| Abstract | Monocytes are a heterogeneous population with two distinct subsets, regular (CD14Bright+) and proinflammatory (CD14Dim+). Little is known about the intracellular heat shock protein (HSP) 72 expression in these subsets during exertional heat stress.
PURPOSE: The purpose of this study was to examine the impact of exertional heat stress on HSP72 expression in CD14⁺ monocyte subsets in trained and untrained individuals.
METHODS: Maximum oxygen consumption expressed per kg of lean body mass (LBM) and activity history was used to divide subjects into trained (TR, n=8, VO₂peak = 68 ± 2 mL·kgLBM⁻-min⁻¹) and untrained (UT, n=6, VO₂peak = 49 ± 1 mL·kgLBM⁻ ¹-min⁻ ¹) groups. Subjects walked at 4.5 km-h¹ with 2% elevation in a climatic chamber (40°C; 30% R.H.) wearing military biological and chemical protective clothing until specific end-point criteria were attained. Venous blood samples were collected at baseline, rectal temperatures of 38.0°C, 38.5°C, 39.0°C and at exhaustion. Whole blood samples were analyzed immediately by flow cytometry for spontaneous intracellular HSP72 immunofluorecence staining in monocyte subsets CD14Dim+ and CD14Bright+. Similar analyses were performed at baseline and exhaustion following in vitro heat shock (42°C, 2 hr).
RESULTS: Total CD14⁺ and CD14⁺Bright counts (×10⁹cells/L) were significantly increased in TR at exhaustion compared to UT, whereas UT subjects levels of CD14⁺Dim were significantly increased compared to TR at 39.0°C and exhaustion (0.12 ± 0.04 vs. 0.076 ± 0.004 ×10⁹cells). Percentages of CD14+ cells positive for intracellular HSP72 expression were significantly elevated in TR compared to UT during in vivo heat stress; however, following in-vitro heat shock (42°C, 2hr), only CD14Dim+ subset was significantly elevated compared to UT.
CONCLUSIONS: These findings reveal an increased level of proinflammatory (CD14Dim+) monocytes in the circulation in UT versus TR during heat stress. Furthermore, UT had a blunted HSP response in their proinflammatory monocyte subset upon subsequent in-vitro heat shock. Combined, these findings imply a cytoprotective adaptation in highly trained individuals, which may help contribute to increased thermotolerance. |
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