Histopathological study of protective immunity against murine salmonellosis induced by killed vaccine

Dynamics of growth and dissemination of Salmonella in vivo

The last decade has witnessed increasing research on dissemination of bacterial pathogens in their hosts and on the processes that underlie bacterial spread and growth during organ colonization. Here, we discuss work on the mouse model of human typhoid fever caused by Salmonella enterica serovar Typhimurium. This has revealed the use of several routes of systemic dissemination that result in colonization and growth within the spleen and liver, the major sites of bacterial proliferation. We also highlight techniques that enable in vivo analysis of the infecting population at the spatiotemporal and single cell levels. These approaches have provided more detailed insights into the events underlying the dynamics of Salmonella replication, spread and clearance within host organs and tissues.

Contrasting roles of SPARC-related granuloma in bacterial containment and in the induction of anti-Salmonella typhimurium immunity

The role of matricellular proteins in bacterial containment and in the induction of pathogen-specific adaptive immune responses is unknown. We studied the function of the matricellular protein secreted protein, acidic and rich in cysteine (SPARC/osteonectin) in the dissemination of locally injected Salmonella typhimurium and in the subsequent immune response. We show that SPARC was required for the development of organized acute inflammatory reactions with granuloma-like (GL) features and for the control of bacterial spreading to draining lymph nodes (DLNs). However, SPARC-related GL also inhibited dendritic cell (DC) migration to the DLNs and limited the development of adaptive immune response, thus conferring increased susceptibility to the pathogen. In SPARC-deficient mice, both DC migration and antigen-specific responses were restored against bacteria, leading to protective anti–S. typhimurium immunity. This highlights a new function of matricellular proteins in bacterial infection and suggests that initial containment of bacteria can have drawbacks.

Comparative histopathology in mouse typhoid among genetically diverse mice

Genetically resistant CBA and A/J mice and susceptible BALB/c and C57BL/6 mice were challenged with either an identical infective dose or a minimal lethal dose of Salmonella typhimurium. The histopathological progression of the disease was examined in tissue sections prepared by the JB-4 Plus resin embedding method and compared between the resistant and susceptible mice. In a fatal disease, the lesions in both animal hosts began with focal abscesses within the first three days post infection. Mononuclear cell infiltration started by day 4 and transformed the lesions into granulomata. Well-formed granulomata were evident by day 7 and persisted in sublethally infected resistant mice. Massive bacterial proliferation and extensive tissue degeneration marked the terminal stage of a lethal challenge. There were no distinguishable features that would identify the tissue response to infection in a resistant host from a susceptible one, except that the lesions in the sublethally infected resistant mice advanced slower and were discrete and self-limiting.

Recurrent Salmonella enteritidis sepsis and hepatic tuberculosis

A 33 year old woman with recurrent Salmonella enteritidis sepsis is described. Penicillins, ceftriaxone, ciprofloxacin, and chloramphenicol could not eradicate the salmonellas but a combination of high dose ciprofloxacin and ceftriaxone for the eighth episode successfully cured the infection. The combination of ciprofloxacin and ceftriaxone may be a valuable therapeutic regimen in patients with recurrent salmonella sepsis. Prolonged intrahepatic cholestasis resulting from granulomatous hepatitis in this patient improved considerably with empiric ursodeoxycholic acid treatment. A liver biopsy specimen showing non-caseating epitheloid granulomas was positive for mycobacterial DNA by polymerase chain reaction. Repeated bronchoscopy with multiple biopsies eventually revealed caseating granulomas with acid fast bacilli in the lung biopsy specimens. Therefore, tuberculosis was diagnosed as the underlying disease and the cause of granulomatous hepatitis in this patient and tuberculostatic treatment was started. Polymerase chain reaction for mycobacterial DNA may be helpful in the differential diagnosis of hepatic granulomas when routine histological examination and culture of biopsy specimens are not diagnostic. Tuberculosis should be considered as one of the diseases predisposing to recurrent salmonella sepsis.

Correlation of antibody titres induced by vaccination with protection in mouse typhoid

The ELISA method was used to titrate the humoral immune response in vaccinated mice. When mice were given two doses of a heat-killed salmonella vaccine 6 days apart, there was a steady but low-level increase of antibody synthesis. In contrast, if a booster vaccination was administered 21 days after the primary inoculation, the anamnestic response produced a significantly greater antibody titre, rapidly reaching its peak within 10 days. Such a heightened humoral response induced in the genetically susceptible C57BL/6J mice also coincided with effective protection against an otherwise lethal challenge with Salmonella typhimurium.

