Malgorzata Garstka

Controlled intracellular release of peptides from microcapsules enhances antigen presentation on MHC class I molecules.

To understand the time course of action of any small molecule inside a single cell, one would deposit a defined amount inside the cell and initiate its activity at a defined moment. An elegant way to achieve this is to encapsulate the molecule in a micrometer-sized reservoir, introduce it into a cell, remotely open its wall by a laser pulse, and then follow the biological response by microscopy. The validity of this approach is validated here using microcapsules with defined walls that are doped with metallic nanoparticles so as to enable them to be opened with an infrared laser. The capsules are loaded with a fluorescent antigenic peptide and introduced into mammalian cultured cells where, upon laser-induced release, the peptide binds to major histocompatibility complex (MHC) class I proteins and elicits their cell surface transport. The concept of releasing a drug inside a cell and following its action is applicable to many problems in cell biology and medicine.

Calreticulin‐dependent recycling in the early secretory pathway mediates optimal peptide loading of MHC class I molecules

Calreticulin is a lectin chaperone of the endoplasmic reticulum (ER). In calreticulin‐deficient cells, major histocompatibility complex (MHC) class I molecules travel to the cell surface in association with a sub‐optimal peptide load. Here, we show that calreticulin exits the ER to accumulate in the ER–Golgi intermediate compartment (ERGIC) and the cis‐Golgi, together with sub‐optimally loaded class I molecules. Calreticulin that lacks its C‐terminal KDEL retrieval sequence assembles with the peptide‐loading complex but neither retrieves sub‐optimally loaded class I molecules from the cis‐Golgi to the ER, nor supports optimal peptide loading. Our study, to the best of our knowledge, demonstrates for the first time a functional role of intracellular transport in the optimal loading of MHC class I molecules with antigenic peptide.

Peptide-receptive major histocompatibility complex class I molecules cycle between endoplasmic reticulum and cis-Golgi in wild-type lymphocytes.

Prior to binding to a high affinity peptide and transporting it to the cell surface, major histocompatibility complex class I molecules are retained inside the cell by retention in the endoplasmic reticulum (ER), recycling through the ER-Golgi intermediate compartment and possibly the cis-Golgi, or both. Using fluorescence microscopy and a novel in vitro COPII (ER-to-ER-Golgi intermediate compartment) vesicle formation assay, we find that in both lymphocytes and fibroblasts that lack the functional transporter associated with antigen presentation, class I molecules exit the ER and reach the cis-Golgi. Intriguingly, in wild-type T1 lymphoma cells, peptide-occupied and peptide-receptive class I molecules are simultaneously exported from ER membranes with similar efficiencies. Our results suggest that binding of high affinity peptide and exit from the ER are not coupled, that the major histocompatibility complex class I quality control compartment extends into the Golgi apparatus under standard conditions, and that peptide loading onto class I molecules may occur in post-ER compartments.

Tapasin dependence of major histocompatibility complex class I molecules correlates with their conformational flexibility

Major histocompatibility complex (MHC) class I molecules present cell internally derived peptides at the plasma membrane for surveillance by cytotoxic T lymphocytes. The surface expression of most class I molecules at least partially depends on the endoplasmic reticulum protein, tapasin, which helps them to bind peptides of the right length and sequence. To determine what makes a class I molecule dependent on support by tapasin, we have conducted in silico molecular dynamics (MD) studies and laboratory experiments to assess the conformational state of tapasin‐dependent and ‐independent class I molecules. We find that in the absence of peptide, the region around the F pocket of the peptide binding groove of the tapasin‐dependent molecule HLA‐B∗44:02 is in a disordered conformational state and that it is converted to a conformationally stable state by tapasin. This novel chaperone function of tapasin has not been described previously. We demonstrate that the disordered state of class I is caused by the presence of two adjacent acidic residues in the bottom of the F pocket of class I, and we suggest that conformational disorder is a common feature of tapasin‐dependent class I molecules, making them essentially unable to bind peptides on their own. MD simulations are a useful tool to predict such conformational disorder of class I molecules.—Garstka, M. A., Fritzsche, S., Lenart, I., Hein, Z., Jankevicius, G., Boyle, L. H., Elliott, T., Trowsdale, J., Antoniou, A. N., Zacharias, M., Springer, S. Tapasin dependence of major histocompatibility complex class I molecules correlates with their conformational flexibility. FASEB J. 25, 3989–3998 (2011). www.fasebj.org

The mechanism of action of tapasin in the peptide exchange on MHC class I molecules determined from kinetics simulation studies

To understand the mechanism of action of the chaperone protein tapasin, which mediates loading of high-affinity peptides onto major histocompatibility complex (MHC) class I molecules in the antiviral immune response, we have performed numerical simulations of the class I-peptide binding process with four different mechanistic hypotheses from the literature, and tested our predictions by laboratory experiments. We find - in agreement of experimental and theoretical studies - that class I-peptide binding in cells is generally under kinetic control, and that tapasin introduces partial thermodynamic control to the process by competing with peptide for binding to class I. Based on our results, we suggest further experimental directions.