“Neisseria meningitidis group B bacteria evade the human immune system by encasing its bacterial cell surface in PSA,” explains Dr. Moe. “Because PSA is highly expressed during fetal development, the immune system does not recognize the PSA capsule as foreign and doesn’t attack the bacteria as ‘other’ the way it would attack a normal foreign body.”
While no evidence existed prior to this study that Leishmania would express PSA or NeuPSA, recent research demonstrated that that they make a sticky, protective layer, or biofilm, on their outer surface.
"No one really has known what the composition of the biofilm is, but when researchers extracted the biofilm and used it as a vaccine, it was able to protect against Leishmania infection," explains Dr. Moe.
“No one really has known what the composition of the biofilm is, but when researchers extracted the biofilm and used it as a vaccine, it was able to protect against Leishmania infection,” explains Dr. Moe.
Dr. Moe’s lab had already discovered that vaccines containing NeuPSA elicited antibodies that were protective against N. meningitidis groups B and C, and Dr. Moe knew as well that one of the most distinctive characteristics of NeuPSA is its stickiness. He thought that PSA and NeuPSA might be part of the biofilm produced by Leishmania.
"Our first goal was just to find out, does Leishmania make PSA and NeuPSA?" says Dr. Moe. "And in fact our study shows that they do make both these things, but that most of the PSA is inside the cell, not on the outer surface where we had expected it to be."
This was only the first step in the groundbreaking study however. Upon further investigation of the presence of PSA or NeuPSA in Leishmania, Drs. Iovannisci and Moe discovered that when the Leishmania come together to form flower-like clusters, a stage called rosettes, the rosettes expressed PSA and NeuPSA on their surface.
A fluorescence micrograph of a rosette of Leishmania major promastigotes. PSA is marked by green, and NeuPSA by red fluorescence.
Rosettes have been known for decades, but scientists have always believed that they were a totally meaningless structure, despite a resemblance to mating stages in other microorganisms.
"Researchers argued that flagella, the thin whip-like structures on the exterior of the cells, were just getting entangled," says Dr. Iovannisci.
Not so, according to the new study. Most researchers incubate their leishmanial cultures at 25 degrees Celsius or higher. Drs Iovannisci and Moe discovered, however, that when Leishmania are cultured at a very specific temperature – 20 degrees Celsius, they abundantly form rosettes – the surfaces of which are covered in NeuPSA.
“No one has ever identified NeuPSA or PSA in Leishmania before, and no one has ever shown their expression in a stage specific manner in any other organism before,” says Dr. Iovannisci. “In addition, showing that PSA and NeuPSA are expressed only on the outer surface of the cells of the rosettes demonstrates that rosettes are in fact a real stage in the lifecycle of the organism.”
Identifying a sexual stage in the Leishmania lifecycle provides the key to understanding the genetics of the parasite – and, ultimately, to finding novel treatments or cures.
"What identifying this sexual reproductive stage means is that researchers now will be able to study how the genetics of the parasite works in the natural setting, and to more easily construct strains that have genetic characteristics of interest in terms of testing for new avenues of treatment," says Dr. Iovannisci.
"The fact that Leishmania express a unique carbohydrate on their surface also provides a new means of intervening in the lifecycle process with novel vaccines, therapeutic drugs and so forth," says Dr. Moe.
“We have essentially opened up a whole new approach to investigating and treating this invasive parasite.”