Two separate growth phases during the development of Leishmania in sand flies: implications for understanding the life cycle

Abstract

The life cycle of Leishmania alternates between two main morphological forms: intracellular amastigotes in the mammalian host and motile promastigotes in the sand fly vector. Several different forms of promastigote have been described in sandfly infections, the best known of these being metacyclic promastigotes, the mammal-infective stages. Here we provide evidence that for Leishmania (Leishmania) mexicana and Leishmania (Leishmania) infantum (syn. chagasi) there are two separate, consecutive growth cycles during development in Lutzomyia longipalpis sand flies involving four distinct life cycle stages. The first growth cycle is initiated by procyclic promastigotes, which divide in the bloodmeal in the abdominal midgut and subsequently give rise to non-dividing nectomonad promastigotes. Nectomonad forms are responsible for anterior migration of the infection and in turn transform into leptomonad promastigotes that initiate a second growth cycle in the anterior midgut. Subsequently, leptomonad promastigotes differentiate into non-dividing metacyclic promastigotes in preparation for transmission to a mammalian host. Differences in timing, prevalence and persistence of the four promastigote stages were observed between L. mexicana and L. infantum in vivo, which were reproduced in cultures initiated with lesion amastigotes, indicating that development is to some extent governed by a programmed series of events. A new scheme for the life cycle in the subgenus Leishmania (Leishmania) is proposed that incorporates these findings.

Figure 1

Developmental profiles of L. mexicana and L. infantum in vivo and in vitro. Flies were infected with tissue amastigotes of L. mexicana (A) or L. infantum (B) and the relative proportions of amastigotes (■-■), procyclic promastigotes (Δ-Δ), nectomonad promastigotes (▲-▲), leptomonad promastigotes (◇-◇), and metacyclic promastigotes (◆-◆) determined. Similarly, in vitro cultures were initiated with tissue amastigotes of L. mexicana (C) or L. infantum (D). Each experiment was repeated and the results combined to yield the data shown.

Figure 2

Occurrence of dividing life cycle stages and nectomonad forms (non-dividing) in vivo. The number of procyclic promastigotes (Δ-Δ), nectomonad promastigotes (▲-▲) and leptomonad promastigotes (◇-◇) per fly for L. mexicana infections (A) and L. infantum infections (B) in Lutzomyia longipalpis. Combined data from two independent experiments with each species.

Figure 3

Revised life cycle of Leishmania (Leishmania) species in Lutzomyia longipalpis. Replication of parasites occurs at three points: amastigotes in macrophage phagolysosomes; procyclic promastigotes in the abdominal midgut; and leptomonad promastigotes in the thoracic midgut. These growth cycles are linked by various non-dividing or transmission stages as shown.