Date of Award
Master of Science
Molecular Biology Microbiology and Biochemistry
Salmonella enterica serovar Typhimurium (S. typhimurium) is a gram-negative bacteria capable of infecting a variety of warm-blooded vertebrate hosts. In humans, consumption of S. typhimurium through contaminated food or water typically leads to an acute but self-limiting gastroenteritis and oftentimes an enlargement of the spleen (splenomegaly). Splenomegaly has been attributed to the expansion of phagocytes, B and T lymphocytes, and immature CD71+Ter119+ red blood cells (RBCs). The spleen is an important organ with distinct roles in RBC recycling, the capture of blood-borne pathogens, and as a site of initiation of the adaptive immune response. The spleen has a characteristic tissue architecture composed of three compartments. The white pulp (WP), largely populated by B and T lymphocytes is surrounded by the red pulp (RP), which primarily contains F4/80+ macrophages. The border between the WP and RP, the marginal zone (MZ), is populated by MOMA+ and MARCO+ macrophages which are important for the capture of blood-borne pathogens. This precise organization of the spleen allows for efficient antigen capture and activation of adaptive immunity, due to the close proximity of antigen presenting cells and lymphocytes. It is known that Salmonella spp. infections delay the adaptive immune response in comparison to other bacteria, such as Listeria monocytogenes. Therefore, we investigated the effect of an attenuated S. typhimurium strain (÷9088) on in situ splenic organization. We utilized four-color immunofluorescence microscopy (IFM) in combination with flow cytometry to characterize the in situ changes of spleen architecture and cell population profiles during S. typhimurium infection in mice. Within the first week of infection, splenomegaly is evident and after three weeks of infection, the spleen comprises over 5% of the mouse's total body weight. During this time S. typhimurium has not been cleared from the spleen and mice are anemic with decreased pack cell volume. We confirmed previous studies that reported extramedullary erythropoiesis was a major cause of splenomegaly. However, we also show that RP F4/80+ macrophages significantly expand and take over the WP regions of the spleen, increasingly co-localizing with immature (CD71+Ter119+) and mature (CD71-Ter119+) RBC subsets. As a result of these dramatic changes in cell proportions and their in situ distribution, the splenic architecture becomes unrecognizable. The boundary between WP and RP is lost as proportions of MOMA+ macrophages of the MZ are reduced following infection. Likewise, B and T cell zones of the WP are also drastically reduced, most likely due to their increased co-localization with F4/80+ macrophages. As a result of infection, the increased cellularity of splenomegaly results in changing proportions of cell populations, potentially disrupting the link between infection and immune response. Together, these data provide further insight into the disease process of S. typhimurium infection in mice. Understanding how the changes in splenic architecture affect the adaptive immune response has implications for the design of more effective Salmonella-based vaccines and therapies.
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