Molecular imaging of influenza and other emerging respiratory viral infections

Abstract

Research on the pathogenesis and therapy of influenza and other emerging respiratory viral infections would be aided by methods that directly visualize pathophysiologic processes in patients and laboratory animals. At present, imaging of diseases, such as swine-origin H1N1 influenza, is largely restricted to chest radiograph and computed tomography (CT), which can detect pulmonary structural changes in severely ill patients but are more limited in characterizing the early stages of illness, differentiating inflammation from infection or tracking immune responses. In contrast, imaging modalities, such as positron emission tomography, single photon emission CT, magnetic resonance imaging, and bioluminescence imaging, which have become useful tools for investigating the pathogenesis of a range of disease processes, could be used to advance in vivo studies of respiratory viral infections in patients and animals. Molecular techniques might also be used to identify novel biomarkers of disease progression and to evaluate new therapies.

Figure 1.

Radiographs and histopathologic examination of lung tissues at autopsy of patients fatally infected with pandemic swine-origin H1N1 influenza. A, Chest radiograph on day 5 of illness, showing multiple, bilateral opacities (arrows). B, CT of the same patient, showing multifocal, patchy ground-glass opacities. C, CT of a different patient, showing areas of denser consolidation consistent with bacterial pneumonia. D, Section of trachea showing inflammation of submucosal glands. E, Section of lung showing diffuse alveolar damage with hyaline membrane formation. F, Section of lung showing a massive infiltrate of neutrophils, consistent with bacterial pneumonia. (From [12] and [14], with permission.)

Figure 2.

Chest radiograph (A) and CT (B) of a 46-year-old woman with SARS on day 12 of hospitalization. The radiograph shows bilateral, multifocal opacities (arrows), which appear on CT as multiple subpleural areas of ground-glass attenuation. (Courtesy of Dr. Narinder Paul, Toronto.) The similarity of these changes to those seen in swine-origin H1N1 influenza (Figure 1) illustrates the nonspecificity of radiographic findings in viral infections of the respiratory tract.

Figure 3.

Posterior-anterior and lateral radiographs of a cynomolgus macaque 6 days after infection with SARS coronavirus by the intranasal and intrabronchial routes, showing multiple areas of pulmonary opacification (arrows). (From [18].)

Figure 4.

Sequential radiographs of a 22-year-old man with H5N1 avian influenza. The film on hospital admission (A) shows bilateral perihilar patchy opacities with air bronchograms. Two days later (B), there was bilateral multifocal consolidation. On day 4, the patient showed worsening respiratory failure with features suggestive of ARDS. (From [3], with permission.)

Figure 5.

SPECT/CT images of the thorax of a mouse 6 weeks after low-dose aerosol infection with recombinant M. tuberculosis, in which a bacterial TK gene is under the control of the strong, constitutive P_hsp60 promoter. The probe is 1-(2'deoxy-2'-fluoro-β-D-arabinofuranosyl) - 5-[¹²⁵I]-iodouracil (¹²⁵I-FIAU), which is preferentially phosphorylated by the bacterial TK over the host cell enzyme to produce SPECT signal localizing to pulmonary granulomas (arrows). (From [30].)

Figure 6.

CT (upper row) and ¹⁸FDG-PET images of the lungs of a human volunteer before and 24 hours after an intrabronchial installation of endotoxin. The subtraction image shows an area of tracer retention representing an acute inflammatory response. Red indicates the highest and blue the lowest level of activity. (From [39], with permission.)

Figure 7.

Chest CT (A), ¹⁸FDG-PET scan (B), and fused image (C) of a patient with severe swine-origin H1N1 influenza and ARDS. The PET image shows increased glucose metabolism, both in areas that contain infiltrates by CT and in regions of apparently normal aeration. Red indicates the highest and blue the lowest level of radiotracer activity. (Courtesy of Giacomo Bellani.)

Figure 8.

Identification by MRI of areas of inflammation in the lungs of a mouse 24 and 48 hours after an intratracheal inoculation of lipopolysaccharides, based on the uptake of ¹⁹F-labeled emulsified perfluorocarbons, which are phagocytized by monocytes and macrophages. A and B, ¹H gradient echo images of the thorax. C, ¹⁹F MRI. D, Superimposed images, with sites of perfluorocarbon uptake highlighted in red. (From [67], with permission.)

Figure 9.

A, correlation of photon flux with viral titers in the organs of interferon-α receptor knockout mice infected intranasally with a recombinant vaccinia virus encoding firefly luciferase. The animals were killed on day 7 after infection, and tissue samples were analyzed for luciferase activity and viral titer. B, BLI of the infected mice, showing the progression of infection over time (days). Red indicates the highest and blue the lowest level of radiotraceractivity. (From [75], with permission.)

Figure 10.

Ex vivo imaging of the distribution of a GFP-encoding influenza A virus in the lungs of mice 2 days after intranasal inoculation, without or with oseltamivir therapy. In the treated animals, the signal is almost entirely low-level (blue) and is localized principally to the central large airways, while the photon flux is more intense and diffuse in the untreated mice. (From [31], with permission.)

Figure 11.

Response of “reporter mice” homozygous (left) or heterozygous (right) for firefly luciferase under the control of an IFN-β promoter to infection with an NS1-deficient influenza A virus. Red indicates the highest and blue the lowest level of photon flux. (From [81], with permission.)