MURINE CUTANEOUS LEISHMANIASIS: RESISTANCE IN RECONSTITUTED NUDE MICE AND SEVERAL F1 HYBRIDS INFECTED WITH LEISHMANIA TROPICA MAJOR

After cutaneous injection of promastigotes of an isolate of the intramacrophage protozoan parasite, Leishmania tropica major, mouse strains develop chronic cutaneous lesions or show a resolving pattern of disease. On this basis, they can be classified as resistant (e.g. CBA/H and C57BL/6) or susceptible (e.g. BALB/c, BALB/c.H-2^b and BALB/c.H-2^k). Hypothymic nude (nu/nu) mice of either BALB/c, CBA/H or C57BL/6 genotype are susceptible to chronic disease. However, nude mice of these genotypes, including BALB/c, are resistant to chronic cutaneous leishmaniasis when injected at the time of parasite challenge with small numbers of H-2 compatible lymphoid cells from normal mice. Nude mice remain susceptible when injected with fully H-2 incompatible cells. Using cells from H-2 mutant mice for reconstitution of resistance in C57BL/6.nu/nu mice, evidence was obtained that I region compatibility is necessary for cells to mediate host-protective effects. Cells from chronically-diseased BALB/c mice do not have protective effects in BALB/c.nu/nu mice at any cell dose and will abrogate the resistance-promoting effect of lymphoid cell populations from chronically-diseased BALB/c.H-2^k and BALB/c.H-2^b mice can be demonstrated when assayed at certain cell doses in H-2 compatible CBA/H.nu/nu and C57BL/6.nu/nu mice, respectively. The data suggest that chronically-diseased (genetically-susceptible) mice contain a mixture of resistance-promoting and disease-promoting T cells in their peripheral lymphoid organs and that expression of the resistance-promoting subset can occur in nude mice of resistant genotype. Previous data have indicated that Lyl⁺2^? T cells are efficient mediators of both T cell-dependent activities. No evidence for the operation of disease-promoting or resistance-promoting antibodies in perpetuation or resolution of disease has been obtained in extensive serum transfer experiments. Some discrepancies exist in the literature on the question of the dominance of susceptibility or resistance in F₁ hybrid mice. A re-examination of susceptibility/resistance in F₁ hybrids between BALB/c and several other parental strains was undertaken using cloned pathogenic promastigotes derived from a heterogeneous L. t. major isolate in order to reduce effects of parasite heterogeneity in the analysis. Resistance was dominant in some but not all F₁ hybrids, with most showing a delayed healing pattern of disease relative to the resistant parental strain. Despite the use of genetically-homogeneous parasites, the analysis was complicated by variability within groups of F₁ hybrid mice as well as between males and females and between F₁ hybrids of reciprocal crosses. A hypothesis based on antigen and H-2 expression on infected macrophages is advanced to account for the balance between the effects of resistance-promoting and disease-promoting Lyl⁺2^? T cells in mice of various genotypes.