Mild hyperthermia influences macromolecular dynamics at the T cell surface thus increasing the efficiency of stimulatory signals
Zynda, Evan Robert
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There is growing recognition of the fact that although the effects of fever-range hyperthermia may be less extreme than that of heat shock temperatures, they may be more biologically significant. Our lab recently demonstrated that pre-exposure to mild hyperthermia (39.5°C) can increase IL-2 production by stimulated CD4+ T lymphocytes. This thermally induced effect is correlated with the aggregation of plasma membrane signaling domains. The primary goal of this thesis research has been to elucidate the molecular mechanism by which temperature affects microdomain organization and results in cytokine production. The literature implies that the cell surface is the primary vehicle through which temperature imposes various immune responses. Two cell surface properties of particular interest are plasma membrane fluidity and cytoskeletal reorganization; both are responsive to heat shock temperatures and both are altered during optimal T cell activation. This led us to investigate the effects of physiologically relevant temperature shifts on the T cell plasma membrane fluidity, and assess whether thermally-induced changes in membrane order are capable of inducing macromolecular reorganization at the plasma membrane, thereby enhancing the efficiency of T cell stimulatory signals. The work presented here indicates that mild hyperthermia is able to compensate for less-than-optimal levels of signal 2 in the presence of signal 1 by altering the macromolecular dynamics at or near the T cell surface. We established that T cell plasma membrane fluidity is sensitive to physiologically relevant temperature shifts; warmer temperatures correlate with an increase in fluidity. In addition, we provide evidence that slight increases in T cell plasma membrane fluidity - both thermally- and chemically-induced - are associated with the redistribution of various cytoskeletal proteins, allowing for aggregation of membrane microdomains and an increased rate of receptor cap formation upon stimulation. The resulting proximity of signaling domains leads to increased stimulatory efficiency. Interestingly, stimulation though CD28 alone induced a similar increase in T cell plasma membrane fluidity, and was also associated with macromolecular redistributions similar to what is described above, while stimulation through the TCR alone had no such effects. These results suggest that mild hyperthermia could be useful in the early stages of the immune response, when there are sub-optimal levels of stimulatory signals present. Continued research in the field is important, as it will aid researchers in gaining a better understanding of how the thermal element of fever can regulate signaling pathways involved in the adaptive immune response. Results from these investigations can have important implications on the usefulness of mild hyperthermia for enhancing immunotherapy in cancer. A perspective future direction may be to employ advanced microscopy techniques, with 1nm spatial, and 1ìs temporal resolution, to more closely dissect and quantify membrane protein dynamics (localization, diffusion, and interaction) in response to mild hyperthermia and/or stimulatory signals. Such techniques would allow researchers to measure associations between proteins-protein and protein-lipid interactions in real time in intact cells. Results from such studies would vastly expand our basic knowledge of the regulation of lipid domains, the cytoskeleton, and other signaling molecules, and can help to better understand their roles in natural immune function.