Dendritic cells play a decisive role in initiating adaptive immune responses. They continuously sample their environment through phagocytosis and endocytosis, and possess the distinctive capacity for "cross-presentation": unlike most cells, they can activate CD8+ T lymphocytes—the effectors of antiviral and antitumor immunity—by presenting peptides derived from extracellular proteins. This process requires a proteolytic machinery capable of cleaving complex antigens into appropriately sized fragments, typically peptides of eight or nine amino acids, the only ones well stabilized within the binding groove of major histocompatibility complex class I (MHC-I) molecules. Insulin-regulated aminopeptidase (IRAP), carried by a specific population of storage endosomes marked by Rab14, had previously been identified as a factor recruited very early to newly formed antigen-containing compartments, where it regulates their maturation and, ultimately, cross-presentation.
Using IRAP-deficient (IRAP−/−) dendritic cells, the authors clarified how this protein modulates the dynamics of phagosome maturation. In its absence, phagosomes acquire late endosomal markers more rapidly, display a lower luminal pH and increased hydrolytic activity, which translates into enhanced microbicidal capacity toward phagocytosed yeasts, fungi, and bacteria. This accelerated maturation is accompanied by greater deposition of lipidated LC3 on the phagosomal membrane, suggesting that fusion with IRAP storage endosomes normally "bypasses" the signals leading to LC3 association.
The study also reveals a link between vesicular trafficking originating from the endoplasmic reticulum–Golgi intermediate compartment (ERGIC) and the formation or stability of the IRAP compartment: silencing of the SNARE Sec22b strongly destabilizes IRAP-marked storage endosomes, offering a possible explanation for the accelerating effect of this silencing on phagosomal maturation. Most importantly, the authors dissect the dual role of IRAP, acting both as a finishing ("trimming") peptidase and as a structural component of endosomes. Using an IRAP mutant lacking protease activity and pharmacological inhibition, they show that it is the expression of IRAP, and not its proteolytic activity, that is required for the formation of storage endosomes and for the phagosomal maturation characteristic of dendritic cells. Conversely, proteolytic activity remains required for fully efficient cross-presentation, with cells expressing inactive IRAP behaving like IRAP−/− cells.
Taken together, this work identifies IRAP as a key factor in cross-presentation, tailoring peptides to the MHC-I binding groove while protecting them from premature destruction associated with overly rapid phagosome maturation. IRAP+Rab14+ endosomes would thus exert a dual role in optimizing cross-presentation, and their availability could contribute to regulating the capacity of dendritic cells to prime CD8+ T lymphocyte responses.