Abstract
Cellular responses to viral infection and non-self tissues depend on presentation of antigenic peptides by polymorphic MHC class I molecules. The regulatory mechanisms controlling MHC class I expression are fundamental to effective immune responses, and in the pig are not fully understood. In this study the cellular responses of SLA class I genes to cytokines were studied by measuring SLA class 1 expression at the cell surface and also by fusing the promoters of the classical genes SLA-1, -2 and -3 and non-classical genes SLA 6 and -7 independently to a luciferase reporter gene. Cell surface expression of SLA class I was measured on Max cells, Shimozuma cells and porcine aortic endothelial cells isolated from inbred pigs of the d/d haplotype and outbred pigs of an unknown haplotype. IFN-alpha and -gamma treatment increased SLA class I expression on d/d PAECs and Max cells but not on outbred PAECs or Shimozuma cells. Analysis of SLA class I promoter activity in Max cells showed constitutive activity of SLA-1, -2, -3, -6, -7 and MIC-2 promoters. In summary, classical SLA promoters were responsive to IFNs and co-expression of the transcription factors IRF-1, NF- KB p65 and CIITA. In contrast to their human counterparts, combined treatment with TNF-alpha and IFN-alpha/-gamma had no synergistic effect on classical SLA class I promoter activity. SLA-1 responded to TNF-alpha and co-expression of the transcription factors IRF-3, -7 and -9. Non-classical promoters were not induced by IFN-alpha or -gamma or CIITA. SLA-7 was responsive to TNF-alpha and co-expression of IRF-1 but not co-expression of NF-kappaB. Interestingly, basal expression of MIC-2 remained unaffected by cytokines or co-expression of transcription factors. These results suggest locus-specific responses of SLA class I genes to cytokines and transcription factors, which can be explained in part by sequence variation in three key SLA class I promoter motifs: ISRE, Enhancer A and SXY. Furthermore, this study has demonstrated that the porcine virus CSFV decreases SLA class I surface expression early during infection and that this effect correlates to decreased constitutive activity of SLA-1, -2 and -7 promoters.