Early adaptive humoral immune responses and virus clearance in humans recently infected with pandemic 2009 H1N1 influenza virus

Abstract

Few studies on the humoral immune responses in human during natural influenza infection have been reported. Here, we used serum samples from pandemic 2009 H1N1 influenza infected patients to characterize the humoral immune responses to influenza during natural infection in humans. We observed for the first time that the pandemic 2009 H1N1 influenza induced influenza A-specific IgM within days after symptoms onset, whereas the unit of IgG did not changed. The magnitude of influenza A-specific IgM antibodies might have a value in predicting the rate of virus clearance to some degree. However, the newly developed IgM was not associated with hemagglutination inhibition (HI) activities in the same samples but correlated with HI activities of subsequently collected sera which were mediated by IgG antibodies, indicating that IgM was critical for influenza infection and influences subsequent IgG antibody responses. These findings provide new important insights on the human immunity to natural influenza infection.

Figure 1. Influenza A specific IgM and IgG in serum samples from 2009 pandemic H1N1 infected patients and non-influenza related sources.

The units of IgM (a) and IgG (b) antibodies to influenza A in sera were determined by commercial ELISA kits. 131 pandemic 2009 H1N1 influenza infected samples, 100 samples collected in Sep. 2008, 3 acute RSV infected samples, 8 acute parainfluenza infected samples, 11 acute adenovirus infected samples and samples from 60 HIV+ subjects taken in Sep. 2009 and Jun. 2010 were also used in this experiment. In the box plot, the whiskers represent 10–90th percentile.

Figure 2. Cross sectional analysis of the associations between IgM and genders, viral loads, nationalities and ages.

a) Mann-Whitney U test was used to test the difference of IgM between 58 female and 71 male H1N1 infected patients; b) The association between 109 paired data of IgM and viral loads in nasopharyngeal swab was analyzed using spearman rank correlation; c) The difference of grouped samples by nationality was tested by Kruskal-Wallis test; d) Age distribution of IgM. All data were derived from samples of the 2nd day post symptom onset.

Figure 3. Time course of IgM in week of H1N1 infection.

a) IgM grouped by days since of illness; b) Longitudinal kinetic of IgM in 73 paired subjects who had seral samples with multiple time-points.

Figure 4. Initial viral load in nasopharyngeal swab and serum IgM affect virus clearance.

Proportions of patients with undetectable viral loads in days since admitted to hospital were graphed with the days post symptom onset. Patients were divided into quartiles by viral load (a) or unit of IgM (b), 1st and 4th quartiles were showed. Each quartile had 27 subjects; c) The viral loads of patients in groups of 1st and 4th quartiles of IgM unit were compared.

Figure 5. Time course of HI titers against homologous and heterologous virus strains in week of H1N1 infection.

a) HI titers against 2009 H1N1 grouped by days since of illness; b) Longitudinal kinetic of HI titers against 2009 H1N1 in 73 subjects. Paired two samples from each subject, including one was collected within 3 days after the illness onset and the other was collected during day 4–10, were determined for their HI titers in parallel and compared; c) HI titers against H3 in 25 paired subjects infected with 2009 H1N1.

Figure 6. Association between IgM and HI titers and the contribution of IgM to HI activities.

The association of early IgM and HI titers in early days (a) or late days (b) since influenza infection was shown. The depletion of IgM (c) or IgG (d) affect HI titers against 2009 H1N1 infeluenza virus.