Opening opportunities for Kd determination and screening of MHC peptide complexes

X-ray spectroscopy meets native mass spectrometry: probing gas-phase protein complexes

Gas-phase activation and dissociation studies of biomolecules, proteins and their non-covalent complexes using X-rays hold great promise for revealing new insights into the structure and function of biological samples. This is due to the unique properties of X-ray molecular interactions, such as site-specific and rapid ionization. In this perspective, we report and discuss the promise of first proof-of-principle studies of X-ray-induced dissociation of native (structurally preserved) biological samples ranging from small 17 kDa monomeric proteins up to large 808 kDa non-covalent protein assemblies conducted at a synchrotron (PETRA III) and a free-electron laser (FLASH2). A commercially available quadrupole time-of-flight mass spectrometer (Q-Tof Ultima US, Micromass/Waters), modified for high-mass analysis by MS Vision, was further adapted for integration with the open ports at the corresponding beamlines. The protein complexes were transferred natively into the gas phase via nano-electrospray ionization and subsequently probed by extreme ultraviolet (FLASH2) or soft X-ray (PETRA III) radiation, in either their folded state or following collision-induced activation in the gas phase. Depending on the size of the biomolecule and the activation method, protein fragmentation, dissociation, or enhanced ionization were observed. Additionally, an extension of the setup by ion mobility is described, which can serve as a powerful tool for structural separation of biomolecules prior to X-ray probing. The first experimental results are discussed in the broader context of current and upcoming X-ray sources, highlighting their potential for advancing structural biology in the future.

RNA to Rule Them All: Critical Steps in Lassa Virus Ribonucleoparticle Assembly and Recruitment

Lassa virus is a negative-strand RNA virus with only four structural proteins that causes periodic outbreaks in West Africa. The nucleoprotein (NP) encapsidates the viral genome, forming ribonucleoprotein complexes (RNPs) together with the viral RNA and the L protein. RNPs must be continuously restructured during viral genome replication and transcription. The Z protein is important for membrane recruitment of RNPs, viral particle assembly, and budding and has also been shown to interact with the L protein. However, the interaction of NP, viral RNA, and Z is poorly understood. Here, we characterize the interactions between Lassa virus NP, Z, and RNA using structural mass spectrometry. We identify the presence of RNA as the driver for the disassembly of ring-like NP trimers, a storage form, into monomers to subsequently form higher order RNA-bound NP assemblies. We locate the interaction site of Z and NP and demonstrate that while NP binds Z independently of the presence of RNA, this interaction is pH-dependent. These data improve our understanding of RNP assembly, recruitment, and release in Lassa virus.

The Enigmatic Nature of the TCR-pMHC Interaction: Implications for CAR-T and TCR-T Engineering

The interaction of the T-cell receptor (TCR) with a peptide in the major histocompatibility complex (pMHC) plays a central role in the adaptive immunity of higher chordates. Due to the high specificity and sensitivity of this process, the immune system quickly recognizes and efficiently responds to the appearance of foreign and altered self-antigens. This is important for ensuring anti-infectious and antitumor immunity, in addition to maintaining self-tolerance. The most common parameter used for assessing the specificity of TCR-pMHC interaction is affinity. This thermodynamic characteristic is widely used not only in various theoretical aspects, but also in practice, for example, in the engineering of various T-cell products with a chimeric (CAR-T) or artificial (TCR-engineered T-cell) antigen receptor. However, increasing data reveal the fact that, in addition to the thermodynamic component, the specificity of antigen recognition is based on the kinetics and mechanics of the process, having even greater influence on the selectivity of the process and T lymphocyte activation than affinity. Therefore, the kinetic and mechanical aspects of antigen recognition should be taken into account when designing artificial antigen receptors, especially those that recognize antigens in the MHC complex. This review describes the current understanding of the nature of the TCR-pMHC interaction, in addition to the thermodynamic, kinetic, and mechanical principles underlying the specificity and high sensitivity of this interaction.

High-Throughput, Quantitative Analysis of Peptide-Exchanged MHCI Complexes by Native Mass Spectrometry

Immune monitoring in cancer immunotherapy involves screening CD8+ T-cell responses against neoantigens, the tumor-specific peptides presented by Major histocompatibility complex Class I (MHCI) on the cell surface. High-throughput immune monitoring requires methods to produce and characterize small quantities of thousands of MHCI-peptide complexes that may be tested for a patient's T-cell response. MHCI synthesis has been achieved using a photocleavable peptide that is exchanged by the neoantigen; however, assays that measure peptide exchange currently disassemble the complex prior to analysis─precluding direct molecular characterization. Here, we use native mass spectrometry (MS) to profile intact recombinant MHCI complexes and directly measure peptide exchange. Coupled with size-exclusion chromatography or capillary-zone electrophoresis, the assay identified all tested human leukocyte antigen (HLA)/peptide combinations in the nanomole to picomole range with minimal run time, reconciling the synthetic and analytical requirements of MHCI-peptide screening with the downstream T-cell assays. We further show that the assay can be "multiplexed" by measuring exchange of multiple peptides simultaneously and also enables calculation of Vc50, a measure of gas-phase stability. Additionally, MHCI complexes were fragmented by top-down sequencing, demonstrating that the intact complex, peptide sequence, and their binding affinity can be determined in a single analysis. This screening tool for MHCI-neoantigen complexes represents a step toward the application of state-of-the-art MS technology in translational settings. Not only is this assay already informing on the viability of immunotherapy in practice, the platform also holds promise to inspire novel MS readouts for increasingly complex biomolecules used in the diagnosis and treatment of disease.