Role of lipopolysaccharide and IL-4 in control of transcription of the class II A alpha gene
At the time of publication, Ellen Gravallese was not yet affiliated with the University of Massachusetts Medical School
The class II (Ia) MHC molecules are cell surface proteins that regulate the activation of T cells. B lymphocyte expression of class II molecules has been shown to be influenced by a number of external stimuli. It has been previously demonstrated that treatment of these cells with IL-4 leads to an increase in class II gene transcription at 18 h as well as to an increase in steady state class II mRNA. It has also been previously demonstrated that LPS treatment of splenic B cells from athymic mice results in a decrease in steady state mRNA encoding the A alpha class II protein. This decrease persists for at least 18 h. Nuclear run-on transcription assays now demonstrate that although steady state mRNA levels for A alpha are decreased by LPS treatment of athymic mouse lymphocytes, LPS does not decrease A alpha gene transcription, but rather modestly activates transcription of this class II gene. LPS and IL-4 have been demonstrated to be synergistic stimuli for a number of genes. Costimulation of splenic lymphocytes from athymic mice with IL-4 plus LPS leads to activation of transcription, but the increase in transcription is no more than that seen with IL-4 stimulation alone. However, in costimulated lymphocytes, steady state A alpha-encoding mRNA levels are intermediate between the increased levels seen with IL-4 stimulation and the decreased levels seen with LPS stimulation. Therefore, LPS and IL-4 act nonsynergistically in class II gene transcription and the effects of LPS in decreasing steady state mRNA are most likely posttranscriptional. An IL-4-inducible and an LPS-inducible DNA-binding protein have been previously identified in splenic lymphocytes from athymic mice. Both nuclear binding proteins form complexes with the same DNA fragments from a control region of the A alpha gene. These nucleoprotein complexes comigrate under nondenaturing conditions and display identical patterns of binding with a panel of oligonucleotide competitors. Oligonucleotides representing protein binding sites of the IL-4 and LPS-induced DNA-binding proteins cross-compete for protein binding. Therefore, the binding proteins induced by LPS and IL-4 are likely related, and may function at different efficiencies as activators of A alpha gene transcription.