Antigenic Determinants of Salmonella for Production of “Clearance” Factor in Mouse Typhoid

The “clearance” factor produced in the peritoneal cavity of mice immunized with killed vaccines prepared from Salmonella typhimurium or S. enteritidis was identified as the specific antibodies elicited by the O side chain of the cell wall polysaccharides in the organisms used as immunogens. After immunization of mice with vaccines prepared from virulent Salmonella strains, complement-dependent antibacterial antibodies in the serum and “clearance” factors in the peritoneal cavity were found to appear coincidentally, to last for more than one year, and to have the same specificity against the virulent bacterial strains. The relationship between the complement-dependent antibacterial antibodies and “clearance” factor, and the mechanisms of bactericidal action of these antibacterial agents in experimental typhoid were discussed.     

Role of peritoneal macrophages in the development of an experimental Salmonella infection

The effect produced on the course of Salmonella infection in mice by the removal of peritoneal macrophages with agarose has been studied. Peritoneal macrophages have been shown to control the multiplication of faintly virulent and virulent S. typhimurium strains in the spleen of mice. In immune mice the elimination of the virulent strain of the causative agent of superinfection may occur without the control of peritoneal macrophages.     

Evaluation of the Lon-deficient Salmonella strain as an oral vaccine candidate

We evaluated the efficacy of CS2022 (the Lon protease-deficient mutant strain of Salmonella enterica serovar Typhimurium) as a candidate live oral vaccine strain against subsequent oral challenge with a virulent strain administered to BALB/c and C57BL/6 mice. CS2022 persistently resided in the spleen, mesenteric lymph nodes, Peyer's patches, and cecum of both strains of mice after a single oral inoculation with 1 x 10(8) colony-forming units. Finally, CS2022 almost disappeared from each tissue sample by week 12 in BALB/c mice, whereas CS2022 still resided in each tissue type at week 12 after inoculation of C57BL/6 mice. A significant increase in the serovar Typhimurium lipopolysaccharide-specific secretory immunoglobulin A (s-IgA), as measured for one of the mucosal immune responses, was detected in bile and intestinal samples of both strains of immunized mice at week 4 after immunization. In addition, the expression of gamma interferon mRNA in the spleens of both strains of immunized mice, especially those of C57BL/6 mice, was significantly increased at week 4 after immunization and was boosted during the following 5 days after the challenge was administered to the mice. Furthermore, peritoneal macrophages isolated from immunized mice at week 4 after immunization exhibited an increase in intracellular killing activity against both virulent and avirulent Salmonella. The present results suggested that salmonellae-specific s-IgA on the mucosal surfaces induced by immunization with CS2022 generally prevented mice from succumbing to an oral challenge with a virulent strain. Simultaneously, CS2022 promoted the protective immunity associated with macrophages in both strains of mice.     

An oral Salmonella vaccine promotes the down-regulation of cell surface Toll-like receptor 4 (TLR4) and TLR2 expression in mice

A single oral immunization with the Lon-protease-deficient Salmonella enterica serovar Typhimurium (strain CS2022) induced protective immunity in mice against a subcutaneous challenge with virulent Listeria monocytogenes as well as virulent Salmonella serovar Typhimurium. The populations of cell surface Toll-like receptor 4 (TLR4) and TLR2 on peritoneal macrophages decreased at week 6 after immunization. This population decrease was not reversed after a challenge with either Salmonella or Listeria. These results suggest that oral immunization with CS2022 induced immune tolerance correlated with the down-regulation of cell surface TLR expression. This down-regulation may in part account for the development of cross-protection against a Listeria challenge by immunization with CS2022.     

Correlation of in vitro properties of Rhodococcus (Corynebacterium) equi with virulence for mice

