The RTX toxins are a group of calcium-dependent, pore-forming cytolysins secreted by several species of Gram-negative bacteria. They share common structural and functional features, the most prominent of which is a series of glycine-rich nonapeptide repeat units found at the C-terminal end of the proteins from which the designation 'Repeats in ToXin' is derived. These units bind calcium which is important for the subsequent interaction of the toxin with a mammalian target cell. All the toxins are able to form pores in eukaryotic cells membranes which promote osmotic swelling and eventual cell lysis. However, the toxins vary in their degree of cytolytic activity towards different mammalian cells; some will attack red blood cells and leukocytes from a variety of mammalian species whereas others are more species-specific. Their principal role as virulence factors is to modulate the host immune response. At high concentrations they can kill immune effector cells at the primary site of infection which can lead to host tissue damage and a secondary inflammatory response. At sub-lytic concentrations they can promote the release of inflammatory mediators from granulocytes and monocytes, damage endothelial cells and compromise the humoral immune system by inhibition of lymphocyte proliferation and antigen presentation by macrophages.
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