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Can MP-SPR play a role in cancer vaccine target selection?

von Réka Enz

Unleashing the Power of Precision: Next-Generation Pipeline for Personalized Oncolytic Cancer Vaccines

Written by: Jonas Schäfer

Peptides in the major histocompatibility complex (MHC) I exposed on the cellular surface are the contact points between cytotoxic CD8+ T cells and tumor cells. The identification of those peptides as targets for customized therapeutic vaccines is key to generating and stimulating specific antitumor adaptive immune responses.

Yet, preparation of individual therapeutics deploying MHC-I-restricted peptides is challenging due to two factors. The identification of peptides capable to initiate an immune response, and the choice of the right excipient to raise the antitumor immune response.

A multinational group of scientists around Vincenzo Cerullo recently published a study presenting a streamlined pipeline to generate personalized cancer vaccines based on immunogenic oncolytic adenovirus which they characterized using multi-parametric SPR.

The complete journey they pursued can be divided into six phases which are well summarized and displayed in the following scheme.

Figure 1: Schematic of the proposed pipeline to generate personalized cancer vaccines based on immunopeptidomics. © Feola et al, 2022

In short, they started to isolate the MHC-I peptides from the surface of tumor cells via affinity chromatography (Step 1) and analyzed the resulting complexes by mass spectroscopy (Step 2). The list of identified peptides was evaluated in silico following two main approaches: A) using the HEX software or B) RNAseq analysis (Step 3). The functionality/immunogenicity of the 26 most promising targets was tested in mice which were pre-immunized with subcutaneous injection of the candidates (Step 4). The six peptides inducing higher frequencies of T-cell-specific response were tested for their interaction with their previously developed PeptiCRAd platform (Step 5). Finally, the oncolytic efficiency of the peptide loaded PeptiCRAd particles was addressed by injection into CT26 tumor lesions in the flanks of model mice (Step 6).

While this study presents applications of a broad methodology, we would like to focus specifically on the role of multi-parametric SPR in the validation of the peptide candidates to virus binding (Step 5).

To fully understand this step, it is necessary to know the so-called PeptiCRAd platform.
It was developed in 2016 by Capasso et al. (DOI: 10.1080/2162402X.2015.1105429). PeptiCRAd is short for peptide-coated conditionally replicating adenovirus. Thus, in contrast to the majority of adenovirus based therapeutic strategies, the tumor peptides are not expressed by the virus but are adsorbed to the viral capsid. The adsorption is caused by electrostatic interactions of the negatively charged capsid and the tumor-specific epitopes which must first be modified with a poly-lysine moiety at the N-terminus to increase the net charge.

In the current publication the characterization of this interaction was performed with an MP-SPR Navi 220A system, developed and manufactured by our partner BioNavis Ltd. To immobilize the negatively charged viral particles a standard sensor coated with 10 nm SiO2 was functionalized with APTES to present a positively charged sensor surface. MP-SPR analysis of the peptide/oncolytic adenovirus interaction is schematically illustrated in the following figure:

Figure 2: Schematic MP-SPR sensogram of a peptide/oncolytic adenovirus interaction. © Feola et al, 2022

With excitation of surface plasmons by a 670 nm laser, they were able to evaluate the affinity of the different peptide candidates to the adjuvant and furthermore addressed the number of peptides per adenovirus-particle. The corresponding sensograms for each of the peptides are presented in the next figure.

Figure 3: Exemplary MP-SPR analysis of the interactions between the immobilized adenovirus particles with the different poly-lysine-modified target peptides. The results are presented as minimum angular position (in degrees) versus time (in seconds). © Feola et al, 2022

These data show that all peptides interacted with the virus particles at equilibrium point. However, in dissociation process the number of retained peptides at the capsid surface was highest for the candidates 1, 2, 6 and 7. Based on these findings pairs of candidates were loaded to the viral particles (building PeptiCRAd 1, 2, and 3) and tested in vivo. Both in vitro and in vivo experiments showed that PeptiCRAd 1 influenced the CD8+ T-cell population and indicated that peptides 1 and 2 are the most promising targets for oncolytic therapeutics.

The study of Feola et al. highlights the unique potential of MP-SPR to performed adsorption analysis even in complex interaction systems. The adenoviruses used as ligands have an average diameter of around 100 nm, thus, these measurements wouldn´t have been possible with classic SPR due to the limitation of detection depth. Moreover, it gives an idea of the broad variety of sensor surfaces available and how these can be customized further.

In summary this publication presents a conclusive way to prepare individualized cancer vaccines based on adenovirus particles as carrier systems. The six identified peptides presented promising oncolytic activity in mice bearing different cancer lesions. Given the general difficulty in these always somewhat ambiguous selections, the results will have to be taken with the usual grain of salt.