Phosphoglycan repeat-deficient Leishmania mexicana parasites remain infectious to macrophages and mice

The human pathogen Leishmania synthesizes phosphoglycans (PGs) formed by variably modified phosphodisaccharide [6-Galbeta1-4Manalpha1-PO(4)] repeats and mannooligosaccharide phosphate [(Manalpha1-2)(0-5)Manalpha1-PO(4)] caps that occur lipid-bound on lipophosphoglycan, protein-bound on proteophosphoglycans, and as an unlinked form. PG repeat synthesis has been described as essential for survival and development of Leishmania throughout their life cycle, including for virulence to the mammalian host. In this study, this proposal was investigated in Leishmania mexicana using a spontaneous mutant that was fortuitously isolated from an infected mouse, and by generating a lmexlpg2 gene deletion mutant (Deltalmexlpg2), that lacks a Golgi GDP-Man transporter. The spontaneous mutant lacks PG repeats but synthesizes normal levels of mannooligosaccharide phosphate caps, whereas the Deltalmexlpg2 mutant is deficient in PG repeat synthesis and down-regulates cap expression. In contrast to expectations, both L. mexicana mutants not only retain their ability to bind to macrophages, but are also indistinguishable from wild type parasites with respect to colonization of and multiplication within host cells. Moreover, in mouse infection studies, the spontaneous L. mexicana repeat-deficient mutant and the Deltalmexlpg2 mutant showed no significant difference to a wild type strain with respect to the severity of disease caused by these parasites. Therefore, at least in Leishmania mexicana, PG repeat synthesis is not an absolute requirement for virulence.