Host Antiviral Responses Against Avian Infectious Bronchitis Virus (IBV): Focus On Innate Immunity Part 3

4.1. IBV Infection Triggered Interferon Activation

IFNs, including type I, type II, and type III IFNs [71], are multifunctional in the innate immune system. 

Type III interferon (IFN) is an important immunoregulatory factor that has antiviral, antitumor, and immunomodulatory effects. Recent studies have found that type III IFN also has a positive impact on cognitive function and memory improvements.

In terms of cognitive function and memory, the main role of type III IFN is to promote the growth and connection of nerve cells, improve the balance of neurotransmitters, and enhance the immunity of neurons, thereby making brain cells more active and agile, and improving memory and thinking. ability. In addition, type III IFN can also promote the adaptability of neural stem cells to the environment, increase the formation and growth of new neurons, help the brain stay young and healthy, and prevent neurodegenerative diseases such as Alzheimer's disease.

In addition, type III IFN can also reduce the decline of memory and cognitive function by regulating the interaction between the immune system and the nervous system, inhibiting the brain's inflammatory response, and protecting neurons from inflammatory damage.

It is important to note that the effects of type III IFN are closely related to dose. An appropriate amount of type III IFN can help improve cognitive function and memory, but an excessive supply of type III IFN can trigger an inflammatory response, damage nerve cells and neural networks, and cause memory decline.

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In general, type I IFNs (IFN-α, IFN-β, etc.) and type III IFNs (IFN-λ) have been proven for antiviral activity, while type II IFNs (IFN-γ) could activate T cells and macrophages [72]. The type I IFN functions as a powerful antiviral mechanism, which is involved in host response after IBV infection. 

It was shown that IFN-α could inhibit respiratory Beaudette or Gray IBV strains both in vitro and in vivo [73]. 

In vitro studies showed that the induction of IFN-β is in an MDA5-dependent manner [56]. At an early infection stage (9 hp), IFN-β was upregulated when infected with a neuropathogenic IBV strain [55]. However, when respiratory M41 IBV strain was used, the expression of IFN-β in CEK cells was delayed until 12 dpi, while accessory protein 5b was involved in the induction of host shutoff that resulted in a reduction of IFNs [74]. 

Furthermore, the respiratory Beaudette IBV strain was shown to interfere with IFN-β-induced translocation of STAT1 and STAT1 phosphorylation in Vero cells at late stages of infection (18 dpi), ref. [74] suggesting respiratory IBV-mediated inhibition of IFN signaling in a time-dependent manner. 

The differences in IFN expression between respiratory and neuropathogenic IBV infections require future work, which may help to understand the mechanisms underlying the tissue tropism of different IBV strains. In vivo, studies showed a more complex result, in that the expression level of IFN-α was significantly upregulated at 1 dpi in spleens after virulent respiratory IBV infection [62], while in tracheas the upregulation of IFNs was not observed at 3 dpi [61]. 

Furthermore, chickens vaccinated with attenuated respiratory M41 or LDT3 posed stronger type I IFN levels, respectively [61]. The virulence of IBVs might be the reason for the differences in IFN levels. Consistent with the results in PRR expression, these results also suggest it is important that the virulence of IBV strains be taken into consideration in field control of the virus. 

Similarly to type I IFNs, at an early stage of infection (12 dpi), after inoculation with the respiratory Conn IBV strain, IFN-γ was significantly downregulated in the tracheas and lungs of the infected chicken [26]. At 2–3 dpi, when inoculated with respiratory M41 IBV strain, IFN-γ was induced in the tracheas and lungs [35,75]. 

Though the antiviral activity of IFN-γ against IBV has not been fully characterized, based on results observed in avian influenza virus (AIV) infected chickens, it was suggested that IFN-γ might indirectly interfere with IBV replication through the initiation of ISG-encoded ribonuclease L (RNase L) [76]. 

For expression of ISGs, studies showed that in different systems including chicken embryos (6 hp), tracheas (3 dpi), and kidneys (5–6 dpi), upregulation of ISGs was presented in transcriptional analysis after infection with different respiratory IBV strains [61]. 

To summarize, though the responses of IFNs after IBV infection vary in a strain-dependent and time-dependent manner, in general, activation of IFNs is restrained at a very early stage of IBV infection to allow viral replication. 

Upregulation of IFNs is often observed with activated ISGs after infection is established when the innate immunity responds for viral clearance. Therefore, early intervention and activation of IFNs are critical in control of the disease.

4.2. IBV Infection Triggered Other Cytokine and Chemokine Activation

Other cytokines and chemokines are also crucial regulators of innate immune responses against viral infection. For instance, correlated with recruited macrophages, the production of IL-1β was involved in reducing IBV viral loads in the respiratory tract [27]. 

In addition, upregulation of IFN-α, IFN-γ, and IL12 at 12 hpi, upregulation of IFN-γ, IL-8, and macrophage inflammatory protein (MIP)-1β at 48 hpi, and upregulation of IFN-γ and IL-6 at 72 hpi were also observed, and the upregulation of these cytokines was associated with inhibition of respiratory IBV Ark99 replication [77]. 

Depending on the IBV strain, it was reported that proinflammatory cytokine expression was induced differently in different tissues. In the tracheas, at an early infection stage (1–3 dpi), expression of IL-1β, IL-10R2, IL-6, and LITAF was elicited after inoculation with either respiratory or neuropathogenic IBV strains [61]. 

The expression of IL-1β was initially downregulated (12 hp) and sharply increased as the IBV infection progressed in the tracheas when chickens were inoculated with the respiratory Conn IBV strain [26]. 

