Imaging of immune cell trafficking patterns refines development of cell-based therapies
Authors: Christopher Contag
Journal: Clinical Cancer Research (2008)

ED04-01   The unique capabilities of immune cells to traffic to tumor targets  in vivo  and recognize malignant cells offer significant opportunities for the development of new therapeutic strategies for treating cancer. The graft vs. tumor effect in bone marrow transplantation is well recognized and there have been advances in both understanding and utilizing this effect in control of disease progression. Despite these advances, effective use of immune cell therapy for control tumor growth has not been achieved.  In vivo  imaging approaches can be used to monitor cell survival, assess  in vivo  trafficking patterns and assess the effects of immune cell therapy on tumor targets (1, 2). Revealing immune cell behavior in the living body through imaging will lead to insights that can be used to refine immune cell therapies and develop new approaches for control of malignancy.  Given the complexity of immune cell trafficking and the use of these patterns to deliver other therapies, and the requirement that delivery methods be evaluated in living animals, effective development of these approaches requires methods of study that refine animal models of cancer and accelerate their analyses. The tools being developed in the field of molecular imaging have addressed this previously unmet need and have enhanced the discovery and development of novel therapies for cancer. These imaging methods utilize molecular probes to label the cancer cell, the immune cell delivery vehicle and/or the therapeutic agent such that their numbers and location can be determined in animal models over time (3). Such imaging tools offer the power of repeated measures without sacrificing the animal yielding improved data while offering a significant saving in the numbers of animals required and the cost of the study.  Among the molecular imaging methods described to date,  in vivo  bioluminescence imaging (BLI) has emerged as one of the most versatile and sensitive methods for  in vivo  studies of biology and therapeutic responses in animal models. As such BLI has been used to advance our understanding of a number of chemotherapeutic agents and biological therapies for cancer and other diseases. Expression of luciferases can be used to create to light-emitting cells, which can be studied in correlative cultures assays and then used in animal models where a low intrinsic background signal from the host animal provides significant signal to noise ratios. This relatively inexpensive and easy to use imaging modality, that doesn’t require radioactivity and uses substrates that are relatively nontoxic, is ideally suited to small animals, such as mice and rats. These laboratory rodents are small enough to allow transmission of light through their tissues from luciferase-expressing cells or organs to the surface, where the photons can then be detected by sensitive camera systems that are based on charge-coupled devices. The response of the cells expressing luciferase, or the expression of luciferase by a promoter of interest, can thus observed in the complex environment of the living body.  We have used this imaging approach to develop immune cell therapies and to optimize a combined biological therapy comprised of immune cell therapy and engineered viruses that were designed to be tumor selective in their replication; oncolytic viruses (4). Similar to chemotherapies, development of biological therapies is based on targeting the unique biology of cancer cells to achieve an effective therapeutic index against malignant versus normal cells. By necessity, measurements of therapeutic indices are performed in cell culture, but these assays can not be used to develop  in vivo  delivery tools that effectively transport therapeutic agents to the tumor target in living subjects. It is not surprising, therefore, that despite the development of extremely potent anticancer drugs and biological therapies, their designs lack mechanisms for crossing biological barriers for directed delivery to malignant cells  in vivo . Selective delivery of a therapeutic agent to a target cell can further enhance the therapeutic index  in vivo  due to selective accumulation of the agent within the tumor. This can both effectively reduce exposure of normal tissues to toxic agents and achieve a higher effective concentration at the tumor site and imaging of cell migration and efficacy guides development of these therapeutic strategy.  The combination of cytokine induced killer (CIK) cells as the carrier vehicle and oncolytic vaccinia virus as its biological payload fall into the category of cell-based combination therapies that may offer additive or synergistic therapeutic benefits (4). CIK cells have been shown to display impressive tumor-trafficking potential following systemic delivery in preclinical models, with the majority of the cells detected in the tumor by 72h after intravenous delivery. CIK cells were developed as a tumorical population of immune cells that have characteristics of both T cells and natural killer (NK) cells. Although these cells display cytolytic ability in correlative cell culture assays and  in vivo  within the tumor microenvironment, large numbers of CIK cells are typically required to achieve efficient tumor clearance-effector to target ratios of 10 or 20 to one. Oncolytic vaccinia viruses have been developed as highly lytic agents with good tumor selectivity, but they are limited in their biodsitribution. By utilizing CIK cells as a carrier vehicle to deliver oncolytic vaccinia to the tumor we overcome the deficiencies of each therapy as a single agent (inefficient tumor delivery of vaccinia and limited tumor cell killing by CIK cells), while retaining their benefits.  Blending an active targeting mechanism with a biologically active delivery vehicle (e.g., an immune cell) increases the therapeutic effect of the targeted agent through active concentration at the tumor site. Such approaches rely on cell types that are recruited by, or naturally traffic to, the tumor microenvironment, and these act as carrier vehicles to deliver a therapeutic agent or payload to the tumor. Tumor-targeting cells that have been used for such directed therapy include hematopoietic or immune cells, or precursor or stem cells. Since these cells have their own biology that can be modified and exploited for improved therapies, understanding and utilizing the biology of the vehicle will be essential for optimizing these approaches. The use of immune cells as carrier vehicles to deliver oncolytic virus strains to tumors has the potential to become a major weapon in the fight against cancer.   References  1. Negrin RS, Contag CH.  In vivo  imaging using bioluminescence: implications for understanding graft-versus-host disease. Nat Rev Immunol 2006;6:484-90.  2. Shachaf CM, Kopelman A, Arvanitis C, Beer S, Mandl S, Bachmann  MH, et al. MYC inactivation uncovers stem cell properties and induces the state of tumor dormancy in hepatocellular cancer. Nature 2004;431:1112-7.  3. Contag CH.  In vivo  pathology: Seeing with molecular sensitivity and cellular resolution in the living body .  Ann Rev Pathol Mech Dis 2007;2:277-305.  4. Thorne SH, Negrin RS, Contag CH Synergistic antitumor effects of immune cell-viral biotherapy. Science. 2006;311:1780-4.