Benefits of Antibiotics During Viral Infections: Immuno‐Modulating Properties of Tulathromycin in Porcine Reproductive and Respiratory Syndrome

Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive‐strand RNA virus that grows in primary alveolar macrophages and causes acute pneumonia in pigs. PRRSV is a major concern in the swine industry with a total cost of productivity losses estimated at $600 million annually in the U.S. alone. However, due to its high antigenic variability, and poorly understood immunopathogenesis, there is currently no treatment to control PRRSV infection. The common occurrence of PRRSV infection with bacterial infections, including Actinobacillus pleuropneumoniae , begs the question of the value of antibiotics for the treatment of the disease it causes. Tulathromycin, a macrolide used for the treatment and prevention of respiratory disease in pigs and cattle, has been shown to exhibit potent immuno‐modulating properties [1]. We hypothesize that tulathromycin attenuates the detrimental effects of PRRSV in porcine macrophages. This may help characterize novel mechanism through which an anti‐microbial agent may deliver clinical benefits in the context of a viral infection. Aims 1) Determine if Tulathromycin has direct anti‐viral effects in PRRSV‐infected porcine macrophages. 2) Identify new immuno‐modulating effects of Tulathromycin in PRRSV‐infected macrophages. Methods Porcine monocytes were isolated from peripheral blood of healthy piglets. Seven days‐old monocyte‐derived macrophages were treated with Tulathromycin (1 mg/ml) or untreated (control) and incubated for 1h at 37°C. Macrophages were then cultured in media alone, or with PRRSV (MOI of 1) for 2h to 24h. (1) Extracellular and intracellular Viral titers were measured at 2h, 24h, and 48h post‐infection. (2) Macrophage activation was monitored by morphological changes and confirmed with IL‐8 ELISA and ROS production assays at 2h, 6h, 12h or 24h. (3) Phagocytosis properties were assessed by measuring zymosan particles (1 μg/mL) or 3μM opsonized latex beads (ratio beads: cells, 10:1) engulfment assay (4) Cell death ELISA, Annexin V immunostaining and lactate dehydrogenase assay were performed to evaluate the effects of Tulathromycin on PRRSV‐induced necrosis and apoptosis. Results (1) Tulathromycin did not change PRRSV particle forming unit (pfu) in macrophages at any time of infection (2.1×10 4 PFU/mL with tulathromycin versus 1.9×10 4 PFU/mL for control) (2). Exposure to PRRSV increased macrophage‐induced production of IL‐8 and ROS by more than four times compared to control. Pre‐treatment with tulathromycin significantly attenuated PRRSV‐induced macrophage activation (i.e. 1.85 times compared to untreated macrophages). (3) Macrophage phagocytosis of both zymosan and latex beads was impaired by PRRSV. Tulathromycin was able to restore phagocytic functions in PRRSV‐infected macrophages to control values. (4) Finally, Tulathromycin reduced viral induced necrosis but synergized with the virus to induce apoptosis. Conclusion Our results demonstrate that Tulathromycin attenuates macrophage‐driven inflammatory response induced by PRRSV, restores PRRSV‐induced phagocytic impairment, and inhibits cell necrosis. These effects occur in the absence of a direct anti‐viral activity. Together, these data demonstrate the potential clinical benefits of Tulathromycin in the context of PRRSV‐induced pneumonia through the modulation of a viral‐induced macrophage‐driven inflammation. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .