Molecular Imaging of Infection: The First 50 Years.

Despite significant advances in the understanding of microorganisms and an increased availability of antimicrobial therapy, infection remains a major cause of morbidity and mortality. The diagnosis can be challenging and imaging studies often are used for confirmation and localization. For nearly 50 years, molecular imaging agents have played an important role in the diagnosis of infection. Gallium-67 citrate was perhaps the first molecular imaging agent used for diagnosing and localizing infection. Poor imaging characteristics, along with a lack of specificity, and the long (usually 48-72 hours) interval between administration and imaging motivated investigators to search for alternatives. Currently the role of 67Ga is limited to differentiating acute tubular necrosis from interstitial nephritis and as an alternative to 18F-FDG for indications, such as sarcoid, spondylodiscitis, and fever of unknown origin, when the latter is not available. The development, in the mid-1970s, of techniques for radiolabeling leukocytes that subsequently migrate to foci of infection was a significant advance and labeled leukocyte imaging still has a preeminent role in molecular imaging of infection. There are significant disadvantages to in-vitro labeled leukocyte imaging. Efforts devoted to developing in-vivo leukocyte labeling methods, however, met with only limited success. Over the past 20 years 18F-FDG has established itself as the molecular imaging agent of choice for fever of unknown origin, vasculitis, sarcoid, and spondylodiscitis. As useful as these agents are, their uptake is based on the host response to infection, not infection itself. Previous attempts at developing infection specific agents, including radiolabeled antibiotics, antibiotics, and vitamins like biotin were limited by poor results and/or limited availability and so investigators continue to focus on developing infection specific molecular imaging agents. Initial results with radiolabeled nucleoside analogs, sugars, and amino acids, and a renewed interest in radiolabeled antibiotics for both diagnosis and monitoring treatment are exciting and hold great promise for the future.

An unmet clinical need: The history of thrombus imaging.

Robust thrombus imaging is an unresolved clinical unmet need dating back to the mid 1970s. While early molecular imaging approaches began with nuclear SPECT imaging, contrast agents for virtually all biomedical imaging modalities have been demonstrated in vivo with unique strengths and common weaknesses. Two primary molecular imaging targets have been pursued for thrombus imaging: platelets and fibrin. Some common issues noted over 40 years ago persist today. Acute thrombus is readily imaged with all probes and modalities, but aged thrombus remains a challenge. Similarly, anti-coagulation continues to interfere with and often negate thrombus imaging efficacy, but heparin is clinically required in patients suspected of pulmonary embolism, deep venous thrombosis or coronary ruptured plaque prior to confirmatory diagnostic studies have been executed and interpreted. These fundamental issues can be overcome, but an innovative departure from the prior approaches will be needed.

Bacteriophage imaging: past, present and future.

The visualization of viral particles only became possible after the advent of the electron microscope. The first bacteriophage images were published in 1940 and were soon followed by many other publications that helped to elucidate the structure of the particles and their interaction with the bacterial hosts. As sample preparation improved and new technologies were developed, phage imaging became important approach to morphologically classify these viruses and helped to understand its importance in the biosphere. In this review we discuss the main milestones in phage imaging, how it affected our knowledge on these viruses and recent developments in the field.

Imaging Immunity in Lymph Nodes: Past, Present and Future.

