The central goal of this study was to investigate, whether the phenomenon of “endotoxin tolerance” or “LPS tolerance”, the reduced responsiveness to endotoxin after a previous confrontation with LPS, is inducible in neuro-glial structures of the afferent somatosensory system. For this purpose, we used mixed neuro-glial primary cultures from the rat DRG and SDH. We induced tolerance to LPS using a long-term stimulation with a moderate LPS dose (1 µg/ml LPS for 18 h, LPS1) followed by a subsequent challenge with a short term stimulation with a high LPS dose (10 µg/ml for 2 h, LPS10) and investigated whether this group shows altered LPS10-induced inflammatory gene expression, altered cytokine formation and altered activation of inflammatory transcription factors. Indeed, our data suggest the manifestation of LPS tolerance within both investigated structures.
Expression profiles of inflammatory genes and formation of cytokines
At the mRNA and the protein levels, the most pronounced tolerance effect was determined for TNF-α. In both investigated neuro-glial structures we observed a highly significant reduction of LPS10-induced expression of TNF-α after pre-treatment with LPS1. This result is in line with studies on peripheral macrophages demonstrating that a drastic reduction of TNF-α induction is a key read-out for endotoxin tolerance under in vivo or in vitro conditions [
9‐
12,
35,
36]. Indeed, the LPS-tolerance effect in our experiments was much more pronounced for TNF-α compared to IL-6 (Figs.
1,
2 and
3). Other studies suggested that LPS tolerance is not simply a reduction of TNF-α expression and formation, but rather a re-programming of macrophages from a pro- to an anti-inflammatory state [
9,
11]. The operation of such mechanisms in our DRG and SDH primary cultures is indicated by a tendency for increased expression of IL-10 in the LPS1/LPS10 compared to the PBS/LPS10 group (Figs.
1 and
2). Even better evidence for such a switch from a pro- to an anti-inflammatory state in LPS tolerance is provided by the calculations of the relative IL-10/TNF-α expression ratios, which were significantly enhanced in DRG and SDH primary cultures (Figs.
1f and
2e). These calculations are in line with the data from other publications, which show an up-regulation of IL-10 in a state of LPS tolerance [
12,
13,
37,
38]. The importance for IL-10 in this context is evident in IL-10-deficient mice, which lose the ability to regulate LPS-induced expression of tolerance related molecules [
39]. In further studies, high IL-10/TNF-α ratios were associated with a better outcome in children with malaria infections [
40], with a reduced infection susceptibility in severe burn injury patients [
33] or with improved outcome in experimentally induced myocardial infarction [
32]. Transferred to our measurements, these studies suggest that a state of LPS tolerance, accompanied by increased IL-10/TNF-α expression ratios, could have some beneficial or even protective value for a subsequent tissue damage or confrontation with another severe inflammatory stimulus within the afferent somatosensory system.
We also measured the expression of TLR-4, which is the cognate receptor for LPS in monocytes / macrophages and also in cells from neuro-glial structures within the CNS [
41‐
43]. Some authors reported that decreased TLR-4 expression in peritoneal macrophages [
17] or changes within the TLR-4-evoked signaling cascade [
44,
45] are critical for the manifestation of LPS tolerance. Investigations of TLR-4 expression under conditions of LPS tolerance have not been performed in structures from the peripheral or central nervous system, according to our knowledge. Interestingly, our data on TLR-4 expression partially support the view of some contribution but not a critical role in LPS tolerance. Namely, in primary cultures from the SDH all cells, which were confronted with LPS, irrespective of dose and duration, show a significant down-regulation of TLR-4 expression when compared with naïve cultures (Fig.
2d). A non-significant tendency for this effect can also be seen in primary cultures from DRG (Fig.
1d). This effect of a reduced TLR-4 expression could, however, be rather due to the exposure with an agonist, e.g. LPS alone and a contribution of this effect on the manifestation of LPS tolerance is questionable since all groups with the exception of naïve cells showed similar responses (PBS/LPS10, PBS/LPS1, LPS1/LPS10).
Nuclear translocation of inflammatory transcription factors
Microglial cells from the SDH and macrophages from the DRG seem to be the major targets for LPS and respond to this stimulus with formation of cytokines, namely TNF-α (Fig.