To study the virulence of Rhodococcus (Corynebacterium) equi, seven ATCC strains of different serotypes were tested for their LD50 in mice, clearance of the organism from the lungs and spleen following intravenous or intratracheal inoculation, and in vitro interaction with murine peritoneal macrophages. Strains ATCC 33704 and 33705 were virulent for mice and multiplied in the lungs and spleen, resulting in death of the animal in 5 days. The other five strains were avirulent for mice. The number of bacteria in the lungs and spleen of mice given these five strains decreased immediately. Pulmonary clearance of strains ATCC 33703, 33706, and 33707 was significantly more rapid than that of the virulent strains ATCC 33704 and 33705 12 hr after inoculation. Complete clearance of the avirulent strain ATCC 33707 occurred by day 14, while that of virulent ATCC 33704 and 33705 strains occurred by day 30. The virulent strains ATCC 33704 and 33705 were resistant not only to phagocytosis but also to intracellular killing by macrophages. Strains ATCC 33702 and 33706 were rapidly killed by macrophages although they were rather resistant to phagocytosis. Strain ATCC 33703 was easily phagocytized though resistant to killing by macrophages. The most avirulent strains, ATCC 33707 and 6939, were easily phagocytized and rapidly killed by macrophages. These results indicate that virulence appeared to be related to the ability of the organisms to resist clearance from the lungs and spleen and to resist phagocytosis and intracellular killing by macrophages.     

The influence of autophagy on mouse inflammatory responses caused by Salmonella enterica serovar Typhimurium with spv genes

An investigation into the effects of Salmonella plasmid virulence genes (spv) on autophagy, apoptosis, and inflammation was carried out in mice, using a strain of Salmonella enterica serovar Typhimurium (S. typhimurium) SR-11 carrying spv. Strain BRD509 without spv was used as a control. Results showed that the expression of autophagy protein Beclin-1 in the livers and spleens in the SR-11 group was lower than that in the BRD509 group, while the apoptosis protein, Caspase-3, was higher in the SR-11 group. Inflammatory cytokine levels [interleukin 12 (IL-12) and interferon γ (IFN-γ)] were higher in the SR-11 group compared with those in the BRD509 group since 4 d post-infection. In addition, we found an increase in severe pathological changes and larger viable bacterial amounts in livers and spleens in the SR-11 group. After intervention with autophagy agonist rapamycin (RAPA), Beclin-1 expression increased in both groups, while Caspase-3 expression was different between the two groups: Caspase-3 decreased in the SR-11 group but increased in the BRD509 group. Moreover, RAPA decreased cytokine levels, bacterial quantity and organ-related injury in the SR-11 group whereas RAPA increased cytokine levels and aggravated organ injury in the BRD509 group. Results from these studies suggest that S. typhimurium with spv genes may exacerbate infection by inhibiting autophagy and affecting the production of inflammatory cytokines. RAPA-enhanced autophagy may improve the secretion of cytokines in order to protect the host from damaging by Salmonella infection. Our study suggests that the regulation of cellular autophagy may play a role in the prevention and control of certain infectious diseases.     

Effect of Specific Immune Mouse Serum on the Growth of Salmonella enteritidis in Nonvaccinated Mice Challenged by Various Routes

Salmonella enteritidis was injected intravenously, intraperitoneally, or subcutaneously into specific pathogen-free mice. The number of organisms in the blood, liver, spleen, peritoneal cavity, and draining inguinal lymph node was determined by daily enumeration. Opsonization of the organism with hyperimmune serum increased the rate of phagocytosis, resulting in rapid blood clearance together with an alteration in the relative numbers of organisms accumulating in the liver and spleen. Serum treatment also brought about a substantial increase in the number of bacteria killed during the first 60 min of the infection. However, the survivors of this initial period of inactivation then multiplied rapidly in the liver and spleen, ultimately resulting in the death of the animal from a generalized infection. Attempts to passively protect mice with hyperimmune serum were uniformly negative. The effects of treatment of the virulent S. enteritidis with hyperimmune serum were consistent with the general thesis that cellular rather than humoral factors play the major role in the expression of an effective antibacterial immunity against salmonella infections.     

LPS regulation of the immune response: Bacteroides endotoxin induces mitogenic, polyclonal, and antibody responses in classical LPS responsive but not C3H/HeJ mice

