An independent study published by the MD Anderson Cancer Center in Houston TX cites using HemaCare PBMCs to take a closer look at the SARS-CoV-2 virus to identify which of its peptide sequences would make the most effective vaccine targets.
Early in the COVID-19 pandemic, the primary goal for scientists across the globe was to save lives by developing a safe, working vaccine as quickly as possible. To that end, they succeeded admirably; with several vaccines approved, doctors finally have a powerful way to fight back against this deadly disease.
Now, nearly two years later, scientists know much more about SARS-CoV-2, the virus that causes COVID-19, and about human immune responses to both the disease and the available vaccines. This knowledge has given researchers the chance to develop a new generation of COVID-19 vaccines and treatments. The goal now is to create vaccines that are effective against a wider range of viral strains and impart significantly longer lasting protection from infection and serious disease.
The research group at MD Anderson Cancer Center have been searching for a shrewder way to identify short protein sequences (peptides) from the SARS-CoV-2 virus that are capable of stimulating a strong immune response in the human body. Such sequences make exceptional targets for vaccine development.
Identification of meaningful SARS-CoV-2 target sequences is complicated. While computer algorithms have been designed to recognize potential target peptides, these peptides often fail to elicit strong T cell responses when actually put to the test. To sidestep this complication, the MD Anderson scientists hypothesized that one of the best ways to identify SARS-CoV-2 peptides capable of eliciting a strong immune response would be to isolate viral peptides bound to an immune cell’s “human leukocyte antigen (HLA) complex.
HLA is a protein complex on the immune cell’s surface that recognizes and binds to foreign antigen peptides. The HLA complex is what allows the human immune system to distinguish “self” from “non-self”. Different HLA variations exist in the human population, and it is these differences that determine whether immune cells can be donated between one person and the next.
The research team knew that once HLA-bound SARS-C0V-2 peptides were isolated, their sequences could be analyzed directly using mass spectrometry. The immunogenicity of the identified peptides could then be evaluated against cytotoxic T cells in culture.
Healthy donor PBMCs representing four different HLA types were acquired from HemaCare as a source of antigen-presenting cells and T cells. The capacity to secure donors with unique HLA subtypes would be fundamental to the analysis of any peptide-specific immune cell response.
To begin their study, the scientists used two highly conserved peptides from the SARS-CoV-2 virus that are not located on the viral surface. They purposely chose internal viral peptides that could only be presented to the immune system once the virus was fragmented and the peptide fragments became bound to immune cell HLAs. For the purpose of this study, the research team chose SARS-CoV-2 peptides from membrane glycol-protein (MGP) and non-structure protein-13 (NSP13).
The two chosen SARS-CoV-2 peptides were overexpressed in healthy donor PBMCs representing four different HLA types that are collectively expressed in 75% of the general population. Following expansion of the SARS-CoV-2 peptide expressing cells in culture, HLA complexes were isolated via immunoprecipitation, after which HLA-bound peptides were eluted from the immune complexes. The detached peptides were then analyzed by mass spectrometry to determine their sequence.
For the second part of the study, the researchers chose five candidate HLA derived peptide sequences, based on their suitability as predicted by sequence analysis, and proceeded to examine their immunogenicity. The immunogenicity of the five peptides would be validated on the basis of their ability to promote a peptide specific T cell response.
Mature, antigen-presenting dendritic cells were derived from healthy donor PBMCs that had never been exposed to the SARS-CoV-2 virus. After being stimulated with each of the candidate peptides, these antigen-presenting cells successfully induced the generation of peptide-specific cytotoxic T cells, as validated by flow cytometry. Following purification and expansion, each of the peptide-specific cytotoxic T cells lines were capable of recognizing and killing SARS-CoV2-expressing target cells.
This data represents proof-of-concept for mass-spectrometry enabled identification of highly conserved SARS-CoV-2 peptides derived from HLA complex elution coupled to direct validation of immunogenicity through the generation of SARS-CoV-2 specific cytotoxic T cells. Highly conserved SARS-CoV-2 peptides with proven immunogenicity should make excellent candidates for second generation vaccine development.
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- Pan K. et al. Immunogenic SARS-CoV2 Epitopes Defined by Mass Spectrometry. BioRxiv. Jul 2021.