The peptide repertoire presented by classical as well as non-classical MHC I molecules is altered in the absence of the ER aminopeptidase associated with antigen processing (ERAAP). To characterize the extent of these changes, peptides from cells lacking ERAAP were eluted from the cell surface and analyzed by high-throughput mass spectrometry. We found that the majority of peptides found in WT cells were retained in the absence of ERAAP. In contrast, a subset of “ERAAP-edited” peptides was lost in WT cells, and ERAAP-deficient cells presented an unique “unedited” repertoire. A substantial fraction of MHC-associated peptides from ERAAP-deficient cells contained N-terminal extensions and had a different molecular composition than those from WT cells. We found that the number and immunogenicity of peptides associated with non-classical MHC I was increased in the absence of ERAAP. Conversely, only peptides presented by classical MHC I were immunogenic in ERAAP-sufficient cells. Finally, MHC I peptides were also derived from different intracellular sources in ERAAP-deficient cells.

SummaryTriple-negative breast cancers (TNBCs) display a complex spectrum of mutations and chromosomal aberrations. Chromosome 5q (5q) loss is detected in up to 70% of TNBCs, but little is known regarding the genetic drivers associated with this event. Here, we show somatic deletion of a region syntenic with human 5q33.2–35.3 in a mouse model of TNBC. Mechanistically, we identify KIBRA as a major factor contributing to the effects of 5q loss on tumor growth and metastatic progression. Re-expression of KIBRA impairs metastasis in vivo and inhibits tumorsphere formation by TNBC cells in vitro. KIBRA functions co-operatively with the protein tyrosine phosphatase PTPN14 to trigger mechanotransduction-regulated signals that inhibit the nuclear localization of oncogenic transcriptional co-activators YAP/TAZ. Our results argue that the selective advantage produced by 5q loss involves reduced dosage of KIBRA, promoting oncogenic functioning of YAP/TAZ in TNBC.

By regulating protein degradation, constitutive proteasomes (CPs) control practically all cellular functions. In addition to CPs, vertebrates express immunoproteasomes (IPs). The major nonredundant role ascribed to IPs is their enhanced ability to generate antigenic peptides. We report that CPs and IPs differentially regulate the expression of >8000 transcripts in maturing mouse dendritic cells (DCs) via regulation of signaling pathways such as IFN regulatory factors, STATs, and NF-κB. IPs regulate the transcription of many mRNAs and maturation of a few of them. Moreover, even when engineered to present optimal amounts of antigenic peptide, IP-deficient DCs are inefficient for in vivo T cell priming. Our study shows that the role of IPs in DCs is not limited to Ag processing and reveals a major nonredundant role for IPs in transcription regulation. The dramatic effect of IPs on the transcriptional landscape could explain the various immune and nonimmune phenotypes observed in vertebrates with IP deficiency or mutations.