Effects of fever-range temperature on Ag-specific CD8+ T cell activation, differentiation and effector function
Mace, Thomas Anthony
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T lymphocytes normally become activated through interactive signals generated at the plasma membrane in response to antigen presentation. This activation is accompanied by the aggregation of cholesterol domains in the plasma membrane which are enriched with signaling molecules, an event critical for optimal activation and function. CD8 + T cell activation and function have been primarily studied in vitro using a standard temperature of 37°C. However, under physiological conditions, normal body temperature can range from as low as 29-38°C in the skin and periphery, while fever and hard exercise can bring core temperature up to 40°C. In this study, we investigated how different physiologically relevant temperatures (33°, 37°, and 39°C) can affect CD8 + T cell activation, differentiation and function and we also explored potential molecular mechanisms by which a shift in temperature can regulate CD8 + cells. A greater percentage of naive Ag-specific CD8 + T cells from Pmel-1 mice incubated at 39.5°C differentiate into effector cells more efficiently than those cells incubated at lower temperatures (33° or 37°C). Raising core body temperature of mice to 39.5°C using whole-body hyperthermia (WBH) prior to activation also resulted in a greater number of naive Pmel-1 CD8 + T cells to differentiate into effector CD8 + T cells capable of secreting IFN-γ in vivo. We found this mild in vitro heating of CD8 + T cells at 39.5°C increased plasma membrane fluidity, induced the clustering of cholesterol dependent-microdomains, and enhanced the rate of Ag-specific conjugate formation with an APC. Importantly, this effect is also seen in vivo as we also observed a 3 fold increase in the number of CD8 + T cells with lipid raft aggregates isolated from WBH mice compared to those isolated from normothermic mice (37°C). We also report that important signaling components involved in Ag-specific activation, TCRβ and CD8 co-receptor, become clustered and aggregate following an elevation in temperature (39.5°C) compared to lower control temperatures (33 and 37°C). We next determined how temperature directly affected Ag-specific CD8 + T cell effector function. We found that effector Pmel-1 CD8 + T cells exposed to 39.5°C in vitro resulted in enhanced IFN-γ production and greater target cell killing when compared to lower temperatures. Once again we discovered that temperature mediated its affects through the membrane as GM1 clustering was enhanced and incubation at higher temperatures prior to activation with mitogens which bypass TCR signaling had no effect on IFN-γ production. Furthermore, we observed higher temperatures enhance CD8 + T cell signaling proximal of the TCR and also result in an increase in IFN-γ gene transcription. These experiments suggest that it is important to consider the temperature at which experiments are performed because immune cells do not exist at a static temperature, but at a gradient of temperatures. CD8 + T cell activation and their subsequent effector functions do not occur solely at 37°C, but could occur at a range of lower and higher temperatures depending upon their position in the body and the physiological state (i.e., presence of a fever). It is important to understand more completely how these ranges of temperatures affect Ag-specific CD8 + T cell activation, differentiation, and effector responses. Overall, these findings can help us to understand better the role of physiological temperatures in CD8 + T cell function and may assist in the rational design of thermal therapy protocols which could be implemented in the clinic for patients that are in need of improved effector CD8 + cell function.