Role of TNFR2 expressed by regulatory T cells (Treg) in immunosupression induced by sepsis – TRIS

Sepsis causes more than 5 million deaths worldwide with an estimated cost of $ 17 billion annually in the USA. It is now assumed that most of the death is due to sepsis-induced immune suppression (SIIS) for which there is no FDA-approved specific therapeutic option. This SIIS renders septic patients prone to new secondary infections and alters their long-term functional status. The patients who die during the late phase of sepsis have usually an immune system that does not recover. However, the mechanisms of the SIIS are still poorly known.
Severe alteration of the lymphocyte compartment is a hallmark signature of SIIS. A dramatic decrease of circulating CD4+ T cells and the persistence of an increased percentage of CD4+FoxP3+ regulatory T cells (Treg) cells were predictive of secondary nosocomial infections and poor outcomes in septic patients. The mouse models of sepsis recapitulate these observations with decreased number of CD4+ T cells and increased proportion of Treg. There is good evidence that Treg, which play a major role in immune suppression, impair the CD4+ T cell compartment during sepsis. We propose that one of the major mechanisms of SIIS is that TNF, released in high amount during sepsis, boosts expansion and suppressive activity of Tregs through the TNF receptor 2 (TNFR2) and NF-?B signaling, which is the working hypothesis of our research project.
We have developed a new and original mouse model of sepsis consisting in blood injection of Staphylococcus aureus. In this model, we observed lymphopenia and increased proportion of Foxp3+ Treg that exhibit an activated signature (high expression of CTLA4), leading to SIIS and impaired immune response to secondary infection in the lung. We recently obtained strong evidence that Treg get activated during sepsis by the TNF/TNFR2 axis. Indeed, transcriptomic analyses suggested that these Treg are activated by TNFR2. SIIS was no more observed in TNFR2 knock-out mice, and TNFR2+ Treg from septic patients exhibited signs of activation (CTLA4 expression) (Journal of Infectious Disease, 2020, in press).
To get further insight into the role of TNFR2+ and Treg in SIIS, our proposal will address the following specific aims: 1. We will perform deep and unbiased profiling of TNFR2+/- Treg in patients during severe sepsis (D1 and D4) at the single cell level using Cellular Indexing of Transcriptome and Epitopes by Sequencing (CITE-seq) of blood CD4+ T cells. 2. We will study the role of TNFR2 and its downstream NF-kB signaling pathway expressed by Treg in SIIS by using conditional knockout mice that we have been recently developed, which are animal models not available by other groups. 3. We will study the sensitivity to TNF of Treg from septic patients and the therapeutic effect of human TNFR2 antagonist on SIIS in humanized knock-in mice expressing the extracellular portion of human TNFR2 fused to the mouse intracellular TNFR2 tail.
We expect to characterize in details Treg molecular remodeling and to identify gene programs associated with TNFR2+ in Treg during the course of sepsis in patients. Mouse studies will generate causative links to validate our working hypothesis and to test the therapeutic efficacy of a new TNFR2 antagonist. If we succeed in these studies, the current understanding of the pathophysiology of sepsis will be deeply modified. Preferential expression of TNFR2 by Treg cells is ideal because targeting this receptor could reduce the toxicity observed with untargeted inhibition of Treg such as anti-CTLA4 in cancer. Finally, this project should increase our understanding of SIIS and may lead to innovative therapeutic strategies.