Brucellosis 2003 proceedings - PHIDIAS

Keynote Lectures
recognition depends on the O-polysaccharide moiety and on MHC-II but not MHC-I
molecules as demonstrated by the use of inhibitory antibodies as well as MHC-II and
MHC-I deficient mice. LPS T cell recognition is restricted to the same MHC-II as the
APCs. The low frequency of LPS-reactive T cells as compared to mitogen reactive T
cells is consistent with a specific LPS T cell recognition. This important finding
highlights a new role of low endotoxic LPS as immunomodulator of the immune
response. Therefore, LPS-cell membrane complexes may be important entities
involved in the pathogenesis and in the control of immune response during infection.
A STEALTHY INTRACELLULAR BUG NAMED Brucella: ASPECTS OF
INTERACTION OF THE PATHOGEN WITH THE MACROPHAGE HOST CELL.
Stephan KÖHLER. INSERM U-431, Montpellier, France.
The macrophage is the major host cell of the intracellular pathogen Brucella.
Intramacrophagic multiplication depends on a certain number of factors that are
essential, various classes of mutants therefore yield typical growth curves. Under
nonopsonic conditions, brucellae enter the macrophage via lipid rafts located in the
cytoplasmic membrane. This mode of entry is limited to smooth bacteria possessing
an intact LPS O side chain. The two-component system BvrR/BvrS, affecting the
outer membrane composition, is also involved in phagocytosis. In the following step,
the rapid acidification of the early phagocytic vacuole by a vacuolar ATPase is
crucial for survival of Brucella, as pH neutralization results in complete eradication of
the bacteria. In this obviously harsh environment, Brucella proteins involved in stress
response such as DnaK, and later Lon and HF-1, are essential. Another major
characteristic event in macrophage infection by brucellae is the inhibition of
phagosome-lysosome fusion, due to the impairment of recognition between the two
compartments. The LPS O antigen is involved in this phenomenon, and only
phagosomes containing rough Brucella fuse rapidly with lysosomes. The inhibition of
phagosome-lysosome fusion enables the bacteria to activate specific virulence genes
such as virB, triggered by the acidic environment and nutrient limitation. The type IV
secretion system VirB participates in the establishment of the final replicative niche,
as the corresponding mutants are rapidly eliminated from the infected macrophages.
Once in its final niche, Brucella has to adapt to the environment encountered in this
compartment, and a certain number of bacterial genes specifically induced within the
macrophage have been evidenced. In parallel, several approaches by signaturetagged
transposon mutagenesis and, more recently, by large-scale Tn5 mutagenesis
yielded a more profound insight into the conditions the intracellular pathogen has to
face: a nutrient-poor compartment devoid of amino acids and nucleic acid bases,
characterized by an acidic pH in the early phase of infection, and by low oxygen
tension. The set of genes that are essential for intracellular multiplication of brucellae
has been termed "virulome". Furthermore, in the course of infection, intracellularly
multiplying brucellae prevent apoptosis of macrophages by interference with the IFNgamma
apoptotic pathway. In addition to specific adaptation to the intracellular
environment by formation of the replicative niche we termed "brucellosome", this
behavior may be part of a general strategy of Brucella favoring its development within
the macrophage host cell.
Brucellosis 2003 International Research Conference
41