Yin and yang

The team has also shown that other exportome proteins are involved in remodeling erythrocytes. Instead of slipping smoothly through the microvasculature, the deformed, rigid sacs tend to “catch” in the microvasculature, presenting opportunities for PfEMP1 to bond with complementary proteins on the walls of blood vessels.

Cowman says other Plasmodium species don’t appear to require PfEMP1 and other exportome proteins for cytoadherence.

In P. falciparum, as many as 100 genes specify variants of PfEMP1 protein; each binds a complementary protein expressed on the endothelium of microscopic blood vessels in a specific organ or tissue. This complex system of yin-yang bonds specifies the organ in which the parasite will find refuge. By switching PfEMP1 variants, the parasite plays hide-and-seek with the immune system.

To mount any PfEMP variant on an erythrocyte’s surface, the parasite must deliver other exportome proteins through two membranes – its own, and the host cell’s – to be assembled into the sticky knob proteins.

The parasite proteins are transported through host erythrocytes that lack the specialised trafficking machinery of normal cells, then inserted into the erythrocyte membrane.

They form a highly organised skeleton that anchors PfEMP1 to the cell’s surface. Cowman says this transport and trafficking system for parasite proteins is unique in cell biology.

KAHRP (Knob-Associated Histidine-Rich Protein), identified by Cowman and WEHI colleague Brendan Crabb in 1997, is a key component of the knobs. Its binding with the membrane skeleton rigidifies the infected cells, causing them to lodge in tiny capillaries, obstructing them and restricting blood flow.

To determine which exportome genes and proteins were essential for PfEMP1 trafficking and function, Cowman’s team made single-gene knockout lines of the parasite – a technique he and Crabb pioneered in the late 1990s – and screened them for functional defects.

“We knocked out about 55 exportome genes, and 35 of them seemed to be essential – the parasite couldn’t do anything without them,” Cowman says.

“When we put all 55 knockout lines through a whole heap of function screens, we identified eight that are required for trafficking and function.

“These are clearly very important biologically to the parasite, which means they are potential new targets for therapeutics. It also raises the possibility of developing drugs that would create attenuated forms of living parasites, by disrupting knob assembly.”

Cowman says these non-adhering variants would be flushed into the bloodstream and carried through the spleen, where they would be exposed to antibody-secreting B-cells.

The attenuated parasites would not be able to cause disease, and would trigger the full defensive repertoire of the immune system, including antibody-secreting B cells and cytotoxic T-cells.

The attenuated parasites would effectively be transformed into a live vaccine, specific to whatever P. falciparum strain had infected that person, and eliciting an immune response unique to that individual.

“It doesn’t make sense to try to develop a vaccine targeting the exportome proteins, because most are expressed internally,” he says. “But we can make drugs that inhibit their function, preventing knob assembly.”

The beauty of this approach, says Cowman, is that the drugs wouldn’t kill the parasite – they would simply allow the immune system to have a long look at all of its surface proteins, not just a few that are can randomly switched so the parasite presents a constantly shifting target to the immune system.

The discovery could obviate the need to develop a vaccine, or vaccines, capable of protecting millions of genetically unique individuals in the world’s tropical regions against whatever unique regional or local strain of the P.falciparum they might encounter – even multi-drug resistant strains.