Type I interferon co-stimulation enhances γδ intraepithelial lymphocyte IFNγ production

Intraepithelial lymphocytes (IEL) expressing the γδ T cell receptor (TCR) constantly survey the intestinal epithelium, facilitating a localized effector response to limit pathogen invasion. Despite constant γδ TCR triggering in vivo, these IELs remain immunologically quiescent, suggesting that a co-stimulatory signal is required to overcome their activation threshold. Type I interferon (IFN) is a family of pro-inflammatory cytokines that are critical for host defense against bacterial and viral infection. Activated γδ IELs limit viral replication in intestinal epithelial cells by producing type I IFNs, such as IFNα; however, the role of IFNα in mediating γδ IEL antiviral effector function remains unknown. To determine whether IFNα directly activates these sentinel lymphocytes, murine small intestinal γδ IELs were isolated, sorted, and stimulated ex vivo with 1μg/mL αCD3, 10 ng/mL IFNα, or both for 5 h. Following treatment with αCD3 and IFNα, we observed a 27%±9.3 increase in IFNγ+ γδ IELs (p<0.0001) and a 2.5-fold increase in IFNγ mean fluorescence intensity (MFI)(p<0.0001) compared to αCD3 stimulation. Interestingly, IFNα treatment alone was not sufficient to induce IFNγ production. To determine the threshold of TCR signaling necessary for this enhanced IFNγ production, we treated γδ IELs with suboptimal concentrations of αCD3 (0.01 or 0.1 μg/mL), to mimic constant TCR triggering in vitro.At 0.1 μg/mL αCD3, IFNα induced a 27%±7.2 increase in frequency of IFNγ+ γδ IELs (p<0.01) and a significant increase in MFI of γδ IEL IFNγ production over αCD3 treatment alone (p<0.001). With a low level of TCR priming (0.1 μg/mL αCD3), a low dose of IFNα (0.1 ng/mL) was able to stimulate a 22%±6.8 increase in IFNγ+ γδ IELs compared to αCD3 treatment alone. However, higher concentrations of IFNα failed to further enhance γδ IEL IFNγ production. Stimulation of γδ IELs with IFNα ex vivo was sufficient to induce STAT1 and STAT4 phosphorylation; therefore, we asked whether STAT1 or STAT4 is required for IFNα-induced IFNγ production. To this end, we treated WT, STAT1 KO, or STAT4 KO γδ IELs with 1 μg/mL αCD3, 10 ng/mL IFNα, or both and found that the IFNα-induced increase in IFNγ production was abrogated in STAT4-, but not STAT1-deficient γδ IELs (p<0.0001). To assess changes in STAT1 and STAT4 expression in γδ IELs during acute viral infection, WT mice were infected orally with 1.5x106 PFU murine gamma herpesvirus (MHV). We observed a 16%±3 increase in the frequency of γδ IELs 3 days post-infection compared to controls, which corresponded with a decrease in total STAT1 (MFI: 905±95 vs 638±102, p<0.01). Taken together, our data demonstrate that γδTCR signaling is required for IFNα-induced IFNγ production in a STAT4-dependent manner. These findings suggest that IFNα signaling in conjunction with constant TCR triggering promotes the rapid activation and expansion of γδ IELs, which may enhance their effector response to microbial pathogens. © FASEB.