Adoptive Lymphocyte Transfer as an Adjunct to T Cell Activating Therapies when Treating Brain Tumors

Abstract

Brain tumors arise in a unique environment, shielded by the restrictive blood-brain/tumor barrier, and embedded within an immunosuppressive micro-environment. As a result of these significant challenges, treatment outcomes for patients with any type of malignant brain cancer are poor. Just 33% of malignant brain cancer patients survive for more than 5 years following diagnosis. Of tumors that directly arise in the brain (i.e., primary), glioblastoma is the most common and most aggressive type, with even worse outcomes. A patient with newly diagnosed glioblastoma survives on average for just 12-15 months post diagnosis. Recurrence is universal and the disease remains uniformly lethal. This is despite a highly aggressive treatment regimen of maximal surgical resection, alkylating chemotherapy and radiotherapy. Despite advances in surgical techniques and newer therapies like tumor-treating fields and dendritic cell vaccines, no substantial improvements in survival have been achieved. Disappointingly, glioblastoma has also proven resistant to immunotherapeutic approaches which have been transformative in other hard-to-treat malignancies. While immune checkpoint inhibitors (ICIs) have revolutionized the treatment landscape for other malignancies like melanoma and lung cancer, trials of efficacy have been unsuccessful in glioblastoma. It has become increasingly clear that any new treatment must address glioblastoma’s many overlapping mechanisms of resistance and protection. In addition to the previously mentioned restrictive blood-brain barrier and immunosuppressed tumor micro-environment, tumors of the Central Nervous System (CNS) induce sequestration of lymphocytes in the bone marrow, further reducing the population of immune cells that can be directed against tumor. Glioblastoma also exhibits extraordinary intra-tumoral heterogeneity, making simultaneous targeting of the entire tumor near-impossible to achieve. To address the urgent clinical need for change, innovative new approaches that address multiple aspects of treatment resistance are required. This dissertation will outline an entirely new combination treatment paradigm, in which T cell activating therapies are co-administered following adoptive transfer of autologous lymphocytes (Adoptive Lymphocyte Transfer or ALT). This combination harnesses the exquisite potency of T cell engagers, which can activate lymphocytes in the presence of tumor at picomolar concentrations, while also ensuring that the tumor micro-environment is populated by functional immune cells. Repopulation via this combination is achieved using immune cells which do not have pre-induced antigen specificity, and which can therefore interact with endogenous antigen presenting cells that have processed tumor neo-antigens. Chapter 1 provides an overview of novel T cell activating therapies, describing the ongoing development of a brain bi-specific T cell engager targeting the glioblastoma specific antigen EGFRvIII. Chapter 2 outlines the processes of T cell entry into the brain in both the healthy and pathological state, as well as describing markers of interest in facilitating T cell entry. Chapter 3 reports on our new findings of using IL-7 to generate a T cell product that is capable of accumulating in established intracranial tumors, and how pre-administration of these autologous lymphocytes enhances the anti-tumor efficacy of T cell activating therapies. We also include preliminary assessments of safety when using this combination. In Chapters 5 and 6 we report the findings of our mechanistic analyses to elucidate how IL-7 boosts the ability of T cells to accumulate in intracranial tumors. We find that IL-7 enhances the expression of VLA-4, a key migratory integrin involved in T cell entry across the blood-brain barrier. We also establish that expression of VCAM-1 (the endothelial binding partner for VLA-4) is increased in the context of intracranial murine glioma. Additional mechanistic analysis via bulk RNA sequencing reveals that IL-7 increases transcription of genes involved in migratory integrin expression including CD9. We also find that IL-7 increases transcription of S1PR1 encoding for Sphingosine-1-Phosphate Receptor 1 (also known as S1P1). This G Protein-Coupled Receptor (GPCR) is lost from the surface of T cells during tumor induced sequestration of lymphocytes in bone marrow, and its restoration is protective against this phenomenon. Finally in Chapters 7 and 8, we outline a clinical dosing strategy and a first-in-human trial design for our brain bi-specific T cell engager (BRiTE), which we will evaluate first as a monotherapy, before subsequently combining with ALT. Key objectives of this proposed study include assessing safety, drug pharmacokinetics and measuring preliminary indicators of efficacy. In summary, we describe an entirely novel treatment approach that delivers highly potent T cell engaging therapies to the brain, while also ensuring the presence of non-specific functional T cells that can be redirected against tumor. This work will form the basis for a clinical trial evaluating combination T cell activating and adoptive lymphocyte therapy in patients with newly diagnosed or recurrent glioblastoma.