Furthermore, expression of IL-6 was upregulated by p38 phosphorylation during IBV infection [78]. In kidneys, regulation of these cytokines was not significantly affected following infection with respiratory IS/885/00-like (885), M41, and neuropathogenic QX-like IBV strains [79]. Chickens infected with the neuropathogenic IBV strain of the KIIa genotype presented upregulated mRNA levels of IL-6 and IL-1β at 1 dpi in tracheas and kidneys, whereas chickens infected with the respiratory IBV strain of the ChVI genotype showed comparatively mild upregulated mRNA expression of these cytokines [80]. 

In addition, in the splenic immune system, the expression levels of IL-7 and IL-18 were significantly upregulated at 1 dpi after respiratory IBV infection [62]. Chemokines orchestrate the migration of cells during immune surveillance. Mass IBV strain stimulated gene expression of CXCR4, CCR6, chemokine-like receptor 1/CHEMR23, and Matrix metalloproteinase (MMPs) from an early phase of viral infection (1 dpi) in tracheas [58]. 

These chemokines may play a role in the migration of activated T cells, which may further contribute to the elimination of the virus. Based on these observations, after IBV infection occurs, the innate immunity is activated, resulting in the recruiting of innate cells at the infection sites and the upregulation of different PPRs, cytokines, chemokines, etc. However, downregulation of PPRs (TLR7), IFNs (IFN-β, IFN-γ), and other cytokines (IL-1β) were still observed at a very early stage of infection (12 hp) by respiratory IBV strains, suggesting inhibition of the innate immunity is important to establish successful IBV infections, which may reflect common strategies the coronaviruses might take to avoid detection by the host innate immunity. 

Because most studies were carried out using respiratory IBV strains, it would be important to gain more information about the antiviral host responses using the neuropathogenic IBV strains, which may help to develop vaccination strategies and other intervention programs. A general description of the innate immune responses against IBV infection is shown in Figure 1.

5. Apoptosis Triggered by IBV Infection

Apoptosis is one of the primary mechanisms that animals use to combat viral infections. It can also facilitate virus spread at a later stage of infection [81]. There have been reports about IBV-induced apoptosis both in vivo [55] and in vitro [28,82]. 

It is suggested that the IBV ORF1b region is responsible for triggering apoptosis [83]. In mammalian cells, the Bcl 2 family of proteins, including proapoptotic (Bax and Bak) and anti-apoptotic (Mcl 1, Bcl 2, and Bcl XL) proteins, modulated IBV-induced apoptosis at an early stage of IBV infection [55]. 

In IBV M41-infected HD11 and PBMCs-Mφ cells, a decreased expression of Bcl-2 accompanied by increased expression of Bcl-2-associated X (Bax) suggests viral replication provokes apoptosis at 48 hpi [28]. At a late stage of infection, apoptosis was demonstrated to facilitate IBV replication. Consider IBV Beaudetteinfected DF-1 cells for instance [84]. 

In these cells, the mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) pathway was activated; this pathway is negatively regulated by phosphatase DUSP6 [84]. Furthermore, the unfolded-protein response (UPR) sensor IRE1α-XBP1 pathway was also activated at late stages.

6. Perspectives in IBV Control

Since first being documented in the United States in 1931, IBV has become endemic throughout the poultry industry [10]. It has been suggested that other avian species might play a role in the spread of IBV worldwide [86]. 

For instance, a partial nucleotide sequence of coronavirus isolated from parrots (E. roratus) showed 100% homology with the IBV GI-13 lineage [87]. Whether wild birds and these avian coronaviruses contribute to the spread of IBV requires further evidence.

Alongside research on vaccination and prevention measures, in recent years more attention has been focused on understanding the early immune responses after IBV infection, as this would expand our knowledge of the pathology of the virus, which in turn could benefit the development of prevention and control strategies. 

Innate immunity contributes to a network by utilizing PPRs to detect conserved PAMPs, where different components such as IFNs and proinflammatory cytokines play essential roles in antiviral activity. Several reviews regarding chicken immune responses to IBV infection are recommended for a comprehensive understanding of the virus-host immunity interaction [55]. 

Given the vast diversity of IBV strains, innate immune responses evoked by IBV infection vary in a strain-dependent and time-dependent manner. Still, early intervention and activation of innate immunity are essential for the control of the disease. To evoke an early innate immune response, agonists of PRRs and IFNs have drawn more attention to novel vaccine design. In addition, the population diversity of the virus also contributes to the enhancement of host immunity, as a more diverse viral population in the vaccine induces stronger innate immune responses [88]. 

Therefore, for a more comprehensive understanding of the IBV-host innate immunity interaction and future development of prevention and control strategies, IBV population structure, the diversity of viral genome, and culture system, as well as the condition of the host animals should be taken into consideration. 

Though the information on IBV-host innate immunity interaction is still limited due to the lack of experimental measures in chickens, it is well-recognized that innate immunity contributes not only to prevention strategy development but also to the pathogenicity of the virus. For effective control of the virus, early enhancement of the host's innate immunity is critical. 

Furthermore, because the chicken's innate immunity acts in a strain-dependent and time-dependent manner after IBV infection, early diagnosis of the IBV strain is also important for better control of the virus. Further investigation is required to explore differences in immune response triggered by different IBV strains with differing genotypes and pathogenicity.

Author Contributions: Manuscript preparation, Y.Z.; Revision, Y.Z., Z.X.; Supervision, Y.C.; Funding Acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding: This study was supported by the Doctoral Initiative Project of the Natural Science Foundation of Guangdong Province (18zxxt49), and the Guangdong Basic and Applied Basic Research Foundation (2019B1515210026).

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Conflicts of Interest: The authors declare that they have no financial or personal relationships with other people or organizations that could influence the work. There is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in this review. The authors do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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