Objective
The objective of this proposal is to provide a non-invasive imaging assay that monitors and characterizes immune responses that mediate islet graft rejection. Immune responses to islet graft are complex and requires coordination between diverse immune cell populations, and across many different tissues. To fully evaluate the factors that contribute to anti-graft immune cell killing, we need tools that are non-invasive and quantitative. We thus, propose to use dual-isotope SPECT imaging to monitor the whole-body distribution of innate and adaptive immune cells during allograft rejection. To achieve this objective, we are using targeted radiolabeled probes that detect T cells and macrophages. In Aim 1, we will validate the ability of probes to measure T cell and macrophage infiltration during immune-mediated graft rejection. It is important to not only visualize the distribution of these cells but also measure their activation. In Aim 2, we will develop a novel SPECT agent capable of distinguishing activated from resting macrophages. In Aim 3, we will image the interactions between macrophages and T cells, evaluate the timing of infiltration to the graft site, and measure how immunosuppression affects macrophage activation and influences the bio-distribution of both cell types. The tools developed by us can be leveraged to study the underlying mechanism of immune rejection and accelerate the development of β-cell replacement therapeutics.
Background Rationale
Islet transplant from organ donors remains the only clinically approved treatment proven to reverse insulin dependence in patients with T1D. However, within the first 3 years, the majority of patients are no longer insulin-independent. A major obstacle to long-term islet survival is the patient’s immune system. From the initial implantation, the donor islets themselves trigger the host’s inflammatory response. The innate immune system, comprised of macrophages, dendritic cells, and neutrophils rapidly surround the graft environment, becomes activated, and triggers T-cell-directed β-cell killing. To preserve graft function and survival, the clinical intervention involves the continuous suppression of the patient’s immunity. However, these treatments have failed to maintain the long-term survival of islets grafts and immune-mediated destruction persists. There is mounting evidence that macrophages are the key driver of graft rejection and their activation facilitates β-cell death. While the majority of immunosuppressive therapies target the proliferation and activation of T cells, macrophages continue to be recruited to the graft site. It is thus important to monitor and modulate anti-graft T cells response while also increasing our understanding of the impact of macrophage infiltration and activation on islet graft survival. To achieve this, tools that are capable of detecting the location of these cells in the body, monitoring how their migration change during graft transplant, and quantifying their activation, over time are needed.
Molecular imaging has been successfully used to track immune cells. Nuclear imaging using Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT), allows quantitative visual measurement of the whole-body distribution of immune cells. However, only the γ-ray detecting technology of SPECT can be used to monitor the interactions between two different immune cell populations. This approach is similar to multi-color fluorescence imaging but uses γ-rays to generate a 3-dimensional image of the location of immune cells in the body. Our goal is to use multi-isotope SPECT to image and characterize the interactions between macrophages and T cells that result in islet graft rejection. The working hypothesis governing the immune-mediated killing of graft transplant is as follows: β-cell stress triggers macrophage infiltration to the graft site upon activation, and macrophages trigger the infiltration of T cells to the graft site. We will use our multi-isotope SPECT approach to image and quantify dynamic processes over time. Furthermore, measuring T cells and macrophages at the same time may prove to be a more accurate and predictive biomarker of islet graft rejection. Non-invasive immune imaging of host innate and adaptive immune responses can further our understanding of the mechanisms of immune rejection, provide biomarkers, and inform treatment planning for targeted inventions that rescue islet grafts
Description of Project
Islet transplantation is a promising therapy for patients with Type 1 diabetes (T1D). However, the full potential of islet replacement therapy has yet to be realized due to persistent pressure of the patient’s immune system that ultimately leads to loss of ß-cell function in ~90 % of transplant recipients. There are currently no available tools that directly measure anti-graft immune responses, limiting our understanding of the mechanisms that drive islet graft rejection.
Beyond T1D, the immune system plays a critical role in many diseases, most prominently in cancer biology. The success of a new class of medicines known as immune checkpoint inhibitors (ICIs) has spurred research to develop new diagnostic imaging tools that detect immune cell distribution in the body. Nuclear imaging, an approach that uses radioactive tags (probes) that bind to cells of interest, provides a way to visualize and quantify immune cell distribution throughout the body in a living subject. Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT) are highly translatable nuclear imaging technologies and are widely used for diagnosis and monitoring patient response to therapy. To better understand the immune response to ICIs, our team has developed probes that are capable of binding specifically to T cells. These probes have been used to successfully measure tumor T cell infiltrates and image the global response of T cells to ICIs. These imaging probes can be used to monitor T-cell distribution and migration to islet graft transplants. T cells, however, are not the only immune cells that hamper long-term graft survival. Macrophages are another key contributor to islet graft rejection. It is thus important to not only monitor both T cells but also track macrophage distribution and monitor changes in macrophage activation.
To do this, we are proposing to develop a dual-isotope approach that can image macrophages and T cells at the same time in a single SPECT scan. We anticipate that dual-isotope will provide more detailed insights into the factors that lead to islet graft rejection. In this project we will visualize the interactions between T cells, resting and activated macrophages; monitor the timing of graft infiltration, and measure how each of these immune cell types responds to immunosuppression.
Anticipated Outcome
The SPECT imaging probes evaluated in this proposed work will specifically bind to T cells and macrophages, allow functional measurement of macrophage activation, and enable visualization of the interactions between macrophages and T cells. We anticipate the multi-isotope SPECT approach will allow real-time longitudinal measurement of changes in macrophage and T cell concentrations and location during islet graft rejection and how immunosuppression changes these dynamics. Lastly, we will identify the most robust dual-isotope combination (A) T cell and macrophage infiltration, (B) Macrophage infiltration and activation, or (C) macrophage activation and T cell infiltration that serves as the predictive early marker of graft rejection
Relevance to T1D
The curative potential of β-cell replacement therapies is significantly hampered by the immune cells, that infiltrate, activate, and induce β-cell death, compromise function, and facilitate transplant rejection. It is thus, critical to have diagnostic tools that allow the non-invasive, longitudinal, whole-body quantification of the immune response. To achieve this goal, we are proposing to develop imaging probes that specifically measure T cell and macrophage response to islet graft transplant. The ability to visualize the interactions between these two immune cells at the same time with multi-isotope SPECT can provide much need mechanistic insights into the timing of the global and tissue-specific immune response. In the long term, these clinically translatable imaging probes can be used to evaluate the efficacy of novel therapeutic approaches that seek to replace islet graft transplantation.