Recent studies have suggested that lipopolysaccharide (LPS) derived from gram-negative organisms such as Bacteroides, which are not members of the Enterobacteriaceae, stimulate B cells from the classic LPS-hyporesponsive C3H/HeJ mouse. In the present study, purified, phenol-water-extracted LPS from Bacteroides fragilis ATCC 25285 (B-LPS) was tested for its ability to induce in vivo and in vitro responses in classic LPS-responsive C3H/HeN, LPS-hyporesponsive C3H/HeJ, and (C3H/HeN X C3H/HeJ)F1 hybrid mice. B-LPS induced mitogenic responses in both C3H/HeN and C3H/HeJ spleen cell cultures when cells were cultured under standard conditions, i.e., 8 X 10(5) cells/well. Interestingly, when lower spleen cell numbers were tested with B-LPS, a typical responsive-nonresponsive pattern developed in which good mitogenic responses were induced by B-LPS in C3H/HeN cultures and in which low responses in C3H/HeJ spleen cell cultures were evident. In vivo immunization of mice with B-LPS resulted in high antibody responses in C3H/HeN, intermediate responses in F1, and low responses in C3H/HeJ mice. When purified splenic B cells were incubated with B-LPS, both mitogenic responses and polyclonal immunoglobulin M (IgM) synthesis occurred in C3H/HeN cultures, whereas intermediate responses were noted in F1 cultures and no response was seen in B cell cultures from C3H/HeJ mice. Furthermore, in vitro TNP-B-LPS responses were induced in C3H/HeN spleen cells or purified B cell cultures, and intermediate anti-TNP PFC responses occurred in F1 spleen cells or purified B cell cultures. The toxicity of B-LPS was tested in galactosamine-sensitized mice. The LD50 values for B-LPS in classic LPS-responsive C3H/HeN and C57BL/6J mice were 0.6 microgram and 1.1 microgram, respectively; F1 hybrid mice were approximately 15-fold more resistant, whereas C3H/HeJ mice gave an LD50 of 1650 micrograms. This study shows that phenol-water preparations of B-LPS are biologically active and induce responses in the classic LPS-responsive but not in the LPS-hyporesponsive C3H/HeJ mouse strain.     

Heat-labile antigens of Salmonella enteritidis. II. Mouse-protection studies

Collins, F. M. (University of Adelaide, Adelaide, South Australia), and Margaret Milne. Heat-labile antigens of Salmonella enteritidis. II. Mouse-protection studies. J. Bacteriol. 92:549-557. 1966.-A number of extracts prepared from a virulent and an avirulent strain of Salmonella enteritidis were used to immunize mice. Living and alcohol-killed whole-cell vaccines were also used to compare the relative protective value of the various preparations. All mice were challenged intravenously with 100 to 1,000 ld(50) of S. enteritidis. Daily counts of the liver, spleen, and blood populations of vaccinated and control mice revealed that the challenge organism was rapidly eliminated only in those mice which had been immunized with a living vaccine. Immunization with extracts resulted in rapid clearance of S. enteritidis from the blood, but, after a delay of 24 to 48 hr, the bacterial populations increased until a maximal liver and spleen population of approximately 5 x 10(8) was reached. Between 55 and 100% of the immunized animals died, compared with 95 to 100% of the controls. With all four extracts, it was only the first antigenic fraction eluted from diethylaminoethyl cellulose which had any detectable effect on host resistance. The ineffectiveness of vaccines prepared with the various extracts or with whole killed bacteria relative to the protection observed after immunization with living organisms is discussed.     

Antibody Response and Protection Induced by Immunization with Smooth and Rough Strains in Experimental Salmonellosis

The antibody response of mice to a smooth strain of Salmonella typhimurium was shown previously to be extremely rapid and potent. As measured by the complement-mediated bactericidal reaction, it was also found to be highly specific as well as reproducible. Experiments which studied the effects of antigen type (live or heat-killed), antigen dose, and the route of immunization indicated that the most rapid and highest antibody response was achieved with live, smooth organisms injected by the intraperitoneal route. Living vaccines of rough strains of either S. typhimurium or S. enteritidis induced antibodies directed against the corresponding smooth organisms. The response to the rough strains was apparently due to antibody production rather than to the simple release of preformed natural antibody. The duration of protection conferred by the rough strain vaccines was closely correlated with the endotoxic content of the immunizing strain. Smooth heat-killed vaccines and a rough live vaccine protected against homologous but not heterologous challenge. In contrast, immunization with a smooth live vaccine protected mice against both homologous and heterologous challenge infections. Protection was not due to a local effect in the peritoneal cavity, since mice were also protected against subcutaneous challenge. The secondary antibody response, induced in immunized animals by the virulent challenge infection, was demonstrated to be rapid and potent, and hence a factor to be considered in protection.     

Salmonella-mediated mucosal cell-mediated immunity.

