de­ of a practical system of genetic manipu­ flexibility. This plasmid and the equally
f
o curious chlamydiaphage found in some
velop after infection, and prevention lation is only in its infancy. (Binet and

al of reinfection are much more difficult Maurelli, 2009; Kari et al., 2011). strains (Storey et al., 1989) represent
n tasks (Byrne, 2010). Most chlamydial the entire complement of extrachro­
r mosomal elements carried by Chla-
u infections are asymptomatic and thus The chlamydial plasmid
o go undiagnosed and untreated, which In most microbial systems, autono­ mydia spp. There is a single example of
e J can lead to chronic inflammation and mously replicating plasmids function as an integrated plasmid in Chlamydia suis
(Dugan et al., 2004), but this is clearly
h irreversible tissue and organ pathology. tools of genetic variation and are built
T the exception and not the rule.
In some countries and communities, with replicative backbones associated
Nucleotide sequence identity among
limited access to medical care and in­ with a variety of functional cassettes
plasmids from Chlamydia spp. ranges
fection screening technologies further encoding niche­specific genes or gene
from 69 to 99%, with very high identity
hampers timely diagnosis and treat­ sets. Often these plasmids can be hori­
within C. trachomatis (Thomas et al.,
ment (Centers for Disease Control zontally transferred, which leads to
1997; Seth­Smith et al., 2009). Sequence
and Prevention [CDC], 2011). Repeat rapid distribution or reassociation of
comparisons suggest that the plasmids
infections are common, and the risk phenotypes among strains (Koonin et al.,
evolved in parallel with the different
of developing the serious sequelae of 2001). The spread of antibiotic resis­
chlamydial species. The C. trachomatis
ocular (e.g., blindness) or urogenital tance is often a function of sharing
plasmid carries a set of five open reading
infection (e.g., infertility) increases of plasmids (or other laterally trans­
frames (ORFs), which share identity
with multiple infections (Darville and ferred genetic elements) among other­
with episomal maintenance genes com­
Hiltke, 2010). wise susceptible strains. Such plasmid
mon to other plasmids, and a set of
transfer is a major and evolving prob­
lem in infectious disease worldwide.
D.D. Rockey is at the Department of Biomedical
© 2011 Rockey This article is distributed under the terms of an
Sciences, Oregon State University College of Many serious diseases are directly the Attribution–Noncommercial–Share Alike–No Mirror Sites license
Veterinary Medicine, Corvallis, OR 97331 result of virulence factors carried by plas­ for the first six months after the publication date (see http://www
.rupress.org/terms). After six months it is available under a Crea-
mids, and variation in plasmid structure tive Commons License (Attribution–Noncommercial–Share Alike
CORRESPONDENCE
3.0 Unported license, as described at http://creativecommons.org/
D.D.R.: [email protected] is often associated with different disease licenses/by-nc-sa/3.0/).
The Rockefeller University Press $30.00
J. Exp. Med. Vol. 208 No. 11 2159-2162 2159
www.jem.org/cgi/doi/10.1084/jem.20112088
However, clinical isolates with a dele­
tion in the target sequence used in
some diagnostic kits (the Swedish Vari­
ant) have been identified. This can lead
to false negative test results in patients
(Unemo and Clarke, 2011). Studies
have reported an apparent clonal expan­
sion of C. trachomatis isolates carrying
this deletion. It is possible that the in­
ability to detect infections caused by
strains carrying the deleted plasmid has
contributed to the rapid expansion of the
variant strains in patient populations.
As mentioned, the lack of routine
genetic tools has been a significant bar­
rier to rapid progress in chlamydial re­
search. Manipulation of the chlamydial
plasmid is a logical approach to intro­
duce DNA into these organisms, but
such efforts have been frustrating and
generally unproductive. However, a very
recent study describes successful trans­
formation experiments using hybrid shut­
tle vectors built with pBR325 and the
entire chlamydial plasmid (Wang et al.,
2011). The vectors are propagated in
both E. coli and C. trachomatis, and the
system was used to generate green fluor­
escent C. trachomatis. This is an exciting
and highly significant development in
the study of chlamydiae.
