A radioattenuated Leishmania major vaccine markedly increases the resistance of CBA mice to subsequent infection with Leishmania mexicana mexicana.

Vaccinating CBA mice with radioattenuated Leishmania major amastigotes but not with radioattenuated L. mexicana mexicana amastigotes rendered them highly resistant to subsequent infection with L. m. mexicana. Unvaccinated CBA mice were highly susceptible to infection with L. m. mexicana producing rapidly growing non-ulcerating cutaneous lesions. Two manifestations of resistance were induced in vaccinated animals depending on the timing of the challenge infection: no lesions appeared at the site of subcutaneous challenge in animals vaccinated four or more weeks previously, while lesions grew rapidly but ulcerated and healed in animals vaccinated less than 3 weeks beforehand. L. major amastigotes were found to be markedly more resistant to gamma irradiation than L. m. mexicana amastigotes both as measured by their ability to infect susceptible strains of mice and to transform and multiply as promastigotes in NNN medium.