SUMMARYNuclear factor kappa B (NF-k B) is a transcription factor pivotal for the development of inflammation. A dysregulation of NF-k B has been shown to play an important role in many chronic inflammatory diseases including rheumatoid arthritis, inflammatory bowel disease and psoriasis. Although classical NFk B, a heterodimer composed of the p50 and p65 subunits, has been well studied, little is known about gene regulation by other hetero-and homodimeric forms of NF-k B. While p65 possesses a transactivation domain, p50 does not. Indeed, p50/p50 homodimers have been shown to inhibit transcriptional activity. We have recently shown that Interleukin-10 exerts its anti-inflammatory activity in part through the inhibition of NF-k B by blocking I k B kinase activity and by inhibiting NF-k B already found in the nucleus. Since the inhibition of nuclear NF-k B could not be explained by an increase of nuclear I k B, we sought to further investigate the mechanisms involved in the inhibition of NF-k B by IL-10. We show here that IL-10 selectively induced nuclear translocation and DNA-binding of p50/p50 homodimers in human monocytic cells. TNF-a treatment led to a strong translocation of p65 and p50, whereas pretreatment with IL-10 followed by TNF-a blocked p65 translocation but did not alter the strong translocation of p50. Furthermore, macrophages of p105/p50-deficient mice exhibited a significantly decreased constitutive production of MIP-2 a and IL-6 in comparison to wild type controls. Surprisingly, IL-10 inhibited high constitutive levels of these cytokines in wt macrophages but not in p105/p50 deficient cells. Our findings suggest that the selective induction of nuclear translocation and DNA-binding of the repressive p50/p50 homodimer is an important anti-inflammatory mechanism utilized by IL-10 to repress inflammatory gene transcription.