Mass Spectrometry Uncovers Hidden Bacterial Targets of Tularemia Immunity

Tularemia, the disease caused by the intracellular bacterium Francisella tularensis, has long occupied an uneasy place in infectious disease research. Although naturally acquired infections are relatively uncommon, the bacterium’s capacity to cause a severe, sometimes fatal pneumonic illness after inhalation of very few organisms has kept it firmly on lists of biodefense concerns for decades. Efforts to develop a vaccine against it have stretched back nearly a century, yet no licensed human vaccine exists today. A central obstacle has been a stubborn gap in basic knowledge: while researchers know that CD4 T cells, a key arm of adaptive immunity, contribute substantially to protection in experimental models, they have not known precisely which bacterial proteins those cells actually recognize. A new study published in PLOS Pathogens by Pavlina Laskova, Marek Link and colleagues now closes part of that gap, using an unbiased, mass spectrometry-driven approach to identify, for the first time at this scale, the molecular targets that infected immune cells display to the immune system.

The technique at the heart of the study is called immunopeptidomics, and it represents a fundamentally different way of hunting for vaccine antigens. Traditional approaches have typically started with the genome or the proteome of a pathogen and then tested candidate proteins one by one to see whether T cells respond to them. Immunopeptidomics inverts that logic. Instead of asking what a pathogen might theoretically present, it asks what infected cells actually present. When a cell is infected by an intracellular bacterium such as F. tularensis, bacterial proteins are degraded inside the cell into short peptide fragments. A subset of those fragments is loaded onto major histocompatibility complex class II, or MHC-II, molecules and shuttled to the cell surface, where they can be surveyed by CD4 T cells. By isolating these MHC-II molecules from infected cells and sequencing the peptides bound to them with high-resolution mass spectrometry, researchers obtain a direct molecular snapshot of the antigen display process itself.

In the new work, the team infected dendritic cells, the professional antigen-presenting cells of the immune system, with the live vaccine strain of F. tularensis, known as LVS. This attenuated strain has been used for decades and provides partial protection against fully virulent strains in animal models, making it a useful system for dissecting the immune response it induces. After allowing time for antigen processing and presentation, the researchers purified MHC-II peptide complexes from the infected cells and analyzed them by liquid chromatography-tandem mass spectrometry. The screen yielded 57 distinct MHC-II peptides derived from 23 different F. tularensis proteins. This is a technically demanding achievement. Bacterial peptides are vastly outnumbered on the cell surface by peptides derived from the host cell’s own proteins, so detecting dozens of pathogen-derived sequences requires careful enrichment, sensitive instrumentation, and rigorous validation to distinguish genuine bacterial ligands from background contamination.

One of the most striking findings to emerge from the peptide catalog was its composition. When the authors cross-referenced the 23 source proteins against existing proteomic data for F. tularensis, they found that most of them belong to the highly expressed fraction of the bacterial proteome. This observation carries real biological weight. Antigen presentation is not a random sampling of everything a pathogen makes; it is shaped by how much of each protein the bacterium produces, how quickly the protein is degraded inside the host cell, and how efficiently the resulting fragments fit into MHC-II binding grooves. The predominance of abundant proteins among the presented peptides suggests that the antigen display machinery preferentially samples the proteins that the bacterium relies on most heavily during infection, which is precisely the subset of molecules a vaccine would ideally target.

Identifying presented peptides, however, is only the first step. A peptide displayed on an infected cell is a candidate antigen, but whether the immune system actually mounts a measurable T cell response against it is a separate question. To address this, the researchers immunized mice with the LVS strain and then tested whether CD4 T cells from the immunized animals responded to each of the identified peptides. The readout they used was the production of interferon-gamma, a signature inflammatory cytokine released by activated CD4 T cells of the Th1 type, which is the helper subset considered most relevant for controlling intracellular bacteria. Out of the full panel of peptides, ten emerged as bona fide immunogenic epitopes, meaning they reliably triggered interferon-gamma responses in T cells from vaccinated mice. The breadth of this response was itself informative, revealing that the immune system’s attention after tularemia vaccination is spread across multiple bacterial proteins rather than concentrated on a single dominant target.

