Hyperthermia differentially regulates TLR4 and TLR2-mediated innate immune response

Abstract

Fever influences multiple parameters of the immune response. However, the mechanisms by which fever manipulates immune response remain undefined. Here we present the evidences that fever range hyperthermia differentially regulates immune response to lipopolysaccharide (LPS) and lipoteichoic acids (LTA) through modulating Toll-like receptor (TLR) signaling. Pretreatment with 39.5 °C temperature enhanced LPS, but not LTA, induced NF-κB activation and TNF-α, IL-6 production in human macrophages. Consistently, expression of TLR4, but not TLR2, was up-regulated by 39.5 °C treatment. The increase in LPS-induced cytokine production was inhibited by TLR4-blocking antibody, indicating the enhancement of LPS-induced cytokine production by 39.5 °C pretreatment was TLR4-dependent. Pretreatment of mice with 39.5 °C temperature also enhanced LPS, but not LTA, induced TNF-α and IL-6 production in vivo. These results support the concept that fever range hyperthermia might activate innate immune response by promoting TLR4 expression and signaling, providing a possible mechanistic explanation for the function of fever in regulating innate immune responses.

Introduction

Fever is a natural reaction during pathogen invading. It is now evident that fever plays an important role in survival of infections in endothermic and ectothermic species [1], [2]. In mice, hyperthermia increases LPS-induced cytokines [3], [4] and nitric oxide production [5]. Numerous reports have documented that hyperthermia is associated with enhancement of the innate and adaptive arms of the immune response [6], [7], [8]. However, how fever modulates host immune responses against pathogens remains to be fully understood.

Toll-like receptors (TLRs), which are broadly expressed on immune cells including macrophages, dendritic cells, neutrophils and lymphocytes, have a crucial role in early host defense against invading pathogens [9]. They are evolutionarily conserved to recognize pathogen associated molecular patterns (PAMPs) from bacteria and viruses [10], [11], [12], [13]. Upon recognition of PAMPs, TLRs activate downstream signal molecules such as NF-κB and MAPK and regulate the production of proinflammatory cytokines and co-stimulators in immune cells, and consequently initiate innate immunity and enhance adaptive immunity [14], [15], [16]. Given the pivotal roles of TLRs in controlling immune responses to microorganisms, understanding how TLR signaling is regulated will obviously facilitate the development of new strategies to control TLR-mediated inflammatory diseases. However, it remains far from understanding how fever modulates TLR signaling.

In this report, we present the evidences that fever range hyperthermia differentially affects TLR4, TLR2 signaling-mediated immune responses in human macrophage-like THP-1 cells. Pretreatment of THP-1 cells with 39.5 °C temperature enhanced LPS-induced NF-κB activation as well as TNF-α and IL-6 production, but did not enhance LTA-induced cytokine production. The enhancement of LPS-induced cytokine production by 39.5 °C pretreatment coincided with up-regulation of TLR4 and was inhibited by TLR4 blocking antibody, demonstrating the effects of 39.5 °C pretreatment on LPS-induced cytokine production was TLR4 dependent. Consistent with these results, pretreatment of mice with 39.5 °C temperature enhanced LPS, but not LTA, induced TNF-α and IL-6 production in vivo. Our results suggested that fever range hyperthermia could manipulate host innate immune responses to PAMPs by modulating TLR signaling, providing a possible mechanistic explanation for the function of fever in regulating immune responses.