Attenuation of porcine deltacoronavirus disease severity by porcine reproductive and respiratory syndrome virus coinfection in a weaning pig model

Porcine deltacoronavirus (PDCoV) is a potentially emerging zoonotic pathogen that causes severe diarrhea in young pigs, with a risk of fatal dehydration. Its pathogenicity on neonatal piglet has been previously reported, however, it is less known if the coinfection with immunosuppressive pathogens can influence PDCoV disease manifestation. Here, a coinfection model of PDCoV and porcine reproductive and respiratory syndrome virus (PRRSV), a global-spread immunosuppressive virus, was set to study their interaction. Weaning pigs in the coinfection group were intranasally inoculated with PRRSV NADC30-like virus and latterly orally inoculated with PDCoV at three day-post-inoculation (DPI). Unexpectedly, compared with pigs in the PDCoV single-infected group, the coinfected pigs did not show any obvious diarrhea, as PDCoV fecal shedding, average daily weight gain (ADWG), gross and microscopic lesions and PDCoV IHC scores consistently indicated that PRRSV coinfection lessened PDCoV caused diarrhea. Additionally, three proinflammatory cytokines TNF-α, IL-1 and IL-6, which can be secreted by PRRSV infected macrophages, were detected to be highly expressed at the intestine from both PRRSV infected groups. By adding to PDCoV-infected cells, these three cytokines were further confirmed to be able to inhibit the PDCoV replication post its cellular entry. Meanwhile, the inhibition effect of the supernatant from PRRSV-infected PAMs could be obviously blocked by the antagonist of these three cytokines. In conclusion, PRRSV coinfection increased TNF-α, IL-1, and IL-6 in the microenvironment of intestines, which inhibits the PDCoV proliferation, leading to lessened severity of diarrhea. The findings provide some new insight into the pathogenesis and replication regulation of PDCoV.猪δ冠状病毒（Porcine deltacoronavirus, PDCoV）是一种潜在的新发人畜共患病原，可引发仔猪严重腹泻，并存在致死性脱水风险。此前已有研究报道其对新生仔猪的致病性，但目前尚不清楚与免疫抑制性病原体混合感染是否会影响猪δ冠状病毒的疾病表现。本研究构建了猪δ冠状病毒与全球流行的免疫抑制性病毒——猪繁殖与呼吸综合征病毒（Porcine reproductive and respiratory syndrome virus, PRRSV）的混合感染模型，以探究二者的相互作用。混合感染组的断奶仔猪先经鼻内接种PRRSV NADC30样毒株，于接种后第3天（day-post-inoculation, DPI）再经口接种猪δ冠状病毒。出乎意料的是，与单一感染猪δ冠状病毒的对照组仔猪相比，混合感染仔猪未出现明显腹泻症状；猪δ冠状病毒粪便排毒量、平均日增重（average daily weight gain, ADWG）、大体与显微病变评分以及猪δ冠状病毒免疫组化（Immunohistochemistry, IHC）评分均一致表明，PRRSV混合感染可减轻猪δ冠状病毒诱发的腹泻症状。此外，在PRRSV感染的两组仔猪肠道中，均可检测到由PRRSV感染巨噬细胞分泌的三种促炎细胞因子——肿瘤坏死因子-α（TNF-α）、白细胞介素-1（IL-1）和白细胞介素-6（IL-6）的高表达。通过向感染猪δ冠状病毒的细胞中添加这三种细胞因子，进一步证实其可在病毒侵入细胞后抑制猪δ冠状病毒的复制。同时，PRRSV感染的猪肺泡巨噬细胞（porcine alveolar macrophages, PAMs）上清液所产生的抑制效应，可被这三种细胞因子的拮抗剂显著阻断。综上，PRRSV混合感染可提升肠道微环境中TNF-α、IL-1及IL-6的水平，进而抑制猪δ冠状病毒的增殖，最终减轻腹泻症状。本研究结果为猪δ冠状病毒的致病机制与复制调控提供了新的研究视角。