Background Adequate concentration of peptide-MHC-II complexes on the surface of antigen-presenting cells (APCs) is required to drive strong priming and activating signals for CD4+ T cells, which are extensively sensitive to antigen expression levels and its subsequent intracellular processing. Antigen that is taken up by activated APCs through endocytic vesicles are subjected to a class of proteases, cathepsins, that stochastically fragment protein material into peptides suitable for loading onto MHC-II.1 Altering the nature of the up-taken protein has been shown to greatly influence antigen processing and the resultant CD4+ T cell activation. Previous work demonstrated that fusion of a model CD4+ T cell antigen peptide to a portion of the transferrin receptor (TFR) as opposed to a cytoplasmic-localizing sequence, facilitated re-localization of the construct to endoplasmic vesicles within APCs for efficient MHC-II presentation and CD4+ T cell expansion.2–4 Additional work is required to better understand the mechanisms that drive MHC-II antigen presentation on APCs, as this would allow for 1) unprecedented resolution of how antigen processing and CD4+ T cell stimulation alters the native helper T cell response in cancer and 2) the design of thoughtful immunotherapies that leverage these natural biological phenomena.
Methods Using a GEMM of lung adenocarcinoma driven by mutant KrasG12D and loss of p53 (KP),5 we initiated tumors in mice that express CD8+ and CD4+ T cell neoantigens. Multiple constructs containing unique B, L, and S cathepsin cleavage sequences flanking the CD4+ neoantigen were installed into these tumors and the antigen-specific T cell response was assessed at 5- and 12-weeks post tumor initiation by flow cytometry and histology analysis.
Results Enhancing the antigen processing of the CD4+ T cell neoantigen with optimized cathepsin cleavage sites resulted in increased Th1 phenotypes of the antigen-specific CD4+ T cells which in turn increased the effector functions of antigen-specific CD8+ T cells (figure 1). Concordantly, there was an increased infiltration of tumor-infiltrating lymphocytes and an overall decrease in tumor burden.
Conclusions Modulating where a neoantigen is localized and how it is processed in APCs alters the activation of CD4+ T cells and the overall anti-tumor immune response. These elements may be universally installed into vaccine-based immunotherapies to improve T cell functionality.
Acknowledgements This research is funded by the Koch Institute for Integrative Cancer Research at MIT.
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