Salmonella enterica causes more severe inflammatory disease in C57/BL6 Nramp1G169 mice than Sv129S6 mice

Salmonella enterica serovar Typhimurium (S. Typhimurium) causes systemic inflammatory disease in mice by colonizing cells of the mononuclear leukocyte lineage. Mouse strains resistant to S. Typhimurium, including Sv129S6, have an intact Nramp1 (Slc11a1) allele and survive acute infection, whereas C57/BL6 mice, homozygous for a mutant Nramp1 allele, Nramp1(G169D) , develop lethal infections. Restoration of Nramp1 (C57/BL6 Nramp1(G169) ) reestablishes resistance to S. Typhimurium; mice survive at least 3 to 4 weeks postinfection. Since many transgenic mouse strains are on a C57/BL6 genetic background, C57/BL6 Nramp1(G169) mice provide a model to examine host genetic determinants of resistance to infection. To further evaluate host immune response to S. Typhimurium, we performed comparative analyses of Sv129S6 and C57/BL6 Nramp1(G169) mice 3 weeks following oral S. Typhimurium infection. C57/BL6 Nramp1(G169) mice developed more severe inflammatory disease with splenic bacterial counts 1000-fold higher than Sv129S6 mice and relatively greater splenomegaly and blood neutrophil and monocyte counts. Infected C57/BL6 Nramp1(G169) mice developed higher proinflammatory serum cytokine and chemokine responses (interferon-γ, tumor necrosis factor-α, interleukin [IL]-1β, and IL-2 and monocyte chemotactic protein-1 and chemokine [C-X-C motif] ligand 1, respectively) and marked decreases in anti-inflammatory serum cytokine concentrations (IL-10, IL-4) compared with Sv129S6 mice postinfection. Splenic dendritic cells and macrophages in infected compared with control mice increased to a greater extent in C57/BL6 Nramp1(G169) mice than in Sv129S6 mice. Overall, data show that despite the Nramp1 gene present in both strains, C57/BL6 Nramp1(G169) mice develop more severe, Th1-skewed, acute inflammatory responses to S. Typhimurium infection compared with Sv129S6 mice. Both strains are suitable model systems for studying inflammation in the context of adaptive immunity.

Hemophagocytic macrophages in murine typhoid fever have an anti-inflammatory phenotype

Histiocytes are white blood cells of the monocytic lineage and include macrophages and dendritic cells. In patients with a variety of infectious and noninfectious inflammatory disorders, histiocytes can engulf nonapoptotic leukocytes and nonsenescent erythrocytes and thus become hemophagocytes. We report here the identification and characterization of splenic hemophagocytes in a natural model of murine typhoid fever. The development of a flow-cytometric method allowed us to identify hemophagocytes based on their greater than 6N (termed 6N+) DNA content. Characterization of the 6N+ population from infected mice showed that these cells consist primarily of macrophages rather than dendritic cells and contain T lymphocytes, consistent with hemophagocytosis. Most 6N+ macrophages from Salmonella enterica serovar Typhimurium-infected mice contain intact DNA, consistent with hemophagocytosis. In contrast, most 6N+ macrophages from control mice or mice infected with a different bacterial pathogen, Yersinia pseudotuberculosis, contain damaged DNA. Finally, 6N+ splenic macrophages from S. Typhimurium-infected mice express markers consistent with an anti-inflammatory M2 activation state rather than a classical M1 activation state. We conclude that macrophages are the predominant splenic hemophagocyte in this disease model but not in Y. pseudotuberculosis-infected mice. The anti-inflammatory phenotype of hemophagocytic macrophages suggests that these cells contribute to the shift from acute to chronic infection.

Chronic murine typhoid fever is a natural model of secondary hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a hyper-inflammatory clinical syndrome associated with neoplastic disorders especially lymphoma, autoimmune conditions, and infectious agents including bacteria, viruses, protozoa and fungi. In both human and veterinary medicine, hemophagocytic histiocytic disorders are clinically important and frequently fatal. HLH in humans can be a primary (familial, autosomal recessive) or secondary (acquired) condition, with both types generally precipitated by an infectious agent. Previously, no mouse model for secondary HLH has been reported. Using Salmonella enterica serotype Typhimurium by oral gavage to mimic naturally-occurring infection in Sv129S6 mice, we characterized the clinical, hematologic and morphologic host responses to disease thereby describing an animal model with the clinico-pathologic features of secondary HLH as set forth by the Histiocyte Society: fever, splenomegaly, cytopenias (anemia, thrombocytopenia), hemophagocytosis in bone marrow and spleen, hyperferritinemia, and hypofibrinogenemia. Disease severity correlates with high splenic and hepatic bacterial load, and we show disease course can be monitored and tracked in live animals. Whereby secondary HLH is known to occur in human patients with typhoid fever and other infectious diseases, our characterization of a viable natural disease model of secondary HLH offers an important means to elucidate pathogenesis of poorly understood mechanisms of secondary HLH and investigation of novel therapies. We characterize previously unreported secondary HLH in a chronic mouse model of typhoid fever, and novel changes in hematology including decreased tissue ferric iron storage that differs from classically described anemia of chronic disease. Our studies demonstrate S. Typhimurium infection of mice is a natural infectious disease model of secondary HLH that may have utility for elucidating disease pathogenesis and developing novel therapies.