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Cell-mediated Immune Responses in the Murine Model
The activation of macrophages to kill intracellular amastigotes is dependent on complex and only partially understood cell-mediated immune
responses. Early studies in rodents revealed that ablation of T helper cells by radiation or chemotherapy resulted in progressive disease in animals that were otherwise capable of mounting protective immune responses against Leishmania. Immunity was restored by the transfer of syngeneic lymphocytes from immune animals. Anti-leishmanial antibodies were not protective.
Bradley, Blackwell and colleagues observed that inbred strains of mice differ in their susceptibility to Leishmania spp. (Plant et al., 1982; Blackwell et al., 1994). They found that the murine Nramp (natural-resistance-associated macrophage protein) locus on chromosome 1, previously known as LshjltyjBcg, governed susceptibility to L. (L.) donovani. Mice that were homozygous for the sensitive allele developed large parasite burdens, while those that were heterozygous or homozygous for the resistant allele spontaneously cleared infection. Among susceptible mice, alleles at H-2 loci
determined whether the strain was able to reduce the parasite burden later in infection. The Nramp locus has pleiotrophic effects. It controls priming and activation of macrophages for antimicrobial activity and the differential expression of the early response gene KC. Low levels of nitric oxide synthase are involved in the signal transduction that controls Nramp expression. The murine model of L. (L.) donovani infection is limited in that susceptible mice do not develop signs of visceral disease. In addition, there does not appear to be an association between Nramp 1 and the development of human visceral leishmaniasis (Blackwell et al., 1997).
Leishmania (L.) major has emerged as the major model for visceralizing Leishmania infection. It disseminates to lymph nodes, liver and spleen in BALB/c and other susceptible strains of mice, which have been extensively studied (Moll and Mitchell, 1988; Locksley and Louis, 1992; Reiner and Locksley, 1993; Scharton-Kersten and Scott, 1995; Noben-Trauth et al., 1996; Scott and Farrel, 1998). In the case of L. (L.) major, multiple loci other than Nramp govern susceptibility to infection.
The outcome of L. (L.) major infection in mice is dependent on complex interactions between potentially protective and disease-enhancing cell-mediated immune responses. The identification of morphologically similar, but functionally distinct CD4+ T helper cell populations in mice led to rapid advances. Leishmania-specific Th1 cells were found to dominate in mice with selfresolving infection and correlated with resistance to re-infection. Th2 cells were dominant in mice with progressive disease. Th1 cells from immune mice secreted IFNy and interleukin-2 (IL-2) in response to leishmanial antigens, whereas Th2 cells from mice with progressive infection produced IL-4, IL-5 and IL-10. As previously discussed, IFNy can activate macrophages to kill amastigotes. It can also inhibit expansion of Th2 cells. In contrast, IL-4 and IL-10 can inhibit proliferation of Th1 cells and activation of macrophages by IFNy.
A key question, which has not been fully resolved, is why Th1 responses become dominant in strains of mice with self-resolving infection and Th2 responses in those with progressive disease. mRNAs for Th1 and Th2 cytokines are present early in infection in both susceptible and
resistant animals. Analysis of the T-cell receptor repertoires in mice infected with L. (L.) major suggests that the same parasite epitopes can drive Th1 responses in immune animals and Th2 responses in animals with progressive disease (Reiner et al., 1993). IL-4 was initially hypothesized to be responsible for inhibiting the development of protective Th1 cells in mice with progressive infection (Reiner and Locksley, 1993), but subsequent studies in which the IL-4 gene was knocked out demonstrated that infection progressed even in the absence of IL-4 (Noben-Trauth et al., 1996).
It now appears that macrophages may play a critical role in the development of Th1 or Th2 responses. In immune animals, secretion of IL-12 precedes and supports the expansion of Leishmania-specific Th1 cells and the production of IFNy (Heinzel et al, 1993, 1995). IL-12 stimulates natural killer (NK) cells, and they in turn produce IFNy, which can activate macrophage microbicidal mechanisms and prime macrophages to produce IL-1 and THFa when they encounter the parasite. The CD40-CD40 ligand signaling process appears to mediate IL-12 secretion (Campbell et al., 1996). In the murine model, CD8+ cytotoxic/suppressor cells may also contribute to protection by secreting IFNy (Murray et al., 1992).
In contrast, transforming growth factor-p (TGFp) stimulates Th2 expansion and inhibits the development of Th1 cells (Barral-Netto,
1992). It has been postulated to play an important role in the progression of disease in susceptible mice. In addition, intracellular amastigotes have been shown to inhibit the secretion of IL-1 and TNFa by infected macrophages and to decrease the expression of HLA Class I and Class II antigens (Reiner et al., 1987). Finally, Leishmania-infected macrophages produce prostaglandins and leukotrienes that may suppress development of protective cell-mediated immune responses (Reiner and Malemud, 1985).