Oral immunization with the recombinant Salmonella typhimurium strain (BRD 847) expressing the C fragment of tetanus toxin (TT) induces brisk Ag-specific mucosal S-IgA and serum Ab responses characterized by strong IgG2a Abs to the encoded antigen. We have constructed an attenuated Salmonella typhimurium (aroA- aroD-) strain that expresses chicken egg albumin (OVA) to further elucidate the role of Salmonella-induced Th1 cell phenotype on mucosal cell-mediated immunity (CMI). Peyer's patches and spleen lymphocytes from mice that received the oral Salmonella-OVA vaccine showed dramatic increases in the percent cell lysis of the H-2b restricted EG7.OVA tumor cell line. These results indicate that a single dose of rSalmonella vaccine antigen vector is required to illicit systemic and mucosal Th1-type responses and CTLs. These results also support the existence of a highly regulated relationship between specific cell-mediated immunity and a branch of the humoral immune system, i.e. mucosal IgA responses.     

Early interactions of Flavobacterium psychrophilum with macrophages of rainbow trout Oncorhynchus mykiss

The early interactions of a low and a highly virulent Flavobacterium psychrophilum strain with head kidney and spleen macrophages of rainbow trout Oncorhynchus mykiss were characterized. The highly virulent strain was killed 5.8 to 11 times less frequently than the low virulent strain. The head kidney macrophages showed a microbicidal activity approximately twice as high as that of the spleen macrophages. A 2- to 3-fold higher production of reactive oxygen species (ROS) was induced by the highly virulent strain than by the low virulent one. The head kidney macrophages produced approximately twice as much ROS as the spleen macrophages. The low virulent strain was killed approximately 10 times more frequently by H2O2 than was the highly virulent strain. In spleen macrophages, the highly virulent strain caused twice as much cytotoxic effects compared to the low virulent strain. In conclusion, virulence in F. psychrophilum appears to be correlated with higher O. mykiss macrophage cytotoxicity and resistance to ROS and, therefore, with enhanced resistance to bacterial killing. Moreover, due to lower ROS production, spleen macrophages have a lower antimicrobial action against F. psychrophilum, compared to head kidney macrophages and, thus, might form a 'safe site' in which bacteria can reside.     

Lung tumor-reactive cytotoxic lymphocytes generated in mixed lymphocyte reaction between C3HfeB/HeN (H-2kb) and C3H/HeN (H-2k) strain mice.

The tumor-associated transplantation antigen expressed by several transplacentally induced lung tumors of C3HfeB/HeN mice (H-2kb haplotype) has previously been shown to exist as a normal tissue alloantigen in mice of known H-2k and H-2a haplotypes. This antigen is not expressed in normal tissues of C3HfeB/HeN mice but is expressed in C3H/HeN mice, the strain from which the C3HfeB/HeN mice were originally derived. The present study indicates that spleen cells from C3HfeB/HeN and C3H/HeN mice respond reciprocally in the mixed lymphocyte reaction. Cytotoxic T lymphocytes specific for the tumor-associated alloantigen can be readily generated in mixed lymphocyte reactions in which spleen cells from C3HfeB/HeN mice are reacted with x-irradiated spleen cells from C3H/HeN or A strain mice. These cells are effective in suppressing the growth of the C3HfeB/HeN-derived lung tumor 85 in x-irradiated syngeneic recipients.     

Suppression of murine cytomegalovirus (MCMV) replication with a DNA vaccine encoding MCMV M84 (a homolog of human cytomegalovirus pp65)

