Murine dendritic cells internalize Leishmania major promastigotes, produce IL-12 p40 and stimulate primary T cell proliferation in vitro.

Metacyclic Leishmania promastigotes (PM), transmitted by sand-fly bite, are likely to interact initially with cells of the dendritic cell (DC) lineage(s) in the epidermis or dermis. Epidermal Langerhans cells internalize L. major amastigotes (AM) and transport them to draining lymph nodes (Moll, H., Fuchs, H., Blank, C. and Röllinghoff, M., Eur. J. Immunol. 1993. 23: 1595) but little is known about the interaction of DC with PM. The present study demonstrates that DC are able to internalize PM and that the fate of the parasites within DC differs from that within macrophages (Mphi). DC took up small numbers of PM which did not differentiate into AM but appeared to be degraded; Mphi internalized large numbers of PM into parasitophorous vacuoles where they differentiated into AM. In response to direct stimulation with PM, DC from both C3H ("resistant" to L. major infection) and BALB/c ("susceptible") up-regulated production of IL-12 p40. In contrast, IL-12 production by Mphi was not detected. DC exposed to either metacyclic PM or PM culture supernatants were also able to stimulate proliferative responses in lymph node T cells from naive mice. These data indicate that DC have the capacity to promote protective Th1 immune responses in Leishmania infection and suggest that DC exposed to PM may be useful in immunotherapy and vaccination.

A protective role for IL-6 during early infection with Toxoplasma gondii.

IL-6 deficient mice were found to be significantly more susceptible to peroral infection with Toxoplasma gondii than their wild-type counterparts as measured by survival, brain cyst burdens and brain pathology at 28 days post infection. The physical manifestations of disease, such as weight loss, were not observed in IL-6 deficient animals until at least seven days later than such changes occurred in wildtype mice. During this early stage of infection IL-6+/+ but not IL-6-/- mice mounted a peripheral blood neutrophilia. Furthermore, between 6-8 days post-infection there was a significant increase in plasma IFN-gamma levels in wild-type but not IL-6 deficient mice. Not until days 18-23 post-infection, concurrent with the majority of deaths in IL-6-/- mice, were plasma IFN-gamma levels substantially and significantly raised in IL-6-/- mice. At this time not only were these plasma IFN-gamma levels 20-fold higher than background but eight-fold greater than peak (6-8 days post-infection) IFN-gamma levels in IL-6+/+ mice. IFN-gamma dependent parasite specific IgG2a levels were also significantly higher in IL-6-/- mice over this period and thereafter. Overall the evidence suggests that in the absence of IL-6 mice are unable to initiate a rapid proinflammatory response against T. gondii, which allows increased parasite growth. Increased mortality in IL-6-/- mice may be directly due to this increased parasite burden and the excessive inflammatory response this induces three weeks post-infection.

The role of IL-4 in adult acquired and congenital toxoplasmosis.

The course of Toxoplasma gondii infection was studied in IL-4-deficient mice from three genetic backgrounds and their wild-type counterparts following peroral inoculation of tissue cysts. Survival rates were significantly reduced in disease-susceptible C57 BL/6 mice and F1 (C57BL/6 x 129Sv) mice deficient in IL-4 compared with wild-type controls. In contrast, this difference was not observed in T. gondii-resistant BALB/c mice. However, brain tissue cyst burdens in IL-4-deficient mice were either equivalent to (C57BL/6 and BALB/c mice) or significantly less (B6/129 mice) than similarly infected wild-type mice. Thus strain-specific differences in the course of T. gondii were demonstrated in the absence of IL-4. The course of T. gondii infection was also compared between B6/129 IL-4-deficient mice and their wild-type counterparts following peroral challenge with 20 tissue cysts on day 12 of pregnancy. Age-matched non-pregnant IL-4-/- and IL-4+/+ mice were also infected to assess the role of IL-4 on T. gondii infection during pregnancy. Disease phenotypes, as measured by mortality, were reversed if infections were initiated during pregnancy compared with non-pregnant infection. Thus significant mortality occurred immediately post partum in IL-4+/+ mothers, while all IL-4-/- mothers survived. Cyst burdens 28 days p.i. were significantly lower in IL-4-/- mothers than IL-4+/+ mothers and both IL-4-/- and IL-4+/+ non-pregnant mice. Congenital disease transmission as measured by foetal death or vertical disease transmission was independent of the presence or absence of IL-4. These studies demonstrate a role for IL-4 in pregnancy-induced immunosuppression and the associated increased susceptibility to T. gondii infection.

Different roles for interleukin-4 during the course of Toxoplasma gondii infection.

The course of Toxoplasma gondii infection from initiation of disease perorally until day 28 postinfection was compared between interleukin-4 (IL-4) gene knockout (IL-4-/-) mice and their wild-type (IL-4+/+) counterparts on a disease-susceptible genetic background. The rate of mortality was significantly greater in mice deficient in Il-4 than in the immunocompetent controls. Although levels of T. gondii-specific spleen cell proliferation measured in vitro were similar between groups at all time points examined throughout infection, the quantities of cytokines released into the culture supernatant differed. Culture supernatants from spleen cells derived from IL-4-deficient mice contained significantly more gamma interferon than those derived from IL-4+/+ mice at day 7 postinfection. Conversely, IL-10 production was significantly greater from the spleen cells derived from wild-type mice at day 28 postinfection. Splenocytes from both groups of mice had a marked inhibition of proliferation in response to soluble tachyzoite antigen as well as reduced proliferation in response to concanavalin A between days 7 and 14 postinfection and marked proliferation on days 21 and 28 postinfection. At day 28 postinfection, histological examination of the brains indicated that IL-4+/+ mice had more severe pathological changes and more cysts than IL-4-/- mice. In addition, although many nonencysted single organisms were present in IL-4+/+ mice within both necrotic lesions and microglial nodules, few nonencysted parasites were found, and no necrotic lesions were present in IL-4-deficient animals. These results suggest that the observed reduction in mortality during the early acute phases of infection may be due to the down-regulatory effects of Il-4 or associated Th2-derived products on proinflammatory cytokines such as gamma interferon. However, the long-term effects of IL-4 are detrimental, possibly because of the ability of this cytokine to inhibit proinflammatory antiparasitic products. This may explain the increased parasite multiplication with cysts observed in the brains of IL-4+/+ mice.