- Recently researchers found out why certain people are having severe covid19 issues by comparing their antibody response.
- IgG and IgA antibody responses are crucial and analysis shows a common viral protein target of the 56mer peptide region
- This standard antibody response is reduced during and aging and that could be the reason for disease severity
- Researchers found out that the IgG antibody showed the most cross-reactivity targeting the 56mer sequence of the viral proteins followed by IgA
- Hospitalized males showed a greater antibody response than hospitalized females
- The greater prevalence of the antibody response in hospitalized patients over non hospitalized ones
- Also, the hospitalized patients show a decrease in general viral antibody cross-reactivity than non hospitalized patients
A recent research paper published in Science highlights the extent of COVID19 severity as researchers found out cross-reactivity across various people.
In a viral epitome profiling method called VirScan, several covid19 patients were screened to detect the humoral response and cross-reactivity.
What’s in the study?
- Understanding humoral responses to SARS-CoV-2 is critical for improving diagnostics, therapeutics, and vaccines.
- Deep serological profiling of 232 COVID-19 patients and 190 pre-COVID-19 era controls using VirScan revealed over 800 epitopes in the SARS-CoV-2 proteome, including 10 epitopes likely recognized by neutralizing antibodies.
- Pre-existing antibodies in controls recognized SARS-CoV-2 ORF1, while only COVID-19 patients primarily recognized spike and nucleoprotein.
- A machine learning model trained on VirScan data predicted SARS-CoV-2 exposure history with 99% sensitivity and 98% specificity; a rapid Luminex-based diagnostic was developed from the most discriminatory SARS-CoV-2 peptides.
- Individuals with more severe COVID-19 exhibited stronger and broader SARS-CoV-2 responses, weaker antibody responses to prior infections, and higher incidence of CMV and HSV-1, possibly influenced by demographic covariates.
How did they do it?
- After serotyping and phylogenetic analysis of the targets of IgG and IgA antibodies separately revealed that the primary responses to SARS-CoV-2 are reactive with peptides derived from spike (S) and nucleoprotein (N).
- These two proteins exhibit significant differential recognition by sera from COVID-19 patients versus pre-COVID-19 era controls, indicating that their recognition is a result of antibody responses to SARS-CoV-2.
- Third-most frequently recognized is the replicase polyprotein ORF1, but, unlike S and N, ORF1 is recognized to a similar extent by sera from COVID-19 patients and pre-COVID-19 era controls.
- This suggests that recognition of SARS-CoV-2 ORF1 is a result of cross-reactions from antibodies elicited by exposure to other pathogens, possibly HCoVs.
- Antibody responses to peptides from membrane glycoprotein (M), ORF3 and ORF9b were occasionally detected in COVID-19 patients.
What antibody study is important?
IgG and IgA are the most abundant isotypes in the blood, while IgA is the principal isotype secreted on mucosal surfaces including the respiratory tract. Altogether the researchers analyzed approximately 550 samples in duplicate, in total assessing ~100 million potential antibody repertoire-peptide interactions.
To more precisely define the immunogenic regions of the SARS-CoV-2 proteome, the researchers examined the specific 56-mer and 20-mer peptides that were detected by VirScan in COVID-19 patients compared to pre-COVID-19 era controls.
They observed strong concordance between the viral regions enriched by the 56-mer and 20-mer fragments, demonstrating the robustness of VirScan.
In many cases the researchers observed recognition of overlapping 56-mer peptides, indicating an epitope in the common region.
What did they find?
The results of the study indicated that the antibodies IgG and IgA recognize the same protein regions with similar frequencies across the population but when IgG and IgA responses were compared within individuals, the researchers observed considerable divergence: many epitopes were recognized by only IgG, only IgA, or both IgG and IgA within an individual patient.
Together, these data suggest that patients raise distinct IgG and IgA antibody responses to SARS-CoV-2, but the regions targeted are largely shared at a population level.
This results in severe cases in some patients.
What did the study reveal about cross-reactivity?
The researchers returned to the question of epitope cross-reactivity by analysing the amino acid protein structure regions of the antibody responses.
