Tracking antigen specific T-cells: Technological advancement and limitations

Nanoscale organization of two-dimensional multimeric pMHC reagents with DNA origami for CD8+ T cell detection

Peptide-MHC (pMHC) multimers have excelled in the detection of antigen-specific T cells and have allowed phenotypic analysis using other reagents, but their use for detection of low-affinity T cells remains a challenge. Here we develop a multimeric T cell identifying reagent platform using two-dimensional DNA origami scaffolds to spatially organize pMHCs (termed as dorimers) with nanoscale control. We show that these dorimers enhance the binding avidity for low-affinity antigen-specific T cell receptors (TCRs). The dorimers are able to detect more antigen-specific T cells in mouse CD8⁺ T cells and early-stage CD4⁺CD8⁺ double-positive thymocytes that express less dense TCRs, compared with the equivalent tetramers and dextramers. Moreover, we demonstrate dorimer function in the analysis of autoimmune CD8⁺ T cells that express low-affinity TCRs, which are difficult to detect using tetramers. We anticipate that dorimers could contribute to the investigation of antigen-specific T cells in immune T cell function or immunotherapy applications.

MHC-peptide multimers are important reagents for detecting antigen specific T cells. Here the authors show that DNA scaffolds can be used to make MHC-peptide multimers and the avidity controlled so that low abundance or T cells with low affinity TCR can be detected using these reagents.

Rapid and efficient generation of antigen‐specific isogenic T cells from cryopreserved blood samples

Clustered regularly interspaced short palindromic repeats/CRISPR‐associated protein 9 (CRISPR/Cas9)‐mediated gene editing has been leveraged for the modification of human and mouse T cells. However, limited experience is available on the application of CRISPR/Cas9 electroporation in cryopreserved T cells collected during clinical trials. To address this, we aimed to optimize a CRISPR/Cas9‐mediated gene editing protocol compatible with peripheral blood mononuclear cells (PBMCs) samples routinely produced during clinical trials. PBMCs from healthy donors were used to generate knockout T‐cell models for interferon‐γ, Cbl proto‐oncogene B (CBLB), Fas cell surface death receptor (Fas) and T‐cell receptor (TCRαβ) genes. The effect of CRISPR/Cas9‐mediated gene editing on T cells was evaluated using apoptosis assays, cytokine bead arrays and ex vivo and in vitro stimulation assays. Our results demonstrate that CRISPR/Cas9‐mediated gene editing of ex vivo T cells is efficient and does not overtly affect T‐cell viability. Cytokine release and T‐cell proliferation were not affected in gene‐edited T cells. Interestingly, memory T cells were more susceptible to CRISPR/Cas9 gene editing than naïve T cells. Ex vivo and in vitro stimulation with antigens resulted in equivalent antigen‐specific T‐cell responses in gene‐edited and untouched control cells, making CRISPR/Cas9‐mediated gene editing compatible with clinical antigen‐specific T‐cell activation and expansion assays. Here, we report an optimized protocol for rapid, viable and highly efficient genetic modification in ex vivo human antigen‐specific T cells, for subsequent functional evaluation and/or expansion. Our platform extends CRISPR/Cas9‐mediated gene editing for use in gold‐standard clinically used immune‐monitoring pipelines and serves as a starting point for development of analogous approaches, such as those including transcriptional activators and/or epigenetic modifiers.

In vivo detection of antigen-specific CD8+ T cells by immuno-positron emission tomography

The immune system’s ability to recognize peptides on major histocompatibility molecules (pMHCs) contributes to eradication of cancers and pathogens. Tracking these responses in vivo could help evaluate the efficacy of immune interventions and improve mechanistic understanding of immune responses. We employ synTacs, dimeric pMHC scaffolds of defined composition, which enable clonal-selective delivery of a variety of signaling, recruitment, and imaging modalities. We show that synTacs, when labeled with positron-emitting isotopes, can non-invasively image antigen-specific CD8 T cells in vivo. We imaged human papillomavirus (HPV16) E7-specific CD8 T cells by positron emission tomography with an HPV16 E7 peptide-loaded synTac in HPV16-positive tumors, following administration of a therapeutic vaccine. We also imaged influenza A virus (IAV) nucleoprotein-specific CD8 T cells in the lungs of IAV-infected mice, using an isotopically labeled flu-specific synTac. It is thus possible to visualize antigen-specific CD8 T cell populations in vivo, which may serve prognostic and diagnostic roles.

Landscape mapping of shared antigenic epitopes and their cognate TCRs of tumor-infiltrating T lymphocytes in melanoma

HLA-restricted T cell responses can induce antitumor effects in cancer patients. Previous human T cell research has largely focused on the few HLA alleles prevalent in a subset of ethnic groups. Here, using a panel of newly developed peptide-exchangeable peptide/HLA multimers and artificial antigen-presenting cells for 25 different class I alleles and greater than 800 peptides, we systematically and comprehensively mapped shared antigenic epitopes recognized by tumor-infiltrating T lymphocytes (TILs) from eight melanoma patients for all their class I alleles. We were able to determine the specificity, on average, of 12.2% of the TILs recognizing a mean of 3.1 shared antigen-derived epitopes across HLA-A, B, and C. Furthermore, we isolated a number of cognate T cell receptor genes with tumor reactivity. Our novel strategy allows for a more complete examination of the immune response and development of novel cancer immunotherapy not limited by HLA allele prevalence or tumor mutation burden.

The immune system is the body’s way of defending itself, offering protection against diseases such as cancer. But to remove the cancer cells, the immune system must be able to identify them as different from the rest of the body. All cells break down proteins into shorter fragments, known as peptides, that are displayed on the cell surface by a protein called human leukocyte antigen, HLA for short. Cancer cells display distinctive peptides on their surface as they generate different proteins to those of healthy cells. Immune cells called T cells use these abnormal peptides to identify the cancer so that it can be destroyed.

Sometimes T cells can lack the right equipment to detect abnormal peptides, allowing cancer cells to hide from the immune system. However, T cells can be trained through a treatment called immunotherapy, which provides T cells with new tools so that they can spot the peptides displayed by HLA on the previously ‘hidden’ cancer cells.

There are many different forms of HLA, each of which can display different peptides. Current research in immunotherapy commonly targets only a subset of HLA forms, and not all cancer patients have these types. This means that immunotherapy research is only likely to be of most benefit to a limited number of patients. Immunotherapy could be made effective for more people if new cancer peptides that are displayed by the other ‘under-represented’ forms of HLA were identified.

Murata, Nakatsugawa et al. have now used T cells that were taken from tumors in eight patients with melanoma, which is a type of skin cancer. A library of fluorescent HLA-peptides was generated – using a new, simplified methodology – with 25 forms of HLA that displayed over 800 peptides. T cells were then mixed with the library to identify which HLA-peptides they can target. As a result, Murata, Nakatsugawa et al. found the cancer targets of around 12% of the tumor-infiltrating T cells tested, including those from under-represented forms of HLA. Consequently, these findings could be used to develop new immunotherapies that can treat more patients.