Autoimmunity: Redoxing PTPN22 activity
A precisely tuned immune system is tremendously important for rapidly sensing and eliminating disease-causing pathogens and generating immunological memory. At the same time, immune cells need to be able to recognize the body’s own cells and distinguish them from foreign invaders. Even small dysregulations can result in the immune system attacking organs and tissues in the body by mistake, leading to conditions known as autoimmune diseases.
The incidence of autoimmune diseases worldwide has increased in recent years, leading scientists to investigate how genetic and environmental factors contribute to these pathologies (Ye et al., 2018). Amongst other findings, research has shown that an enzyme called PTPN22 (short for protein tyrosine phosphatase non-receptor type 22) is a risk factor in multiple autoimmune disorders, including rheumatoid arthritis, diabetes and systemic lupus erythematosus. PTPN22 prevents the overactivation of T-cells (cells of the adaptive immune system) by removing phosphate groups from phosphorylated proteins that are part of the T-cell receptor (TCR) signaling pathway (Figure 1; Bottini et al., 2006; Fousteri et al., 2013; Tizaoui et al., 2021).
Activation of the T-cell receptor is followed by the production of reactive oxygen species (ROS), highly reactive by-products of molecular oxygen, which can oxidize other molecules, including proteins. It is now clear that ROS have important roles in T-cell activation and that defects in ROS production may alter the immune system's responses (Simeoni and Bogeski, 2015; Kong and Chandel, 2018). However, high levels of ROS can also cause oxidative stress, leading to impaired cell activity and even death. Therefore, T-cells must optimally balance ROS production through antioxidative mechanisms and enzymes such as thioredoxin (Patwardhan et al., 2020).
Redox reactions (oxidation and its reverse reaction known as reduction) regulate many proteins, including phosphatases (Tonks, 2005), although how oxidation and reduction modulate PTPN22 activity remained unclear. Now, in eLife, Rikard Holmdahl and colleagues based in Sweden, China, Australia, Austria, France, Russia, Hungary and the United States – including Jaime James (Karolinska Institute) as first author – report that a non-catalytic cysteine may play an important role in the redox regulation of PTPN22 (James et al., 2022). Notably, this regulation was found to modulate inflammation in mouse models with severe autoimmunity.
The team genetically engineered mice that carried a mutated version of PTPN22, in which a non-catalytic cysteine at position 129 was replaced with a serine, preventing that residue from forming a disulfide bond with the catalytic cysteine at position 227 responsible for the enzymatic activity of PTPN22. Notably, this approach was based on a study in which the crystal structure of PTPN22 was examined and an atypical bond was observed between the non-catalytic cysteine at position 129 and the catalytic cysteine residue (C227; Orrú et al., 2009). In vitro experiments performed by James et al. revealed that the mutant enzyme was more sensitive to inhibition by oxidation than its wildtype counterpart. Interestingly, the results also showed that the mutant PTPN22 was less efficient at performing its catalytic role, and that it was less responsive to re-activation by antioxidant enzymes, such as thioredoxin.
To further test the role of cysteine 129 in PTPN22 redox regulation, James et al. used a mouse model that expressed the mutant protein and was susceptible to rheumatoid arthritis. These mice exhibited higher levels of inflammation in response to T-cell activation, which would be expected in animals that cannot downregulate TCR signaling. The mice also displayed more severe symptoms of arthritis, consistent with high immune activity. These effects were not observed when the experiment was repeated in mice that fail to produce high levels of ROS in response to TCR activation, confirming that the initial observations depend on the redox state of PTPN22.
Finally, James et al. performed in vitro experiments on T-cells isolated from mice carrying the mutant PTPN22. They found that when these cells became activated, the downstream targets of PTPN22 showed an increased phosphorylation status, consistent with lower PTPN22 activity.
Taken together, the elegant study of James et al. shows that cysteine 129 is critical for the redox regulation of PTPN22, and that its mutation impacts T-cell activity and exacerbates autoimmunity in mice (Figure 1). What still remains to be determined is why the mutant enzyme has lower catalytic activity, which may be due to the mutation affecting the structural conformation of PTPN22. Additionally, it will be important to assess other cysteines in PTPN22 to determine whether they are also partly involved in its redox regulation.
Understanding how the redox state of PTPN22 regulates the activity of T-cells may help researchers to develop new therapies for treating autoimmune diseases.
References
-
Role of PTPN22 in type 1 diabetes and other autoimmune diseasesSeminars in Immunology 18:207–213.https://doi.org/10.1016/j.smim.2006.03.008
-
Roles of the protein tyrosine phosphatase PTPN22 in immunity and autoimmunityClinical Immunology 149:556–565.https://doi.org/10.1016/j.clim.2013.10.006
-
Regulation of redox balance in cancer and T cellsThe Journal of Biological Chemistry 293:7499–7507.https://doi.org/10.1074/jbc.TM117.000257
-
A loss-of-function variant of PTPN22 is associated with reduced risk of systemic lupus erythematosusHuman Molecular Genetics 18:569–579.https://doi.org/10.1093/hmg/ddn363
-
Redox regulation of regulatory T-cell differentiation and functionsFree Radical Research 54:947–960.https://doi.org/10.1080/10715762.2020.1745202
-
Redox regulation of T-cell receptor signalingBiological Chemistry 396:555–568.https://doi.org/10.1515/hsz-2014-0312
-
The role of PTPN22 in the pathogenesis of autoimmune diseases: a comprehensive reviewSeminars in Arthritis and Rheumatism 51:513–522.https://doi.org/10.1016/j.semarthrit.2021.03.004
Article and author information
Author details
Publication history
Copyright
© 2022, Shumanska and Bogeski
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 633
- views
-
- 109
- downloads
-
- 1
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Further reading
-
- Immunology and Inflammation
- Microbiology and Infectious Disease
Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies can efficiently block parasite transmission. In search for naturally acquired antibodies targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to sexual stages of Pf in the form of gametes and gametocyte extracts. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One mAb, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for Pf circumsporozoite protein (PfCSP), extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf.
-
- Immunology and Inflammation
Trained immunity (TI) is the process wherein innate immune cells gain functional memory upon exposure to specific ligands or pathogens, leading to augmented inflammatory responses and pathogen clearance upon secondary exposure. While the differentiation of hematopoietic stem cells (HSCs) and reprogramming of bone marrow (BM) progenitors are well-established mechanisms underpinning durable TI protection, remodeling of the cellular architecture within the tissue during TI remains underexplored. Here, we study the effects of peritoneal Bacillus Calmette–Guérin (BCG) administration to find TI-mediated protection in the spleen against a subsequent heterologous infection by the Gram-negative pathogen Salmonella Typhimurium (S.Tm). Utilizing single cell RNA-sequencing and flow cytometry, we discerned STAT1-regulated genes in TI-associated resident and recruited splenic myeloid populations. The temporal dynamics of TI were further elucidated, revealing both early and delayed myeloid subsets with time-dependent, cell-type-specific STAT1 signatures. Using lineage tracing, we find that tissue-resident red pulp macrophages (RPM), initially depleted by BCG exposure, are restored from both tissue-trained, self-renewing macrophages and from bone marrow-derived progenitors, fostering long lasting local defense. Early inhibition of STAT1 activation, using specific JAK-STAT inhibitors, reduces both RPM loss and recruitment of trained monocytes. Our study suggests a temporal window soon after BCG vaccination, in which STAT1-dependent activation of long-lived resident cells in the tissue mediates localized protection.