Identification of two proteins binding the 3’UTR of MHC class II mRNAs.

MHC class II molecules are cell-surface glycoproteins that play a central role in the adaptive immune system because they present peptides, derived mainly from extracellular proteins, to the antigen receptor of CD4+ T cells. MHC class II expression is regulated in a precise cell-type specific manner and restricted to thymic epithelial cells (TECs) and to the antigen-presenting cells (APCs), including B cells, cells of the monocyte–macrophage lineage and dendritic cells (DCs). Other cells such as fibroblasts, astrocytes, endothelial cells and epithelial cells, do not express MHC class II molecules unless they are exposed to specific stimuli, particularly IFN-g, that are produced during infection, inflammation or trauma. MHC class II molecules include different isotypes (HLA-DR, HLA-DP and HLA-DQ) composed of two non-covalently associated polypeptides, alfa and beta chains encoded by separate genes, which expression is co-ordinately controlled at level of transcription. Our results indicated that both 3’UTRs of HLA-DRA and HLA-DQA1 mRNA bind the same cytoplasmic complex. procedure we have identified Ebp1 and DRBP76/NF90 as components of this complex by biochemical purification. Both of them are able to bind dsRNAs, indicating that a secondary structure is shared by MHCII mRNAs. The knockdown of these proteins showed a decrease in the MHCII mRNAs accumulation, demonstrating that they are involved in the stabilization of messengers. Moreover, literature data indicated that both proteins could have a role also in the regulation of translation. Ebp1, found in the nucleoli and associated with polysomes, could represent a link between ribosome biosynthesis and cell proliferation. Its interaction with MHCII mRNAs, resulting in a modulation of the MHCII surface density, could give us clarifications about the antigen presentation in tumor cells. DRBP76/NF90 is also involved in mechanisms of stabilization and protein translation of different messengers but it is especially implicated in the modulation of IL-2 mRNA during T cell activation, occurring during the immune response. We hypothesize, according to the ”RNA operon” model that our mRNAs, encoding functionally related proteins, are co-ordinately regulated by their dsRNA structures in the 3’UTR, through the interaction with RNA-binding proteins. These complexes, immediately after transcription, arrange the mRNAs splicing, export, stability and translation to guarantee an equilibrium in the heterodimer synthesis. Our future aim will be to clarify the link between MHCII post transcriptional regulation and the function of antigen presentation of these molecules, both in professional antigen presenting cells and in tumor cells.