In a recent study published in Scientific Reports, researchers assessed the pre-existing cross-protective immune responses against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein and spike (S) protein before the coronavirus disease 2019 (COVID-19) pandemic in individuals residing in North and South America, Europe and Africa in vitro and in vivo.
Background
Researchers initially estimated a high incidence and severity of COVID-19 in the African continent; however, the continent has been relatively spared to date. Several hypotheses have been put forward for explaining the lesser than estimated impact of SARS-CoV-2 infections on the general public health in Africa. However, the role of pre-existing humoral immunity against SARS-CoV-2 needs further investigation.
About the study
In the present study, researchers assessed the pre-existing anti-SARS-CoV-2 S and N titers in sera obtained from residents of North and South America, Europe, and Africa prior to the COVID-19 pandemic (i.e., before November 2019). They also evaluated the neutralization potency and the protective efficacy against the SARS-CoV-2 N protein in vitro and in vivo, respectively.
Anti-SARS-CoV-2 S and N immunoglobulin G (IgG) levels were measured using enzyme-linked immunosorbent assays (ELISA). In addition, anti-SARS-CoV-2 N neutralizing antibodies (nAb) titers were assessed using neutralization assays in vivo among Central Africa donors and in vitro using SARS-CoV-2-infected rodents.
Sera from 121, 43, 146, and 112 individuals from Denmark, Quebec, Gabon (Central Africa), and Brazil were obtained, and anti-BSA (bovine serum albumin) IgG levels were measured with convalescent serum samples from laboratory-confirmed cases of COVID-19 (n=12) from Ontario as positive controls. Further, the team investigated if pre-pandemic cross-protective anti-SARS-CoV-2 immune responses were limited to Gabonese donors, for which, serum samples were obtained from 150 Senegal (West Africa) residents and two confirmed-COVID-19 cases from Senegal before May 2018 were analyzed.
Furthermore, the in vitro SARS-CoV-2 neutralization potency and the in vivo anti-SARS-CoV-2 protective efficacy of Gabonese group sera were tested to investigate if anti-N IgG levels could protect against severe COVID-19. Sera from three confirmed-COVID-19 served as positive controls. Lastly, the ability of humoral antibodies from the Gabon group sera to influence viral load was assessed in SARS-CoV-2-infected human angiotensin-converting enzyme 2 (hACE2) transgenic mice.
The total IgG level in sera of Gabonese donors with or without anti-N antibodies and sera of confirmed-COVID-19 cases (six to eight donors per group) was tested, and pulmonary SARS-CoV-2 titers in mice at five days post-infection (dpi) were evaluated based on the 50% tissue culture infectious dose (TCID50) values.
Results
Anti-SARS-CoV-2 S and anti-SARS-CoV-2 N antibodies were found to be rare across all populations except in Senegal and Gabon, which showed a higher prevalence of anti-SARS-CoV-2 N antibodies. However, the antibodies could not neutralize SARS-CoV-2 in vitro or in vivo. Sera from confirmed cases of COVID-19 and one, two, four, four, and 30 samples from Gabon, Ontario, Denmark, and Brazil showed the presence of anti-BSA IgG titers.
Sera of all COVID-19 patients had detectable anti-S IgG levels whereas anti-S IgG was undetectable or rare in sera obtained from Ontario, Brazil, and Denmark and 10% (n=12) samples from Gabon samples had detectable anti-S IgG levels. However, the anti-S IgG levels were low. On the contrary, high anti-N IgG levels were observed with detectable levels in 82% (n=9) of confirmed COVID-19 cases and 17% (n=20), 12% (n=1), 2.5% (n=3), 3.7% (n=4) and serum samples from Gabon, Ontario, Denmark and Brazil, respectively.
Notably, the anti-N IgG levels were of significantly higher magnitude in Gabon samples in comparison to those from Ontario, Brazil, and Denmark. Unlike in COVID-19 cases, sera from the Gabon group rarely had concomitantly detectable anti-SARS-CoV-2 S antibodies and anti-SARS-CoV-2 N antibodies. In contrast to the Gabon group samples, a large fraction of Senegal group samples had detectable and correlated anti-S IgG and anti-N IgG levels. The findings indicated that the cross-protective anti-SARS-CoV-2 immunity could be due to different immunological mechanisms in Gabon and Senegal residents or due to age-based differences since Senegal residents were younger.
Notably, none of the serum samples obtained from Gabonese donors had detectable anti-S IgG levels. The nAb titers in sera of COVID-19 cases ranged between 320 and 28, whereas no SARS-CoV-2 neutralization (titers below 20) was detected for Gabonese donor samples independent of the absence or presence of anti-N IgG antibodies.
Pulmonary SARS-CoV-2 titers in mock-treated mice were 1.8× 1010 and reduced to 7.4× 103 TCID50/ml on treatment with purified IgG from confirmed COVID-19 cases. Contrastingly, there were no significant differences in pulmonary SARS-CoV-2 titers of mock-treated rodents (1.8× 1010 TCID50/ ml), rodents treated with purified IgG from Gabon group sera with detectable anti-N IgG (1.4× 109 TCID50/ ml) or without detectable anti-N IgG (5.8× 108 TCID50/ ml) and rodents in which commercial anti-N IgG antibodies were administered (3.6× 109 TCID50/ ml).
Conclusion
Overall, the study findings showed that cross-reactive anti-SARS-CoV-2 N immunity was prevalent in Africa before the COVID-19 pandemic. However, the pre-existing immunity does not impact SARS-CoV-2 fitness in mice, indicating that other immunological defense mechanisms may be involved in humans. In Africa, seroprevalence studies against the SARS-CoV-2 N protein may overestimate the circulation of SARS-CoV-2.