4). TLR-4-mediated production of TNF-α, IL-6 and other cytokines has an impact on cellular elements within the afferent somatosensory system. This impact can be assessed by the accumulation of several inflammatory transcription factors in nuclei of LPS- or cytokine-stimulated cells [
4,
5]. Inflammatory transcription factors belong to the intracellular signaling molecules, which may be involved in the refractoriness of macrophages’ responses to repeated stimulation with LPS. In this context, special attention was directed to NFκB. This transcription factor is directly activated within the LPS-TLR-4-stimulated signaling pathway and translocates into the nucleus of LPS-stimulated cells leading to the expression and formation of pro-inflammatory cytokines with TNF-α as initial mediator [
46]. However, this function of NFκB depends on its p65 protein subunit, which forms a heterodimer with a p50 protein subunit. One of the suggested mechanisms for the manifestation of LPS tolerance is a replacement of the NFκB heterodimeric form p50-p65 by the homodimeric form p50-p50, which lacks a transactivation domain and exerts an inhibitory influence on inflammatory gene expression [
10]. This switch causes a drastic change of the LPS-activated genes, resulting in hyporesponsiveness of TNF-transcription and more pronounced transcription of genes with anti-inflammatory capacities including IL-10 [
11,
12,
15,
46,
47]. The enhanced IL-10/TNF-α expression ratios reported here (Figs.
1 and
2) indicate that similar changes occur in primary cultures from rat DRG and SDH primary cultures after induction of LPS tolerance. The antibody, which we employed to quantify the strength of nuclear NFκB immunoreactivity (Fig.
5), is directed against the p65-subunit of NFκB. The reduced nuclear NFκB immunoreactivity, which we determined in the “LPS tolerant” group (LPS1/LPS10 compared to PBS/LPS10) supports the view that a switch from the p50-p65 heterodimer of NFκB to the p50-p50 homodimer may also occur in microglial cells of a given central nervous structure, here the SDH.
STAT3 was the next inflammatory transcription factor, which we investigated with regard to its LPS-induced accumulation in nuclei of cells from the afferent somatosensory system under condition of LPS tolerance. The activation of STAT3 after stimulation with LPS is mediated by a family of LPS-induced cytokines, the “gp130 cytokine receptor family” with IL-6 as its most prominent member [
48]. In CNS structures STAT3 seems to participate in various inflammatory responses [
49] with astrocytes as a main cellular target for STAT3-activating signal molecules [
50,
51]. Thus, activation of astrocytes in the spinal dorsal horn is suggested to play a role in chronic pain states [
51]. On the other hand it was reported that inhibition of STAT3 signaling in brain astrocytes reduced experimental brain metastases [
52]. In line with these studies, LPS-induced activation of STAT3 could exclusively be shown in astrocytes from SDH primary cultures (Fig.
6). In the context of our study, we were interested in the relative STAT3 expression in GFAP positive astrocytes in our SDH primary culture in the state of LPS tolerance. The mean strength of nuclear STAT3 signals was enhanced after treatment with either a moderate dose or a high dose of LPS (Fig.
6), suggesting that either LPS itself or the pro-inflammatory cytokines released after LPS treatment led to an increased STAT3 activation. In tolerant SDH astrocytes the mean strength of nuclear STAT3 signals decreased, indicating attenuated activation of STAT3. As STAT3 seems to play a role in mediating the anti-inflammatory effects of IL-10 [
53], we suggest a fast increase of its activation in astrocytes (2 h) after exposure to LPS and a subsequent decrease after 20 h, which is supported by the work from other groups [
54,
55]. The functional relevance of the observed LPS tolerance effect concerning STAT3 activation in astrocytes of the rat SDH still has to be determined. Astrocytes are not present in primary cultures of rat DRG [
4]. Interestingly, we detected nuclear STAT3 immunoreactivity in DRG neurons in line with previous studies [
4] and an influence of acute stimulation with LPS10, which caused an increase of nuclear STAT3 signals (Fig.
9). The increase of nuclear STAT3 immunoreactivity was abolished in the state of LPS tolerance (Fig.
9). This observation may have functional consequences for the sensitization of primary nociceptive neurons upon inflammatory stimulation. It was reported that inflammatory STAT3 activation in DRG neurons sensitizes peripheral nociceptors via upregulation of TRPV1 [
56]. A state of LPS tolerance might therefore have some beneficial effect with regard to the manifestation of inflammatory pain by reducing the inflammation-evoked sensitization of nociceptive neurons. Unfortunately, we could not detect a down-regulation of TRPV1-expression under conditions of LPS tolerance (Fig.
