number of cattle is persistently infected with Shigatoxin-producing
Escherichia coli (STEC) and thereby represent a source of infections
of man with human pathogenic STEC strains (syn. enterohemorrhagic
E. coli (EHEC)). Intervention strategies to effectively prevent
human EHEC infections must be aimed at the limitation of bovine
STEC infections, therefore. At the beginning of the project, the
mechanisms that form the basis of the persistence were poorly understood.
In the course of the past funding periods we were able to prove
that the production of Shiga toxins (Stxs) plays an important role
in that Stxs suppress the local and systemic immune response in
cattle. Stx1 accelerates the transcription of il-4 in intraepithelial
lymphocytes (IEL) in vitro and eventually blocks the activation
of the IEL. In addition Stx1 induces the transcription of certain
chemokine genes in intestinal epithelial cells and mucosal macrophages,
newly identified target cells for Stx in cattle. As a consequence,
the integrity of the local immune defense is disturbed to an extend
that changes in the IEL composition can be observed in calves after
experimental STEC infection. Since Stx1 blocks the activation of
peripheral lymphocytes in early phases in vitro, the toxin is capable
of delaying the development of a systemic immune response against
STEC antigens in calves orally infected with the bacteria. We therefore
conclude that an intervention strategy that prevents the establishment
of a persistent STEC infection in cattle has to efficiently block
the immunosuppressive effect of Stx at the time of the first infection.
This blockade needs to be effective prior to binding of the toxin
to its cellular receptor, because we have evidence that sole binding
of the enzymatically inactive B-subunit is cabable of suppressing
bovine lymphocytes. Therefore, the objectives of the project in
the upcoming funding period are to investigate receptor derived
signals by the use of alternate ligands and to evaluate their meaning
relative to the cytoplasmic inhibition of the protein synthesis.
Another focus will be put on the characterisation of the Stx1 effects
on antigen-presenting macrophages. Finally, it will be examined
by biochemical and functional methods 1) against what part of the
toxin molecules the antibodies are directed that naturally occur
in the serum and mucosal secretions of cattle upon STEC infections,
2) at what age of calves these antibodies appear first and 3) why
they are unable to prevent the persistence of the STEC infection.
Obtained findings will be eventually used to develop a vaccination
strategy efficacy of which will be tested in calves.