Influence of parasite load on the ability of type 1 T cells to control Leishmania major infection

Abstract

BALB/c mice infected with Leishmania major developed a type 2 immune response which failed to control parasite replication. We found that scid mice that received splenocytes from BALB/c mice that had been infected for 3 weeks with L. major (a type 2 cell population) and that were subsequently infected with L. major were protected when they were treated with interleukin 12 (IL-12). In contrast, IL-12 was ineffective at protecting BALB/c mice infected for 3 weeks, suggesting that a high parasite load regulates the development of protective immunity. To determine how this regulation operates, we performed a series of adoptive transfers of naïve, type 1 or type 2 splenocytes into scid mice. The recipient scid mice were infected either for 5 weeks prior to cell transfer (and thus had a high parasite load) or at the time of cell transfer. scid mice that were infected for 5 weeks and received a type 1 cell population were able to cure their lesions. However, when 5-week-infected scid mice received both type 1 and 2 cell populations, they were unable to control their infections. In contrast, the same type 1 and 2 cells transferred to naïve scid mice, which were subsequently infected, provided protection. In addition, we found that naïve cells mediated protection in scid mice with established lesions. These results show that high parasite numbers do not block type 1 protective responses or the development of type 1 responses. Instead, the influence of a high parasite load is dependent on the presence of a type 2 cell population.

FIG. 1.

IL-12 treatment of L. major-infected scid mice reconstituted with a type 2 cell population. (A) scid mice received 10⁸ spleen cells from BALB/c mice infected for 3 weeks with L. major by intravenous injection (type 2 population) and were simultaneously infected in the footpad with 10⁶ L. major parasites. Mice were then treated with 0.2 μg of IL-12 intralesionally six times during the first 2 weeks of infection. (B) BALB/c mice that were infected for 3 weeks with L. major were treated with 0.2 μg of IL-12 intralesionally six times between weeks 3 and 5, and the course of infection was monitored. Each data point represents the mean lesion size ± the standard error of the mean for five mice. This experiment was performed three times with similar results.

FIG. 2.

Adoptive transfer of type 1 cells into chronically infected scid mice or naïve scid mice that were subsequently infected with L. major. BALB/c mice were infected in the footpad with 10⁶ L. major parasites. Mice were then treated with 0.2 μg of IL-12 intralesionally six times during the first 2 weeks of infection to create a healing BALB/c mouse. After 6 weeks, 8 × 10⁷ splenocytes were transferred intravenously into naïve scid mice that were subsequently infected in the footpad with 10⁶ L. major parasites (A) or scid mice that had been infected for 5 weeks with L. major (B). The course of infection was monitored by measuring lesion size. Each data point represents the mean lesion size ± the standard error of the mean for five mice. This experiment was performed three times with similar results.

FIG. 3.

Adoptive transfer of naïve cells, type 1 cells, type 2 cells, or a mixture of type 1 and type 2 cells into chronically infected scid mice or naïve scid mice that were subsequently infected with L. major. Type 1 cells were taken from healed BALB/c mice as described in the legend to Fig. 2, and type 2 cells were taken from 3-week-infected BALB/c mice. scid mice that were naïve and infected at the time of cell transfer (A) or had been infected for 5 weeks with L. major (B) received either 6 × 10⁷ naïve splenocytes, 6 × 10⁷ type 1 cells, 6 × 10⁷ type 2 cells, or a mixture of 6 × 10⁷ type 1 cells and 6 × 10⁷ type 2 cells. The course of infection was monitored by measuring lesion size. Each data point represents the mean lesion size ± the standard error of the mean for five mice. This experiment was performed two times with similar results. scid mice that received no cells developed an uncontrolled infection (data not shown).

FIG. 4.

Parasite quantitiation in the spleens of scid mouse recipients of naïve, type 1, type 2, or mixed type 1 and type 2 cells. At the termination of the experiment described in the legend to Fig. 3 (10 weeks postinfection), the numbers of parasites in the spleens were assessed as described in Materials and Methods. scid mice that received cells at the time of infection (A) or 5 weeks after infection (B) are shown. Each dot represents the result for an individual animal. Mixed type 1 and type 2 cells provided significant protection to scid mice when they were given at the initiation of the infection but not after 5 weeks of infection.