Moderate Exercise Training Attenuates Inflammation in Transgenic Sickle Cell Mice

Sickle cell disease (SCD) is a monogenic disorder characterized by severe systemic and vascular inflammation. There is strong evidence that moderate exercise training reduces inflammation in healthy people as well as in multiple diseases conditions. The aim of this study was to characterize the effects of moderate exercise training in a humanized transgenic mouse model of SCD (Townes) in mice expressing exclusively human sickle hemoglobin (SS). The SS mice display many of the features of human SCD including systemic inflammation and chronic anemia. We designed a treadmill training protocol of 1hr/day, 5 days a week for 8 weeks. Prior to the training, the maximal oxygen uptake (V̇O₂max) was assessed by an exhaustive incremental exercise. SS mice (n=22) had lower V̇O₂max (mean ± SEM: 7,068 ml/kg/hr ± 133 vs. 8,407 ml/kg/hr ± 224.5, p<0.0001) and lower maximal speed (14.3 m/min ± 0.7 vs. 29.3 m/min ± 1.7, p<0.0001) than control AA mice expressing exclusively normal human hemoglobin (n=12). Together these results indicate SS mice have an inherently reduced aerobic exercise capacity; nonetheless, they tolerated a training regimen at 60% of V̇O₂max. After eight weeks of training, SS mice had a lower hematocrit (20.3% ± 0.8% vs. 24.4% ± 1.8%, p=0.04; n=8) than untrained SS littermates that were exposed to all other experimental conditions except training. There were only moderate changes in red blood cell count (Trained: 4.8.10⁶/µL ± 0.2, Untrained: 5.7.10⁶/µL ± 0.5, p>0.05) or total hemoglobin (Trained: 5.6 g/dL ± 0.2 vs 6.3 g/dL ± 0.4, p>0.05) between the two groups suggesting the low hematocrit in the trained mice was probably due to increased plasma volume, which is a normal physiological response to exercise training. The spleen/body mass ratio was significantly lower in the trained SS mice compared to the untrained littermates (5.7 mg/g body weight ± 0.3 vs. 7.0 mg/g body weight ± 0.9, p=0.04). Importantly, the spleen was less congested in the trained SS mice, and the magnitude of congestion in all animals (trained and untrained) correlated very strongly with the relative spleen mass (r=0.7, p=0.009). Exercise training significantly increased the percent of oxyhemoglobin in the venous blood of SS mice (Trained: 39% ± 4.3, Untrained: 22.3 ± 4.1, p=0.016). And, there was a corresponding decrease in the percent carboxyhemoglobin (Trained: 2.3% ± 0.7, Untrained 6.2% ± 0.5, p=0.001). These data suggest moderate exercise training may reduce the proportion HbS that is deoxygenated in the venous circulation. In addition, exercise training significantly reduced the peripheral blood white cell count in the SS mice (Training: 14.7.10³/µL ± 0.9, Untrained: 21.6.10³/µL ± 3.1, p=0.03); this change was reflected by a corresponding lower lymphocyte count in the trained mice (12.10³/µL ± 0.8 vs. 17.3.10³/µL ± 2.3, p=0.028). Finally, running decreased the liver expression of heme oxygenase-1 (HO-1) mRNA (p=0.04) and the plasma concentration of macrophage inflammatory protein-1β (MIP-1β, p=0.03). Interestingly, HO-1 is implicated in metabolic inflammation and MIP-1β promotes endothelial adhesion of CD8+ T cell through a VCAM-1 dependent pathway. Together with the reductions in leukocyte counts, we suggest that moderate exercise training may reduce sequestration of sickle erythrocytes/congestion, resting blood deoxygenation and inflammation in SCD.