Germinal centers form in the body’s lymphatic tissues shortly after vaccination or infection. Once inside a germinal center, B cells undergo rapid mutations and, through a process of natural selection, only B cells with antibodies that most effectively bind to their target antigens survive. Figuring out how germinal centers work is critical to understanding immunity and developing more effective vaccines.
A new study “Clonal replacement sustains long-lived germinal centers primed by respiratory viruses” in Cell shows why some germinal centers persist for months rather than weeks, providing insights that could inform future vaccine design.
“The duration of GC reactions spans a wide range, and long-lasting GCs (LLGCs) are potentially a source of highly mutated B cells. We show that rather than consisting of continuously evolving B cell clones, LLGCs elicited by influenza virus or SARS-CoV-2 infection in mice are sustained by progressive replacement of founder clones by naive-derived invader B cells that do not detectably bind viral antigens,” write the investigators.
“Rare founder clones that resist replacement for long periods are enriched in clones with heavily mutated immunoglobulins, including some with very high affinity for antigen, that can be recalled by boosting. Our findings reveal underappreciated aspects of the biology of LLGCs generated by respiratory virus infection and identify clonal replacement as a potential constraint on the development of highly mutated antibodies within these structures.”
“The goal of the germinal center is to generate high-affinity plasma cells and memory B cells, that it then exports” says Renan V.H. de Carvalho, PhD, a postdoctoral fellow in the laboratory of Gabriel D. Victora at The Rockefeller University.
In mice, most germinal centers shut down after a few weeks, having accomplished their goal of producing high-affinity B cells. But those that form in response to certain respiratory infections, including the flu, can stay in business for more than six months, roughly a quarter of a mouse’s normal lifespan.
de Carvalho and his colleagues wanted to understand why these germinal centers are so long-lived, and what precisely goes on inside them. For the study, the researchers first infected mice with influenza and SARS-CoV-2 viruses, waited for them to form germinal centers, and then sequenced the antibody genes of B cells harvested from those centers over the course of 24 weeks.
Rather than continuously evolving at a steady clip, antibody optimization peaked after 12 weeks and then apparently regressed, even as the center remained active. This drop-off was due to the continuous introduction of unevolved “naïve” B cells into the germinal centers, researchers later found.
As weeks turned into months, a more complete picture began to form: the founder B cells that had initially seeded the long-lived germinal centers were being gradually replaced by naïve ones, so that only a tiny fraction of late germinal centers was made of the descendants of the B cells that started them.
Old- vs new-school
These new B cells did not behave like the original ones in the germinal center. Subsequent experiments showed that, while the naïve B cells also underwent evolution inside the germinal centers, they did not produce antibodies that could bind to flu or SARS-CoV-2 antigens.
“We used to think of infection-induced germinal centers as a single reaction targeting antigens from a particular pathogen,” de Carvalho says. “Apparently it’s not, at least in the case of these long-lived germinal centers.”
But the few original B cells that remained on site were enough to produce efficient immunity against the initial pathogen. When the researchers re-exposed the mice to flu antigens three months after they were first infected—effectively mimicking a repeat infection or booster shot—they demonstrated that many of the memory B cells which began producing antibodies were descended from the few founder cells that lingered in germinal centers for many months, and not their naïve replacements.
“Even though they constitute a small fraction of the total number of cells later on, the founder cells that stay in the germinal center for a long time are still doing their job,” notes Carvalho. But just how well those founder B cells do their jobs, and whether naïve recruits cramp their style and reduce their efficacy, remains to be seen. Future studies from the Victora lab will address this question.
The findings already have implications for our general understanding about how germinal centers operate. Understanding the dynamic between founder and naïve B cells could help researchers leverage long-lived germinal centers to produce more effective antibodies against dangerous respiratory viruses, like the flu and SARS-CoV-2.