This time they examined antibody responses to the triple-alanine scanning library. For each 56-mer peptide spanning the SARS-CoV-2 proteome, this library contained a collection of scanning mutants: the first mutant peptide encoded 3 alanines instead of the first 3 residues, the second mutant peptide contained the 3 alanines moved one residue downstream, and so on
Antibodies that recognize the wild-type 56-mer peptide will not recognize mutant versions of the peptide containing alanine substitutions at critical residues; thus, the location of the linear epitope can be deduced by looking for “antibody footprints,” indicated by stretches of alanine mutants missing from the pool of immunoprecipitated phage.
The first and last triple-alanine mutations to interfere with binding are expected to start two amino acids before the first residue essential for the antibody binding, and end two amino acids after the last.
With respect to cross-reactivity, IgG from COVID-19 patients recognized more 56-mer peptides from the common HCoVs HKU1, OC43, 299E, and NL63, than IgG from pre-COVID-19 era controls.
This difference is primarily driven by a dramatic increase in recognition of S peptides from the HCoVs and is likely a result of cross-reactivity of antibodies developed during SARS-CoV-2 infection
The reason for severity in certain patients?
An important goal is to uncover serological correlates of COVID-19 severity.
To this end, we compared cohorts of COVID-19 patients who had (H) or had not (NH) required hospitalization. Using both VirScan and the COVID-19 Luminex assay, the researchers found a striking and somewhat counterintuitive increase in recognition of peptides derived from the SARS-CoV-2 S and N proteins among the H group, with more extensive epitope spreading.
Whether this is a cause or a consequence of severe disease is not clear.
Individuals whose innate and adaptive immune responses are not able to quell the infection early may experience a higher viral antigen load, a prolonged period of antibody evolution and epitope spreading. Consequently, these patients might develop stronger and broader antibody responses to SARS-CoV-2 and could be more likely to have hyperinflammatory reactions such as cytokine storms that increase the probability of hospitalization.
The researchers noticed that hospitalized males had stronger antibody responses to SARS-CoV-2 than hospitalized females.
This may indicate that males in this group are less able to control the virus soon after infection and is consistent with reported differences in disease outcomes for males and females
Exposure History Showed Greater Prevalence in Hospitalized Patients
VirScan allowed us to examine viral exposure history, and this revealed two striking correlations.
First, the seroprevalence of CMV and HSV-1 was much greater in the H group compared to the NH group. The demographic differences in our relatively small cohort of H versus NH COVID-19 patients make it impossible for us to determine with certainty if CMV or HSV-1 infection impacts disease outcome or is simply associated with other covariates such as age, race and socioeconomic status.
While CMV prevalence does slightly increase with age after 40, its prevalence also differs greatly among ethnic and socioeconomic groups . CMV is a chronic herpes virus that is known to have a profound impact on the immune system: it can skew the naïve T-cell repertoire , decrease T and B cell function , and is associated with higher systemic levels of inflammatory mediators (35) and increased mortality of people over 65 years of age. The effects of CMV on the immune system could potentially impact COVID-19 outcomes.
The second striking correlation we observed was a significant decrease in the levels of antibodies targeting ubiquitous viruses such as Rhinoviruses, Enteroviruses, and Influenza viruses, in COVID-19 H patients compared with NH patients. When they examined only the CMV+ or HSV-1+ individuals in the two groups, we found that the strength of the antibody response to CMV and HSV-1 peptides was also reduced in the H group. They examined the effects of age on viral antibody levels in a pre-COVID-19 era cohort and found a diminution with age in the antibody response against viral peptides differentially recognized between the H and NH groups, consistent with previous studies on the effects of aging on the immune system.
This inferred reduced immunity during aging could impact the severity of COVID-19 outcomes.
Limitations of the Study
In correlative analyses such as these, it is difficult to draw strong conclusions about causality given the demographic differences in the NH versus H groups. The NH group is younger and has a higher percentage of Caucasians and females (average age 42, 66% female) compared to the H group (average age 58, 42% female) (fig. S2), consistent with well-documented demographic skews in severely-affected COVID-19 patients
However, even if age and other demographic factors are covariates, CMV seropositivity and age-related reduction in antibody titers against viral antigens as described here could still impact the severity of infection.
Larger Study Needed
To test these hypotheses, a much larger cohort of COVID-19 patients with severe and mild disease that could be matched for age, race and sex is required. Such future studies have the potential to enhance our understanding of the biological mechanisms underlying variable outcomes of COVID-19.
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Source: Science
