The T cell protein cytotoxic T lymphocyte antigen 4 (CTLA4) was identified as a crucial negative regulator of the immune system over 15 years ago, but its mechanisms of action are still under debate. It has long been suggested that CTLA4 transmits an inhibitory signal to the cells that express it. However, not all the available data fit with a cell-intrinsic function for CTLA4, and other studies have suggested that CTLA4 functions in a T cell-extrinsic manner. Here, we discuss the data for and against the T cell-intrinsic and -extrinsic functions of CTLA4.

The emerging role of CTLA4 as a cell-extrinsic regulator of T cell responses

Treg and CTLA-4: two intertwining pathways to immune tolerance.

Costimulation is a concept that goes back to the early 1980’s when Lafferty and others hypothesized that cell surface and soluble molecules must exist that are essential for initiating immune responses subsequent to antigen exposure. The explosion in this field of research ensued as over a dozen molecules have been identified to function as second signals following T-cell receptor engagement. By 1994, it seemed clear that the most prominent costimulatory pathway CD28, and functionally related costimulatory molecules such as CD154, were the major drivers of a positive immune response. Then the immunology world turned upside down. CD28 knockout mice, which were, in most cases, immunodeficient, led to increased autoimmunity when bred into the non-obses diabetic background. Another CD28 family member, cytotoxic T-lymphocyte-associated protein 4, which was presumed to be a costimulatorymolecule on activated T cells, turned out to be critical in down regulating immunity. These results, coupled with the vast suppressor cell literature which had been largely rebuked, suggested that the immune system was not poised for response but controlled in a way such that regulation was dominant. Over the last decade, we have learned that these costimulatory molecules play a key role in the now classical CD4+CD25+Foxp3+ regulatory T cells (Tregs) that provide critical control of unwanted autoimmune responses. In this review, we discuss the connections between costimulation and Tregs that have changed the costimulation paradigm.

Regulating the regulators: costimulatory signals control the homeostasis and function of regulatory T cells

An impressive clinical success has been observed in treating a variety of cancers using immunotherapy with programmed cell death-1 (PD-1) checkpoint blockade. However, limited response in most patients treated with anti-PD-1 antibodies remains a challenge, requiring better understanding of molecular mechanisms limiting immunotherapy. In colorectal cancer (CRC) resistant to immunotherapy, mismatch-repair-proficient or microsatellite instability-low (pMMR-MSI-L) tumors have low mutation burden and constitute ~85% of patients. Here, we show that inhibition of N6 -methyladenosine (m6 A) mRNA modification by depletion of methyltransferases, Mettl3 and Mettl14, enhanced response to anti-PD-1 treatment in pMMR-MSI-L CRC and melanoma. Mettl3- or Mettl14-deficient tumors increased cytotoxic tumor-infiltrating CD8+ T cells and elevated secretion of IFN-γ, Cxcl9, and Cxcl10 in tumor microenvironment in vivo. Mechanistically, Mettl3 or Mettl14 loss promoted IFN-γ-Stat1-Irf1 signaling through stabilizing the Stat1 and Irf1 mRNA via Ythdf2. Finally, we found a negative correlation between METTL3 or METTL14 and STAT1 in 59 patients with pMMR-MSI-L CRC tumors. Altogether, our findings uncover a new awareness of the function of RNA methylation in adaptive immunity and provide METTL3 and METTL14 as potential therapeutic targets in anticancer immunotherapy.

m 6 A RNA methyltransferases METTL3/14 regulate immune responses to anti-PD-1 therapy.

https://www.cell.com/trends/immunology/pdf/S1471-4906(11)00099-8.pdf

Cell-autonomous and -non-autonomous roles of CTLA-4 in immune regulation.

Objective

CD4+CD25+ regulatory T cells (known as Treg cells) suppress unwanted and autoreactive T cell responses. Treg cells express the costimulatory molecule CTLA-4 intracellularly, but the mechanisms by which Treg cells exploit CTLA-4 signaling remain unclear. The present study was undertaken to investigate the role of CTLA-4 in controlling the homeostasis and suppressive function of Treg cells.



Methods

Murine Treg cells were analyzed by flow cytometry for coexpression of CTLA-4 and typical Treg cell–expressed molecules, and the influence of CTLA-4 on T cell proliferation, suppression, and apoptosis was investigated by in vitro assays. To analyze the importance of CTLA-4 in Treg cell–mediated suppression in vivo, wild-type Treg cells were transferred into CTLA-4–deficient mice displaying lymphoproliferation, and survival was monitored over time.



