Sequence similarity and structural homologies are involved in the autoimmune response elicited by mouse hepatitis virus A59

Interleukin-17A (IL-17A) is involved in antibody specificity to conformational epitopes

The specificity of antibodies (Ab) is essential for the precise recognition of foreign or dangerous molecules. We have shown that mice infected with non-pathogenic Lactate Dehydrogenase Elevating Virus (LDV) inoculated with human growth hormone (hGH) or Ovalbumin (OVA), exhibit modified specificity of anti-hGH or anti-OVA Ab associated with the secretion of IFN-γ, IL-13, and IL-17. Cytokines are directly or indirectly involved in the isotypes, specificity, and affinity of Ab. Accordingly, here we investigated the effect of IL-17 neutralization on Ab specificities to OVA and Diphtheria Toxoid (DTx) in a mouse model of viral infection. Thereby, we employed an anti-cytokine "auto-vaccination" with an OVA/IL-17A complex or a Monoclonal Ab (MAb) anti-IL-17A (MM17/F3). Competitive ELISA assays were used to estimate the quality of the humoral immune response and the amount of Abs to conformational versus linear antigenic determinants. Results indicated that the OVA/IL-17A complex increased Abs levels to conformational epitopes of OVA, while LDV prolonged antibodies for a longer period. Mice treated with MM17F3 MAb showed an increase in Abs to conformational epitopes of OVA. A similar effect, confirmed by a competitive Western-blot assay, was produced by LDV. Moreover, an increased level of IgM, IgG1, and IgG2a was found in infected animals. Similarly, MAb anti-IL-17A treatment increased the proportion of Ab to conformational epitopes of DTx in uninfected mice, while LDV decreased this parameter. In conclusion, our findings demonstrate a correlation between IL-17A neutralization and a change in Ab specificity to OVA or DTx, presenting a novel strategy for obtaining Abs with higher specificity.

Antigen epitopes of animal coronaviruses: a mini-review

Coronaviruses are widespread in nature and can infect mammals and poultry, making them a public health concern. Globally, prevention and control of emerging and re-emerging animal coronaviruses is a great challenge. The mechanisms of virus-mediated immune responses have important implications for research on virus prevention and control. The antigenic epitope is a chemical group capable of stimulating the production of antibodies or sensitized lymphocytes, playing an important role in antiviral immune responses. Thus, it can shed light on the development of diagnostic methods and novel vaccines. Here, we have reviewed advances in animal coronavirus antigenic epitope research, aiming to provide a reference for the prevention and control of animal and human coronaviruses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s44149-023-00080-0.

Development of a T Cell-Based COVID-19 Vaccine Using a Live Attenuated Influenza Vaccine Viral Vector

The COVID-19 pandemic emerged in 2020 and has caused an unprecedented burden to all countries in the world. SARS-CoV-2 continues to circulate and antigenically evolve, enabling multiple reinfections. To address the issue of the virus antigenic variability, T cell-based vaccines are being developed, which are directed to more conserved viral epitopes. We used live attenuated influenza vaccine (LAIV) virus vector to generate recombinant influenza viruses expressing various T-cell epitopes of SARS-CoV-2 from either neuraminidase (NA) or non-structural (NS1) genes, via the P2A self-cleavage site. Intranasal immunization of human leukocyte antigen-A*0201 (HLA-A2.1) transgenic mice with these recombinant viruses did not result in significant SARS-CoV-2-specific T-cell responses, due to the immunodominance of NP₃₆₆ influenza T-cell epitope. However, side-by-side stimulation of peripheral blood mononuclear cells (PBMCs) of COVID-19 convalescents with recombinant viruses and LAIV vector demonstrated activation of memory T cells in samples stimulated with LAIV/SARS-CoV-2, but not LAIV alone. Hamsters immunized with a selected LAIV/SARS-CoV-2 prototype were protected against challenge with influenza virus and a high dose of SARS-CoV-2 of Wuhan and Delta lineages, which was confirmed by reduced weight loss, milder clinical symptoms and less pronounced histopathological signs of SARS-CoV-2 infection in the lungs, compared to LAIV- and mock-immunized animals. Overall, LAIV is a promising platform for the development of a bivalent vaccine against influenza and SARS-CoV-2.

