Macrophage migration inhibitory factor (MIF) is a pleiotropic immune modulator that plays a critical role in several inflammatory conditions such as infection, sepsis and autoimmune diseases. MIF is present in cytoplasmic pools in a plethora of immune and non-immune cells and is rapidly released in response to inflammatory stimuli. MIF levels are elevated in a number of diseases like rheumatoid arthritis were increased concentrations in serum, synovial fluid and tissue are correlated with disease severity. In order to find out how MIF action is controlled in vivo, MIF interacting proteins (MIPs) that are able to limit MIF´s strong pro-inflammatory potential were searched for. Following co-immunoprecipitation experiments with lysates from NIH 3T3 cells ribosomal protein S19 (RP S19) was identified as a novel MIP. In pull-down experiments with wild-type and mutant MIF proteins RP S19 was shown to directly interact with MIF in vitro. Cysteine 60 of MIF but not proline 2 was shown to be important for interaction. Because RP S19 is released in inflammatory lesions by apoptotic cells, it was investigated if RP S19 binding can alter biological key functions of the cytokine. Because MIF proline 2 is not decisive for interaction, RP S19 could only moderately inhibit MIF s proline 2 dependent tautomerase activity. In a chemotaxis assay (Transwell-system) MIF stimulated migration of human peripheral blood derived monocytes, and pre-incubation of MIF with RP S19 significantly inhibited MIF s chemotaxis-promoting activity. Moreover, MIF s glucocorticoid overriding activity on TNF secretion by monocytes was abrogated by pre-incubation with RP S19. Therefore, RP S19 was identified as the first endogenous inhibitor that is able to limit and control MIF s strong pro-inflammatory activities.
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