Hypothermia (HT) has been associated with both beneficial and detrimental consequences in various pathophysiological states. While HT is generally thought to have anti-inflammatory and cytoprotective effects, we have previously shown that moderate in vitro HT prolongs TNF-alpha production by LPS-stimulated mononuclear phagocytes, in part by prolonging TNF-alpha gene transcription and activation of the pleiotropic transcription factor NF-kappaB. In this study, we have further characterized the effect of moderate (32 degrees C) and marked (28 degrees C) HT in human monocytic THP-1 cells by showing that even short (2 h) exposure to HT followed by a return to normothermic conditions for 22 h resulted in augmented and prolonged production of TNF-alpha. Production of heat shock protein 72 and activation of heat shock factor 1 are not affected by HT in these studies, suggesting that the effect is not part of a generalized stress response. Using immunoblotting, we have shown that HT augments phosphorylation of IKK-beta and IKK-alpha (up to an 8-fold increase at 28 degrees C and a 3.6-fold increase at 32 degrees C vs. 37 degrees C). Furthermore, nuclear accumulation of NF-kappaB p65 was significantly prolonged in hypothermic cells (1.4- and 2.5-fold more nuclear p65 at 2 and 4 h at 28 vs. 37 degrees C). Reexpression of IkappaB-alpha, which contributes to the termination of NF-kappaB-dependent transcription, was delayed several hours in HT-exposed cells. Thus we have shown that clinically relevant HT alters both cytosolic and nuclear events responsible for NF-kappaB activation and deactivation. Enhanced NF-kappaB activation may contribute to the immunomodulatory effects of HT in various clinical settings.