Coronavirus-induced autoimmunity

The pandemic of Coronavirus disease 2019 (COVID-19), caused by a new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spotlighted the link between viral infection and autoimmunity. In this review, we focus on coronavirus-induced autoimmunity based on evidence from experimental animal models, SARS-CoV infection with in vitro studies of molecular mimicry and COVID-19 with several clinical reports of autoimmune manifestations of this disease. Further studies will be needed to better characterize the role of SARS-CoV-2 in the development of autoimmunity.

Valinomycin as a potential antiviral agent against coronaviruses: A review

Human coronaviruses (HCoVs), including severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), have been resulting in global epidemics with heavy morbidity and mortality. Unfortunately, there are currently no specific medicines that can better treat these coronaviruses. Drug repurposing is an effective and economical strategy for drug discovery from existing drugs, natural products, and synthetic compounds. In this review, the broad-spectrum antiviral activity of valinomycin (VAL), especially its activity against coronaviruses such as SARS-CoV, MERS-CoV, human coronavirus OC43 (HCoV-OC43), were summarized, it highlights that VAL has tremendous potential for use as a novel antiviral agent against SARS-CoV-2.

The autoimmune response elicited by mouse hepatitis virus (MHV-A59) infection is modulated by liver tryptophan-2,3-dioxygenase (TDO)

In a previous work we demonstrated that inhibition of mouse indoleamine 2,3-dioxygenase (IDO) by methyltryptophan (MT) exacerbated the pathological actions of mouse hepatitis virus (MHV-A59) infection, suggesting that tryptophan (TRP) catabolism was involved in viral effects. Since there is a second enzyme that dioxygenates TRP, tryptophan-2, 3-dioxygenase (TDO), which is mainly located in liver, we decided to study its role in our model of MHV-infection. Results showed that in vivo TDO inhibition by LM10, a derivative of 3-(2-(pyridyl) ethenyl) indole, resulted in a decrease of anti- MHV Ab titers induced by the virus infection. Besides, a reduction of some alarmin release, i.e, uric acid and high-mobility group box1 protein (HMGB1), was observed. Accordingly, since alarmin liberation was related to the expression of autoantibodies (autoAb) to fumarylacetoacetate hydrolase (FAH), these autoAb also diminished. Moreover, PCR results indicated that TDO inhibition did not abolish viral replication. Furthermore, histological liver examination did not reveal strong pathologies, whereas mouse survival was hundred percent in control as well as in MHV-infected mice treated with LM10. Data presented in this work indicate that in spite of the various TDO actions already described, specific TDO blockage could also restrain some MHV actions, mainly suppressing autoimmune reactions. Such results should prompt further experiments with various viruses to confirm the possible use of a TDO inhibitor such as LM-10 to treat either viral infections or even autoimmune diseases triggered by a viral infection.

Retention of anergy and inhibition of antibody responses during acute γ herpesvirus 68 infection

The majority of the human population becomes infected early in life by the gammaherpesvirus EBV. Some findings suggest that there is an association between EBV infection and the appearance of pathogenic Abs found in lupus. Gammaherpesvirus 68 infection of adult mice (an EBV model) was shown to induce polyclonal B cell activation and hypergammaglobulinemia, as well as increased production of autoantibodies. In this study, we explored the possibility that this breach of tolerance reflects loss of B cell anergy. Our findings show that, although anergic B cells transiently acquire an activated phenotype early during infection, they do not become responsive to autoantigen, as measured by the ability to mobilize Ca2+ following AgR cross-linking or mount Ab responses following immunization. Indeed, naive B cells also acquire an activated phenotype during acute infection but are unable to mount Ab responses to either T cell-dependent or T cell-independent Ags. In acutely infected animals, Ag stimulation leads to upregulation of costimulatory molecules and relocalization of Ag-specific B cells to the B-T cell border; however, these cells do not proliferate or differentiate into Ab-secreting cells. Adoptive-transfer experiments show that the suppressed state is reversible and is dictated by the environment in the infected host. Finally, B cells in infected mice deficient of CD4+ T cells are not suppressed, suggesting a role for CD4+ T cells in enforcing unresponsiveness. Thus, rather than promoting loss of tolerance, gammaherpesvirus 68 infection induces an immunosuppressed state, reminiscent of compensatory anti-inflammatory response syndrome.

