
==== Front
Function (Oxf)
Function (Oxf)
function
Function
2633-8823
Oxford University Press

39251388
10.1093/function/zqae039
zqae039
Perspectives
AcademicSubjects/SCI00960
AcademicSubjects/MED00772
AcademicSubjects/SCI01360
AcademicSubjects/SCI01270
Defining Cytokine Responsive and Non-responsive Human β-cells
https://orcid.org/0000-0003-1152-2691
Stephens Samuel B Fraternal Order of Eagles Diabetes Research Center, University of Iowa, Iowa City, IA, 52242, USA
Department of Internal Medicine, Division of Endocrinology and Metabolism, University of Iowa, Iowa City, IA, 52242, USA
Department of Anatomy and Cell Biology, University of Iowa, Iowa City, IA, 52242, USA

Address correspondence to S.B.S. (e-mail: samuel-b-stephens@uiowa.edu)
2024
09 9 2024
09 9 2024
5 5 zqae03903 9 2024
03 9 2024
05 9 2024
17 9 2024
© The Author(s) 2024. Published by Oxford University Press on behalf of American Physiological Society.
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

beta-cells
cytokines
inflammation
islets
single-cell RNA-seq
National Institutes of Health 10.13039/100000002 R01 DK140093 U.S. Department of Defense 10.13039/100000005 W81XWH-20-1-200
==== Body
pmcA Perspective on “Single Cell RNAseq Analysis of Cytokine-Treated Human Islets: Association of Cellular Stress With Impaired Cytokine Responsiveness”

Type 1 diabetes (T1D) is an autoimmune disease that is caused by the near complete ablation of the pancreatic β-cell population. While T1D is both polygenic and heritable, greater than 85% of cases are spontaneous with no known family history.1,2 T-cell mediated attack of β-cells is recognized as a critical step in T1D progression, which is supported by recent successes of the anti-CD3 T-cell immunotherapy, teplizumab, which can delay, but not prevent, disease onset.3 Longitudinal studies of high-risk patients have provided further insight into the stages of T1D development and demonstrated that early β-cell dysfunction can be present up to 5 years prior to clinical diagnosis.4 Currently, the mechanisms contributing to β-cell dysfunction during this asymptomatic period are not well understood, but inflammatory signals released by T-cells and macrophages, including proinflammatory cytokines, interleukin-1β (IL-1β), and interferon-gamma (IFN-γ), are widely thought to elicit β-cell damage and contribute to disease evolution.1,5

Early studies identified the upregulation of nitric oxide synthase 2 (NOS2) by IL-1β and IFN-γ and the production of nitric oxide (NO) as critical mediators of the deleterious effects of cytokine exposure in β-cells.5 While neuronal NOS (nNOS, NOS1) and endothelial NOS (eNOS, NOS3) produce nanomolar levels of NO that regulate physiological processes, such as synaptic transmission, smooth muscle relaxation, and vasodilation, inflammatory activation of NOS2 in β-cells produces micromolar quantities of NO. This level of NO leads to inhibition of mitochondrial oxidative metabolism and insulin secretion, elicits ER stress and DNA damage, and prolonged exposure to cytokines and NO can result in β-cell death. Despite these negative effects, the primary role of cytokines in promoting β-cell damage has been challenged, in part, because the actions of NO are fully reversible in β-cells for up to 36 h.5 Indeed, cytokines and NO stimulate protective gene expression in β-cells, including antiviral and antimicrobial responses,6 and protect β-cells from DNA damage induced apoptosis and viral replication.7,8 These latter mechanisms may rely on temporary inhibition of mitochondrial oxidation and depletion of cellular energy stores by NO signaling to invoke a transient inactive metabolic state as a defense mechanism. Thus, immune cell release of IL-1β and IFN-γ may be intended to protect β-cells from environmental threats rather than cause cellular damage and promote cell death. Collectively, these studies highlight the complex and seemingly paradoxical roles of cytokine and NO signaling in β-cells and demonstrate the need for a more detailed understanding of the direct β-cell responses to inflammatory signals.

Strong support exists for the role of NO in mediating cytokine action in rodent models, whereas the contribution of NO to cytokine signaling in human islets has been controversial.5 To address this, a recent study in Function6 analyzed the effects of cytokines (IL-1β and IFN-γ) and NO on human islet gene expression using single-cell RNA sequencing and compared these data to mouse islets treated similarly. Unlike mouse islets in which IL-1β activated Nos2 expression in 29% of β-cells and 20% of endocrine non-β-cells (α-, δ-, and PP-cells), only 1.6% of human β-cells and 1% of endocrine non-β-cells expressed detectable cytokine-induced NOS2 mRNA. Responses for other markers of IL-1β and IFN-γ signaling, including SOD2, ICAM1, IRF1, CXCL10, GBP2, and GBP5, were also dampened across all human endocrine islet cell types. Further examination of gene expression within the small population of NOS2 positive human β-cells revealed strong induction of other IL-1β and IFN-γ regulated genes resembling mouse β-cell responses to cytokines. To define parameters that may prevent human islet cells from responding to cytokines, examination of NOS2 negative β-cells demonstrated high expression of genes encoding ribosomal proteins, including RPL5, and heat shock response chaperones, such as HSPA1A. This latter observation is consistent with increased expression of heat shock chaperones in cytokine treated mouse Nos2 negative β-cells as well as the ability of heat shock to blunt cytokine and NO signaling.9 Importantly, RPL5 expression was negatively correlated with other IL-1β and IFN-γ regulated genes in both β-cell and endocrine non-β-cell populations.6

Based on this new study,6 several important topics should be considered. First, gene expression patterns in NOS2 positive human β-cells are similar to Nos2 expressing mouse β-cells. This data indicate that while the overall numbers of cytokine responsive cells are dramatically lower in human islets, direct cytokine signaling may be conserved between species. Second, dampened cytokine signaling is negatively correlated with increased expression of heat shock chaperone genes and other cell stress markers in both mouse and human β-cells.6 This observation is consistent with previous reports showing that heat shock can not only blunt cytokine and NO signaling, but also attenuate streptozotocin toxicity in rodent islets.9 While the mechanisms by which cell stress responses dampen cytokine signaling is not clear, evaluation of the state of cellular stress should be carefully considered when interpreting human (or mouse) β-cell responses to cytokine exposure. Potentially, donor cause of death and/or disposition of tissue pre- or post-isolation may be contributing factors.6 Third, while β-cells are the primary islet cell target for T-cell killing in T1D,1 the other endocrine non-β-cells also respond to cytokine signaling via NOS2 induction in both mouse and human islets.6 Importantly, α-cell and δ-cell mass are either unchanged or increased in T1D pancreata,10 highlighting a major disconnect between cytokine exposure, NO signaling, and cell death in other islet cell types. This notion is supported by the lack of death response genes observed in cytokine treated human or mouse islet α-, β-, or δ-cells.6 In addition, α-cell dysfunction and poor response to glycemic changes remain a major problem combatting hypoglycemia in T1D.1 These ongoing defects may be related to prolonged cytokine and NO suppression of α-cell metabolic activity and glucose sensing as well as the absence of β-cell paracrine regulation. In conclusion, this new study6 reveals additional complexities between immune cell mediators and islet cell functions that may be critical to understanding the evolution of β-cell defects that contribute to T1D development.

Funding

This work was supported by Breakthrough Type 1 Diabetes (SRA-2024-1553), the National Institutes of Health (R01 DK140093), and the United States Department of Defense (W81XWH-20-1-200) to S.B.S.

Conflict of Interest

No conflict of interest.
==== Refs
References

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