Leishmania mexicana: temperature sensitivity of isolated amastigotes and of amastigotes infecting macrophages in culture.

Different temperature requirements for intracellular growth of Leishmania spp. may account for the predominantly peripheral or visceral localization of the parasites in the vertebrate host. We show here that Leishmania mexicana amazonensis, an agent of cutaneous disease, is growth restricted in vitro at 37.5 C and viable at 34 C. This temperature sensitivity has been observed in both infected macrophage cultures and isolated amastigotes. Counts of the percentage of infected macrophages and of the number of amastigotes per infected cell documented that at 34 C the parasites progressively proliferated for at least 11 days. In contrast, no amastigotes were found in infected cells 48 to 72 hr after the cultures were shifted to the restrictive temperature of 37.5 C. This effect was reversible within the first 24 hr of temperature shift. Incorporation of [3H]uracil into parasite RNA was used to monitor the viability of intracellular and of isolated amastigotes. The trichloroacetic acid-precipitable incorporation of [3H]uracil by infected macrophages was markedly reduced as early as 6 hr after the monolayers were placed at 37.5 C. This decrease was also reversible within the first 24 hr at the restrictive temperature. In order to determine whether the restrictive temperature could directly affect the parasites, isolated amastigotes were incubated at either 34 C or 37.5 C and assayed for [3H]uracil incorporation. The results indicated that by 3 hr of incubation at the restrictive temperature the incorporation was significantly decreased and remained low for at least 9 hr.