Our immune system clears invading pathogens through the activation of immune cells that recognize pathogen-derived signals. An additional role of the immune system is to provide long-lasting protective memory to avoid additional microbe invasions upon exposure to the same pathogen. Antibodies play a critical role in the establishment of long-lasting immunity. The generation of antibody-forming cells depends on the exchange of information between different cell types in specialized immune niches within lymph nodes. In these microanatomical sites, the precursors of antibody-forming cells respond to pieces of the pathogenic microbe or vaccine agents. The responding cells express on their surface an antibody that can recognize specifically the stimulating pathogen or vaccine and differentiate into antibody-forming cells. Whereas this process was extensively studied in the context of immune responses in peripheral lymph nodes in response to invading microbes and vaccination, less was known about how antibodies emerge in gut lymphoid organs. These are specialized structures that are scattered throughout the small intestine and host the precursors of antibody-forming cells and also supporting immune cells. As opposed to typical lymph nodes, the gut lymphoid organs have a different structure and they do not passively drain antigens from tissues as occurs in typical lymph.
In the current ERC-funded study we examined how antibody-mediated immune responses emerge in gut lymphoid organs and whether the same steps occur as those described for immune responses in peripheral lymph nodes. Since gut lymph nodes have an unconventional structure and host unique immunological niches we developed advanced imaging approaches in order to visualize them. This technique allows the visualization of all the immunological niches in intact organs and the detection of rare cells that respond to oral vaccination or a pantheon. Using this approach we show that precursors of antibody-forming cells encounter the vaccine or antigens in a unique niche with gut lymph nodes that are located close to the gut lumen. Followed initial activation, the immune cells transit to a different immunological niche where a selection of immune cells that express high-affinity protective antibodies takes place. We demonstrated that antibody affinity has no role in the niche where immune cell interacts with microbe-derived antigen, however, only cells that carry high-affinity antibodies progress to the next niche. We have identified the regulatory cells within the activation niche that controls immune cell expansion and allow the transit to the second niche. Furthermore, we show that the molecular mechanisms that control immune cell translocation between niches and cell-cell interaction in the gut are not necessarily similar to the one described in lymph nodes. Accordingly, we found that transgenic mice that are unable to support antibody immune response in peripheral lymph nodes can generate antibodies in the gut. Collectively, through the development of new imaging approaches, we have uncovered the cellular dynamics and molecular regulation that mediate antibody-mediated immune responses in the gut. These events are not similar to those that occur in peripheral lymph nodes. Understanding how antibodies are generated in the gut is expected to lead to better design of oral vaccinations.