Within that set of ten epitopes, a clear hierarchy became apparent. Five peptides, derived from the antigens FTL_1678, DacD, DnaK, FTL_1498, and MinD, stood out as dominant epitopes, each eliciting a response in at least 80 percent of the immunized animals. The identities of these proteins are intriguing. DnaK is a well-characterized molecular chaperone, a highly conserved protein that helps other bacterial proteins fold correctly, and its abundance inside the bacterium likely explains both its prominence in the MHC-II peptide pool and its immunogenicity. DacD is an enzyme involved in bacterial cell wall metabolism, MinD participates in the machinery that positions the cell division plane, and FTL_1678 and FTL_1498 are proteins of less certain function annotated by their gene numbers in the F. tularensis genome. The fact that two of the five dominant targets are proteins of unknown function underscores how much of the protective immune response to this pathogen has remained invisible to researchers until now.

The logical next question was whether these newly discovered epitopes could be assembled into something therapeutically useful. The team constructed a peptide vaccine containing the dominant epitopes and tested it in mice. The results were encouraging but nuanced. The peptide formulation proved immunogenic, successfully priming CD4 T cell responses against the included epitopes. When vaccinated mice were subsequently challenged with the fully virulent F. tularensis FSC200 strain, the vaccine contributed to the early control of bacterial replication, reducing the bacterial burden during the initial phase of infection. However, the epitope cocktail alone was not sufficient to confer complete protection against the virulent challenge. This outcome is consistent with a growing recognition in the tularemia field that effective immunity against this organism likely requires the coordinated action of multiple immune mechanisms, including CD4 T cells, CD8 T cells, antibodies, and innate effector programs, and that a peptide vaccine targeting a handful of CD4 epitopes represents one component of a more complex protective recipe rather than a standalone solution.

Even with that caveat, the practical implications of the study extend well beyond the immediate vaccine result. The identified epitopes provide researchers with precise molecular reagents for the first time. With known peptide sequences and the knowledge that they are both naturally presented and immunogenic, investigators can now build tetramer or other staining tools to directly count and characterize antigen-specific CD4 T cells in blood and tissues during vaccination studies. Such tools have transformed the study of T cell biology in viral systems, where epitopes are well mapped, but have been largely unavailable for F. tularensis. The epitopes could also serve as benchmarks for comparing vaccine candidates, allowing researchers to measure whether a new formulation actually expands responses to the same protective targets that the live vaccine strain induces. In this sense, the study delivers a toolkit as much as a vaccine lead.

Methodologically, the work also offers a template that can be applied to other intracellular pathogens where the antigenic targets of protective T cells remain obscure. Immunopeptidomics sidesteps the biases of prediction algorithms, which rely on inferred binding motifs and often miss real ligands, and instead reports the actual peptide repertoire as processed by living cells. As mass spectrometry sensitivity continues to improve, the approach is likely to reveal progressively deeper layers of the pathogen-derived peptidome, including peptides from low-abundance proteins and noncanonical sources. For F. tularensis specifically, the identification of conserved and abundant proteins as dominant CD4 T cell targets raises the possibility that immune recognition focuses on functions the bacterium cannot easily alter without fitness costs, a property that would make these antigens comparatively stable vaccine components.

The study leaves open questions that will shape the next phase of research. Whether the same epitopes are presented and recognized in the context of infection with fully virulent strains, rather than the attenuated LVS used for screening, remains to be confirmed, as does their relevance to human immune responses, since the current work was conducted in mice with mouse MHC-II molecules. It also remains to be determined which combinations of epitopes, delivery formats, and adjuvants can convert the early control of bacterial replication observed here into durable, complete protection. What the study establishes, however, is a concrete starting point: a validated list of the bacterial proteins that infected cells actually display, and a demonstration that several of them drive reproducible CD4 T cell immunity. For a pathogen that has resisted vaccine development for generations, that list may prove to be the most valuable map produced in the field in years.

Subject of Research: Identification of immunogenic CD4 T cell epitopes of Francisella tularensis by MHC-II immunopeptidomics in mice

Article Title: Immunopeptidomics reveals conserved and abundant Francisella tularensis proteins as immunogenic CD4 T cell antigens in mice

Article References: Laskova, P., Porkertova, S., Ballek, O., Balonova, L., Pavloskova, J., Neuwirth, A., Pavkova, I., Fabrik, I., Lukac, P., Vannucci, L., Filipp, D., Stulik, J., & Link, M. (2026). Immunopeptidomics reveals conserved and abundant Francisella tularensis proteins as immunogenic CD4 T cell antigens in mice. PLOS Pathogens, 22(9), e1014597. https://doi.org/10.1371/journal.ppat.1014597

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014597

Keywords: Francisella tularensis, tularemia, immunopeptidomics, CD4 T cells, MHC class II, epitopes, vaccine development, mass spectrometry, dendritic cells, interferon-gamma, biodefense, intracellular bacteria