IL-2–mTORC1 signaling coordinates the STAT1/T-bet axis to ensure Th1 cell differentiation and anti-bacterial immune response in fish

Abstract

Utilization of specialized Th1 cells to resist intracellular pathogenic infection represents an important innovation of adaptive immunity. Although transcriptional evidence indicates the potential presence of Th1-like cells in some fish species, the existence of CD3+CD4+IFN-γ+ T cells, their detailed functions, and the mechanism determining their differentiation in these early vertebrates remain unclear. In the present study, we identified a population of CD3+CD4-1+IFN-γ+ (Th1) cells in Nile tilapia upon T-cell activation in vitro or Edwardsiella piscicida infection in vivo. By depleting CD4-1+ T cells or blocking IFN-γ, Th1 cells and their produced IFN-γ were found to be essential for tilapia to activate macrophages and resist the E. piscicida infection. Mechanistically, activated T cells of tilapia produce IL-2, which enhances the STAT5 and mTORC1 signaling that in turn trigger the STAT1/T-bet axis-controlled IFN-γ transcription and Th1 cell development. Additionally, mTORC1 regulates the differentiation of these cells by promoting the proliferation of CD3+CD4-1+ T cells. Moreover, IFN-γ binds to its receptors IFNγR1 and IFNγR2 and further initiates a STAT1/T-bet axis-mediated positive feedback loop to stabilize the Th1 cell polarization in tilapia. These findings demonstrate that, prior to the emergence of tetrapods, the bony fish Nile tilapia had already evolved Th1 cells to fight intracellular bacterial infection, and support the notion that IL-2–mTORC1 signaling coordinates the STAT1/T-bet axis to determine Th1 cell fate, which is an ancient mechanism that has been programmed early during vertebrate evolution. Our study is expected to provide novel perspectives into the evolution of adaptive immunity. Author summary Differentiation of T helper (Th) cells provides the immune system with specialized T-cell subsets, which ensures a more effective immune response and reduces energy expenditure, thus representing a key innovation of adaptive immunity. Although leukocytes, CD4-1+ lymphocytes, or CD3+ T cells of some bony fish have been reported to express IFN-γ at the transcriptional level, indicating that Th1-like cells exist in these early vertebrates, the CD3+CD4-1+IFN-γ+ T-cell population has not been identified in cold-blooded animals. Here, we demonstrated the existence of CD3+CD4-1+IFN-γ+ Th1 cells in Nile tilapia, explored their immunological function in resisting Edwardsiella piscicida infection, and investigated their differentiation mechanisms. Our findings suggest that prior to the emergence of tetrapods, Nile tilapia already utilized well-evolved Th1 cells to fight intracellular bacterial infection and that the mechanisms underpinning Th1 cell differentiation in mammals have been programmed early during vertebrate evolution.