p38 mitogen-activated protein kinase (MAPK) regulates cytokines in arthritis and is, in turn, regulated by MAPK kinase (MKK) 3 and MKK6. To modulate p38 function but potentially minimize toxicity, we evaluated the utility of targeting MKK3 by using MKK3 ؊/؊ mice. These studies showed that TNF-␣ increased phosphorylation of p38 in WT cultured synoviocytes but that p38 activation, IL-1, and IL-6 expression were markedly lower in MKK3 ؊/؊ synoviocytes. In contrast, IL-1 or LPS-stimulated p38 phosphorylation and IL-6 production by MKK3 ؊/؊ synoviocytes were normal. Detailed signaling studies showed that NF-B also contributes to IL-6 production and that TNF-␣-induced NF-B activation is MKK3-dependent. In contrast, LPS-mediated activation of NF-B does not require MKK3. To determine whether this dichotomy occurs in vivo, two inflammation models were studied. In K͞BxN passive arthritis, the severity of arthritis was dramatically lower in MKK3 ؊/؊ mice. Phospho-p38, phospho-MAPK activator protein kinase 2, IL-1, CXC ligand 1, IL-6, and matrix metalloproteinase (MMP) 3 levels in the joints of MKK3 ؊/؊ mice were significantly lower than in controls. Exogenous IL-1 administered during the first 4 days of the passive model restored arthritis to the same severity as in WT mice. In the second model, IL-6 production after systemic LPS administration was similar in WT and MKK3 ؊/؊ mice. Therefore, selective MKK3 deficiency can suppress inflammatory arthritis and cytokine production while Toll-like receptor 4-mediated host defense remains intact.cytokines ͉ inflammation ͉ rheumatoid arthritis ͉ fibroblast ͉ signal transduction