Results

A strong correlation between expression of forkhead box P3 and ex vivo expression of CTLA-4 in Treg cells was observed. Inhibition of CTLA-4 signaling in Treg cells during in vitro stimulation increased cell cycling and led to enhanced activation-induced cell death (AICD), which was mediated by CD95/CD95 ligand–induced activation of caspases. Blockade of CTLA-4 signaling resulted in impairment of the suppressive capacity of Treg cells. Despite these effects, high amounts of Treg cells persisted in CTLA-4–deficient mice. Results of transfer experiments in CTLA-4–deficient mice showed that the mice had a significantly prolonged lifespan when CTLA-4–competent Treg cells were injected.



Conclusion

Expression of CTLA-4 on Treg cells serves to control T cell proliferation, to confer resistance against AICD, and to maintain the suppressive function of Treg cells.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.24181

CTLA-4 (CD152) controls homeostasis and suppressive capacity of regulatory T cells in mice.

Performance evaluation of the Roche Elecsys Anti-SARS-CoV-2 S immunoassay.

CD8 T cell expansion and cytokine production is needed to generate an effective defense against viral invasion of the host. These features of CD8 T lymphocytes are regulated, especially during primary responses, by positive and negative costimulation. We show in this study that surface expression of CD152 is highly up-regulated on activated CD8 T lymphocytes during primary immune responses, suggesting a prominent regulatory role. Indeed, production of the proinflammatory cytokine IFN-γ, but not TNF-α, by CD8 T cells was inhibited by CD152 engagement. The inhibition was regulated independent of proliferation and IL-2 production, but dependent on the quality of the TCR signaling. We show that signals induced by CD152 on activated CD8 T lymphocytes reduce the frequency of IFN-γ high -expressing cells. Our data also show that in activated CD8 T cells, the CD152-mediated inhibition of cytokine production is more pronounced than inhibition of their proliferation.

/pdf/high-ifn-g-production-of-individual-cd8-t-lymphocytes-is-4d7tjra41o.pdf

High IFN-γ Production of Individual CD8 T Lymphocytes Is Controlled by CD152 (CTLA-4)

Outcome prediction by serum calprotectin in patients with COVID-19 in the emergency department.

Background The Elecsys® Anti-SARS-CoV-2 S immunoassay (Roche Diagnostics International Ltd, Rotkreuz, Switzerland) has been developed for the in vitro quantitative detection of antibodies to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein. We evaluated the performance of this assay using samples from seven sites in Germany, Austria, and Switzerland. Methods Anonymized frozen, residual serum, or plasma samples from blood donation centers or routine diagnostic testing were used for this study. For specificity and sensitivity analyses, presumed negative samples collected before October 2019 and SARS-CoV-2 PCR-confirmed single or sequential samples were tested, respectively. The performance of the Elecsys Anti-SARS-CoV-2 S immunoassay was also compared with other commercial immunoassays. Results The overall specificity (n=7880 pre-pandemic samples) and sensitivity (n=240 PCR-positive samples [≥14 days post-PCR]) for the Elecsys Anti-SARS-CoV-2 S immunoassay were 99.95% (95% confidence interval [CI]: 99.87–99.99) and 97.92% (95% CI: 95.21– 99.32), respectively. Compared with seven other immunoassays, the Elecsys Anti-SARS-CoV-2 S assay had comparable or greater specificity and sensitivity. The Elecsys Anti-SARS-CoV-2 S immunoassay had significantly higher specificity compared with the LIAISON® SARS-CoV-2 S1/S2 IgG, ADVIA Centaur® SARS-CoV-2 Total, ARCHITECT SARS-CoV-2 IgG, iFlash-SARS-CoV-2 IgM, and EUROIMMUN Anti-SARS-CoV-2 IgG and IgA assays, and significantly higher sensitivity (≥14 days post-PCR) compared with the ARCHITECT SARS-CoV-2 IgG, iFlash-SARS-CoV-2 IgG and IgM, and EUROIMMUN Anti-SARS-CoV-2 IgG assays. Conclusion The Elecsys Anti-SARS-CoV-2 S assay demonstrated a robust and favorable performance across samples from multiple European sites, with a very high specificity and sensitivity for the detection of anti-S antibodies.

/pdf/performance-evaluation-of-the-roche-elecsys-anti-sars-cov-2-3pj7atb5wu.pdf

J. Kolja Hegel

Papers

CTLA-4 (CD152) controls homeostasis and suppressive capacity of regulatory T cells in mice.

Performance evaluation of the Roche Elecsys Anti-SARS-CoV-2 S immunoassay.

High IFN-γ Production of Individual CD8 T Lymphocytes Is Controlled by CD152 (CTLA-4)

Outcome prediction by serum calprotectin in patients with COVID-19 in the emergency department.

Performance evaluation of the Roche Elecsys Anti-SARS-CoV-2 S immunoassay