HLA-DR3 transgenic mice expressing Nur77-GFP reveal early and robust activation of T cells by superantigens during Staphylococcus aureus pneumonia.

The role of superantigens (SAg) in the immunopathogenesis Staphylococcus aureus pneumonia has been shown by several indirect means. To directly demonstrate activation of T cells in vivo during staphylococcal pneumonia by SAg and to study the kinetics of T cell activation, HLA-DR3 + .Nur77-eGFP + mice were used. In these mice, rapid expression of green fluorescent protein (GFP) in T cells occurs only upon activation through the TCR, but not following other inflammatory stimuli. Experimental pneumonia was induced using isogenic strains of S. aureus either producing the SAg staphylococcal enterotoxin B (SEB), or not producing any SAg. Mice were sacrificed at different time points and expression of GFP was determined by flow cytometry. As SEB primarily activates CD4 + and CD8 + T cells bearing TCR Vβ8, the expression of GFP within the TCR Vβ8 + sub-populations was examined. Strong upregulation of GFP in the TCR Vβ8 + sub-population was appreciable in both CD4 + and CD8 + T cell subsets (20–25%) even as early as 4 hours only in mice infected with S. aureus strain producing SEB (SEB+SA), but not those infected with the isogenic strain that does not produce SEB (SEB-SA). Serum cytokine/chemokine levels mirrored this finding. By 24 hours, 80–90% of the CD4 + and CD8 + T cells bearing TCR Vβ8 + expressed GFP in mice infected with SEB+SA, compared to only 1–2% in mice infected with SEB-SA. The pattern of expression of CD69 expression, an early T cell activation marker, was distinct from that of GFP. Our novel study directly demonstrated T cell activation and established their activation kinetics in vivo during staphylococcal pneumonia. These results might be useful for timing of administration of novel drugs aimed at suppressing T cell activation during pneumonia.