Induction of Interferon-γ and Tissue Inflammation by Overexpression of Eosinophil Cationic Protein in T Cells and Exosomes

T cells play a critical role in the pathogenesis of systemic lupus erythematosus (SLE). Serum-derived exosomes are increased in SLE patients and are correlated with disease severity. This study was undertaken to investigate whether T cell–derived exosomal proteins play a role in SLE pathogenesis.

Scientists from Taiwan characterized proteins in T cell–derived exosomes from SLE patients and healthy controls by MACSPlex exosome analysis and proteomics. To study the potential pathogenic functions of the exosomal protein identified, we generated and characterized T cell–specific transgenic mice that overexpressed that protein in T cells.

They identified eosinophil cationic protein (ECP, also called human RNase III) as overexpressed in SLE T cell–derived exosomes. T cell–specific ECP–transgenic mice (n = 5 per group) displayed early induction of serum interferon-γ (IFNγ) levels (P = 0.062) and inflammation of multiple tissue types. Older T cell–specific ECP–transgenic mice (n = 3 per group) also displayed an increase in follicular helper T cell and plasma B cell numbers, and in autoantibody levels (P < 0.01). Single-cell RNA sequencing showed the induction of IFNγ messenger RNA (P = 2.2 × 10-13) and inflammatory pathways in ECP-transgenic mouse T cells. Notably, adoptively transferred ECP-containing exosomes stimulated serum autoantibody levels (P < 0.01) and tissue IFNγ levels in the recipient mice (n = 3 per group). The transferred exosomes infiltrated into multiple tissues of the recipient mice, resulting in hepatitis, nephritis, and arthritis.

A key finding of this study was the identification of one novel T cell exosomal protein, ECP, which plays an important role in SLE pathogenesis. ECP overexpression in T cells resulted in enhancement of inflammatory responses and T cell activation. Notably, ECP-containing exosomes from T cells targeted several tissues (such as the liver, kidney, and joint) of the recipient mice, leading to tissue inflammation. These data suggest that ECP-overexpressing T cells or ECP-containing exosomes may act as a causal factor in SLE.

One of the notable findings of this study is that T cell–derived exosomal ECP contributes to autoimmune diseases. This is the first study to show that ECP is an exosomal protein. Extracellular ECP stimulation using ECP recombinant protein induces cell death, including necrosis and apoptosis; however, T cell development in Lck-ECP–transgenic mice was not affected. The data suggest that ECP recombinant protein acts differently from exosomal and intracellular ECP. In support of this notion, soluble ECP levels were not increased in the sera of human SLE patients. These findings also suggest that exosomal ECP and ECP-overexpressing T cells contribute to autoimmune responses through cell death–independent pathways. Furthermore, adoptive transfer of ECP-containing exosomes induced autoantibody production and inflammation expansion; it is likely that the T cell–derived exosomes from Lck-ECP–transgenic mouse T cells contain other inflammatory molecules in addition to ECP proteins.

 

Sherry