Highlight: Is too much salt bad for your guts?


Whether eating too much table salt in our diet is bad for our health has long been debated. Links have been proposed to several cardiovascular diseases. But, a recent expert committee for the Institute of Medicine concluded that the data do not support such a link [1], keeping the discussion going. Two recent publications in Nature, however, suggest that too much dietary salt might impact our immune system instead and potentially increase the likelihood of autoimmune diseases.

CD4 T lymphocytes can differentiate in specialized subsets that promote or help diverse immune responses. Called T helper (Th) cells, particular subsets are named after prominent cytokines they produce, e.g. IL-17 in the case of Th17 T cells. Th17 cells are important for protection of the body against many bacterial and fungal infections and they are prevalent in the intestinal tissue were they are believed to aid the barrier function of the gut to keep the intestinal bacteria were they belong [2]. However, the “too much of a good thing” proverb applies to lymphocytes too and in the case of Th17 T cells this is exemplified by their pathogenic involvement in several autoimmune diseases. Therefore, the control of cell number and function of Th17 cells requires a delicate balance.

It was known that there is a cross talk between the gut lumen and the Th17 cell response. For example, a few years back it was shown that the frequency of a common bacterium within the gut microbiota could influence the prevalence of Th17 cells in the intestinal tissue [3]. The two new studies demonstrate that table salt (sodium chloride, NaCl) is a surprising new factor on the list to influence the frequency and function of Th17 cells [4-6].Lymphocyte activation

Adding 40 mM NaCl – a level found in the intestinal tissue of animals after feeding of a high salt diet – to in vitro cultures augmented the differentiation of Th17 cells [4, 5]. Similar to in vitro, feeding mice with a high salt diet increased the frequency of Th17 cell in the intestinal tissue, but not in the lymph nodes or the spleen. In both settings (in vitro and in vivo) the resulting Th17 cells were capable of producing large amounts of pro-inflammatory cytokines. By analysis of the mRNA expression, both reports characterized the MAP-kinase p38, NFAT5 (nuclear factor of activated T cells 5) and SGK1 (serum glucocorticoid-regulated kinase-1) as critical molecular players in sensing NaCl and mediating its effect. The elimination of any of these factors from the T cells, either by genetic ablation or by impeding the expression by means of RNA-interference (shRNA), blocked the increased Th17 cell differentiation in the presence of NaCl. Although all three proteins are part of the same pathway, SGK1 appeared to be central in the regulation of the NaCl induced effect. Although this finding is surprising, the results are in line with the known function of SGK1 in sodium transport and homeostasis [7]. SGK1 expression was not only induced by increased NaCl concentrations, but also by the cytokine IL-23, which has a critical role in stabilizing and reinforcing the TH17 phenotype [2]. As NaCl also increased the expression of the IL-23 receptor this established a positive feedback loop that strengthened the Th17 cell differentiation. Importantly, both groups also showed that raising the levels of dietary salt could augment the severity of EAE (experimental autoimmune encephalomyelitis), a mouse model for the autoimmune disease multiple sclerosis [4-6].

In summary, these reports demonstrate that high levels of salt in the diet could make mice susceptible to a form of autoimmune disease that involves pathogenic Th17 T cells. The data suggest that high concentration of NaCl might be an environmental risk factor for autoimmune diseases. However, it should be pointed out that high concentration of NaCl did not induce autoimmune responses by itself, as the EAE animal model requires the immunization with a know self-antigen. Autoimmunity is a complex interplay of numerous genetic pre-disposing and environmental factors. In this regard these new reports [4, 5] suggest that high dietary salt concentrations might tilt the balance a bit towards autoimmunity in genetically predisposed individuals.

However, the reality will likely be more complicated – as it usually is. For example, it will be critical to show that the correlation between dietary NaCl and Th17 cells is valid also in humans. Furthermore, with this knew knowledge other factors might come to light soon. For example, SGK1 expression is also stimulated by several hormones including endogenous steroids like stress hormones [7], suggesting that the induction of Th17 cell might be augmented by stress as well. Therefore, these intriguing new reports [4, 5] will surely spur now the required research to clarify these points. Till then, going slow on sodium-laden junk food might be generally a justified suggestion.                                 


[1] Strom, Brian (2013). Sodium Intake in Populations: Assessment of Evidence. Washington, DC: The National Academies Press: The Institute of Medicine.

[2] Weaver, C. T., Elson, C. O., Fouser, L. A. & Kolls, J. K. The Th17 pathway and inflammatory diseases of the intestines, lungs, and skin. Annu Rev Pathol 8, 477–512 (2013).

[3] Ivanov, I. I. et al. Induction of Intestinal Th17 Cells by Segmented Filamentous Bacteria. Cell 139, 485–498 (2009).

[4] Kleinewietfeld, M. et al. Sodium chloride drives autoimmune disease by the induction of pathogenic TH17 cells. Nature (2013). doi:10.1038/nature11868.

[5] Wu, C. et al. Induction of pathogenic TH17 cells by inducible salt-sensing kinase SGK1. Nature (2013). doi:10.1038/nature11984.

[6] O’Shea, J. J. & Jones, R. G. Autoimmunity: Rubbing salt in the wound. Nature 496, 437–439 (2013).

