UF researchers devise new measures of flu immunity

Man and woman peer at birds in cages at a market in southern China.
A market in southern China within a FluScape study site; this photo was taken by Derek Cummings before the pandemic when uncovered faces in public spaces were customary.

Two University of Florida researchers have devised original metrics by which to evaluate the “shape” of individual immunity profiles against influenza over time and across different ages. Their new study, which published today in PLOS-Pathogens, could be used to deepen our understanding of how past exposures to flu affect our body’s response to new or currently circulating strains, as well as to seasonal flu vaccines.

The human immune system is remarkably complex. It recognizes disease-causing pathogens and in response pumps out custom-built disease-fighting proteins called antibodies. Exposures that occur during youth are thought to shape a person’s immune responses over his or her lifetime.

The immune system can fire off protective antibodies even when a person who was exposed to an infectious agent doesn’t become sick. Scientists can test an individual’s immune response repertoire to learn which diseases they’ve been exposed to in the past. Reading these histories is referred to as studying a person’s antibody profile.

Analyzing antibody profiles of various strains of common flu, to look for patterns across ages and over time, is one of the goals of the FluScape project spearheaded by UF preeminence professor Derek Cummings. He is the new paper’s senior author and a professor of biology in UF’s College of Liberal Arts and Sciences Department of Biology. Bingyi Yang, a postdoctoral associate in the same department, is the paper’s first author. Cummings heads the UF Infectious Disease Dynamics laboratory, where Yang is a member; they are both affiliated with UF’s Emerging Pathogens Institute.

The influenza virus can frequently change its surface molecules, or antigens, to successfully evade detection by the immune system of a person that has been infected with influenza before. Such viruses are said to have antigenic variation, and the flu virus is a champion at this among human pathogens. Each new flu strain requires the body to produce a new strain-specific variation of flu antibody. This is why health providers recommend people receive annual flu vaccines, to protect against new strains our bodies may not recognize.

Researchers can study past flu exposures in an individual by measuring the amount of antibody they have to a panel of influenza viruses that have circulated at different points in time during that person’s lifetime. If an individual’s serum produces antibodies against the tested flu strain, it tells researchers the person was exposed to the strain in the past.

But it’s a lot more complicated to characterize someone’s comprehensive antibody profile — the collection of all their responses to a group of flu strains — in a way that allows for measurable comparisons between individuals, across age groups and over time.

In their new study, Yang and Cummings worked with international collaborators from China and the United Kingdom to collect serum samples, paired in time, from 777 study participants aged two to 86 in southern China. The same individuals were sampled in both 2010 and 2014, with their serum tested at both time points against a panel of 21 strains of influenza type A subtype H3N2. The variant strains of H3N2 were selected to represent isolates of seasonal flu virus every two to three years since this subtype first emerged in humans in 1968. 

By analyzing the same individuals in different years, the researchers were able to key in on individual immune response changes. Fluctuations were expected due to annual exposures to new flu strains.

The six panels above show individual profiles of hemagglutination inhibition assay(HAI) titers for participants aged six, 18, 28, 43, 64 and 85 years old.
The six panels above show individual profiles of hemagglutination inhibition assay(HAI) titers for participants aged six, 18, 28, 43, 64 and 85 years old. The HAI test is useful for characterizing the strength of an individual’s immune response against specific viruses, such as influenza.The light blue and red vertical lines indicate the time points in 2010 and 2014 when participants’ serum was tested. The gray shaded area depicts their baseline (2010) HAI titers, and the purple shaded area represents their follow-up (2014) HAI titers.

The researchers then created three original metrics to analyze different features of how an individual’s immune system reacted to these flu strain variations in antibody tests. One metric gives a snapshot of the overall levels of antibody-mediated immunity (panels B-D below), and a second offers a picture of the scope and breadth of antibody-mediated immune response (panels F-H below). The third metric describes the temporal center of mass of their H3N2 immunity, which gives an indication of what year’s H3N2 viruses their immunity best targets (panels J-L below).

The team hypothesized that their new metrics would “capture biologically relevant properties of the immune response to H3N2.”

Panels that depict scatter plots with trend lines of the three new metrics the researchers created to characterize immunity profiles.
Panels B-D, F-G, and J-L depict scatter plots with trend lines of the three new metrics the researchers created to characterize immunity profiles. Blue plots represent the baseline sampling in 2010, red plots represent the follow-up sampling in 2014, and purple plots represent differences in metrics between the two visits.

They found that the overall shape of a person’s antibody profile remained somewhat constant across time, but there were noticeable differences attributable to the flu strains circulating in the year the samples were taken. There were also distinct differences between age groups.

Their results show that people accumulate immunity to various strains of flu up to age 20, then there is a decrease of immunity until ages 40-50, after which it rises again. They suggest that people build a portfolio of immunity up until their twenties and reap the benefits of greater protection through the next couple of decades when they see lower rates of infection. But because they do not get infected as frequently, their antibody portfolio decays, leading to subsequent increases in antibody after their 40s and 50s.

“We think that people in their seventies may have higher antibody titers because they are becoming infected more frequently or other aspects of their immune responses are declining,” Yang says. “They have higher titers because they are getting infected and those infections last longer. But many mechanisms may contribute to this phenomenon.”

It could also be that advanced age is itself a byproduct of excellent immunity. 

“Alternatively, it may be due to a survivors’ effect,” Yang says. “Unfortunately by this age, some people in this cohort have sadly passed away, and those who survive may be the ones who have a more robust immune system overall.”

The team also found that flu antibody measures increased significantly over the four years of the study.

“Our results suggest that our entire study population is varying in the amount of antibody to H3N2 they have over time,” says Yang. “This may be due to variation in the incidence of infections from year to year and contribute to that variation because the population’s immunity is derived from those exposures,” Yang says.

Another finding was that people who had higher immunity levels to previously exposed strains were more likely to experience seroconversion, or the process of making new antibodies, to recently circulating strains. The mechanism for this is unclear, Yang says.

“It might be the cross-reactivity to newer strains conferred from immunity to older strains is not enough to protect people from new infection,” she says. “Or it might be some people are good at stimulating their immune responses, so they are always good at generating antibodies.”

Despite the complexity of the data, Cummings and Yang hope the new metrics will offer a tool to uncover deeper patterns related to age and the cross-reactivity of strain-specific antibodies being activated to fight a different but closely related one.

“Our results suggest that our immunity to influenza comes from life course exposures that could possibly affect future exposure risks,” Cummings says. “It motivates us to understand more about antibody profiles, to examine the impacts of flu vaccine, and to predict future epidemics.”

Future research will include additional influenza subtypes to elicit more patterns in the search to understand how a person’s past exposures affect how they will respond to new flu exposures.

EPI Explainer: What is an antibody profile?

Think of it this way: over a lifetime, an individual’s immune system builds a reference library of the infectious agents with which it has interacted. Each “book” in this library is a pathogen-specific antibody. Because many viruses mutate frequently and have several strains that are related, some “books” are variations on an antibody theme and are specific to these strains. An antibody profile is akin to a bibliography but with some added dimensions; it tells researchers which strains a person has been exposed to, but it also tells researchers about the strength of an individual’s response to fight a specific pathogen.

Written by: DeLene Beeland