Uric acid and HMGB1 are involved in the induction of autoantibodies elicited in mice infected with mouse hepatitis virus A59

We have shown that mice infected with mouse hepatitis virus A59 develop autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH). Because it has been proposed that the immune system is stimulated by alarm signals called damage-associated molecular patterns or alarmins, we investigated the participation of uric acid and high-mobility group box protein 1 (HMGB1) in the autoimmune response elicited by mouse hepatitis virus (MHV). Mice subjected to MHV infection had increased plasmatic uric acid concentration that significantly decreased after 20 days of daily treatment with allopurinol and, simultaneously, autoAb to FAH were undetected. Furthermore, this autoAb disappeared after 30 days of treatment with ethyl pyruvate, along with a substantial reduction in serum HMGB1 concentration. Both results indicated a remarkable relationship between the autoimmune process induced by the virus and uric acid and HMGB1 liberation. Unexpectedly, it was found that allopurinol and ethyl pyruvate inhibited the release of both uric acid and HMGB1. Because HMGB1 is activated through binding to interleukin 1β, and that this cytokine is produced by the NLRP3 inflammasome that could be stimulated by uric acid, we propose that both alarmins could be acting in concert with the induction of the autoAb to FAH in MHV-infected mice.

Autoimmune hepatitis-like disease in C57BL/6 mice infected with mouse hepatitis virus A59

Mouse hepatitis virus A59 (MHV A59) induces autoantibodies (autoAb) to fumarylacetoacetate hydrolase (FAH), a soluble cytosolic enzyme present in the liver and kidneys, in various mouse strains. The aim of this work was to amplify and diversify the autoimmune response restricted to FAH through the use of the exogenous adjuvant called PADRE. Accordingly, C57BL/6 mice were chosen, because these animals respond to PADRE better than other mouse strains. Results presented herein indicate that, surprisingly, C57BL/6 mice developed signs of autoimmune hepatitis-like disease (AIH), including transient hypergammaglobulinemia, elevated transaminases, autoAb directed against different liver proteins and hepatic cellular infiltrates, indicating that a new model of experimental AIH could be generated by a viral inoculation. Furthermore, PADRE administration amplified the MHV effect, extending the duration of hypergammaglobulinemia and increasing the binding of autoAb as well as the degree of hepatic infiltrates. However, the adjuvant did not expand the time of the symptoms. Additionally, since plasmatic uric acid and high-mobility group box protein 1 (HGMB1) concentrations augmented in MHV- and/or PADRE-treated mice, it is suggested that both alarmins were probably involved in the spreading of the immune response induced by the viral infection and the adjuvant administration.

Fine specificity of autoantibodies induced by mouse hepatitis virus A59

We have shown that mice infected with mouse hepatitis virus A59 (MHV-A59) develop autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH). The autoAb recognized conformational as well as linear antigenic determinants in the enzyme, and the autoimmune response was not entirely restricted to molecular mimicry and/or epitope spreading. Since the N- and C-terminal portions of the enzyme were the most reactive with autoAb, the fine specificity of these Ab was investigated. Immobilized 15-mer linear peptides (overlapping by 14 amino acids) spanning the N-terminal FAH sequence 1-49 were recognized by Ab from MHV-infected mice. The pattern of reactivity indicated the existence of two major epitope cores (i.e., sequences 9-23 and 30-44), and sequence comparison permitted the identification of two minimal epitopes, DSDFPIQ (amino acids 9-15) and IGDQILD (amino acids 36-42). Mutational analysis of sequences 9-23 and 30-44 indicated that residues 9-12 (DSDF) from the first major N-terminal epitope, and residue 36 (I) from the second, were the key amino acids energetically important for Ab contact. Interestingly, those residues were inside the two minimal epitopes previously predicted. The C-terminal portion of the enzyme (sequence 390-419) presented only one major epitope, located between residues 390 and 409. In this case, the minimal epitope had nine amino acids, CQGDGYRVG, corresponding to the FAH sequence 396-404 that outlines a loop specific for the enzyme. Data indicated that neither the FAH minimal epitopes nor the key residues important for binding to Ab from MHV-infected mice have their counterparts in the viral proteins. However, location of the energetically important residues in the tertiary structure of the enzyme originates a virtual conformational epitope. Such hypothetical B-cell epitopes could be present in any viral protein, originating a cross-reaction leading to the autoimmune response induced by MHV.