The cytotoxic T-lymphocyte (CTL) response against the murine cytomegalovirus (MCMV) immediate-early gene 1 (IE1) 89-kDa phosphoprotein pp89 plays a major role in protecting BALB/c mice against the lethal effects of the viral infection. CTL populations specific to MCMV early-phase and structural antigens are also generated during infection, but the identities of these antigens and their relative contributions to overall immunity against MCMV are not known. We previously demonstrated that DNA vaccination with a pp89-expressing plasmid effectively generated a CTL response and conferred protection against infection (J. C. Gonzalez Armas, C. S. Morello, L. D. Cranmer, and D. H. Spector, J. Virol. 70:7921-7928, 1996). In this report, we have sought (i) to identify other viral antigens that contribute to immunity against MCMV and (ii) to determine whether the protective response is haplotype specific. DNA immunization was used to test the protective efficacies of plasmids encoding MCMV homologs of human cytomegalovirus (HCMV) tegument (M32, M48, M56, M82, M83, M69, and M99), capsid (M85 and M86), and nonstructural antigens (IE1-pp89 and M84). BALB/c (H-2(d)) and C3H/HeN (H-2(k)) mice were immunized by intradermal injection of either single plasmids or cocktails of up to four expression plasmids and then challenged with sublethal doses of virulent MCMV administered intraperitoneally. In this way, we identified a new viral gene product, M84, that conferred protection against viral replication in the spleens of BALB/c mice. M84 is expressed early in the infection and encodes a nonstructural protein that shares significant amino acid homology with the HCMV UL83-pp65 tegument protein, a major target of protective CTLs in humans. Specificity of the immune response to the M84 protein was confirmed by showing that immunization with pp89 DNA, but not M84 DNA, protected mice against subsequent infection with an MCMV deletion mutant lacking the M84 gene. The other MCMV genes tested did not generate a protective response even when mice were immunized with vaccinia viruses expressing the viral proteins. However, the M84 plasmid was protective when injected in combination with nonprotective plasmids, and coimmunization of BALB/c mice with pp89 and M84 provided a synergistic level of protection in the spleen. Viral titers in the salivary glands were also reduced, but not to the same extent as observed in the spleen, and the decrease was seen only when the BALB/c mice were immunized with pp89 plus M84 or with pp89 alone. The experiments with the C3H/HeN mice showed that the immunity conferred by DNA vaccination was haplotype dependent. In this strain of mice, only pp89 elicited a protective response as measured by a reduction in spleen titer. These results suggest that DNA immunization with the appropriate combination of CMV genes may provide a strategy for improving vaccine efficacy.     

Lipopolysaccharide and mouse virulence of Salmonella: O antigen is important after intraperitoneal but not intravenous challenge

Abstract The quality of the O-antigenic polysaccharide part of the cell wall lipopolysaccharide (LPS) is a virulence determinant in Salmonella strains: isogenic derivatives with antigen O-4,12 have been shown to be more virulent than those with O-6,7 when given intraperitoneally (i.p.) to mice. The O-6,7 LPS activates complement by the alternative pathway more efficiently than does O-4,12. We show here that the O-6,7 (but not O-4,12) bacteria were rapidly killed in the peritoneal cavity of the mice, resulting in approx. 100-fold reduced numbers of bacteria reaching the liver; the subsequent rate of growth of the bacteria was not affected. After intravenous challenge, both O-6,7 and O-4,12 sister strains survived equally well in the liver and spleen and were of approximately equal virulence. We suggest that the rapid activation of complement by the O-6,7 LPS leads to the killing of these bacteria by the peritoneal cells and thereby to reduced virulence.     

Mechanisms of killing of measles virus-infected cells by human lymphocytes: interferon associated and unassociated cell-mediated cytotoxicity

The recent interest in natural killer (NK) cells in immunosurveillance and the ability of infection with certain organisms to modulate NK activity led us to examine the influence of Toxoplasma gondii infection on mouse NK cells. Infection of BALB/c mice with 5 × 10³ virulent Toxoplasma intraperitoneally (ip) resulted in significantly enhanced NK activity in peritoneal exudate cells (PC) and in spleen cells (SC). Intravenous (iv) and subcutaneous (sc) challenge of BALB/c mice with Toxoplasma also resulted in enhanced natural killer (NK) activity in PC and SC. In BALB/c mice, as well as in other strains (A/J, C57BL/6, C3H/HeJ, CeH/HeN, [A/J × C3H]F₁), peak augmentation of PC and SC NK activity was observed 3 days following ip Toxoplasma challenge. Administration of silica to mice abolished Toxoplasma-induced NK cytotoxicity. BALB/c mice chronically infected with Toxoplasma had significantly higher endogenous NK activity than did controls in PC but not in SC. Chronically infected BALB/c mice boosted with virulent Toxoplasma ip exhibited significantly enhanced NK activity in PC but not in SC. Thus, acute and chronic infection with Toxoplasma modulates NK activity in addition to macrophage activation and thereby provides a system that should facilitate study of the relative contribution of NK cells and activated macrophages in resistance to tumor growth and spread.     

