Our immune system is always on the alert, detecting and eliminating pathogens and cancer cells. Cellular control mechanisms cause diseased cells to present antigens on their surface as signals to the immune system. To analyse the complex processes required for antigen processing and transport in real time, a German team has developed a “cage” that opens with light to release trapped antigens at a specific location and time, the journal reports. Applied Chemistry.
In our cells, both endogenous and foreign proteins are constantly broken down into small fragments and transported to the endoplasmic reticulum (ER), a branched system of membrane-enclosed channels, by the transporter associated with antigen processing (TAP). There, the supramolecular peptide loading complex PLC controls the loading of MHC I (major histocompatibility complex class I) with antigenic peptides. Certain peptides are preferentially loaded onto MHC I, further processed for immune surveillance (antigen processing), and presented on the cell surface. Peptides originating from normal endogenous proteins remain immunologically inconspicuous (unless misdirected autoimmune reactions occur).
Despite many new insights, the mechanistic principles of antigen translocation, dynamic PLC assembly and the interplay between different PLC subunits in the “quality control” of peptide-MHC complexes remain generally unknown. To further analyse antigen processing, Ralph Wieneke, Robert Tampé and their team at the University of Frankfurt am Main (Germany) have developed a photostimulated antigen delivery system that can be used to precisely study antigen flux. Antigens are released (antigen burst) on demand from a “caged” inactive state by the application of light. The advantage of light stimulation is that it can be precisely dosed at limited times and locations and is noninvasive, allowing experiments to be performed on living cells.
The team used a peptide derived from an HIV antigen as a model. They used a linker to attach the epitope (section of an antigen) to biotin and then the biotin to a bulky protein called streptavidin. In this state, the epitope is protected to the point that it can no longer be recognized by the antigen processing transporter (TAP). The linker contains a group that can be cleaved off by light. When irradiated with UV light, the peptide epitope is immediately released from its “cage.” It is then recognized by the TAP and transported across the ER membrane and loaded onto MHC I via the PLC.
This method is versatile, as demonstrated by a variety of scenarios, such as tracking antigen transport by the native human PLC TAP into the ER membrane of a human lymphoma cell line. According to Wieneke and Tampé, “Our goal is to use the light-activated system to follow the antigen processing pathway through different cellular compartments and gain insight into the kinetics of various immunological processes.” live.”