Nasal IgA Provides Protection against Human Influenza Challenge in Volunteers with Low Serum Influenza Antibody Titre

Abstract

In spite of there being a number of vaccines, influenza remains a significant global cause of morbidity and mortality. Understanding more about natural and vaccine induced immune protection against influenza infection would help to develop better vaccines. Virus specific IgG is a known correlate of protection, but other factors may help to reduce viral load or disease severity, for example IgA. In the current study we measured influenza specific responses in a controlled human infection model using influenza A/California/2009 (H1N1) as the challenge agent. Volunteers were pre-selected with low haemagglutination inhibition (HAI) titres in order to ensure a higher proportion of infection; this allowed us to explore the role of other immune correlates. In spite of HAI being uniformly low, there were variable levels of H1N1 specific IgG and IgA prior to infection. There was also a range of disease severity in volunteers allowing us to compare whether differences in systemic and local H1N1 specific IgG and IgA prior to infection affected disease outcome. H1N1 specific IgG level before challenge did not correlate with protection, probably due to the pre-screening for individuals with low HAI. However, the length of time infectious virus was recovered from the nose was reduced in patients with higher pre-existing H1N1 influenza specific nasal IgA or serum IgA. Therefore, IgA contributes to protection against influenza and should be targeted in vaccines.

Keywords: Human Infection Challenge study; IgA; influenza; nasal; vaccine.

FIGURE 1

Human influenza infection in clinical volunteers. Patients were inoculated intranasally with 3.5 × 10⁶ TCID₅₀ influenza A/California/2009 (H1N1). Samples were collected daily and assessed for influenza by culture or rtPCR, and disease signs and symptoms. Cultured virus presented as days virus positive (A), influenza RNA rtPCR data presented as area under the curve plotted over the time course of infection (B), signs and symptoms are presented as cumulative total (C). Correlation plots of days virus positive against detectable Influenza RNA (D) or signs and symptoms (E). (A–C) Are presented as binned frequencies. Data presented are combined from two studies, n = 47 volunteers.

FIGURE 2

Infection with influenza increases H1 serum IgG antibody responses, but IgG does not correlate with protection. Healthy adult human volunteers were infected intranasally with different doses of H1N1 influenza (low dose: 3.5 × 10⁴ TCID₅₀, Medium dose: 3.5 × 10⁵ TCID₅₀ and high dose: 3.5 × 10⁶ TCID₅₀). H1 specific IgG responses were measured by ELISA in serum on day-1 and day 29 of infection and presented as absolute values (A), fold change based on the infecting dose (B). HAI was assessed at day-1 and day 29 (C) and compared with IgG titre on day 29 (D). IgG ELISA titre was compared to days culture positive (E) and cumulative signs and symptoms (F). (A–D) Represent data from a single study n = 29 volunteers, (E,F) Represent data from two studies combined n = 47.

FIGURE 3

Infection with influenza increases H1 serum IgA antibody responses. Healthy adult human volunteers were infected intranasally with different doses of H1N1 influenza. H1 specific IgA responses were measured by ELISA in serum on day-1 and day 29 of infection and presented as absolute values (A) and fold change based on the infecting dose (B). Serum IgA and IgG were compared at day-1 (C) and day 29 (D). Data from a single study n = 29 volunteers.

FIGURE 4

H1N1 specific IgA in nasal wash and serum correlate weakly with protection. H1N1 specific IgA and total IgA were measured in the nasal wash samples collected on day-1 prior to infection (A). H1N1 specific IgA responses were compared between nasal wash and serum (B). Days culture positive virus was recoverable compared to serum IgA (C) and nasal IgA (D). Data pooled from two studies, n = 47 volunteers.

FIGURE 5

H3N2 specific antibody does not correlate with protection. H3N2 specific IgG (A), HAI (C) and IgA (D) were measured in serum on day-1 and day 29, fold change in IgG response compared between H1N1 and H3N2 IgG (B). Days culture positive virus was recoverable compared to H3N2 specific serum IgA (E) and nasal IgA (F). Data pooled from two studies, n = 47 volunteers.