The chlamydial plasmid appears to
be critical for growth in vivo (Russell
et al., 2011), but it is not required for
growth in vitro. Multiple laboratories
have described phenotypic changes as­
sociated with plasmid­deficient strains
as compared with closely related wild
type strains. Creation of plasmid­deficient
isogenic strains began with the careful
Figure 1. Chlamydial growth and plasmid structure. The obligately intracellular chlamydiae
work of Akira Matsumoto, who devel­
develop within a host vacuole termed the inclusion. The two developmental forms have different
oped a plaque­purification technique for
functions in growth. Elementary bodies (EB) are infectious but minimally metabolically active,
chlamydial strains (Matsumoto et al.,
whereas reticulate bodies (RB) grow and divide, but cannot infect. As shown in a magnified reticulate
body, the chlamydial plasmid has eight ORFs, which encode plasmid maintenance and chlamydia- 1998) and noted that unlike wild­type
specific functions. ORF5 encodes Pgp3, which is secreted from the bacterium and the inclusion and C. trachomatis strains, three of his plaque­
accumulates in the cytosol of the host cell. Pgp3 is immunogenic in many host species. The chla- purified clones of C. trachomatis did not
mydial plasmid is also associated with the accumulation of glycogen granules (purple) in the lumen accumulate glycogen within the inclu­
of the inclusion and, as discussed in the text, functions in virulence in animal model systems. sion. Each glycogen­negative clone was
then shown to lack the chlamydial
three genes encoding proteins of The chlamydial plasmid also has a plasmid. Subsequent investigations of
unknown function, each of which is practical side. It is a common target for plasmid­negative strains (O’Connell
chlamydia specific. Human strains of nucleic acid amplification test (NAAT)– and Nicks, 2006; Carlson et al., 2008;
Chlamydophila pneumoniae lack plasmid, based diagnostics of human infections O’Connell et al., 2011) confirmed the
but equine C. pneumoniae strains and (Fredlund et al., 2004). NAAT­based absence of glycogen and examined
other veterinary chlamydial species are tests have been, and remain, a valuable transcription across the chlamydial
plasmid positive. aspect of chlamydial disease control. genome relative to wild­type strains.
2160 Emerging role for conserved chlamydial plasmids | Rockey
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Although the precise scope and nature of and it is hypothesized that Pgp3 serves question remains unaddressed, it appears
plasmid­chromosome cross­regulation is a similar function. that some aspect of chlamydial plasmid
not fully understood, transcription of a Chlamydial vaccine studies are com­ biology is strongly selected for during
set of chlamydial chromosomal genes is plicated by the possibility that the de­ successful infection or transmission of
negatively affected by the absence of bilitating pathology associated with the pathogen within a host population
plasmid. One C. trachomatis gene that is chlamydial ocular and urogenital infec­ (Russell et al., 2011). It is likely that this
down­regulated in the plasmid­negative tions represents collateral damage of competitive advantage tips the scale to­
strains is glgA, a glycogen biosynthetic innate and/or adaptive immune re­ ward pathogenesis in human infections.