1). However, other studies revealed effects of inflammatory mediators on the TRPV1 channel particularly via posttranslational mechanisms (e.g. phosphorylation) that lead to enhanced membrane availability without observing changes in transcription [
57,
58]. For example, phosphorylation of TRPV1 at a single tyrosine residue under the influence of nerve growth factor is followed by insertion of TRPV1 channels into the surface membrane [
59]. Thus, the process of sensitization may rather involve enhanced translocation of TRPV1 channels into the membrane of nociceptors than increased mRNA expression [
60]. A possible effect of LPS tolerance on sensitization of nociceptive neurons might therefore rather involve an influence of modified intracellular signaling pathways and thereby a modification (reduction) of TRPV1 trafficking to the plasma membrane [
58]. Therefore, we cannot exclude the possibility that there is some inhibitory effect on exaggerated nociception in LPS-tolerant DRG neurons despite the lack of an influence on TRPV1 expression. Other experimental approaches are required to evaluate translocation of TRPV1 from the cytosol to the plasma membrane, especially under the influence of LPS tolerance.
Finally, we investigated a third transcription factor, NF-IL6, which has pro-inflammatory properties during the early stage of LPS-induced systemic inflammation, but exerts anti-inflammatory effects during the later stage of the inflammatory process [
61]. Here we show (Figs.
7 and
8) that the LPS induced increase of nuclear NF-IL6 immunoreactivity is blunted in the state of LPS tolerance in ED1-positive macrophages (DRG) and microglial cells (SDH). The long-term exposure to the low LPS dose (LPS1) evoked a prolonged increase of nuclear NF-IL6 immunoreactivity, which was still present after a subsequent short-term exposure to the solvent PBS in DRG primary cultures (Fig.
7). This effect was also observed in microglial cells from SDH, although to a smaller degree (Fig.
8). Nevertheless, there was a striking difference between both cultures with regard to nuclear NF-IL6 activation. Our results show a significant decrease in tolerant (LPS1/LPS10) DRG macrophages compared to the LPS1/PBS group. Such an effect was absent in primary cultures from the rat SDH. The reduction of the strength of nuclear NF-IL6 signals in LPS-tolerant DRG macrophages after short-term stimulation with LPS10 can be possibly explained by active down-regulation of NF-IL6 by cytokines distinct from IL-10 [
62]. Furthermore, a breakdown of the inflammatory transcription factor NF-IL6 due to LPS tolerance could be another plausible explanation for these results. To investigate whether an active downregulation or a breakdown of the transcription factor NF-IL6 plays a role in the manifestation of LPS tolerance, further experiments, e.g. the use of NF-IL6 knockout mice, would be needed.
As previously reported [
4], we also detected nuclear NF-IL6 immunoreactivity in DRG neurons (Fig.
10), which was enhanced in naïve cells by short-term stimulation with LPS. The pre-exposure with LPS1 again caused a long-lasting increase of neuronal NF-IL6 activation. In contrast to macrophages, the LPS1-induced nuclear NF-IL6 activation in neurons was not attenuated by the short-term stimulation with LPS10. In contrast to STAT3 [
56,
63], no information from literature is available for a sensitizing effect of NF-IL6 on nociceptive DRG neurons. This question could be addressed by the use of DRG primary cultures from NF-IL6-deficient mice in future studies.
The impact of LPS tolerance in structures of the peripheral and central nervous system
There are just a few studies, in which the manifestation of LPS tolerance was investigated in structures from the nervous system [
22‐
24,
34]. In these studies, tolerance effects were detected in microglial cells and perivascular macrophages. Our present study on structures of the afferent somatosensory system supports this evidence in so far that the most pronounced tolerance effects were also observed in microglial cells (SDH) and macrophages (DRG). However, we present data showing that LPS tolerance to some degree also develops in astrocytes from the SDH and in neurons from DRG. In the brain, the transient hyporesponsiveness to LPS in the state of tolerance was interpreted as a self-protecting mechanism of the brain against an excessive formation of key mediators of the inflammatory response, for example reactive oxygen species (ROS), nitric oxide via induction of inducible nitric oxide synthase (iNOS) or pro-inflammatory cytokines, such as TNF-α [
22‐
24]. LPS tolerance might thus be a mechanism, which limits the activation of innate immunity within the brain and thereby protects this sensitive tissue from exaggerated immune responses via depressing the activity of transcription factors with pro-inflammatory capacities [
23,
64]. Still, there are some indications for modified properties of neurons in the state of LPS tolerance, which still have to be determined with regard to their functional relevance. In some recent studies, a state of LPS tolerance was, indeed shown to have positive effects on stress-induced behavioral abnormalities [
65] or on neuropathology and cognitive deficits in model of status epilepticus [
66]. Models of LPS-pre-conditioning seem thus to have capacities for protective value in central nervous structures. In the case of neurons from the afferent somatosensory systems it will, for example, be of interest to investigate, whether inflammation related changes of neuronal responses can be modified in a state of LPS tolerance or generally in states of immune tolerance.