Hernan R. Chang

Fish oil decreases natural resistance of mice to infection with Salmonella typhimurium.

Mortality rate in mice fed fish oil for 4 weeks was remarkably higher after a very low peroral (PO) challenge with Salmonella typhimurium, as compared with those fed diets rich in either corn oil or hydrogenated coconut oil, or a low fat (chow) diet. None of the surviving mice fed the fish oil diet showed bacterial counts in their spleens, unlike 45.4% to 66.6% of surviving mice fed high fat or low fat diets. The spleens of mice fed fish oil presented the highest number of bacteria 7 days after intraperitoneal infection with the same bacterial strain. Thus, the current studies demonstrate that a diet rich in fish oil decreases host resistance to infection.

Expression of host resistance to Salmonella typhi and Salmonella typhimurium: bacterial survival within macrophages of murine and human origin☆

Cell-association of various strains of Salmonella typhi and Salmonella typhimurium with different populations of macrophages was studied. Macrophages were infected, exposed to gentamicin, washed, and counts of viable bacteria protected from gentamicin killing were made. J774A.1 cells, a continuous macrophage-like cell line, were the most permissive, all strains tested achieving similar high recoveries. Virulent S. typhimurium 779C-Sms, but not avirulent S. typhimurium 779C-SmD, survived well in mouse peritoneal macrophages and human monocyte-derived macrophages. Virulent S. typhi Ty2 were killed by mouse peritoneal macrophages, but were able to survive within human monocyte-derived macrophages. Viable counts of clinical isolates of S. typhi within the human monocyte-derived phagocytes were lower as compared with those of S. typhi Ty2. Phagocytosis of opsonized and non-opsonized virulent S. typhi Ty2 and S. typhimurium 779C-SmS by mouse peritoneal macrophages failed to trigger their respiratory burst as assessed by the intracellular reduction of nitroblue tetrazolium dye (NBT). These experiments support the view that the intracellular survival of Salmonella is in part host dependent and specific in nature. They also suggest that virulence influences the survival and intracellular multiplication of Salmonella within macrophages, and that their ultimate fate within macrophages may not be related to oxygen-dependent mechanisms.

Dietary supplementation with fish oil enhances in vivo synthesis of tumor necrosis factor.

Studies reported here investigate the influence of dietary fat types on cytokine production in response to endotoxin (LPS) challenge. Tumor necrosis factor (TNF) serum levels were markedly higher (by 10-fold) in mice fed chronically a diet rich in fish oil rather than either a diet rich in corn or coconut oil or a low fat diet. This in vivo hyper-responsiveness in LPS-induced TNF production following fish oil consumption concorded with similar exaggerated in vitro TNF release from macrophages exposed to LPS. These data suggest that high consumption of fish oils, by virtue of their high content of omega-3 polyunsaturated fatty acids, can lead to an exaggerated production of mediators of inflammation with potentially adverse consequences on the outcome and severity of infectious diseases.

In vitro and in vivo effects of doxycycline on Toxoplasma gondii.

We investigated the effects of doxycycline on Toxoplasma gondii infections in vitro and in vivo. Resident peritoneal macrophages were infected with the virulent RH strain of T. gondii and exposed to doxycycline at different concentrations. The antitoxoplasmic activity of doxycycline was first assessed with [3H]uracil, which is incorporated by the parasite but not the host cell. The concentration of doxycycline that inhibited 50% of the radioactive uptake was calculated to be 6.4 micrograms/ml (95% confidence limits, 5.07 to 8.06 micrograms/ml); the concentration of doxycycline that inhibited 90% of the radioactive uptake was 14 micrograms/ml. Tetracycline was ineffective up to 40 micrograms/ml. Furthermore, microscopic examination of the infected macrophages after treatment with doxycycline confirmed the inhibition of intracellular growth of T. gondii. Mice acutely infected by the intraperitoneal route with 5 x 10(3) tachyzoites of T. gondii were protected against death with a dose of 300 mg of doxycycline per kg (body weight) administered by the oral route for 10 days, starting 24 h after challenge. When mice were infected with 10(5) tachyzoites of T. gondii and treated 12 days starting 2 h after challenge, the protection and the cure rates were, respectively, 100 and 0% after doxycycline alone (300 mg/kg per day), 0 and 0% after pyrimethamine alone (12.5 mg/kg per day), and 100 and 60% after the combination of these two drugs at the same dosages given above. These results suggest that doxycycline may prove to be useful in the treatment of toxoplasmic infections.

Macrophage oxidative metabolism and intracellular Toxoplasma gondii.

We explored the mechanisms by which Toxoplasma gondii avoids destruction by the oxidative metabolism of normal macrophages. Unelicited murine peritoneal macrophages were infected with T. gondii and exposed to different experimental conditions. As endpoints we used measurement of hydrogen peroxide (H2O2) release and intracellular reduction of nitroblue tetrazolium dye (NBT). Three main observations were made. Firstly, different T. gondii preparations (live or dead, opsonized or not) failed to trigger the respiratory burst. Combined challenges also showed that a primary T. gondii infection was able to block H2O2 release triggered by heat-killed (HK)-Candida albicans. The H2O2 release, however, once triggered by HK-C. albicans, was not inhibited by a subsequent challenge with T. gondii. Secondly, when a respiratory burst was obtained in T. gondii-infected macrophages--for instance by stimulation with phorbol myristate acetate (PMA)--the toxic oxygen metabolites (as determined by the NBT reduction test) did not seem to reach the vacuoles containing the parasite. Thirdly, when a respiratory burst occurred in T. gondii-infected macrophages, the intracellular development of T. gondii did not seem to be affected. In conclusion, we hypothesize that T. gondii is not damaged by the macrophage oxidative metabolism because the parasite fails to encounter toxic oxygen metabolites. The killing of intracellular T. gondii, as it is commonly observed in activated macrophages, does not appear oxygen-dependent.

Activity of epiroprim (Ro 11-8958), a dihydrofolate reductase inhibitor, alone and in combination with dapsone against Toxoplasma gondii.

We examined the effect of epiroprim (Ro 11-8958), a dihydrofolate reductase inhibitor, alone and in combination with dapsone, against Toxoplasma gondii. In vitro, the anti-T. gondii effects of epiroprim and dapsone were observed at nanogram-per-milliliter levels when a 72-h uracil assay and an infection rate of one parasite per 120 macrophages were used. In combination, these drugs exerted a synergistic effect that, however, was only parasitostatic. In a model of acute infection, mice were infected intraperitoneally with 10(4) parasites of the RH strain of T. gondii and were treated for 14 days by gavage (therapy divided into two daily dosages), starting 24 h after infection. Used alone, dapsone and epiroprim, each at a dose of 50 mg/kg of body weight per day, protected 10 and 0% of the mice, respectively. When these drugs were administered simultaneously, a 100% survival rate was observed. Pyrimethamine-sulfadiazine (4 and 250 mg/kg/day, respectively) protected 100% of the mice. A 3-week therapy of chronically infected mice with either epiroprim (50 mg/kg/day), dapsone (50 mg/kg/day), or pyrimethamine (15 mg/kg/day) reduced the numbers of T. gondii cysts and the inflammation in their brains. A combination of epiroprim and dapsone, both at 50 mg/kg/day, further reduced the number of brain cysts in comparison with the number after the corresponding monotherapies. Epiroprim may have a role in the prophylaxis or therapy of human toxoplasmosis, especially when combined with other drugs active against T. gondii, such as dapsone. Images