Comparative antibody response to Salmonella antigens in genetically resistant and susceptible mice

The ELISA was used to titrate the antibody response in mice inoculated with salmonella antigens. The genetically resistant A/J and susceptible C57BL/6J mice were either infected with the virulent or the avirulent Salmonella typhimurium. Alternatively, they were inoculated either once or twice with the heat-killed salmonella vaccine. No appreciable difference could be detected in the relative ability of these two strains of mice to produce antibodies against the lipopolysaccharide antigens of this pathogen under these four conditions.

Recombinant Salmonella typhimurium strains that invade nonphagocytic cells are resistant to recognition by antigen-specific cytotoxic T lymphocytes

To address the question of whether Salmonella-infected nonphagocytic cells could serve as target cells for recognition by antigen-specific, major histocompatibility complex class I-restricted cytotoxic T lymphocytes (CTL), four recombinant Salmonella typhimurium constructs that expressed full-length, or fragments of, influenza A virus nucleoprotein (NP) were made. The bacteria were shown to infect Chinese hamster ovary (CHO) cells. Appropriate major histocompatibility complex restriction molecules, HLA-B27 and H-2 Db, were transfected into CHO cells, which were then infected with recombinant S. typhimurium and used as targets for NP-specific CTL. The cells in which NP was expressed by intracellularly replicating bacteria were not lysed by NP-specific CTL, although they were killed when appropriate influenza A virus or peptides were used. Thus, S.typhimurium bacteria within nonphagocytic cells were resistant to CTL recognition. In contrast to these results, mice infected with recombinant S.typhimurium that expressed fragments of NP in the periplasm were primed for NP-specific CTL responses. The results indicate that CTL responses specific to Salmonella antigens can be generated, but the bacteria may be safe from the CTL attack once they have entered the nonphagocytic cells.

Phenotypic analysis of splenic lymphocytes and immunohistochemical study of hepatic granulomas after a murine infection with Salmonella abortusovis

Infection in mice with an attenuated strain of Salmonella abortusovis (SAO), a specific pathogen for sheep, was used as a convenient model to understand further the induced immunity against SAO. The hypovirulent Rv6 strain, subcutaneously inoculated in salmonella-susceptible BALB/cby (Itys) mice, colonized the spleen and the liver in less than 6 days post-infection (PI) to be cleared after Day 28 PI. Simultaneously, an increase in spleen cell numbers, splenomegaly and hepatic granulomatous lesions developed to a maximum level on Day 9 PI. In spleen of uninfected mice, the number of Thy-1.2+ cells represents twice the number of surface immunoglobulin-positive cells (sIg+). Cytofluorometric analysis of the spleen lymphoid cell subsets showed a significant increase (10 times, P less than 0.05) in the number of sIg+ cells from Day 6 to Day 28 PI compared to control values. The number of Thy-1.2+ cells also significantly increased, to a lesser degree than the sIg+ cells, on Day 2 and on Day 16 PI (twice control values, P less than 0.05), but decreased on Day 6 PI compared to Day 2 PI. The highest L3T4+:Lyt-2+ ratio was observed on Day 2 PI and the lowest on Day 9 PI. On Day 28 PI, the number of sIg+ cells was still greater than the number of Thy-1.2+ cells. The granulomatous lesions were observed in the liver as early as Day 2 PI and their frequency was maximal on Day 9 PI. Immunohistochemical analysis of the granulomatous lesions showed that macrophages (F4/80+, Mac1+) were the basic cells and that L3T4+ cells were the predominant T cells. In well-developed granulomas observed on Day 9 PI, macrophages were in the centre whereas L3T4+ T cells were preferentially located at the periphery. T cells expressing Lyt-2 antigen were rarely detected. Variations in the proportion of lymphoid cells in the spleen and in hepatic granulomatous lesions suggest different and complementary effector mechanisms in induced immunity against SAO.

