The overwhelming susceptibility of BALB/c mice to infection with Leishmania tropica can be substantially reversed by immediately prior sub-lethal irradiation. This is related to radiation dosage, and at 550 rad, causes 60 percent complete cures and only 19 percent (instead of 100 percent) incidence of progressive disease. Irradiation 10 d before infection is only weakly prophylactic, whereas 10 d after is without effect. Control of lesion development is only apparent after the first 30 d, coincident with the analogous onset previously found in resistant strains and adult thymectomized, x-irradiated, bone marrow-reconstituted BALB/c mice. Instead of the specific suppression of DTH characteristic of L. tropica infection in the BALB/c strain, healed irradiated mice express strong anti-leishmanial DTH reactivity and resistance to reinfection. T cells from these mice transfer DTH reactivity which is suppressed by admixture with cells from nonhealed, nonreactive donors.

Irradiated BALB/c mice again develop inexorable disease progression, after its transient arrest, when they are reconstituted with normal T cells. When the T cells are derived from uncontrollably-infected donors, the susceptibility regained is indistinguishable from that of normal mice. B cells do not modify the prophylactic effect of 550 rad, whereas T cells from healed mice confer strong protective immunity throughout the initial phase. Regression or progression of disease correlates completely with DTH reactivity in all these groups.

Although BALB/c mice express an extreme level of genetic susceptibility to L. tropica infection, they are nevertheless capable of mounting a curative cell mediated immune response. That this is ineffective during pathogenesis of the disease was previously associated correlatively with potent specific suppressor T cell generation, which is now shown to be preventable by prior irradiation. Most important, however, a causal role for these cells in vivo has been demonstrated directly by reconstitution.

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