Control of Inflammatory Response by Tissue Microenvironment
Posted on: 13 June 2024 , updated on: 14 June 2024
Preprint posted on 14 May 2024
Categories: cell biology, immunology
Background
Tissue homeostasis relies on several things, including the tight maintenance of physiological pH. In mammals, pH values of blood are usually kept between 7.35 and 7.45 and any alteration from these values is a sign of pathological conditions.
Even though we know conditions where a deviation from standard pH values occurs (i.e. tumors generally establish acidic environments due to hypoxia and switch on metabolism) and molecular sensors of pH levels have been described (such as G-protein coupled receptors, acid sensing ion channels and transient receptor potential cation channel subfamily V member 1); we don’t fully understand how pH levels are monitored intracellularly.
In this study, Wu and colleagues explored how different pH levels impact inflammation in macrophages and found that transcriptional regulation mediated by BRD4 is a novel pH-dependent process that regulates the inflammatory output in these cells.
Key findings of this preprint
- Extracellular pH regulates inflammation in macrophages
Infection is known to reduce pH in the affected organism and, more specifically within macrophages. To study the effect of this pH change on the macrophages inflammatory response, the authors treated bone marrow-derived macrophages (BMDM) with lipopolysaccharide (LPS) after cultivation in standard pH medium (pH 7.4) or acidic pH medium (pH 6.5) (fig.1). Surprisingly, acidic extracellular conditions upregulated the second wave of early inflammatory genes (i.e. Ifnb1, Il23a, …), but downregulated late responsive genes (i.e. Il6, Il1b, …). As a consequence of this, the authors suspected that macrophages might fine-tune their inflammatory response according to what they sense on the extracellular milieu.
Figure 1 – Overview of the stimulation of macrophages after cultivation at different pH levels (left). Quantification of inflammatory genes by real time PCR (right).
- pH controls adaptive immune programmes
Using a deconvolution model to investigate the contribution of the two variables (pH and LPS) and their mutual interactions, the authors found that genes linked to early innate immune activation were insensitive to pH changes, whereas both genes that were antagonized or synergistically induced by pH and LPS, played a role in recruitment and modulation of the adaptive immune response (fig.2).
Figure 2 – Biological processes of genes insensitive or synergically/antagonistically modulated upon LPS treatment.
- A novel transcriptional pH-dependent regulation process
Several attempts to find a candidate pH sensor responsible for the pH-dependent regulation of the inflammatory response in BMDM have failed. Moreover, signal transduction pathways appeared to not be significantly affected by pH changes. These observations indicate that pH-sensitive and pH-insensitive genes are likely regulated at the level of transcription rather than signal transduction. To this end, the authors performed ATAC-seq and CHIP-seq to investigate chromatin modifications and accessibility. No global histone modifications were detected in the LPS and pH conditions, but it was possible to conclude that early response genes are not affected by pH, while the second wave of early genes and late response genes are affected due to the modulation of transcription factors and long-range enhancer activity.
- BRD4 condensates are pH dependent
There is evidence for a pH dependent SWI/SNF chromatin remodelling complexes in the budding yeast. The authors performed an in-silico screening to identify novel potential mammalian candidates that could behave similarly. BRD4 emerged as such a candidate since it contains some histidine residues likely to be protonated at an acidic pH. Indeed, a pH of 6.5 was observed to reduce BRD4 condensates, demonstrating a novel pH-dependent regulation of this protein and its activity (fig.3).
Figure 3 – Immunofluorescence of BRD4 condensates inside the nuclei of macrophages.
- BRD4 fine-tunes the inflammatory response
Acidic intracellular pH repressed part of the inflammatory response in macrophages and concomitantly decreased BRD4 condensates. Also, the inhibition of BRD4 by JQ-1 treatment reduced intracellular pH acidification. Moreover, the glycolytic switch that normally happens in macrophages upon LPS stimulation at physiological pH levels, was diminished at a pH of 6.5. Taken together, these observations suggest a fine-tuning mechanism involving BRD4 activity, metabolism and inflammation in activated macrophages; which regulates long range chromatin interaction at specific genes to control and direct the inflammatory response by integrating signals from the extracellular environment (fig.4).
Figure 4 – Model of BRD4 modulation by pH.
Why I chose this preprint
The immune system is fascinating, yet fundamental mechanisms underlying its functioning are still not well understood. In this preprint, the authors started from a simple yet clever assumption on pH regulation of inflammation and significantly advanced our understanding on this topic.
Questions to the authors
- Is there any documented work that focuses on the acidic pH in the tumor microenvironment and its modulation of anti-tumor immunity?
References
- Levin, L.R., and Buck, J. (2014). Physiological Roles of Acid-Base Sensors. Annu Rev Physiol 77,347–362. 10.1146/annurev-physiol-021014-0718
- Hajjar, S., and Zhou, X. (2023). pH sensing at the intersection of tissue homeostasis and inflammation. Trends Immunol. 44, 807–82 10.1016/j.it.2023.08.008.
doi: https://doi.org/10.1242/prelights.37684
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