Periodic Reporting for period 1 - GUT-MBC (Memory B cell dynamics in the gut mucosa)
Periodo di rendicontazione: 2022-08-01 al 2024-07-31
The immune system has the ability to respond quickly when it encounters a pathogen for the second time thanks to the generation of long-lasting memory cells from B and T lymphocytes. Humoral immunity in particular depends on two main types of B cells: long-lived plasma cells (LLPCs) that produce antibodies, and memory B cells (MBCs), which remain inactive until they encounter the same pathogen again when they quickly produce antibodies. These memory B cells have been extensively studied in primary and secondary lymphoid organs but recently, there has been more focus on how they work in barrier tissues like the lungs and intestines. Recently, our research group found that after flu infection, more than 75% of B cells generated in the lungs became MBCs situated in specific areas near the bronchi to give a rapid protection against future infections in the airways. However, it is still unclear if this also happens in the intestines. In our current study, we are examining how long-term immunity develops in the intestines after an infection with Salmonella typhimurium. Unlike what we observed in the lungs, nearly 90% of these B cells in the intestines become LLPCs, while only about 10% become MBCs. This suggests that while long-term immune response in the lungs relies mainly on MBCs, in the intestines it is primarily driven by LLPCs. We aim to find out whether these different strategies are due to the type of pathogen, the environment of the tissue, or a combination of both. Understanding these two different memory strategies could give us the clues to effectively design and develop nasal and oral vaccines against mucosal pathogens.
I completed the first part of my project focused on understanding how memory B cells develop in the gut after a Salmonella infection. Using a reporter mouse, I tracked these cells by flow cytometry in different organs including spleen, mesenteric lymph nodes, Peyer’s patches and Lamina propria, at different times after oral infection. I analyzed the isotype expression among memory B cell populations as well as the specificity of LLPCs by Elispot. However, the results didn't match what we expected. Unlike in the lungs, I didn’t find a significant number of memory B cells in the lamina propria. Because of this, we decided not to continue with the next phases of the project, which were originally planned to focus on characterizing these B cells. Instead, we shifted our focus to compare the different strategies adopted by the gut and lungs to respond to secondary infections. To do this, we added influenza infection as part of the project and study the immune response in the lungs and lymph nodes in order to be able to compare with salmonella infection in the gut. We discovered that the lungs and gut have different strategies for dealing with secondary infections: the lungs rely mostly on memory B cells, while the gut mainly uses long-lived plasma cells (LLPCs). By infecting mice with different load of bacteria (germ-free vs SPF mice) we found that in the gut, unlike in the lungs, the presence of microbes triggers the production of TGF-β, a cytokine that favors B cells switch to IgA. To go deeper in the analysis of IgA response in both mucosal tissues we used different transgenic mouse models and observed that the expression of IgA on B cells, regardless their affinity, pushes them towards becoming LLPCs by modulating downstream signaling. This means that the microbial status could shape the long-term strategies adopted by each mucosa.
In this study we were focused on study how memory B cell works at the barrier tissues, as lung and gut mucosas. We found that the lungs and gut have different strategies to establish a memory response. While in the lungs a large number of MBCs are produced, in the gut, the long term response is mostly dominated by LLPCs. This difference could be related to the presence of microbiota in the gut that favors a regulatory environment with high concentrations of TGF-β. This cytokine encourages B cells to switch to IgA, isotype that promotes B cells to enter into LLPC program. Current experiments being performed are focused to understand how IgA signaling favors B cell fate.
Understanding how memory response is established in these two important mucosal tissues is crucial to developing vaccines needed to protect against oral and/or nasal infections. Having effective vaccines that can prevent infections will significantly reduce healthcare costs.
Understanding how memory response is established in these two important mucosal tissues is crucial to developing vaccines needed to protect against oral and/or nasal infections. Having effective vaccines that can prevent infections will significantly reduce healthcare costs.