Purpose Asthma exacerbations are frequently associated with viral infections. Many of these viruses cause a marked decrease in antigen-presenting cell MHC class I production. We have previously identified a potential mechanism linking the T-helper type 2 (TH2) cytokine preponderance seen in acute asthma and this viral-mediated event. That is, NKG2A was identified as the only known ligand for MHC class I (ie, HLA-E) expressed by activated TH2 lymphocytes. Therefore, we aimed to measure TH2 cell effector function upon T cell receptor (TCR) stimulation in the presence and absence of HLA-E, intending to imitate the reduction in MHC class I seen with respiratory viral infections.
Methods Human monocytes and TH2 cells were negatively purified from healthy volunteers using previously validated antibody cocktails. TH2 cells were cultured in media containing anti-CD3 and anti-CD28 in the presence (or absence) of autologous monocytes, which served as HLA-E presenters. Four-color flow cytometry was performed. Analyses with FlowJo 5.7 were carried out after back-gating on CD3+CD4+ cells. Paired t-tests measured statistical significance using SPSS 14.
Results TH2 isolates were > 95% pure CD3+CD4+CD8- cells and > 84% pure TH2 cells. Monocyte isolates were > 90% pure. Flow studies of activated TH2 cells showed mean ± SE (mean fluorescence intensity) NKG2A+ of 11.4 ± 0% (15.4 ± 0.1) and intracellular IL-4+ of 4.4 ± 0% (153.4 ± 0.1). In the presence of monocytic HLA-E, activated TH2 lymphocytes showed NKG2A+ of 11.2 ± 0% (35.6 ± 0) (p < .001) and intracellular IL-4+ of 0.04 ± 0.01% (14.1 ± 0) (p < .001). There was no detectable switching from inhibitory NKG2A to the activating lectin NKG2C in any sample. Conclusions: We identified significant suppression of IL-4 expression in activated TH2 cells via NKG2A-HLA-E (MHC class I) binding and ultimate TCR signaling inhibition. Extrapolated to a viral-induced asthma exacerbation scenario, where MHC class I and thus HLA-E are in low abundance, we have shown that TH2 cells would exhibit a relatively robust response. This represents a new aspect of TH1/TH2 balance in the inflammatory response and could be clinically important in viral-induced TH1/TH2-associated diseases like asthma.
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