T helper cell subsets in arthritis and the benefits of immunomodulation by 1,25(OH)₂ vitamin D

The prevalence of vitamin D deficiency has been shown to be increased in many of the common arthritides. Importantly, vitamin D has significant immunomodulatory effects in addition to its role in calcium homoeostasis. Both aspects of its function have a major bearing on joint disease whether as part of an inflammatory arthritis or from wear and tear. While the exact mechanisms still require clarification, there is now compelling evidence that the hormonally active 1,25-dihydroxycholecalciferol vitamin D can reduce the activity of the proinflammatory Th1 and Th17 T cell subsets. Additionally, it is stimulatory of enhanced anti-inflammatory Th2 activity at the same time as promoting T regulatory cell activity. These various actions suggest that correcting vitamin D deficiency should be a important part of the management of all patients with joint disease. For the future, vitamin D analogues with enhanced immunomodulatory properties but with reduced ability to increase calcium are being investigated.