Infection with Salmonella enterica Serovar Typhimurium Leads to Increased Proportions of F4/80+ Red Pulp Macrophages and Decreased Proportions of B and T Lymphocytes in the Spleen

Infection of mice with Salmonella enterica serovar Typhimurium (Salmonella) causes systemic inflammatory disease and enlargement of the spleen (splenomegaly). Splenomegaly has been attributed to a general increase in the numbers of phagocytes, lymphocytes, as well as to the expansion of immature CD71+Ter119+ reticulocytes. The spleen is important for recycling senescent red blood cells (RBCs) and for the capture and eradication of blood-borne pathogens. Conservation of splenic tissue architecture, comprised of the white pulp (WP), marginal zone (MZ), and red pulp (RP) is essential for initiation of adaptive immune responses to captured pathogens. Using flow cytometry and four color immunofluorescence microscopy (IFM), we show that Salmonella-induced splenomegaly is characterized by drastic alterations of the splenic tissue architecture and cell population proportions, as well as in situ cell distributions. A major cause of splenomegaly appears to be the significant increase in immature RBC precursors and F4/80+ macrophages that are important for recycling of heme-associated iron. In contrast, the proportions of B220+, CD4+ and CD8+ lymphocytes, as well as MZ MOMA+ macrophages decrease significantly as infection progresses. Spleen tissue sections show visible tears and significantly altered tissue architecture with F4/80+ macrophages and RBCs expanding beyond the RP and taking over most of the spleen tissue. Additionally, F4/80+ macrophages actively phagocytose not only RBCs, but also lymphocytes, indicating that they may contribute to declining lymphocyte proportions during Salmonella infection. Understanding how these alterations of spleen microarchitecture impact the generation of adaptive immune responses to Salmonella has implications for understanding Salmonella pathogenesis and for the design of more effective Salmonella-based vaccines.鼠伤寒沙门氏菌（Salmonella enterica serovar Typhimurium）感染小鼠可引发全身性炎症性疾病与脾肿大（splenomegaly）。此前研究认为，脾肿大的成因包括吞噬细胞、淋巴细胞数量的普遍增多，以及未成熟CD71+Ter119+网织红细胞的扩增。脾脏的核心生理功能包括回收衰老红细胞（red blood cells, RBCs），以及捕获并清除血液传播的病原体。保持由白髓（white pulp, WP）、边缘区（marginal zone, MZ）与红髓（red pulp, RP）组成的脾脏组织架构的完整性，是启动针对捕获病原体的适应性免疫应答的必要条件。本研究通过流式细胞术（flow cytometry）与四色免疫荧光显微镜术（four color immunofluorescence microscopy, IFM）发现，沙门氏菌诱导的脾肿大以脾脏组织架构、细胞群体比例及原位细胞分布的显著改变为典型特征。脾肿大的主要诱因之一，似乎是未成熟红细胞前体与F4/80+巨噬细胞的显著增多——这类巨噬细胞在血红素结合铁的回收过程中发挥关键作用。与之相反，随着感染进程推进，B220+、CD4+与CD8+淋巴细胞，以及边缘区MOMA+巨噬细胞的比例均出现显著下降。脾脏组织切片可见明显撕裂，且组织架构发生显著改变：F4/80+巨噬细胞与红细胞突破红髓范围，占据了脾脏的绝大多数组织区域。此外，F4/80+巨噬细胞不仅会主动吞噬红细胞，还会吞噬淋巴细胞，这提示它们可能参与了沙门氏菌感染过程中淋巴细胞比例的下降。阐明此类脾脏微架构改变如何影响针对沙门氏菌的适应性免疫应答的产生，对于理解沙门氏菌的致病机制以及开发更高效的基于沙门氏菌的疫苗具有重要参考价值。