Introduce
Red blood cells (RBCs) are the most abundant cell type in the bloodstream and serve for transporting oxygen and carbon dioxide via blood circulatory system. RBCs are differentiated from multipotent hematopoietic stem cells in the bone marrow and circulated in the body for about 100-120 days, and then are recycled by macrophages in spleen and liver (de Back, Kostova, van Kraaij, van den Berg, & van Bruggen, 2014). Although RBCs lack of nucleus and organelles, RBCs still have numerous cell surface receptors for binding chemokines (Darbonne et al., 1991; Fukuma et al., 2003) and nucleic acids (Hotz et al., 2018). Moreover, the receptors could be employed by pathogens to adhere or invade in RBCs. For instance,Plasmodium falciparum invaded RBCs by adhering to glycophorin A (GYPA) (Cowman, Berry, & Baum, 2012) andPlasmodium vivax attached RBCs via Duffy Ag receptor for chemokines (DARC) (Boddey & Cowman, 2013; Horuk et al., 1993). Additionally, HIV-1 persisted on RBCs through binding to DARC and presented virion to CD4+ T cells (He et al., 2008). Although several reports have indicated that certain viruses could bind to RBCs (Beck et al., 2009; Hess et al., 2002; Sutherland et al., 2016), the underlying mechanisms have not been confirmed.
The recent outbreak of porcine epidemic diarrhea (PED) is characterized as vomiting, severe watery diarrhea, dehydration and high mortality in suckling piglets, resulting in a huge loss in swine industry worldwide. As the causative agent of PED porcine epidemic diarrhea virus (PEDV) is an enveloped, single-stranded, and positive-sense RNA virus, belonging to the Coronaviridae family (C. Lee, 2015). Due to immature immune system, suckling piglets are more susceptible to PEDV infection, especially 1-2 days old neonatal piglets. PEDV infection causes severe destruction of intestinal villous enterocytes and villous atrophy in the jejunum and ileum (Jung & Saif, 2015). Moreover, PEDV can enter the blood and induce viremia in suckling piglets (Jung et al., 2014; Wang, Hayes, Byrum, & Zhang, 2016), though the mechanism has been much less well understood.
Due to lack of ribosomes or genomes, mature RBSs are inability of gene transcription and protein synthesis, meaning that RBCs are invulnerable to viral infection. However, several researches have claimed that RBCs might be possible vehicles for virus transmission. For instance, RBCs could bind to HIV-1 virions and enrich virus infectious (Beck et al., 2009). In addition, a recent study has reported that complete genome sequence of the DNA of African swine fever virus (ASFV) was determined from porcine RBCs, suggesting that RBCs might serve as a carrier for virus transmission (Olesen et al., 2018). Our previous study demonstrated that PEDV could be captured by dendritic cells (DCs) from nasal epithelium, transferred to CD3+ T cell and entered the peripheral blood, causing intestine infection (Li et al., 2018). Although PEDV could be carried into the blood by T cells, is it possible that RBCs, as the most abundant cell type, serve as another potential vehicle for PEDV transmission in the blood?
In the present study, the hypothesis for PEDV transmission in the blood by hijacking RBCs was proposed. To verify our hypothesis, neonatal RBCs were infected with PEDV. Subsequently, an animal challenge through autotransfusion with PEDV-loaded RBCs was performed to validate whether PEDV could cause intestine infection through hijacking RBCs in vivo. A co-culture model of PEDV-loaded RBCs and CD3+ T cells was established to determine the mechanism of which PEDV entering the intestinal mucosa from the blood. Moreover, through intranasal challenge with PEDV in newborn piglets, the PEDV-loaded RBCs were found in the nasal capillary, consequently, nasal capillary was speculated to be the possible entry for PEDV binding RBCs. This study was first time to demonstrate that the coronavirus PEDV could spread throughout the body and cause intestinal infection by hijacking RBCs in newborn piglets. Therefore, our study provides a novel insight into the role of RBCs in coronavirus pathogenesis as potential cells for virus transmission.