Leishmania donovani: Physicochemical, immunological, and biological characterization of excreted factor from promastigotes

Leishmanial excreted factor (EF) from promastigote cultures was enriched from the crude product by differential precipitation with ammonium sulfate and perchloric acid, followed by column chromatography; and by boiling EF-antibody complex. Boiling destroyed the antibody, releasing the EF, which retained its ability to precipitate antibody. Enriched EF from Leishmania donovani promastigotes was found to be a highly negatively charged, carbohydrate-like material with a molecular weight approximating to 33,000, when monitored against a series of protein markers by gel filtration. Its ability to precipitate with antibody was unimpaired by boiling, lyophilization, pH changes from 1 to 11, treatment with high concentrations of NaCl, 10% phosphotungstic acid in 10% HCl, 0.6 M perchloric acid, 5% H₂SO₄, acetone, or dioxan. It did not absorb at wavelengths between 220 and 750 nm. Treatment with trypsin, Pronase, neuraminidase, and hyaluronidase did not affect its activity. Biochemical analysis showed that enriched EF contains carbohydrates but, at our level of detection, no protein, lipid, triglycerides, fatty acids, DNA, RNA, pentoses, amino sugars, sialic or uronic acid. Precipitation of EF by antibody was studied and the optimal molecular proportions for complete precipitation determined. EF-antibody complex, prepared at optimal proportions, and EF complexed with methylated bovine serum albumin, like EF alone, did not elicit antibody production in rabbits. EF in 0.5% phenol-saline elicited a delayed skin response of induration and erythema in guinea pigs cured of L. enriettii. Elevated temperature increased the release of EF from promastigotes, while the presence of trypsin acting at 37 C seemed to inhibit this effect slightly. Fractionation of mechanically broken promastigotes, by differential centrifugation and stepwise sucrose gradients, revealed a factor that precipitated rabbit antibody against whole promastigotes. This factor was associated with the soluble, organelle-free fraction and resembled EF when monitored by gel diffusion. This factor did not migrate when the complete extract from the broken promastigotes was run in immunoelectrophoresis. Boiling the extract for 5 min released a factor, which migrated to the anode. This factor appeared to be associated with another component in the promastigote, from which it dissociated on boiling. Boiling hamster tissues infected with leishmanial amastigotes, i.e., spleens containing L. donovani and epididymides containing L. tropica, also released factors similar to EF. These precipitated antibody in the same way, producing precipitation arcs that were continuous with those formed by EF from the homologous promastigotes. EF acted as a conditioner for culture promastigotes. Conditioned cultures showed maximal growth before similar, unconditioned cultures. However, both types of culture produced equal numbers of promastigotes per unit volume by the end of exponential growth.