Enhanced sensitivity of colon tumour cells to natural killer cell cytotoxicity after mild thermal stress is regulated through HSF1-mediated expression of MICA.

PURPOSE: Previously we showed that mild thermal stress increased natural killer (NK) cell-mediated tumour cytotoxicity and that this could be blocked by anti-NKG2D or anti-MICA (major histolocompatability complex (MHC) class I related chain A) antibodies. Here, we investigated the role of the transcription factor heat shock factor 1 (HSF1) in thermal regulation of MICA expression in tumour cells in vitro and in vivo. MATERIALS AND METHODS: Hyperthermia experiments were conducted in vitro and in mice using a target temperature of 39.5 °C. Apoptotic cells and NK cells in situ were visualised by use of the TUNEL assay or expression of NKp46 respectively. Using Colo205 cells, HSF1 message was blocked utilising siRNA while luciferase reporter assays were used to measure the activity of the MICA promoter in vitro. Cell surface MICA was measured by flow cytometry. RESULTS: Following whole body hyperthermia (WBH), tumour tissues showed an increase in NK cells and apoptosis. Mild thermal stress resulted in a transient increase in surface MICA and enhanced NK cytotoxicity of the Colo205 colon cancer cell line. Silencing (mRNA) HSF1 expression in Colo205 cells prevented the thermal enhancement of MICA message and surface protein levels, with partial loss of thermally enhanced NK cytotoxicity. Mutations of the HSF1 binding site on the MICA promoter implicated HSF1 in the thermal enhancement of MICA. Some, but not all, patient-derived colon tumour derived xenografts also exhibited an enhanced MICA message expression after WBH. CONCLUSIONS: Up-regulation of MICA expression in Colo205 cells and enhanced sensitivity to NK cell killing following mild thermal stress is dependent upon HSF1.

Dissecting the role of hyperthermia in natural killer cell mediated anti-tumor responses.

The effects of hyperthermia on natural killer (NK) cell cytotoxicity against tumor cell targets are not yet fully understood. A more complete understanding of these effects could be important for maximizing the clinical benefits obtained by using hyperthermia for cancer therapy. Here, we summarize results in the literature regarding the effects of elevated temperatures on NK cells and our own recent data on the effects of fever-range temperatures. At treatment temperatures above 40 degrees C, (which is above the physiological body temperatures normally achieved during fever or exercise), both enhancing and inhibitory effects on cytotoxic activity of NK cells against tumor cells have been reported. Our own results have shown that fever-range thermal stress (using a temperature of 39.5 degrees C) enhances human NK cell cytotoxicity against tumor target cells. This effect requires function of the NKG2D receptor of NK cells, and is maximal when both NK and tumor cell targets are heated. Reported heat sensitive cellular targets affected by hyperthermia on tumor cells include heat shock proteins, MICA and MHC Class I. In NK cells, plasma membrane reorganization may occur after mild heat stress. We conclude this review by listing several unresolved questions that should be addressed for a more complete understanding of the molecular mechanisms which underlie the effects of thermal stress on the function of NK cells. Altogether, the available data indicate a strong potential for heat-induced enhancement of NK cell activity in mediating, at least in part, the improved clinical responses seen when hyperthermia is used in combination with other therapies.

Enhancement of natural killer (NK) cell cytotoxicity by fever-range thermal stress is dependent on NKG2D function and is associated with plasma membrane NKG2D clustering and increased expression of MICA on target cells.

Circulating NK cells normally experience temperature gradients as they move about the body, but the onset of inflammation can expose them and their targets to febrile temperatures for several hours. We found that exposure of human peripheral blood NK cells and target cells to fever-range temperatures significantly enhances lysis of Colo205 target cells. A similar effect was not observed when NK cell lines or IL-2-activated peripheral blood NK cells were used as effectors, indicating that thermal sensitivity of effectors is maturation or activation state-dependent. Use of blocking antibodies revealed that this effect is also dependent on the function of the activating receptor NKG2D and its ligand MHC class I-related chain A (MICA). On NK cells, it was observed that thermal exposure does not affect the total level of NKG2D surface expression, but does result in its distinct clustering, identical to that which occurs following IL-2-induced activation. On tumor target cells, a similar, mild temperature elevation results in transcriptional up-regulation of MICA in a manner that correlates with increased sensitivity to cytolysis. Overall, these data reveal that NK cells possess thermally responsive regulatory elements, which facilitate their ability to capitalize on reciprocal, stress-induced changes simultaneously occurring on target cells during inflammation and fever.