Cancer cells may evade immune surveillance as a result of defective antigen processing and presentation. In this study, we demonstrate that CD40 ligation overcomes this defect through the coordinated action of the transcription factors NF-kappaB and interferon regulatory factor 1 (IRF-1). We show that unlike interferon signaling, which triggers the STAT1-mediated transcriptional activation of IRF-1, the ligation of CD40 in carcinomas induces the rapid upregulation of IRF-1 in a STAT1-independent but NF-kappaB-dependent manner. The transcriptional activation of IRF-1 is controlled largely by the recruitment of p65 (RelA) NF-kappaB to the IRF-1 promoter following the engagement of a TAK1/IkappaB kinase beta/IkappaBalpha signaling pathway downstream of CD40. NF-kappaB and de novo-synthesized IRF-1 converge to regulate the expression of genes involved in antigen processing and transport, as evident from the sequential recruitment of NF-kappaB and IRF-1 to the promoters of the genes encoding transporter for antigen processing 1 (TAP1), TAP2, tapasin, and low-molecular-mass polypeptides LMP2 and LMP10. Moreover, the RNA interference-mediated knockdown of IRF-1 reduced, whereas the inhibition of NF-kappaB abolished, the effects of CD40 on TAP1 and LMP2 upregulation in carcinoma cells. Collectively, these data reveal a novel "feed-forward" mechanism induced by NF-kappaB which ensures that acutely synthesized IRF-1 operates in concert with NF-kappaB to amplify the immunoproteasome and antigen-processing functions of CD40.
ATP-Binding Cassette Transporters Antigen Presentation