Comparing Tumor-Specific CD8+ T Cells in the Bone Marrow with Tumor Infiltrating Lymphocytes: Implications for Adoptive Cell Therapy in Solid Tumors

Abstract

The variety of diseases encompassed by the term “cancer” result in substantial global disease burden, comprising one of the leading causes of premature mortality. Immunotherapy, comprised largely of immune checkpoint blockade and T cell therapy, has progressed immensely in recent years, becoming an integral strategy in cancer care. Strategies employing adoptive cell therapy (ACT) utilize cells derived from a patient or from a donor to develop a cellular product for infusion. Current ACT approaches have seen the establishment of chimeric antigen receptor-T cells (CAR-T cells) using peripheral blood lymphocytes as well as tumor infiltrating lymphocytes (TILs) with significant clinical results. Alongside marked progress, the limitations of current strategies are also emerging. Notably, current ACT modalities face challenges of persistence following infusion and duration of clinical responses. TIL therapy specifically requires candidate patients to be at an advanced disease stage, decreasing the likelihood of a positive outcome following therapeutic intervention. Additionally, although TILs have seen clinical success in limited settings, they are also a dysfunctional population of T cells. As the quality of an initial T cell population has been linked to improved outcomes following adoptive transfer, this implies that TILs may be a sub-optimal T cell source for broad clinical applications. Due to these limitations, novel approaches are needed. The bone marrow (BM) is an immunological niche that houses T cells with specificity for previously encountered antigens, including tumor-associated antigens from certain solid cancers. BM T cells possess an enhanced reactivity to a variety of hematologic and solid cancers as compared to T cells from the peripheral blood. As TILs are an approved treatment modality for at least two types of solid tumors, t¬¬his project sought to improve our understanding of the biology of tumor-specific BM T cells in the context of solid cancer by comparing them with TILs. In doing so, we intended to further the rationale for using the BM as a source of T cells for ACT against solid malignancies. Herein, we used the B16.OVA murine melanoma model to profile tumor specific CD8+ T cells from the BM and contrast them with TILs. Through immunophenotyping via polychromatic flow cytometry, bulk RNA-sequencing, and a combination of in vitro and in vivo assays, we demonstrate that T cells from the BM exhibit characteristics that could make them a superior source of cells for cellular therapy as compared to TILs; these include a stem-like memory phenotype, improved effector function, enhanced persistence within a tumor-bearing host, and increased tumor infiltration. Additionally, our studies indicate that the BM appears to maintain a stable population of tumor-specific T cells during disease progression. If this holds true in humans, this could make the BM a reservoir of tumor-specific T cells that is accessible in and harvestable from all patients, even at early, pre-metastatic stages of disease. Collectively, the data presented here provide a foundation for further exploring the BM as a source of tumor-specific T cells for ACT in solid malignancies