Inbred strains derived from feral mice reveal new pathogenic mechanisms of experimental leishmaniasis due to Leishmania major

Abstract

Two inbred mouse strains, derived from feral founders, are susceptible to experimental leishmaniasis due to Leishmania major and support a disease of a severity intermediate between those observed in strains C57BL/6 and BALB/c. Mice of the MAI strain develop a severe, nonhealing, but nonfatal disease with no resistance to a secondary parasite challenge. The immunological responses showed a TH2 dominance characterized by an early peak of interleukin-4 (IL-4) and IL-13. However, neutralization of IL-4, which leads to a resistance phenotype in BALB/c mice, has no effect on disease progression in MAI mice. Mice of strain PWK develop a protracted but self-healing disease, characterized by a mixed TH1-plus-TH2 pattern of immune responses in which IL-10 plays an aggravating role, and acquire resistance to a secondary challenge. These features are close to those observed in human cutaneous leishmaniasis due to L. major and make PWK mice a suitable model for the human disease.

FIG. 1.

Lesion progression in mice from inbred strains who were inoculated with L. major amastigotes. (A) Results for mice of nine inbred strains inoculated with 2 × 10⁶ parasites. (B) Lesion progression over 30 weeks of observation in PWK mice inoculated with 2 × 10⁶ parasites. Results are expressed as increases in footpad thickness (in millimeters) and are means ± SD for 8 to 10 individual mice per group.

FIG. 2.

Investigation of the DTHR and resistance to reinfection developed by inbred mice inoculated with a primary challenge of 2 × 10⁶ L. major amastigotes. (A) DTHR footpad test. Groups of MAI, PWK, MBT, SEG, BALB/c, and C57BL/6 mice were injected at week 10 p.i. with LTA in the uninfected contralateral hind footpad. Footpad swelling was measured (in millimeters) with a metric caliper at 24, 48, and 72 h. Results are means ± SD. (B) Course progression of secondary lesions in MAI and PWK mice (derived from wild strains) compared to BALB/c and C57BL/6 mice. Mice were inoculated subcutaneously with 2 × 10⁶ L. major amastigotes and were challenged at week 10 p.i. with the same number of parasites in the contralateral footpad. The progression of the secondary lesion was monitored for 14 additional weeks. Results are expressed as mean lesion sizes for five mice per group (in millimeters) ± SD.

FIG. 3.

Cytokine expression in mice from wild-mouse-derived inbred strains inoculated with 2 × 10⁶ L. major amastigotes. Shown are levels of IL-4 (left), IL-10 (center), and IFN-γ (right) expressed in the supernatants of in vitro LTA-stimulated draining lymph node cells obtained from mice during the established phase of disease at 5, 8, and 15 weeks p.i. Cytokines were analyzed by ELISA. Values are expressed as nanograms per milliliter and are means from four separate experiments. Spontaneous cytokine production was less than 10% when cells were left unstimulated. Diamonds, BALB/c; triangles, MAI; circles, PWK; large squares, MBT; small squares, SEG.

FIG. 4.

Effect of early treatment with an anti-IL-4 or anti-IL-10 MAb on disease progression in MAI and PWK mice(wild-mouse-derived strains). BALB/c and C57BL/6 mice were used as well. Mice (five per group) were treated i.p. with 5 mg of the anti-IL-4 (11B11; rat IgG1) (A) or anti-IL-10 (JES-2A5; rat IgG1) (B) antibody 48 h before infection with L. major GLC94. Control mice of each strain were treated i.p. with control antibodies. Results are expressed as mean sizes of lesions (in millimeters) ± SD.