T-bet protein found essential for maintaining flu-fighting memory B cells

At the surface, the immune response to a flu virus is simple. Some cells recognize the pathogen and send a signal to the immune system, and immune cells produce a potentially lifesaving antibody against the virus. Antigen in, antibody out.

However, details of the intervening steps, as researchers have learned in the past 65 years, are quite complex—certain cells carry the flu antigen to the immune system, specific immune cells respond to the antigen and touch and interact with each other to evoke a response, antibody genes in each responsive B cell undergo many somatic mutations to create a panoply of potential specific antibodies, antibody classes like IgM and IgG are switched, cells producing the least fit antibodies are eliminated, and the survivors that produce the best flu-specific antibody boost their metabolism to produce massive amounts of the antibody protein.

Less well understood, but also complex, is the memory immune response to influenza, the creation of sentinel immune cells that stand on guard against subsequent infections. These long-lived memory cells have reacted to the flu antigen but avoid the irreversible pathway of antibody production. Instead, they wait quiescently in lungs and lymph nodes, primed to quickly transform into antibody-producing cells if a flu virus attacks again.

In a study published in the journal Immunity, Fran Lund, Ph.D., professor of microbiology and director of the University of Alabama at Birmingham Immunology Institute, and colleagues describe six subsets of memory B cells—including one subset that produces the transcription factor T-bet. Using detailed genetic analysis and manipulation, they show that continuous T-bet expression in these cells is key to preserving the protective memory response. In a mouse influenza model, they found that T-bet expression was required for the persistence of lung and lymph node memory B cells that have rapid differentiation potential to become antibody-producing plasma cells.

T-bet is a transcription factor, one of the control proteins in the cell nucleus that can turn specific groups of genes on and off. Cohorts of different transcription factors orchestrate diverse sets of genes that alter cell function and differentiation.

Previous studies had shown some association of T-bet expression with attributes of human, vaccine-specific memory B cells and with a long-lived humoral response to infection by mouse, virus-specific germinal center B cells.

To better understand T-bet-expressing memory B cells, UAB researchers infected mice with flu virus. Thirty days after infection, they isolated mature memory B cells that were specific for response to the influenza NP antigen and used single-cell sequencing to identify the gene expression of each cell. Gene-expression data of these individual cells comprised seven distinct clusters.

One cluster was excluded as developmentally distinct, and the other six were analyzed in detail, including core transcriptional regulators, the B-cell receptor repertoires and the functional attributes of each the gene expression in each cluster. T-bet was highly expressed and upregulated in cluster 2. Cluster 2 was also enriched for expression of genes reported to be upregulated in flu vaccine-elicited, T-bet-expressing human effector memory B cells. Enriched genes for protein synthesis in cluster 2 cells suggested a shift from the memory B cell identity program toward the antibody-producing program. While only the cluster 2 gene expression showed effector memory B cell characteristics, other clusters showed distinct stem-like, tissue surveilling or inflammatory characteristics.

Researchers used constitutive or inducible deletions of the T-bet gene from B cells to show that T-bet identifies flu infection-elicited lymph node and lung memory B cells that have rapid differentiation potential to antibody-secreting cells, and that T-bet expression by lung memory B cells was needed for a secondary lung antibody-secretion response following a second exposure to flu virus. In the future, the authors hope to use these data to design new ways to induce T-bet expression in human memory B cells to elicit memory cells that will reside at local sites of infection and can provide early protection from infection.