[7] Lang, F. & Shumilina, E. Regulation of ion channels by the serum- and glucocorticoid-inducible kinase SGK1. The FASEB Journal 27, 3–12 (2013).

RORγt+ TREGS: A unique subset of TREGS that specifically promote Colorectal Cancer

tregThe role of CD4+ FoxP3+ regulatory T cells (TREGS) in colorectal cancer (CRC) has continued to be unclear.  TREGS act to suppress inflammatory mechanisms that are associated with tumor progression and can thus act to suppress the development of cancer.  However, TREGS also function to inhibit anti-tumor T cell responses, thereby promoting cancer escape from immune surveillance.  Many studies have been published on the frequencies of TREGS in the peripheral blood and tumors of CRC patients, but there is yet to be a consensus regarding the relationship between TREGS and disease outcome.  In a report by Blatner et. al, the expression of RORγt in a subset of CD4+ FoxP3+ T cells was found to specifically mediate pathogenic pro-tumor activity compared with RORγtCD4+FoxP3+ TREGS in CRC patients.

CD4+ FoxP3+ cells have been classified into three functional populations based on the expression of CD45RA and FoxP3: CD45RA+FoxP3int, CD45RAFoxP3int, and CD45RAFoxP3high.  The CD45RAFoxP3high population exhibits the most suppressive activity of these subsets.  In the study by Blatner et. al, the CD45RAFoxP3high population was found to be specifically expanded in peripheral blood mononuclear cells (PBMCs) and within the tumor of CRC patients and increased with cancer stage.  Because IL-17 expressing CD4+ FoxP3+ cells have been described in the gut and enhanced in patients with CRC and Crohn’s disease, the authors examined CD4+ FoxP3+ populations for expression of the TH17 transcription factor, RORγt. 

In CRC patients, a large fraction of all three subsets of TREGS in PBMCs and in the tumor were found to express RORγt.  Interestingly, when TREG populations were sorted from healthy donors versus CRC patients, CRC patient TREGS  retained suppressive activity over T cell proliferation but had lost their ability to suppress mast cell degranulation.  Expression of IL-17 was also found in a large percentage of CRC TREGS, in a fashion mutually exclusive from IL-10 expression.

To further explore the role of RORγt in CRC, APC∆468 polyposis-prone mice were crossed with mice deficient in RORγt.  RORγt-/-APC∆468 mice were highly resistant to polyp development, had reduced expansion of splenic proinflammatory macrophages, myeloid-derived suppressor cells (MDSCs) and polyp-associated mast cells, compared with RORγt+APC∆468 mice.  Interestingly, the effect of RORγt deficiency in APC∆468 mice was not phenocopied by the loss of IL-17.  Instead, although IL-17 deficiency reduced the frequency of polyps, mast cell recruitment to polyps was enhanced, and ultimately IL-17-/-APC∆468 mice developed invasive lesions.

Overall, this study revealed several fascinating points: CD4+FoxP3+RORγt+ cells appear to be a pathogenic TREG subset that have lost their anti-inflammatory properties and are specifically expanded in CRC patients where they assist in disease progression.  The function of RORγt was not synonymous with IL-17 in TREGS, indicating that other effects of RORγt contribute to the role of these cells in tumor pathogenesis.  Thus, the roles and relationships between FoxP3, RORγt, and IL-17 in TREGS deserve further attention in CRC pathogenesis.  Hopefully, a clearer understanding of this newly identified subset of RORγt+  TREGS and their role in CRC progression will enable much improved methodology for targeting specific TREGS populations in CRC and other disease settings.

Further Reading:

Expression of RORγt marks a pathogenic regulatory T cell subset in human colon cancer.  Blatner NR, Mulcahy MF, Dennis KL, Scholtens D, Bentrem DJ, Phillips JD, Ham S, Sandall BP, Khan MW, Mahvi DM, Halverson AL, Stryker SJ, Boller AM, Singal A, Sneed RK, Sarraj B, Ansari MJ, Oft M, Iwakura Y, Zhou L, Bonertz A, Beckhove P, Gounari F, Khazaie K. Sci Transl Med. 2012 Dec 12;4(164):164ra159. doi: 10.1126/scitranslmed.3004566.

Translational mini-review series on Th17 cells: induction of interleukin-17 production by regulatory T cells.  Afzali B, Mitchell P, Lechler RI, John S, Lombardi G. Clin Exp Immunol. 2010 Feb;159(2):120-30. doi: 10.1111/j.1365-2249.2009.04038.x. Epub 2009 Nov 11.

Inflammation-driven reprogramming of CD4+ Foxp3+ regulatory T cells into pathogenic Th1/Th17 T effectors is abrogated by mTOR inhibition in vivo.  Yurchenko E, Shio MT, Huang TC, Da Silva Martins M, Szyf M, Levings MK, Olivier M, Piccirillo CA. PLoS One. 2012;7(4):e35572. doi: 10.1371/journal.pone.0035572. Epub 2012 Apr 24.

In colorectal cancer mast cells contribute to systemic regulatory T-cell dysfunction.  Blatner NR, Bonertz A, Beckhove P, Cheon EC, Krantz SB, Strouch M, Weitz J, Koch M, Halverson AL, Bentrem DJ, Khazaie K. Proc Natl Acad Sci U S A. 2010 Apr 6;107(14):6430-5. doi: 10.1073/pnas.0913683107. Epub 2010 Mar 22.