A Live Experimental Vaccine against Burkholderia pseudomallei Elicits CD4+ T Cell–Mediated Immunity, Priming T Cells Specific for 2 Type III Secretion System Proteins
Ashraful Haque
.)
2
3
4
Karen Chu
2
3
Anna Easton
2
3
Mark P. Stevens
1
2
Edouard E. Galyov
1
2
Tim Atkins
0
2
Rick Titball
0
2
Gregory J. Bancroft
2
3
0
Defence Science and Technology Laboratories
,
Porton Down
,
United Kingdom
1
Institute for Animal Health
,
Compton, Berkshire
2
Received 12 May 2006; accepted 20 June 2006; electronically published 25 September 2006. Presented in part: 4th World Melioidosis Conference
,
16-18 September 2004
,
Singapore.
Potential conflicts of interest: none reported. Financial support: Defence Science and Technology Laboratory
,
United Kingdom (grants RD032-0469 and RD013-0931417 to G.J.B.
); National Institutes of Health (grant AI-61363
,
subaward 2004-1489, to G.J.B.); Biotechnology and Biological Sciences Research Council (grant C20021 to M.P.S. and E.E.G.)
3
London School of Hygiene and Tropical Medicine
,
London
4
Present affiliation: Queensland Institute for Medical Research
,
Herston, Brisbane
,
Australia.
ical Research
,
300 Herston Rd., Herston, Brisbane, QLD 4006
,
Australia (ashraful
Burkholderia pseudomallei is the etiological agent of melioidosis, a serious human disease for which no vaccine is available. Immunization of susceptible BALB/c mice with the live attenuated mutant B. pseudomallei ilvI (referred to as 2D2) generated significant although incomplete, immunity. Splenic B. pseudomallei-specifi T cells, detected in immunized mice, proliferated and produced interferon-g in vitro in response to dead bacteria. Assessment of T cell antigen specificit indicated that subpopulations of B. pseudomallei-reactive T cells were responsive to BopE, a type III secretion system (TTSS) effector protein, and to a lesser extent to BipD, a TTSS translocator protein. Increased survival of severe combined immunodeficien mice adoptively transferred with T cells from immunized mice, compared with that of naive T cell recipients, demonstrated that immunization with 2D2 generated T cell-mediated immunity. CD4+ and CD8+ cell depletion studies demonstrated that CD4+ cells, but not CD8+ cells, mediated this protection in vivo. Thus, CD4+ T cells can mediate vaccine-induced immunity to experimental melioidosis.
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Burkholderia pseudomallei, the etiological agent of
melioidosis, is a gram-negative, intracellular bacterial
pathogen that is endemic in Southeast Asia and northern
Australia [1]. Clinical manifestations vary from acute,
lethal sepsis to chronic, localized abscess formation to
latent infection, which can reactivate decades later [2].
Mortality rates in acute cases can exceed 40%, with
10%15% of survivors relapsing despite prolonged
antibiotic treatment [2]. B. pseudomallei is classifie as a
class B potential agent for biological warfare and
terrorism. There is no vaccine against B. pseudomallei, and
knowledge of mechanisms of resistance to B.
pseudomallei is limited. A better understanding of immunity
to B. pseudomallei is needed for the generation of
vaccine or immunotherapy strategies against this infection.
Knowledge of mechanisms of resistance to B.
pseudomallei infection has largely derived from murine models
of melioidosis [36]. Our previous work highlighted the
importance of interferon (IFN)g, interleukin (IL)12,
and IL-18 as early mediators of protection [6, 7]. In
addition, we demonstrated a role for T cells in resistance
to primary infection [7]. Studies of melioidosis patients
demonstrated elevated levels of IFN-g, IL-12, and IL-18
in serum samples [8] and have also detected B.
pseudomalleispecifi T cells in the blood [9]. Thus, there is
growing evidence that T cellmediated immune
responses occur during B. pseudomallei infection.
Most examples of protection against B. pseudomallei have
relied on antibodies against flagellin capsule, or lipopolysaccharide
[1013]. However, given the ability of B. pseudomallei to persist
inside host cells [14], effective vaccine-mediated immunity to B.
pseudomallei will most likely require the combined actions of
antibody and T cellmediated immunity. We, and others, have
reported significan protection by immunization with live B.
pseudomallei [15, 16], but in neither case were the immune
mechanisms of protection studied. Two recent studies demonstrating
antibody and cellular immune responses in mice vaccinated with
antigen-pulsed dendritic cells or DNA-encoding B. pseudomallei
flagelli did not assess the direct contribution to protection of
B. pseudomalleispecifi T cells [17, 18]. Thus, to date, the
generation of specifi T cell responses against B. pseudomallei
proteins has not been proven as a vaccine strategy.
We generated immunity to B. pseudomallei by immunizing
genetically susceptible BALB/c mice with a live attenuated
mutant, 2D2 [15]. 2D2 was highly attenuated in mice, was
effectively cleared from various organs, and protected mice in the
short term from subsequent virulent challenge [15]. Here, we
demonstrate that immunization with 2D2 (hereafter, 2D2
immunization) primes B. pseudomalleispecifi CD4+ and CD8+
T cells an (...truncated)