ARTHRITIS & RHEUMATISM Vol. 56, No. 10, October 2007, pp 3215–3225 DOI 10.1002/art.22919 © 2007, American College of Rheumatology Fractalkine Mediates T Cell–Dependent Proliferation of Synovial Fibroblasts in Rheumatoid Arthritis Hirokazu Sawai, Yong W. Park, Xiaowen He, Jo ¨rg J. Goronzy, and Cornelia M. Weyand Objective. In rheumatoid arthritis (RA), synovial fibroblasts proliferate excessively, eventually eroding bone and cartilage. The aim of this study was to examine the mechanisms through which CD4 T cells, the domi- nant lymphocyte population in patients with rheuma- toid synovitis, regulate synoviocyte proliferation. Methods. Fibroblast-like synoviocyte (FLS) lines were established from rheumatoid synovium. CD4 T cells from patients with RA and age-matched control subjects were cultured on FLS monolayers. FLS prolif- eration was quantified by cytometry, using carboxyfluo- rescein succinimidyl ester staining or microscopic enu- meration of PKH26-stained FLS. Surface expression of the fractalkine (FKN) receptor CX 3 CR1 was monitored by fluorescence-activated cell sorting. The induction of CX 3 CR1 and its ligand FKN in FLS was quantified by real-time polymerase chain reaction. Results. The proliferation of FLS was signifi- cantly increased in the presence of CD4 T cells from patients with RA compared with control T cells. CD4,CD28– T cells were particularly effective in sup- porting FLS growth, inducing a 25-fold expansion com- pared with a 5-fold expansion induced by CD4,CD28 T cells. The growth-promoting activity of CD4,CD28– T cells was mediated through CX 3 CR1, a chemokine receptor expressed on both T cells and FLS. Anti- CX 3 CR1 antibodies inhibited T cell production of tumor necrosis factor  (TNF) and suppressed FLS prolifer- ation. TNF amplified the expansion of FLS by enhanc- ing their expression of CX 3 CR1 and FKN. Conclusion. FKN–CX 3 CR1 receptor–ligand inter- actions regulate FLS growth and FLS-dependent T cell function. FLS stimulate autocrine growth by releasing FKN and triggering the activity of their own CX 3 CR1. This growth-promotion loop is amplified by TNF pro- duced by CX 3 CR1-expressing T cells upon stimulation by FKN-expressing FLS. These data assign a critical role to FKN and its receptor in fibroblast proliferation and pannus formation in RA. Hyperplasia of the synovial layer is a principal disease mechanism in rheumatoid arthritis (RA). The expansion of synovial tissue generates pannus, a destruc- tive tumorlike structure that penetrates into the cartilage and subchondral bone, leading to erosion of these structures. In the rheumatoid joint, the hyperplastic membrane is composed of a specialized type of fibro- blasts, fibroblast-like synoviocytes (FLS), which grow in an anchorage-independent manner and are resistant to apoptosis (1). FLS from the rheumatoid synovium have long been recognized as a source of proinflammatory cytokines and proteases, functioning as an amplifier of inflammation and directly contributing to tissue damage (2,3). FLS derived from patients with RA were shown to attach to and invade normal human cartilage in a SCID mouse model (4). It has been suggested that the invasive behavior of FLS from patients with RA correlates with the rate of joint destruction as the disease progresses (5). FLS have also been implicated in regulating the fate of tissue-invasive lymphocytes, placing them in a critical position in the rheumatoid disease process. Specifically, FLS have been described to provide signals to T cells, altering their susceptibility to apoptosis and determining their survival and migration pattern in the inflamed lesions (6–8). Mutuality in the relationship between FLS and T cells has been suggested by the demonstration that activated T cells determine the functional profile of FLS in a contact-dependent manner (9,10). Supported in part by the NIH (grants R01-AR-42527, R01- AI-44142, R01-AR-41974, and R01-AI-57266). Hirokazu Sawai, MD, Yong W. Park, MD, Xiaowen He, PhD, Jo ¨rg J. Goronzy, MD, PhD, Cornelia M. Weyand, MD, PhD: Emory University School of Medicine, Atlanta, Georgia. Drs. Sawai and Park contributed equally to this work. Address correspondence and reprint requests to Cornelia M. Weyand, MD, PhD, Lowance Center for Human Immunology, Emory University School of Medicine, Room 1003 Woodruff Memorial Research Building, 101 Woodruff Circle, Atlanta, GA 30322. E-mail: cweyand@emory.edu. Submitted for publication December 12, 2006; accepted in revised form June 22, 2007. 3215