Superantigens (SAgs) are microbial proteins which have potent effects
on
the immune system. They are presented by
major histocompatibility complex (MHC) class II molecules and interact
with a large number of T cells expressing specific
T cell receptor Vβ domains. Encounter of a SAg leads initially to
the
stimulation and subsequently to the clonal deletion
of reactive T cells. SAgs are expressed by a wide variety of microorganisms
which use them to exploit the immune system
to their own advantage. Bacterial SAgs are exotoxins which are linked to
several diseases in humans and animals. A classical
example is the toxic shock syndrome in which the massive release of
cytokines by SAg-reactive cells is thought to play
a major pathogenic role. The best characterized viral SAg is encoded by
mouse mammary tumour virus (MMTV) and
has proved to have a major influence on the viral life cycle by dramatically
increasing the efficiency of viral infection. In
this paper, we review the general properties of SAgs and discuss the
different types of microorganisms which produce
these molecules, with a particular emphasis on the role played by the
SAg-induced immune response in the course of microbial infections.