University of Massachusetts investigators have identified 25 new naturally processed peptide sequences, derived from 6 distinct HHV-6B proteins, using mass spectrometry. The peptides were able to elicit T-cell responses in vitro, with responding cells producing IFN-γ and TNF-α and displaying a cytotoxic effector function.
Previous HHV-6 viral peptides have been identified by either prediction of MHC-peptide binding or by analysis of sequences within known immunogenic proteins. Unfortunately, only 2-3% of peptides identified by these means were recognized by CD4 T-cells in multiple donors. The Massachusetts group used mass spectrometry of MHC-bound peptides eluted from infected cells to identify the new HHV-6B specific peptides. This method has been used previously to identify antigens for EBV, HSRV, measles, vaccinia, influenza, dengue, West Nile, and hepatitis C and B, but never for HHV-6. While this method is efficient, the authors note that some epitopes were likely missed. These could include HHV-6B-derived peptides that were present in the studied in vitro infected cells but which were below the detection limit of the method or epitopes that are generated in vivo in the antigen-presenting cells APCs relevant during T-cell priming or maintenance in primary and chronic infections.
Functional studies of the recognized peptide epitopes revealed broadly reactive polyfunctional and cytotoxic CD4 T-cell responses. While every peptide was reactive to multiple donors, the responses varied dramatically based on various HLA alleles. A pool of six of the peptides were used to capture the specific T-cell response in HLA-DRB1*03:01 donors specifically. T-cells from these donors, expanded with the pool of peptides, mainly produced the cytokines IFN-γ and TFN-α and displayed a cytotoxic phenotype, with some of them storing granzyme B and able to kill target cells pulsed with the virus derived peptide epitopes.
These identified peptides can be added to the growing list of the HHV-6 viral peptidome alongside other MHC class I (MHC-I) and MHC class II (MHC-II) epitopes derived from HHV-6A/B infected cells that have already been published (Tejada-Simon 2003, Nastke 2012, Martin 2012, Gerdemann 2013, Iampietro 2014, Becerra-Artiles 2015, Martin 2018).
The identification of reactive HHV-6 viral peptides is vital in deciphering the way this virus interacts with our immune system. Characterizing how our T-cells and the viral peptidome interact will likely also be essential in developing more efficient testing and treatment of HHV-6.
Since viral peptide sequences presented by major histocompatibility complexes are the major mechanism by which our T-cells recognize an infected cell, the identification of HHV-6 peptides that interact with these complexes and produce a subsequent T-cell response is imperative.
To read the full paper, click here: Becerra-Artiles et al. 2019.