 Increased numbers of circulating fibrocytes have been found in murine RA models. In a small sample of RA patients six patients, ten controls, peripheral blood fibrocytes had elevated phosphorylation activation compared to controls, and cells were demonstrated in the synovium. Here fibrocyte activation was correlated with arthritis disease activity DAS28. In a murine model, activated circulating fibrocytes where found to migrate to arthritis joints in the preclinical phase of the disease, and to accumulate in the FLS cell layer of the synovium. Adoptive transfer of circulating fibrocytes in this murine model, led to worsening of joint disease indicating that circulating fibrocytes are involved in RA joint pathology, possibly as a FLS-cell precursor. Further, fibrocytes have also been implicated in extra articular manifestations of RA, e.g. pericarditis murine model, atherosclerosis, and interstitial lung disease. 