Pathogenesis and immunity in murine salmonellosis

Salmonella is traditionally described as a facultative intracellular parasite, and host macrophages are regarded as the primary effector cells in both native and acquired immunity in mouse typhoid. This concept has not been unanimously accepted in the literature. Based on cell culture experiments and electron microscopic examinations of infected tissues, we observed that virulent Salmonella typhimurium is killed within polymorphs and macrophages of guinea pigs and mice. In a systemic disease, the organism propagates primarily in the extracellular locations of sinusoids and tissue lesions and within hepatocytes. Hence, it is more likely to be an extracellular pathogen and its virulence is directly related to its antiphagocytic property. The conspicuous absence of macrophages in the primary lesions of murine salmonellosis disputes the likelihood of their significant role in native resistance to the disease. Acquired cellular immunity is expressed as an enhanced antibacterial activity of macrophages facilitated by cytophilic antibodies rather than as an altered antibacterial action of immune macrophages. It is proposed that acquired immunity in murine salmonellosis is a synergistic manifestation of the innate capacity of polymorphs and macrophages to destroy ingested salmonellae, the activated antibacterial functions of macrophages mediated by cytophilic antibodies, the opsonic and agglutinating actions of antiserum, and the accelerated inflammation associated with delayed hypersensitivity to bacterial antigens. Unlike live attenuated vaccines, nonviable vaccines offer a significant, though not a solid, protection against subsequent challenges.

Cellular aspects of the longer-lasting immunity against mouse typhoid infection afforded by the live-cell and ribosomal vaccines

In order to compare the potential of salmonella vaccines prepared from Salmonella typhimurium to provide the longer-lasting protection from the aspects of cell-mediated immunity, groups of mice were immunized with optimal doses of the following preparations: live cells, ribosome-rich extract, acetone-killed cells, and heat-killed cells. At various intervals post-immunization, mouse peritoneal macrophages and splenic T cells were tested for biological activities. The capacity of each vaccine to confer mouse protection against a lethal challenge with S. typhimurium correlated with the degree of macrophage activation engendered by each of them in the early stage of immunization. In the late stage of immunization, the level of mouse protection conferred by each vaccine was found to be based on the capacity of T cells to respond to salmonella antigens, which correlated with the degree of adoptive immunity by T cells. The live-cell and ribosomal vaccines were superior to killed-cell vaccines in inducing the cell-mediated protection. Thus, the longer-lasting immunity provided by the live-cell and ribosomal vaccines can be accounted for by the fact that T cells of mice immunized with both vaccines have the persistent reactivity to salmonella antigens.

Local transfer of delayed-type hypersensitivity after Salmonella infection in mice

An adoptive local transfer system has been used to study the mediators of delayed-type hypersensitivity induced in mice by infection with Salmonella enteritidis 11RX. The cells which transfer this state of hypersensitivity to untreated recipients are nonadherent T lymphocytes with the surface phenotype Lyt 1+2-, and successful transfer requires compatibility at the I-A subregion of the H-2 complex. In these and other respects these cells are indistinguishable from those previously found to be responsible for in vitro lymphokine release upon culture with 11RX antigens.

Effect of iron on antibacterial immunity in vaccinated mice

The effect of iron on resistance to Salmonella typhimurium was investigated in mice inoculated with vaccines prepared from live and avirulent (SL3770) or killed and virulent (SR11 or LT2) bacteria. It has been found that mice vaccinated with SL3770 vaccine develop an immunity which can be neutralized with iron. Iron promoted the development of lethal infections by serving as a growth-essential nutrilite for infecting bacteria and by neutralizing the acquired immunity. The titration of this dual effect of iron showed that more iron was needed to neutralize the immunity in vaccinated animals than to promote bacterial growth in normal animals. In the presence of a sufficient amount of exogenous iron, as few as 10 bacteria caused lethal infections in normal and immune mice with the same effectiveness. This iron-sensitive immunity could be changed to iron-resistant immunity by the immunological stimulation of SL3770-vaccinated mice with a sonicated vaccine prepared from heat-killed SR11 or LT2 bacteria. In distinction to iron-sensitive immunity, iron-resistant immunity could be transferred from SR11- or LT2-stimulated to normal mice with serum. Although effective in the transfer of antibacterial immunity, sera of SR11- or LT2-stimulated mice supported the growth of virulent bacteria as well as did sera of normal mice. The absorption of immune serum with either SR11 or LT2 bacteria removed its protective quality, but the sensitized bacteria remained as infectious as untreated bacteria for iron-treated normal mice. Only in SL3770-vaccinated mice were the immune serum-sensitized bacteria not able to cause the infection in spite of daily treatment with iron. These results suggest that iron-resistant immunity is due to the synergistic action of specific antibody and phagocytes of immunologically stimulated animals.