Discovery about fertilization points way to possible malaria vaccine

Understanding fertilization in microorganisms may lead to a new type of vaccine, called an “altruistic vaccine,” Dr. Snell said. There is no vaccine to prevent malaria, but in the future, vaccinating an infected person to prevent the parasitic cells from completing the fusion step might stop the disease from passing to others.

Although the study involved only single-celled organisms, Dr. Snell said that the use of two different proteins in the two-step fertilization process may be the case in all species. The gene controlling whether egg and sperm can bind would be unique to each species, while the gene for the second step – fusing into a single cell – could be more universal. For example, the researchers found that HAP2, the gene that controlled whether cells fused, is also present in agriculturally important crops such as corn and wheat.

For the parasite that causes malaria, fusion is controlled by a gene not found in mammals, so blocking this step might prove effective in stanching the spread of the disease without harming humans, Dr. Snell said.

The UT Southwestern researchers initially were interested in general methods of reproduction, which are often studied in Chlamy. This single-celled organism can reproduce sexually, not with egg and sperm, but rather with some cells roughly corresponding to “male” and others to “female.”

After realizing that Plasmodium species, including the one that causes malaria in humans, also use “male” and “female” cells to reproduce, Dr. Snell contacted malaria researchers at Imperial College London to test whether HAP2 might be involved in reproduction in that organism and in the spread of malaria.

The British researchers found that blocking HAP2 in Plasmodium cells stops the fusing step. When mutant Plasmodium organisms lacking HAP2 were injected into mice, mosquitoes that bit the mice did not become infected with Plasmodium and therefore could not spread the infection to other mice. This indicates that without HAP2, Plasmodium could not reproduce in a mosquito’s gut, a vital step in the cycle of infection.

In addition to its applicability in fighting disease, the latest study confirms a theory about why different species can’t interbreed, Dr. Snell said. There are rare exceptions in closely related species such as horses and donkeys, which can interbreed to create mules, or lions and tigers that can mix to form ligers, but such hybrids are sterile.