RT-qPCR comparison of mast cell populations in whole blood from healthy horses and those with laminitis.

Inflammatory damage to the digital laminae, a structure responsible for suspension of the distal skeleton within the hoof capsule, results in a painful and often life-threatening disease in horses called laminitis. There can be many diverse causes of laminitis; however, previous work in the horse has suggested that in each case, the inflammation and resulting tissue damage is consistent with the action of mediators released from mast cells (MC), as well as the downstream consequences of their activation. The recent development of molecular genetics tools to characterize cells based on their transcriptional activity makes a new approach for measuring MCs possible. Healthy thoroughbred horses from a variety of age groups were used to assess the amount of variation in KIT (encoding mast cell growth factor receptor) and TPSB2 (encoding mast cell tryptase beta 2) gene expression present in the population and to establish "normal" values. Horses (n=9) with a wider range of body condition scores (3-8), because of a more lax management setting that could predispose them to laminitis, had significantly higher KIT expression in circulating peripheral blood cells than horses under individualized management conditions (n=10) that produced ideal body condition scores (4-6) (mean 2.573-fold, P<0.0005). Likewise, horses affected with acute laminitis (n=11) had elevated expression of TPSB2 (2.760-fold, P=0.0011) relative to control horses (n=15). These data suggest that investigation of MC-related genes KIT and TPSB2 may be effective to assay MC population and activity. More work is needed to refine the diagnostic criteria to better describe at what point MC activation occurs and illustrate the use of gene expression assays in clinical cases of laminitis. Additionally, MC activation is associated with inflammatory disease in several mammalian species and may prove a valuable therapeutic target in the horse.