In a recent study published in the journal Nature Medicine, researchers examine clinical glioblastoma and benign intracranial samples to determine the presence and function of immune cells in the brain.
Study: Cranioencephalic functional lymphoid units in glioblastoma. Image Credit: Gorodenkoff / Shutterstock.com
The neuro-immune barrier
Historically, the brain has been perceived as an ‘immune-privileged’ organ, in which little immunological activities occur within the brain. More recently, researchers have identified the presence of both innate and adaptive immune cells within the choroid plexus, meninges, and dural sinuses. The presence of immune cells at this interface between the central nervous system (CNS) and the rest of the body allows information to be transmitted from the brain through interstitial, cerebrospinal, and lymphatic fluids.
The disruption of the neuro-immune barrier may be implicated in malignant diseases, such as glioblastoma; however, immune checkpoint inhibitors have been associated with limited efficacy in treating glioblastomas. Systemic immunosuppression and intrinsic, adaptive, and acquired immunotherapy resistance may prevent these immunotherapies from successfully reaching brain tumors.
About the study
The present study examined immune cell populations present within cranial bone marrow samples to determine the prevalence and functions of these cells.
Clinical samples were obtained from patients diagnosed with grade four isocitrate dehydrogenase (IDH)-wildtype glioblastoma who did not have a history of chemotherapy or radiation treatment. Postsurgical computed tomography (CT) scans were obtained within 24 hours of sample collection, whereas magnetic resonance imaging (MRI) scans were obtained within 72 hours. Clinical CXC chemokine receptor 4 (CXCR4) radiolabeling was also combined with positron emission tomography-CT (PET-CT) imaging data to visualize and quantify the presence of CXCR4-expressing cells within the cranial bone and tumor tissues.
Primary tissue cultures and single-cell suspensions were isolated from resected tumor tissue, bone, and blood samples. Within the cell suspensions, clusters of differentiation 45 (CD45+), CD4+, and cytotoxic CD8+ T-cells, CD4+ regulatory T (Treg) cells, and mucosal-associated invariant (MAIT) cell levels were measured.
Magnetically activated cell sorting was also performed to isolate CD45+ immune cells from craniotomy-derived bone, peripheral blood mononuclear cells, and glioblastoma tissue. Within-individual proliferative potential of T-cells and tumor reactiveness was determined using expanded cytotoxic T-cells from the tumor, peripheral blood, and cranial bone samples.
Study findings
A total of 19 glioblastoma patient samples were analyzed and compared with samples obtained from five patients with non-malignant intracerebral disease. Active lymphoid tissue populations were identified in cranial bone marrow at the initial diagnosis of glioblastoma tumors.
The combination of CXCR4 radiolabeling with PET-CT imaging was found to be highly effective for investigating immune cell dynamics within brain tissues. High concentrations of CXCR-expressing cytotoxic T-cells were observed within cranial bone marrow, particularly in regions adjacent to glioblastoma tissue. The increased presence of these T-cells surrounding the tumor, rather than within the tumor itself, suggests that these cells may be more actively involved in immune surveillance activities and initiating responses from the bone marrow.
Likewise, tumor-reactive CD8+ T-cells were present at higher concentrations in cranial bone regions near the tumor, thus indicating an active immune response in this area. These cells were positive for sphingosine-1-phosphate receptor 1 (SRPR1), which may indicate a mechanism by which these T-cells left lymphoid organs to reach the cranial environment.
Cranial bone cytotoxic T-cells exhibited higher major histocompatibility complex (MHC)-based tumor reactivity than matching tumor or peripheral blood samples. This observation suggests that tumor-reactive cytotoxic T-cells were more prevalent in the proximal cranial bone. Other immune cells identified within cranial bone marrow samples included CD4+ and CD8+ T-cells, Tregs, and MAIT cells, all exhibiting varying levels of tumor reactivity and persistence in the cranial bone and tumor environments.
Further investigation of the cranial bone subspace showed the entire developmental spectrum of T-cells, ranging from naïve to fully differentiated effector cells. Through the use of diffusion mapping techniques, the observed T-cell differentiation trajectories suggest a well-developed immune response within this region of the cranial bone.
Despite the observation that B-cells are present within glioblastoma tissue, single-cell gene signatures were not detected. The lack of sufficient structural organization of B-cells within these tumors indicates that these immune cells may not be actively involved in the glioblastoma immune response.
Conclusions
The study findings provide important insights into the dynamics of T-cell populations, particularly cytotoxic and memory T-cells, within the brain and how they function in malignant environments. Future studies are needed to gain a deeper understanding of the immune landscape within glioblastomas to ultimately develop targeted and more effective treatments. Utilizing CCCR4 radiolabeling in PET-CT imaging also offers a novel approach to visualize and monitor immune cell dynamics in glioblastoma patients during treatment.
Journal reference:
- Dobersalske, C., Rauschenbach, L., Hua, Y. et al. (2024). Cranioencephalic functional lymphoid units in glioblastoma. Nature Medicine. doi:10.1038/s41591-024-03152-x
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