 Abatacept is designed to target and inhibit a specific molecule involved in costimulation of the inflammatory signal that is thought to be important in RA. While abatacept has been shown to be effective in trials and clinical practice, the exact mechanism of action of abatacept in RA has not been fully elucidated. Understanding these actions is likely to inform both the use of abatacept in RA and lead to increased understanding of inflammation in humans with implications for further therapies. This six month prospective open label study, therefore, aims to investigate the effects of inhibiting costimulation on a variety of important inflammatory cell types and processes in humans with RA. 