Isolated specific IgA against respiratory viruses, Influenza or SARS-CoV-2, present in the saliva of a fraction of healthy and asymptomatic volunteers

Abstract

Objectives: Defense against respiratory viruses depends on an immune response present in the mucosa, as saliva IgA secretes antibodies. During the pandemic, such as influenza or SARS-CoV-2, most infected patients are asymptomatic but retain specific antibodies post-infection. The authors evaluated IgG and IgA antibodies against SARS-CoV-2 and influenza in the saliva of asymptomatic volunteers, validated with controls or vaccinated individuals.

Methods: The authors detected specific antibodies by validated conventional ELISA using natural SARS-CoV-2 antigens from infected Vero cells or capture-ELISA for influenza using natural antigens of the influenza vaccine.

Results: Saliva from influenza-vaccinated individuals had more IgA than paired serum, contrary to the findings for specific IgG. In COVID-19-vaccinated samples, specific IgA in saliva increased after vaccination, but IgG levels were high after the first dose. In saliva from the asymptomatic population (226), anti-Influenza IgG was found in 57.5% (130) of samples, higher than IgA, found in 35% (79) of samples. IgA results were similar for SARS-CoV-2, with IgA present in 30% (68) of samples, while IgG was less present, in 44.2% (100) of samples. The proportion of influenza IgG responders was higher than that for SARS-CoV-2 IgG, but both populations presented similar proportions of IgA responders, possibly due to variable memory B cell survival. For both viruses, the authors found an important proportion (> 10%) of IgA+IgG- samples, suggesting the occurrence of humoral immunity directed to the mucosa.

Conclusion: Specific antibodies for respiratory viruses in saliva are found in either infection or vaccination and are a convenient and sensitive diagnostic tool for host immune response.

Keywords: IgA; IgG; Influenza; SARS-CoV-2; Saliva.

Fig. 1

Detection of IgA (A) and IgG (B) antibodies specific for influenza in the serum (open triangle) and saliva (closed diamond) of vaccinated volunteers. Individual values are expressed as Artificial Units. Significant differences are marked by asterisks between the indicated groups (****p < 0.0001), determined by Bonferroni posttests.

Fig. 2

Detection of specific IgA (A) and IgG (B) antibodies for SARS-CoV-2 in the saliva of volunteers vaccinated with CoronaVac®. Individual values are expressed as Artificial Units. Significant differences are marked by asterisks between the indicated groups (*p < 0.05, **p < 0.01, ****p < 0.0001), determined by Bonferroni posttests.

Fig. 3

Individual detection of IgG (A) and IgA (B) antibodies specific for influenza in the saliva of volunteers. Individual values are expressed as Artificial Units. Significant differences are marked by asterisks between the indicated groups (****p < 0.0001), determined by Bonferroni posttests.

Fig. 4

Percentage of individuals positive for SARS-CoV-2 (black bar) and influenza (gray bar) specific IgG and IgA. Significant differences are marked by asterisks between the indicated groups (**p < 0.01), determined by Chi square tests. NS, not significant.

Fig. 5

Individual detection of specific IgG (A) and IgA (B) antibodies for SARS-CoV-2 in the saliva of volunteers. Individual values are expressed as Artificial Units. Significant differences are marked by asterisks between the indicated groups (****p < 0.0001), determined by Bonferroni posttests.

Fig. 6

Correlation between specific IgG and IgA for SARS-CoV-2 (A) and influenza (B) in the group of positive individuals. Individuals’ values are expressed as Artificial Units. (C) Percentage of IgG+IgA+, IgG+IgA-, and IgG-IgA+ for SARS-CoV-2 (black bar) and influenza (gray bar) in exposed infected individuals, excluding negative samples. All proportions are similar when restricted to any immunoglobulin positive population.