The acidic domain, the human version and the protein interaction of the ETS transcription factor SPI-C

Sammanfattning: Primary infections and vaccination stimulates the immunesystem to produce memory cells that rapidly respond to reexposure of the same antigen. These memory responses are dependent on an adaptive type of immunity that sustains the reactivity of the immune system by long-lived memory immune cells. From an evolutionary perspective adaptive immunity as such appears to be restricted to jawed vertebrates, since non-jawed vertebrates and invertebrates lack recombining receptors and memory responses to infection. One of the two major celltypes of the adaptive immune response is the B-cell. B-cell development is a tightly controlled process that is dependent on the precise transcriptional regulation of intracellular and membrane bound proteins. During the evolutionary transition from non-jawed to jawed vertebrates both the immune systems and the regulatory transcription factors have undergone diversification. One diversified group of transcription factors is the SPI-group of the ETS transcription family, that contains the PU.1, SPI-B, SPI-C and SPI-lamprey proteins. The absence of the PU.1 gene results in a complete block of early B-cell development. SPI-B knock-out mice display reduced ability to terminally differentiate into plasma cells. Both PU.1 and SPI-B activate transcription dependent on acidic domains in the N-terminus of the proteins. Previously it was shown that SPI-C can activate transcription from a kappa-Y Ig light chain promoter element. We scrutinised the SPI-C protein and could map an acidic transactivating region to the N-terminus of the protein. SPI-C, in contrary to SPI-B or PU.1, does not form a ternary complex with PIP on a lambda-B light chain enhancer element in electro-mobility shift assays. In addition, we identified the human version of the SPI-C protein and mapped the human and mouse genes. The human SPI-C was, like the mouse orthologue, expressed in the spleen and B-cell lines but not in T-cells or in the thymus. Phylogenetic analysis shows that human SPI-C is closest related to SPI-C orthologues. In order to identify proteins interacting with the SPI-C transcription factor we employed the yeast two hybrid system. In such a screen we identified a partial STAT6 clone (13:45) as an interactor of SPI-C. To confirm that SPI-C and 13:45 interacted in mammalian cells we coexpressed the proteins in the COS7 cell line and performed co-immunoprecipitations. 13:45 or the full length STAT6 was co-immunoprecipitated with the SPI-C protein. STAT6 binds to the I-epsilon promoter that regulate transcription and class switch recombination to the IgE isotype in a IL-4 dependent manner. STAT6 and SPI-C activates an I-epsilon reporter gene synergistically in an IL-4 dependent manner when ectopically expressed in COS7 cells and may thus regulate the induction of class switch recombination to the IgE isotype and allergy.