Two Distinct Pathways Mediated by PA28 and hsp90 in Major Histocompatibility Complex Class I Antigen Processing

Major histocompatibility complex (MHC) class I ligands are mainly produced by the proteasome. Herein, we show that the processing of antigens is regulated by two distinct pathways, one requiring PA28 and the other hsp90. Both hsp90 and PA28 enhanced the antigen processing of ovalbumin (OVA). Geldanamycin, an inhibitor of hsp90, almost completely suppressed OVA antigen presentation in PA28α−/−/β−/− lipopolysaccharide blasts, but not in wild-type cells, indicating that hsp90 compensates for the loss of PA28 and is essential in the PA28-independent pathway. In contrast, treatment of cells with interferon (IFN)-γ, which induces PA28 expression, abrogated the requirement of hsp90, suggesting that IFN-γ enhances the PA28-dependent pathway, whereas it diminishes hsp90-dependent pathway. Importantly, IFN-γ did not induce MHC class I expressions in PA28-deficient cells, indicating a prominent role for PA28 in IFN-γ–stimulated peptide supply. Thus, these two pathways operate either redundantly or specifically, depending on antigen species and cell type

Immunoproteasome assembly and antigen presentation in mice lacking both PA28α and PA28β

Two members of the proteasome activator, PA28α and PA28β, form a heteropolymer that binds to both ends of the 20S proteasome. Evidence in vitro indicates that this interferon-γ (IFN-γ)-inducible heteropolymer is involved in the processing of intracellular antigens, but its functions in vivo remain elusive. To investigate the role of PA28α/β in vivo, we generated mice deficient in both PA28α and PA28β genes. The ATP-dependent proteolytic activities were decreased in PA28α(–/–)/β(–/–) cells, suggesting that ‘hybrid proteasomes’ are involved in protein degradation. Treatment of PA28α(–/–)/β(–/–) cells with IFN-γ resulted in sufficient induction of the ‘immunoproteasome’. Moreover, splenocytes from PA28α(–/–)/β(–/–) mice displayed no apparent defects in processing of ovalbumin. These results are in marked contrast to the previous finding that immunoproteasome assembly and immune responses were impaired in PA28β(–/–) mice. PA28α(–/–)/β(–/–) mice also showed apparently normal immune responses against infection with influenza A virus. However, they almost completely lost the ability to process a melanoma antigen TRP2-derived peptide. Hence, PA28α/β is not a prerequisite for antigen presentation in general, but plays an essential role for the processing of certain antigens

Hsp90-mediated Assembly of the 26 S Proteasome Is Involved in Major Histocompatibility Complex Class I Antigen Processing*S⃞

Heat shock protein 90 (hsp90) and the proteasome activator PA28 stimulate major histocompatibility complex (MHC) class I antigen processing. It is unknown whether hsp90 influences the proteasome activity to produce T cell epitopes, although association of PA28 with the 20 S proteasome stimulates the enzyme activity. Here, we show that hsp90 is essential in assembly of the 26 S proteasome and as a result, is involved in epitope production. Addition of recombinant hsp90α to cell lysate enhanced chymotrypsin-like activity of the 26 S proteasome in an ATP-dependent manner as determined by an in-gel hydrolysis assay. We successfully pulled down histidine-tagged hsp90α- and PA28α-induced, newly assembled 26 S proteasomes from the cell extracts for in vitro epitope production assay, and we found these structures to be sensitive to geldanamycin, an hsp90 inhibitor. We found a cleaved epitope unique to the proteasome pulled down by both hsp90α and PA28α, whereas two different epitopes were identified in the hsp90α- and PA28α-pulldowns, respectively. Processing of these respective peptides in vivo was enhanced faithfully by the protein combinations used for the proteasome pulldowns. Inhibition of hsp90 in vivo by geldanamycin partly disrupted the 26 S proteasome structure, consistent with down-regulated MHC class I expression. Our results indicate that hsp90 facilitates MHC class I antigen processing through epitope production in a complex of the 26 S proteasome