The primary effect of the Ity locus is on the rate of growth of Salmonella typhimurium that are relatively protected from killing

The Ity locus affects the net increase in numbers of Salmonella typhimurium in the liver and spleen of infected mice. There has been controversy, however, about whether the effects of this locus are due to differential killing of S. typhimurium or differential growth rates of S. typhimurium in mice. Our studies using S. typhimurium aroA mutants, which do not grow in vivo, demonstrate that growth of the infecting salmonella is necessary for the observation of the Ity phenotype. To examine the effects of the Ity locus on the growth and killing of fully virulent salmonella, we infected Ity-congenic mice i.v. with stationary phase S. typhimurium containing a single copy of the plasmid pHSG422. This plasmid exhibits defective replication at body temperature and is diluted out during salmonella growth in vivo. Thus, the frequency of plasmid-containing salmonella recovered from mice provides a measure of salmonella cell divisions in vivo. Inasmuch as the numbers of plasmid-containing salmonella are only slightly affected by bacterial division, any decline in the numbers of plasmid-containing salmonella is an unbiased measure of killing. By infecting mice with these plasmid-containing salmonella we observed that: 1) during the first four h post infection (during blood clearance of injected salmonella) there is about 3-fold more killing of salmonella in Ityr mice than in Itys mice; 2) from 4 to 44 h postinfection (after blood clearance is completed) there is little if any additional killing in either Itys or Ityr mice; and 3) during the first 48 h postinfection there is about 18-fold more growth of salmonella in Itys mice than in Ityr mice. Thus, the major effect of the Ity locus on resistance to salmonella, is the regulation of growth within a "safe" (relatively nonbactericidal) site in the liver and spleen.     

Novel virulence properties of the Salmonella typhimurium virulence-associated plasmid: immune suppression and stimulation of splenomegaly

Mice infected subcutaneously with wild-type Salmonella typhimurium, SR11, developed a significant splenomegaly when compared with mice infected with an equal number of a plasmid-cured strain. Further, the bacterial load in the spleen at 14 days after infection, measured as colony-forming units per gram tissue, was significantly higher in mice infected with the parent strain than in mice infected with the plasmid-cured strain. These data confirm the previously reported plasmid-associated ability of Salmonella to multiply within the spleen. In addition, lymph node cells (LNC) from mice infected with the parent strain had a significantly reduced ability to proliferate in response to concanavalin A, a T-cell mitogen, and to heat-killed S. typhimurium cells when compared with LNC isolated from mice infected with the plasmid-cured strain. Finally, reintroduction of a functional Tn5-tagged 90-kb plasmid into a plasmid-free strain restored its capacity to cause a marked splenomegaly and to suppress lymph node cell proliferation in BALB/c mice. These data demonstrate that the 90-kb plasmid of highly virulent S. typhimurium strains mediates several novel pathogenic properties in infected mice: (1) enhancement of the ability of Salmonella to multiply within the spleen; (2) stimulation of a splenic inflammatory response as displayed by marked splenomegaly; and (3) a general suppression of lymphocyte responsiveness to both T-cell mitogens and specific Salmonella antigens.     

GM-CSF administration augments the survival of ity-resistant A/J mice, but not ity-susceptible C57BL/6 mice, to a lethal challenge with Salmonella typhimurium

Ity resistant A/J mice were challenged with a lethal dose (2 x 10(3) organisms) of Salmonella typhimurium. Infected mice treated with 1 microgram of GM-CSF twice daily showed increased median survival time and had a higher survival fraction than untreated controls. GM-CSF was most effective when given for a brief period (1 to 2 days) after infection. Pretreatment of the mice or delayed treatment with GM-CSF had no effect on the survival of the mice. Studies on the effect of GM-CSF on the bacterial load showed that mice treated with GM-CSF had fewer S. typhimurium in the spleen and peritoneal cavity on day 4 but not on day 2 after infection. GM-CSF treatment of ity-susceptible C57BL/6 mice infected with 10 organisms had no therapeutic effect.     

Role of cyclic 3',5'-adenosine monophosphate in the development of Salmonella infection in an experiment

The changes in cAMP levels in spleen macrophages of mice infected with low-virulent and virulent Salmonella strains and the effect of propranolol on Salmonella reproduction in the spleen, and the outcome of Salmonella-induced infection have been studied. A persistent increase in cAMP levels in spleen macrophages during Salmonella infection caused by virulent Salmonella strains has been demonstrated. Low-virulent Salmonella strains failed to cause the elevation of cAMP levels in spleen macrophages. Propranolol injection to mice prevented intensive Salmonella reproduction in the spleen and diminished the animal mortality rate.