In Vivo Tracking of Streptococcal Infections of Subcutaneous Origin in a Murine Model

Sugar Shock: Probing Streptococcus pyogenes Metabolism Through Bioluminescence Imaging

Streptococcus pyogenes (S. pyogenes) can thrive in its host during an infection, and, as a result, it must be able to respond to external stimuli and available carbon sources. The preclinical use of engineered pathogens capable of constitutive light production may provide real-time information on microbial-specific metabolic processes. In this study, we mapped the central metabolism of a luxABCDE-modified S. pyogenes Xen20 (Strep. Xen20) to its de novo synthesis of luciferase substrates as assessed by the rate of light production in response to different environmental triggers. Previous characterization predicted that the lux operon was under the myo-inositol iolE promotor. In this study, we revealed that supplementation with myo-inositol generated increased Strep. Xen20 luminescence. Surprisingly, when supplemented with infection-relevant carbon sources, such as glucose or glycine, light production was diminished. This was presumably due to the scavenging of pyruvate by L-lactate dehydrogenase (LDH). Inhibition of LDH by its inhibitor, oxamate, partially restored luminescent signal in the presence of glucose, presumably by allowing the resulting pyruvate to proceed to acetyl-coenzyme A (CoA). This phenomenon appeared specific to the lactic acid bacterial metabolism as glucose or glycine did not reduce signal in an analogous luxABCDE-modified Gram-positive pathogen, Staph. Xen29. The Strep. Xen20 cells produced light in a concentration-dependent manner, inversely related to the amount of glucose present. Taken together, our measures of microbial response could provide new information regarding the responsiveness of S. pyogenes metabolism to acute changes in its local environments and cellular health.

Pan-caspase inhibition as a potential host-directed immunotherapy against MRSA and other bacterial skin infections

Staphylococcus aureus causes most skin infections in humans, and the emergence of methicillin-resistant S. aureus (MRSA) strains is a serious public health threat. There is an urgent clinical need for nonantibiotic immunotherapies to treat MRSA infections and prevent the spread of antibiotic resistance. Here, we investigated the pan-caspase inhibitor quinoline-valine-aspartic acid-difluorophenoxymethyl ketone (Q-VD-OPH) for efficacy against MRSA skin infection in mice. A single systemic dose of Q-VD-OPH decreased skin lesion sizes and reduced bacterial burden compared with vehicle-treated or untreated mice. Although Q-VD-OPH inhibited inflammasome-dependent apoptosis-associated speck-like protein containing caspase activation and recruitment domain (ASC) speck formation and caspase-1-mediated interleukin-1β (IL-1β) production, Q-VD-OPH maintained efficacy in mice deficient in IL-1β, ASC, caspase-1, caspase-11, or gasdermin D. Thus, Q-VD-OPH efficacy was independent of inflammasome-mediated pyroptosis. Rather, Q-VD-OPH reduced apoptosis of monocytes and neutrophils. Moreover, Q-VD-OPH enhanced necroptosis of macrophages with concomitant increases in serum TNF and TNF-producing neutrophils, monocytes/macrophages, and neutrophils in the infected skin. Consistent with this, Q-VD-OPH lacked efficacy in mice deficient in TNF (with associated reduced neutrophil influx and necroptosis), in mice deficient in TNF/IL-1R and anti-TNF antibody-treated WT mice. In vitro studies revealed that combined caspase-3, caspase-8, and caspase-9 inhibition reduced apoptosis, and combined caspase-1, caspase-8, and caspase-11 inhibition increased TNF, suggesting a mechanism for Q-VD-OPH efficacy in vivo. Last, Q-VD-OPH also had a therapeutic effect against Streptococcus pyogenes and Pseudomonas aeruginosa skin infections in mice. Collectively, pan-caspase inhibition represents a potential host-directed immunotherapy against MRSA and other bacterial skin infections.

Imaging Cardiovascular and Lung Macrophages With the Positron Emission Tomography Sensor 64Cu-Macrin in Mice, Rabbits, and Pigs

BACKGROUND

Macrophages, innate immune cells that reside in all organs, defend the host against infection and injury. In the heart and vasculature, inflammatory macrophages also enhance tissue damage and propel cardiovascular diseases.

METHODS

We here use in vivo positron emission tomography (PET) imaging, flow cytometry, and confocal microscopy to evaluate quantitative noninvasive assessment of cardiac, arterial, and pulmonary macrophages using the nanotracer 64Cu-Macrin-a 20-nm spherical dextran nanoparticle assembled from nontoxic polyglucose.

RESULTS

PET imaging using 64Cu-Macrin faithfully reported accumulation of macrophages in the heart and lung of mice with myocardial infarction, sepsis, or pneumonia. Flow cytometry and confocal microscopy detected the near-infrared fluorescent version of the nanoparticle (VT680Macrin) primarily in tissue macrophages. In 5-day-old mice, 64Cu-Macrin PET imaging quantified physiologically more numerous cardiac macrophages. Upon intravenous administration of 64Cu-Macrin in rabbits and pigs, we detected heightened macrophage numbers in the infarcted myocardium, inflamed lung regions, and atherosclerotic plaques using a clinical PET/magnetic resonance imaging scanner. Toxicity studies in rats and human dosimetry estimates suggest that 64Cu-Macrin is safe for use in humans.

CONCLUSIONS

Taken together, these results indicate 64Cu-Macrin could serve as a facile PET nanotracer to survey spatiotemporal macrophage dynamics during various physiological and pathological conditions. 64Cu-Macrin PET imaging could stage inflammatory cardiovascular disease activity, assist disease management, and serve as an imaging biomarker for emerging macrophage-targeted therapeutics.