Astrocytomas are among the most common brain tumors that are usually fatal in their malignant form. They appear to progress without significant impedance from the immune system, despite the presence of intratumoral T cell infiltration. To date, this has been thought to be the result of T cell immunosuppression induced by astrocytoma-derived cytokines. Here, we propose that cell contact-mediated events also play a role, since we demonstrate the in vivo expression of

Expression of tissue-specific homing molecules directs antigen-experienced T cells to particular peripheral tissues. In studies using soluble antigens that focused on skin and gut, antigen-presenting cells (APCs) within regional lymphoid tissues were proposed to be responsible for imprinting homing phenotypes. Whether this occurs in other sites and after physiologic antigen processing and presentation is unknown. We define in vivo imprinting of distinct homing phenotypes on monospecific T cells responding to antigens expressed by tumors in intracerebral, subcutaneous, and intraperitoneal sites with efficient brain-tropism of CD8 T cells crossprimed in the cervical lymph nodes (LNs). Multiple imprinting programs could occur simultaneously in the same LN when tumors were present in more than one site. Thus, the identity of the LN is not paramount in determining the homing phenotype; this critical functional parameter is dictated upstream at the site of antigen capture by crosspresenting APCs.

Peptides presented at the cell surface reflect the protein content of the cell; those on HLA class I molecules comprise the critical peptidome elements interacting with CD8 T lymphocytes. We hypothesize that peptidomes from ex vivo tumour samples encompass immunogenic tumour antigens. Here, we uncover >6000 HLA-bound peptides from HLA-A*02(+) glioblastoma, of which over 3000 were restricted by HLA-A*02. We prioritized in-depth investigation of 10 glioblastoma-associated antigens based on high expression in tumours, very low or absent expression in healthy tissues, implication in gliomagenesis and immunogenicity. Patients with glioblastoma showed no T cell tolerance to these peptides. Moreover, we demonstrated specific lysis of tumour cells by patients' CD8(+) T cells in vitro. In vivo, glioblastoma-specific CD8(+) T cells were present at the tumour site. Overall, our data show the physiological relevance of the peptidome approach and provide a critical advance for designing a rational glioblastoma immunotherapy. The peptides identified in our study are currently being tested as a multipeptide vaccine (IMA950) in patients with glioblastoma.

Immune checkpoint blockade (ICB) is currently evaluated in patients with glioblastoma (GBM), based on encouraging clinical data in other cancers, and results from studies with the methylcholanthrene-induced GL261 mouse glioma. In this paper, we describe a novel model faithfully recapitulating some key human GBM characteristics, including low mutational load, a factor reported as a prognostic indicator of ICB response. Consistent with this observation, SB28 is completely resistant to ICB, contrasting with treatment sensitivity of the more highly mutated GL261. Moreover, SB28 shows features of a poorly immunogenic tumor, with low MHC-I expression and modest CD8 + T-cell infiltration, suggesting that it may present similar challenges for immunotherapy as human GBM. Based on these key features for immune reactivity, SB28 may represent a treatment-resistant malignancy likely to mirror responses of many human tumors. We therefore propose that SB28 is a particularly suitable model for optimization of GBM immunotherapy.