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. 2021 Jul 29;10(8):956.
doi: 10.3390/pathogens10080956.

Immunodominant Cytomegalovirus Epitopes Suppress Subdominant Epitopes in the Generation of High-Avidity CD8 T Cells

Kirsten Freitag  1 , Sara Hamdan  1 , Matthias J Reddehase  1 , Rafaela Holtappels  1
Affiliations

Immunodominant Cytomegalovirus Epitopes Suppress Subdominant Epitopes in the Generation of High-Avidity CD8 T Cells

Kirsten Freitag et al. Pathogens. .

Abstract

CD8+ T-cell responses to pathogens are directed against infected cells that present pathogen-encoded peptides on MHC class-I molecules. Although natural responses are polyclonal, the spectrum of peptides that qualify for epitopes is remarkably small even for pathogens with high coding capacity. Among those few that are successful at all, a hierarchy exists in the magnitude of the response that they elicit in terms of numbers of CD8+ T cells generated. This led to a classification into immunodominant and non-immunodominant or subordinate epitopes, IDEs and non-IDEs, respectively. IDEs are favored in the design of vaccines and are chosen for CD8+ T-cell immunotherapy. Using murine cytomegalovirus as a model, we provide evidence to conclude that epitope hierarchy reflects competition on the level of antigen recognition. Notably, high-avidity cells specific for non-IDEs were found to expand only when IDEs were deleted. This may be a host's back-up strategy to avoid viral immune escape through antigenic drift caused by IDE mutations. Importantly, our results are relevant for the design of vaccines based on cytomegaloviruses as vectors to generate high-avidity CD8+ T-cell memory specific for unrelated pathogens or tumors. We propose the deletion of vector-encoded IDEs to avoid the suppression of epitopes of the vaccine target.

Keywords: CD8 T cells; antigen presentation; antigenic peptides; cytomegalovirus; epitope(s); immunodominance; immunotherapy; protective immunity; vaccine design.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript or in the decision to publish the results.

Figures

Figure 1
Figure 1
Cytofluorometric determination of viral epitope-specific CD8+ T-cell subpopulations in the time course after intraplantar infection of immunocompetent BALB/c mice with mCMV-BACW. (A) Spleen, representing a lymphoid site. (B) Lungs, representing an extra-lymphoid site. (Top panels), gating strategy to define and quantitate viral epitope-specific subpopulations iTEM, DPEC, EEC, cTEM, and TCM. (Bottom panels), time course of the response to IDEs IE1 and m164, as well as non-IDE m18, separated into the subpopulations indicated. To average individual variation, cells isolated from 5–10 mice per time, depending on cell yield, were pooled for the analysis.
Figure 2
Figure 2
Impact of IDE antigenicity deletion on the long-term response to non-IDE m18 by CD8+ T-cell subpopulations iTEM, DPEC, EEC, cTEM, and TCM. Intraplantar infection was performed with recombinant mCMVs lacking or expressing functional IDEs, that is viruses ΔIDE and rev-IDE, respectively. To average individual variation, cells isolated from 5–10 mice per time, depending on cell yield, were pooled for the analysis.
Figure 3
Figure 3
Detection and avidity distributions of epitope-specific IFNγ+CD8+ T cells. Immunocompetent BALB/c mice were infected via the intraplantar route with viruses ΔIDE (gray-shaded bars) or rev-IDE (black bars). At 40 weeks after infection, functional CD8+ T cells specific for the indicated four IDEs were quantitated based on IFNγ secretion in an ELISpot assay. Stimulation in the assay was achieved by P815 mastocytoma cells exogenously loaded with the respective synthetic peptides at the loading concentrations indicated. (Left column) Cumulative avidity plots. Bars represent the most probable numbers (MPN) of cells responding in the assay, as calculated by intercept-free linear regression. MPN values sum up all cells that respond to ≤ the test concentration indicated. Error bars represent the 95% confidence intervals. EC50 values were calculated from the MPN and the upper and lower confidence limit values. (Right column) Avidity distributions deduced from the cumulative avidities by plotting the response increments. The dotted line operationally separates cells with non-protective avidity, equivalent to >10−9 M peptide loading concentration (to the left), from cells with protective avidity, equivalent to ≤10−9 M peptide loading concentration (to the right). To average individual variation, cells isolated from 10 mice were pooled for the analysis.
Figure 4
Figure 4
Avidity distributions of functional IFNγ+CD8+ T cells specific for non-IDE m18, depending on the absence or presence of IDEs. Immunocompetent BALB/c mice were infected via the intraplantar route with viruses ΔIDE (gray-shaded bars) or rev-IDE (black bars), respectively. At the indicated times after infection, CD8+ T cells specific for non-IDE m18 were quantitated based on IFNγ secretion in an ELISpot assay. Stimulation in the assay was achieved by P815 mastocytoma cells exogenously loaded with synthetic m18 peptide at the loading concentrations indicated. (Left column) Cumulative avidity plots and EC50 values. (Right column) Avidity distributions. For further details, see the legend to Figure 3. The dotted line operationally separates cells with non-protective avidity, equivalent to >10−9 M peptide loading concentration (to the left), from cells with protective avidity, equivalent to ≤10−9 M peptide loading concentration (to the right). To average individual variation, cells isolated from 5–10 mice, depending on cell yield, were pooled for the analysis.
Figure 5
Figure 5
Avidity distributions of CD8+ T cells specific for a panel of non-IDEs dependent upon the absence or presence of non-IDEs. For more explanation, see the legends of Figure 3 and Figure 4.
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