PERSPECTIVES known as GVHD2,3. Severe GVHD limits the INNOVATION overall effectiveness of HCT and precludes the application of this life-saving therapy to In vivo imaging using other clinical settings, such as for the treat- ment of severe autoimmune disorders4 or for the induction of tolerance to organ transplan- bioluminescence: a tool for tation. The risks of GVHD are substantial, such that 20–60% of patients will develop probing graft-versus-host disease this complication following allogeneic HCT, depending on disease-related factors, such as Robert S. Negrin and Christopher H. Contag the stage of disease, the age of the recipient and the degree of genetic disparity between Abstract | Immunological reactions have a key role in health and disease and are the donor and recipient. The reasons why complex events characterized by coordinated cell trafficking to specific locations some patients develop severe GVHD after throughout the body. Clarification of these cell-trafficking events is crucial for HCT, whereas others do not, remain unclear. improving our understanding of how immune reactions are initiated, controlled Given the substantial risk of GVHD, applica- and recalled. As we discuss here, an emerging modality for revealing cell tion of current HCT procedures requires that the donor is a fully matched histocompatible trafficking is bioluminescence imaging, which harnesses the light-emitting sibling or unrelated donor. Unfortunately, properties of enzymes such as luciferase for quantification of cells and uses low- many patients that need HCT are unable to light imaging systems. This strategy could be useful for the study of a wide range secure a well-matched donor, resulting in of biological processes, such as the pathophysiology of graft-versus-host and lethal consequences for the patient due to graft-versus-leukaemia reactions. disease progression. Therefore, the study of GVHD and GVL reactions provides insight into both normal and pathological immune Normal immune function is crucial for as for genetic and acquired immune deficien- reactions and has significant implications for maintaining health in a hostile environ- cies1. HCT involves the transfer of the entire the development of new and more effective ment containing many potential pathogens. haematopoietic and immune systems from a strategies for clinical management of disease. To protect against pathogens, immune donor to a recipient. The procedure involves responses must be both rapid and sustained. pretreatment of the patient with high doses Experimental models of HCT In addition, effector cells of the immune of chemotherapy, with or without irradiation, HCT has been widely studied in both rodent system provide immune surveillance against to eliminate malignant or defective and canine models, and these studies have malignancy and promote tissue remodelling haemato poietic cells. This is followed by the been crucially important in developing the and repair. The importance of a normal transfer to the patient (recipient) of donor- theoretical basis of HCT. Initial studies in immune response is further exemplified derived haematopoietic cells, which home mice established the concept of allogeneic by the observation that patients with many to the bone marrow and re-establish immune responses and GVHD. Mouse diseases, including cancer and infectious and haematopoiesis. After HCT, all of the haema- studies have also been crucial for studying auto immune diseases, have dysfunctional topoietic cells in the recipient, including allorecognition, and for exploring effector-cell immune responses. Therefore, a greater professional antigen-presenting cells (APCs) populations and mechanisms, owing to the understanding of normal and pathological and immune effector cells, are of donor defined genetics and availability of strains immune responses will not only provide origin. The functional consequences for the that lack key effector molecules. Early stud- insights into basic biological mechanisms but recipient of the new donor-derived immune ies in dogs showed that in some litter-mates also will aid in the development of effective system are dramatic and include the ability to long-term engraftment occurred, whereas treatments for a range of diseases. reject the underlying malignancy or replace in others GVHD developed. The outbred In this Innovation article, we discuss damaged haemato poietic-cell populations canine model has been useful in the develop- how bioluminescence imaging (BLI) can be with normal cells. For example, the damaged ment of preparative regimens for transplan- used to analyse aspects of complex immune red blood cells in patients with thalassaemia tation, which were translated to the clinic5,6, reactions in living animals. We use graft-versus- or sickle-cell disease can be replaced with and to study the use of drug prophylaxis leukaemia (GVL) reactions and graft-versus-host healthy cells. The rejection of malignancy is for both acute and chronic GVHD in large disease (GVHD) as examples of what BLI can known as the GVL effect. The antigens on animals. More recent studies have led to teach us about clinically relevant immune the tumour cells that are recognized by the the development of non-myeloablative HCT, responses. donor leukocytes are largely unknown but in which the intensity of the preparative they include major and minor histocompatibility regimen of the recipient is reduced and Haematopoietic-cell transplantation antigens (depending on the genetic disparity replaced with immuno suppressive Nowhere in clinical medicine is the impor- between the donor and the recipient) as well medications to prevent graft rejection. This tance of effective immune responses clearer as potential tumour-specific antigens. strategy of HCT is associated with reduced than after allogeneic haematopoietic-cell Despite its success in promoting the rejec- transplant-related morbidity and mortality7. transplantation (HCT). Over the past several tion of malignancy, allogeneic HCT carries Non-myeloablative HCT has been widely decades, HCT has emerged as an effective the significant risk that the donor-derived applied in the clinic; however, GVHD and often life-saving treatment for a broad immune cells will recognize and respond to remains a serious complication of the array of haematological malignancies, as well recipient tissues and result in a syndrome therapy with an associated mortality rate 484 | JUNE 2006 | VOLUME 6 www.nature.com/reviews/immunol © 2006 Nature Publishing Group PERSPECTIVES Box 1 | Molecular imaging in immunology: watching and waiting for an immune response small animals also has several advantages over other approaches: it can usually accommodate Imaging modalities. Two-photon intravital microscopy offers resolution of cells in vivo11, but it is constrained by small fields of view and motion artefacts. Non-invasive measures of immunological multiple animals in a single image at each processes in vivo have been accomplished using positron emission tomography (PET)49–51, single time point; it is user-friendly so dedicated photon emission computed tomography (SPECT)52, magnetic resonance imaging (MRI)53, imaging technicians are not required; and bioluminescence imaging (BLI) and fluorescence imaging (FLI)49,54–56. Ultrasound and X-ray it is less expensive than the instrumenta- computed tomography (CT) provide anatomical information, and when used in combination with tion required for SPECT, MRI and PET. other modalities, this information improves localization of the signals obtained by PET, SPECT The use of two-photon intravital microscopy or optical imaging57. PET, SPECT, ultrasound MRI and CT have potential clinical uses, and therefore offers high-resolution images of cell–cell are useful in translational studies. interactions in tissues11, but is constrained Optical methods. BLI and FLI can be used to refine and accelerate studies of animal models, but by a limited field of view and can be severely they have limited clinical application. Imaging times for optical imaging methods are generally hampered by motion artefacts. As a result short, which facilitates the analysis of greater numbers of animals. Optical methods also allow a of these limitations and constraints, only range of image resolutions from microscopic to macroscopic, produce images without the use of certain tissues can be observed using this ionizing radiation, offer the choice of many reporters and dyes, and benefit from user-friendly and approach. 58 59,60 inexpensive instrumentation . Signal-to-noise ratios (SNRs) for BLI are excellent , enabling In the field of optical imaging, there are detection of subtle changes non-invasively, thereby obviating the need to remove overlying tissue. several approaches that have been used to Reporter genes. The use of dyes and contrast agents allows visualization of the early events, but generate whole-body images of biological they are diluted by cell division. To prevent loss of labels during cell division, genes that encode processes in rodents. These include the use reporter proteins can be integrated into the genome. Reporter genes are available for PET, SPECT, of light scatter, absorbance, fluorescence MRI and optical imaging49,57, each with strengths and weaknesses. The radiotracers used for PET and bioluminescence. In this Perspective and SPECT often produce signals from kidney, liver and bladder that can obscure the target tissue, and MRI is generally less sensitive than imaging of reporter-gene expression with PET or SPECT. article,