The autoimmune response induced by mouse hepatitis virus A59 is expanded by a hepatotoxic agent

Mouse hepatitis virus strain A59 (MHV-A59) triggers various pathologies in several mouse strains, including hypergammaglobulinaemia, hepatitis and thymus involution. We reported previously the presence of autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH) in sera from mice infected with MHV-A59.

Long-term MHV-infected mice represented a good model of non-pathogenic autoimmune response since the animals were apparently healthy in spite of the presence of autoAb. The aim of this work was to see whether a severe liver injury, which releases endogenous adjuvants, i.e. danger signals, could elicit a broader spectrum of autoAb and perhaps signs of autoimmune hepatitis. Carbon tetrachloride (CCl₄) was injected into mice 30 days after MHV infection, and serum was assayed for autoAb and total IgG 20 days later. The association of MHV infection with the toxic effects of CCl₄ resulted in hypergammaglobulinaemia and the production of autoAb to various liver and kidney proteins. Histological examination of liver samples showed tissue damages but without significant differences between the animals submitted to MHV + CCl₄ and controls, which were either infected by MHV without CCl₄, or poisoned by CCl₄ in the absence of MHV infection. Those results show that liver injury after viral infection may lead to the spreading of the immune response and to an increase of serum IgG, suggesting that the procedure used herein could simulate the onset of autoimmune hepatitis.

Properties of cryptic epitopes and their corresponding antibodies as indicated by the study of human and ovine growth hormones

Antibodies (Ab) directed to hidden antigenic determinants (cryptotopes) are undesirable because they are not neutralizing. Additionally, we have previously demonstrated a close association between the extent of Ab to cryptic determinants and the expression of autoantibodies (autoAb) under some experimental conditions. Thus, the first objective of this work was to establish the physicochemical characteristics of Ab to cryptotopes and the second one was to examine the structural features of cryptic epitopes themselves. Using human and ovine growth hormones (hGH and oGH) as antigenic models and competition ELISA under different conditions of temperature, pH or ionic strength, we did not find any difference between the binding properties of anti-cryptic epitope antibodies (Ab) and anti-native epitope Ab. Then, using synthetic peptides and tryptic digests and direct and competition ELISAs we studied the structures of cryptic hGH and oGH epitopes. Isolated peptides either in solution or adsorbed on microplates failed to react. Partially digested hGH was recognized only when insolubilized on microplates, and anti-oGH Ab only reacted with a large fragment of the hormone either in solution or insolubilized. These results indicate that, at least in the case of hGH and oGH, cryptic epitopes are not simple linear sequences, as commonly referred without any evidence, but new exposed conformational structures different from those found in the native antigen.

The peptide specificities of the autoantibodies elicited by mouse hepatitis virus A59

Synthetic decapeptides (N = 206) covering the entire sequence of mouse liver fumarylacetoacetate hydrolase (FAH) were used to analyze the specificities of the autoantibodies (autoAb) elicited towards this enzyme in mice infected with mouse hepatitis virus (MHV). These autoAb bound mainly to N- and C-terminal FAH peptides, the most reactive sequences being 1–50 and 390–420, respectively. Surprisingly, although FAH sequence 1–50 shares a high degree of homology with various MHV proteins, the C-terminal portion does not. Moreover, whereas the autoAb reacted with homologous peptides surrounding residues 70, 160 and 360, non-similar sequences around residues 130, 210, 240, 250, and 300 were also recognized, indicating that autoAb were not restricted to epitopes with sequence homologies. There was also a lack of correlation between the amount of anti-MHV or anti-FAH antibodies produced and the reactivity towards the peptides. Moreover, the spectrum of peptides recognized by the autoAb of a given mouse did not change significantly with time, which suggests that the MHV-elicited autoimmune response does not induce an epitope recognition spreading. Finally, anti-FAH Ab produced after immunization with rat liver FAH recognized essentially the same mouse FAH regions than autoAb from MHV-infected mice. Results indicated that the induction of the autoAb is not only related to molecular or structural mimicry, but rather supports the Danger model, in which any aggression, in this case the MHV infection, is susceptible to trigger the production of autoAb.