ABSTRACT The γ isoform of phosphoinositide 3‐kinase (PI3Kγ) has been viewed as restricted to leukocytes mediating the regulation of chemokine‐induced migration and recruitment of neutrophils, monocytes, and macrophages. In line with the observation that PI3Kγ‐deficient mice display defects in adaptive immunity, inhibition of PI3Kγ reduces synovial inflammation in the collagen‐induced arthritis mouse model of inflammatory arthritis [rheumatoid arthritis (RA)], which has been attributed to reduced influx of inflammatory cells. Challenging the concept of leukocyte‐restricted PI3Ky function, we report here a novel, nonredundant function of PI3Kγ as an important regulator of fibroblast‐ induced cartilage destruction during chronic destructive arthritis. We show that in human tumor necrosis factor transgenic mice, the loss of PI3Kγ leads to a milder inflammatory arthritis. Interestingly, PI3Kγ deficiency does not alter the recruitment of inflammatory cells, but significantly reduces cartilage damage through reduced expression of matrix metalloproteinases in fibroblasts and chondrocytes. In vitro analyses demonstrate that the decreased invasiveness of fibroblasts is mediated by reduced phosphorylation of Akt and extracellular signalregulated kinase. Using a PI3Kγ specific inhibitor, these data are confirmed in human synovial fibroblasts from patients with RA who exhibit a disease‐specific up‐regulation of PI3Kγ. Our data indicate that in addition to mediating the recruitment of inflammatory cells, PI3Kγ is an important regulator of fibroblast‐mediated joint destruction in RA and suggest that specific inhibitors of PI3Kγ will interfere with the activation of RA synovial fibroblasts and reduce cartilage destruction in RA.— Hayer, S., Pundt, N., Peters, M. A., Wunrau, C., Kühnel, I., Neugebauer, K., Strietholt, S., Zwerina, J., Korb, A., Penninger'J.'Joosten' L. A. B., Gay, S., Rückle, T., Schett, G., Pap, T. Phosphatidylinositol 3‐kinase‐γ regulates cartilage damage in chronic inflammatory arthritis. FASEBJ. 23, 4288‐4298 (2009). www.fasebj.org