Immune responses occur as a result of stochastic interactions between a plethora of different cell types and molecules that regulate the migration and function of innate and adaptive immune cells to drive protection from pathogen infection. The trafficking of immune cells into peripheral tissues during inflammation and then subsequent migration to draining lymphoid tissues has been quantitated using radiolabelled immune cells over 40 years ago. However, how these processes lead to efficient immune responses was unclear. Advances in physics (multi-photon), chemistry (probes) and biology (animal models) have provided immunologists with specialized tools to quantify the molecular and cellular mechanisms driving immune function in lymphoid tissues through directly visualising cellular behaviours in 3-dimensions over time. Through the temporal and spatial resolution of multi-photon confocal microscopy immunologists have developed new insights into normal immune homeostasis, host responses to pathogens, anti-tumour immune responses and processes driving development of autoimmune pathologies, by the quantification of the interactions and cellular migration involved in adaptive immune responses. Advances in deep tissue imaging, including new fluorescent proteins, increased resolution, speed of image acquisition, sensitivity, number of signals and improved data analysis techniques have provided unprecedented capacity to quantify immune responses at the single cell level. This quantitative information has facilitated development of high-fidelity mathematical and computational models of immune function. Together this approach is providing new mechanistic understanding of immune responses and new insights into how immune modulators work. Advances in biophysics have therefore revolutionised our understanding of immune function, directly impacting on the development of next generation immunotherapies and vaccines, and is providing the quantitative basis for emerging technology of simulation-guided experimentation and immunotherapeutic design.

Retrospective on the development of aequorin and aequorin-based imaging to visualize changes in intracellular free [Ca(2+) ].

In this review, we take a retrospective look at the discovery and utilization of the Ca(2+) -sensitive bioluminescent protein complex, aequorin. We do consider the contribution it has made to our understanding of the natural phenomenon of bioluminescence, but it is in the application of extracted and purified aequorin as a reporter of Ca(2+) dynamics in living cells, which is arguably its major contribution to biological and biomedical science. Following its extraction, purification, and subsequent availability in the mid-1960s, aequorin became the intracellular reporter of choice until it was replaced in the late 1970s by easier-to-use fluorescence-based reporters. From the mid-1980s onwards, however, aequorin-based Ca(2+) imaging underwent a renaissance following the cloning of the aequorin gene and the emergence of routine techniques to target and express it exogenously in plant and animal systems. The development of aequorin as a tool continues as spectral varieties are being developed that allow simultaneous imaging of Ca(2+) dynamics in different cellular organelles and microdomains. We predict that further developments in the use of aequorin, as well as other bioluminescent proteins, will continue, especially in the areas of regenerative medicine and whole organism imaging.

Abass Alavi. A distinguished physician scientist and a pioneer in molecular imaging.

Abass Alavi is a world renowned physician-scientist and has made substantial contributions to development and translation of modern imaging techniques to the day-to-day practice of medicine. Among his accomplishments, the introduction of fluorine-18-fluorodeoxyglucose ((18)F-FDG )-positron emission tomography (PET) has truly revolutionized the field of medicine worldwide. The impact of using (18)F-FDG -PET along with computed tomography (CT) (and soon magnetic resonance imaging-MRI) in managing so many serious diseases and disorders is unparalleled by any other technique in recent history. He has received many awards for his outstanding contributions to the field of molecular imaging. Currently, he is actively involved in conducting research on a full time basis.

Bioluminescence imaging: a shining future for cardiac regeneration.

Advances in bioanalytical techniques have become crucial for both basic research and medical practice. One example, bioluminescence imaging (BLI), is based on the application of natural reactants with light-emitting capabilities (photoproteins and luciferases) isolated from a widespread group of organisms. The main challenges in cardiac regeneration remain unresolved, but a vast number of studies have harnessed BLI with the discovery of aequorin and green fluorescent proteins. First described in the luminous hydromedusan Aequorea victoria in the early 1960s, bioluminescent proteins have greatly contributed to the design and initiation of ongoing cell-based clinical trials on cardiovascular diseases. In conjunction with advances in reporter gene technology, BLI provides valuable information about the location and functional status of regenerative cells implanted into numerous animal models of disease. The purpose of this review was to present the great potential of BLI, among other existing imaging modalities, to refine effectiveness and underlying mechanisms of cardiac cell therapy. We recount the first discovery of natural primary compounds with light-emitting capabilities, and follow their applications to bioanalysis. We also illustrate insights and perspectives on BLI to illuminate current efforts in cardiac regeneration, where the future is bright.