gene. Thus, it is hypothesized that ele­ sponses (Rockey et al., 2009; Farris and In addition, as repeated and/or persistent
ments encoded by the plasmid regulate, Morrison, 2011). Protection, in contrast, chlamydial infections are strongly asso­
and perhaps directly participate in, is thought to be serovar or genovar ciated with severe pathology, it is likely
the process of glycogen accumulation specific, but neither the mechanisms of that inflammatory responses elicited by
within chlamydial inclusions. pathogenesis nor the mechanisms of the bacterium are associated with serious
protection have been clearly elucidated. disease sequelae. Thus, the plasmid may
Linking disease and vaccines Several candidate protective antigens have encode or control expression of proteins
to the chlamydial plasmid been explored (Rockey et al., 2009), but that are responsible for successful bac­
Subsequent in vivo studies using plasmid­ no molecule has emerged as a clear and terial growth and/or of the stimulation
deficient strains demonstrated that the practical choice as a single protective of a deleterious immune response. The
plasmid is a virulence factor that signifi­ antigen. Work in the mouse system sug­ search for deleterious chlamydial antigens
cantly impacts clinical disease in mouse gests that delivery of antigens, or a com­ has a long history, and genus­common
model systems. For example, although bination of antigens, is required to antigens are thought to be major players
plasmid­negative strains grew similarly generate a protective response. Indeed, in this pathogenetic response. Perhaps a
to isogenic wild­type strains, they failed to a live attenuated veterinary chlamydial major immunopathogenetic determinant
activate Toll­like receptor 2–dependent vaccine, generated via chemical muta­ is encoded by the plasmid, or the gene
immune responses either in vivo or genesis, is commercially available and products encoded/controlled by the
in vitro (O’Connell et al., 2011). Other routinely used to reduce the incidence of plasmid allow persistence to a level that
work indicated that plasmid­negative abortion in sheep (Burall et al., 2009). facilitates deleterious responses to other
C. trachomatis strains had significantly In this issue of the Journal of Experi- chlamydial macromolecules. Under the
higher 50% infective dose values relative mental Medicine, Kari et al. describe ex­ latter model, absence of the plasmid
to a wild­type matched isolate (Carlson periments demonstrating that a plasmid­ would allow the bacterium to be cleared
et al., 2008). The plasmid­deficient strains deficient C. trachomatis strain is both before an immunopathogenic response
also functioned as successful attenuated avirulent in ocular infections of ma­ can be generated or amplified. Alterna­
live vaccines in mice, wherein prior caques and functions as a live attenuated tively, glycogen accumulation might be
infection with plasmid­negative strains vaccine in that system. These fascinat­ a metabolic equivalent of a virulence
limited the pathology associated with a ing studies conclusively demonstrate factor, providing energy stores as chla­
subsequent infection by wild­type strains that the plasmid functions as a virulence mydiae fight to grow and survive in a
(O’Connell et al., 2007). factor in C. trachomatis ocular infections, hostile host environment.
The near universal carriage of this consistent with the work conducted in Although the mechanisms of atten­
single plasmid by C. trachomatis has led the genital tract in the murine model uated virulence and/or of immune pro­
to speculation about its importance in system. In addition to the colonization tection are not clearly defined by Kari
human chlamydial infections. Early studies, Kari et al. demonstrated that et al., this study is a seminal contribu­
work demonstrated that chlamydia­ three of the six animals infected with tion to the field of chlamydia patho­
infected individuals produce antibodies the plasmid­negative strain were com­ genesis and forms the basis for continued
specific for at least one plasmid­encoded pletely protected against challenge by an exploration of the role of the plasmid
protein, Pgp3, which is encoded by isogenic and virulent plasmid­positive in the chlamydial infectious process.
ORF5 (Comanducci et al., 1994). Fur­ strain. Analysis of the major histocom­ Future studies, perhaps encouraged by
ther work explored the possible utility patibility complex genotypes of each this work, will need to address the
of Pgp3 as a vaccine candidate (Donati animal demonstrated that all three of challenging issues of antigenic variation
et al., 2003). Recent work has shown the solidly protected animals shared a and host genetic variation in designing
that native Pgp3 is a trimeric molecule common M1 haplotype, suggesting a vaccine strategies.
that is transported to the host cell cytosol potential role for CD4+ T cells in the
during the infectious process (Li et al., observed differential protection. Richard Morrison of the University of Arkansas for
2008; Chen et al., 2010; Fig. 1). Other What role does the plasmid play in Medical Sciences is acknowledged for critical reading
of this manuscript. Editorial comments were also
chlamydial proteins are delivered to the chlamydial infections and why is it main­
provided by members of my laboratory.
cytosol, where they function to turn tained as an extrachromosomal element
The Rockey laboratory is supported by Public
the host cell into a “chlamydia factory,” in this pathogen? Although the latter Health Service awards AI069214 and AI086469.
JEM Vol. 208, No. 11 2161
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2162 Emerging role for conserved chlamydial plasmids | Rockey