Animals receiving flu shots in the same paw for both a first and second dose had better trained memory B cells that bound tighter to the vaccine antigen than did mice who got the doses in separate limbs. 

When the adaptive immune system encounters something foreign for the first time—either by infection or vaccination—it trains its army to recognize and fight the invader. Still, repeated exposures to an antigen are often required to optimize this response. And new data suggests that the optimization process can be fine-tuned: booster shots elicited higher quality memory B cells in mice when they were given in the same limb as the original dose, researchers report today (May 6) in Science Immunology.

According to the study authors’ analyses, the higher-quality memory B cells seen after the same-limb booster are direct descendants of memory B cells trained by the first shot that had stuck around in the lymph node that drains the vaccination site. So in effect, they received two rounds of training in the same place instead of one, which might explain why their antibodies bound more tightly to the antigen. 

"I’m actually pretty excited about the paper," says University of Alabama at Birmingham immunologist Troy Randall, who was not involved in the work. "I think it is an interesting immunology question, but it is also a really practical question: which arm should you get boosted in?"

To test whether matching vaccination locality matters, a team led by scientists at Duke University injected mice with influenza hemagglutinin, the target protein used in flu vaccines. All mice got their first shot in their right back footpad and, one to three months later, a second shot either in the same or in the left back paw. 

When we boost, if we come back on the same site, we may be reactivating a different type of memory B cell than if we go on the other side.

–Tri Phan, Garvan Institute of Medical Research

Regardless of the strategy, all mice had comparable quantities of antibodies and B cells eight days after the second dose, indicating that in both cases, memory B cells generated by the first shot were reactivated by the vaccine antigens. But those that received the shots in the same paw had a higher number of highly mutated B cells that had evolved transmembrane receptors with stronger affinity to the antigen. Masayuki Kuraoka, an immunologist at Duke University and coauthor of the new study, says that the constant update of these receptors is "very important to fight against [the] so-called evolving viruses," such as influenza, SARS-CoV-2, and HIV, which constantly change.

After a vaccine is injected, the pathogen-mimicking antigens within it end up in the nearest lymph node, either by passive draining or with the aid of immune cells. There, a transient structure known as the germinal center forms—a sort of boot camp where the antibody-producing B cells acquire new mutations and go through rounds of selection for tighter binding between antibody and antigen. The trained B cells then act as the immune system’s memory and recirculate in the blood, ready to ramp up production of their antibodies should the antigen be seen again. But recent studies, including some authored by Randall, suggest that there is a subset of memory B cells that stay around in the site where the antigen was first observed and those may contribute to the immune response in following infections.   

Kuraoka and colleagues write the characteristics of the high-quality B cells they observed—and their higher frequency in mice with local boosters—led the team to hypothesize these were the progeny of cells already trained in the primary germinal center boot camp, in the nearest lymph node to the first shot. To test this, they labelled the B cells from the primary immunization with a fluorescent protein and traced their fate after the second shot. 

Their analysis revealed that mice with a local boost had a higher number of labelled cells in the secondary germinal center—the one formed in the nearest lymph node after the booster—than mice boosted in the opposite footpad. Moreover, these labelled cells had a higher number of mutations, placing them in the high-quality range the authors had previously observed. Overall, the results suggest that memory B cells trained after the first round of vaccination that had stayed in that same lymph node later engaged more efficiently in the immune response if a local boost—rather than a distal one—was received. 

Tri Phan, a B cell researcher at the Garvan Institute of Medical Research in Sydney, Australia, not involved the new study but whose team has also reported evidence of the so-called resident memory B cells, says it’s now clear that "memory B cells come in all sorts of different flavors." Based on the findings, he posits "that when we boost, if we come back on the same site, we may be reactivating a different type of memory B cell than if we go on the other side." 

Randall says further research on the importance of vaccination site is warranted, as it may be possible to optimize the location of the germinal center for any given vaccine.  "Where we need to go next is matching the vaccination with the infection," he says. If an infection happens in the lungs, memory B cells in an arm may not be ideal, for example. The challenge is to develop "a vaccine that would either put memory cells in the lung" or stimulate those already present if you were previously infected, he says. 

The new findings were focused on influenza, but the three researchers tell The Scientist that these results can likely be generalized to other vaccines. Kuraoka says that the benefits of choosing the same arm to boost against a particular agent in humans still need to be tested—and groups like Phan’s are currently addressing the question. But for now, he adds, it won’t hurt to get vaccines and boosters in the same arm just in case it does work better in people, too.