The research, led by Yasmina Drissi-El Boukili and Anna Rosanas-Urgell together with colleagues at the Clinical Research Unit of Nanoro and the Institute of Tropical Medicine in Antwerp, examined people naturally infected with Plasmodium falciparum, the deadliest of the malaria parasites that infect humans. The team focused on mutations in the beta-globin gene, HBB, which encodes one of the protein chains of hemoglobin, the oxygen-carrying molecule inside red blood cells. Two variant hemoglobins in particular drew their attention: hemoglobin S and hemoglobin C, both of which are common in West Africa precisely because they confer survival advantages against falciparum malaria.

People who carry one normal copy of the gene and one variant copy produce red blood cells containing a mixture of hemoglobins, a condition known as the HbAS trait for hemoglobin S or HbAC for hemoglobin C. These heterozygous states are famously protective: carriers are far less likely to develop the life-threatening complications of malaria, which is why natural selection has maintained these variants at high frequency in malaria-endemic regions. What has remained far less clear is whether these altered red blood cells affect another dimension of the parasite’s life cycle, namely its ability to commit to producing the sexual stages that make transmission possible.

That commitment process, called sexual conversion, is the opening move of gametocytogenesis. Most parasites circulating in a patient’s blood are asexual forms that multiply relentlessly inside red blood cells, consuming hemoglobin and driving the fevers and anemia of clinical malaria. But a small fraction of parasites instead differentiates into gametocytes, the sexual forms that lie dormant in the bloodstream until a mosquito takes a blood meal and ingests them. Inside the mosquito, these gametocytes transform again into gametes, fertilize, and give rise to the sporozoites that the insect will inject into its next human victim. Without sexual conversion, the entire transmission chain would collapse, which makes the factors that regulate this switch a matter of intense interest for anyone hoping to block the spread of malaria.

Measuring sexual conversion in natural infections, however, has long been technically difficult. To overcome this obstacle, the researchers developed a new ex vivo assay, which they called the evSCA, designed to quantify how frequently parasites drawn directly from infected patients commit to the sexual pathway. Using blood samples collected in Nanoro, a malaria-endemic district in central Burkina Faso, they compared sexual conversion rates among infected individuals grouped by their HBB genotype: those with the standard HbAA hemoglobin, those with the sickle-cell trait HbAS, and those with the HbAC variant.

The results were striking. Sexual conversion rates were significantly higher in infected individuals carrying HbAS or HbAC than in those with wild-type HbAA red blood cells. In other words, the very red blood cells that shield their owners from severe disease appeared to be nudging the parasites inside them toward the sexual pathway at a greater rate. Because gametocytes are the only forms the mosquito can pick up, elevated sexual conversion could in principle raise the number of transmissible parasites in the carrier’s blood, with consequences for how malaria spreads through communities where these hemoglobin variants are common.

To test whether this association reflected a direct effect of the red blood cell environment rather than some other difference between patients, the team turned to a controlled in vitro system. They cultured parasites in red blood cells of different hemoglobin compositions and measured sexual conversion using a reporter parasite line, NF54-gexp02-Tom, in which the expression of a fluorescent marker is tied to the activation of ap2-g, the master regulator gene that triggers sexual commitment. This reporter line allowed the researchers to count precisely how many parasites flipped the genetic switch under each culture condition. Consistent with the ex vivo findings from patient samples, parasites grown in HbAS red blood cells showed higher sexual conversion rates than parasites grown in HbAA cells, supporting the idea that the altered intracellular environment of variant-hemoglobin red blood cells actively promotes the sexual switch.

The researchers also considered an alternative explanation rooted in immunity rather than in the red blood cell itself. Antibodies directed against infected red blood cells can shape which parasite stages survive and flourish in a given host, so differences in antibody responses between HBB genotypes might conceivably account for the observed pattern. To investigate this, the team used flow cytometry to quantify immunoglobulin G and immunoglobulin M responses against antigens on the surface of red blood cells infected with trophozoites, the actively feeding asexual stage, and with stage I gametocytes, the earliest sexual stage. When they compared plasma from individuals with different HBB genotypes, they found no significant differences in these antibody responses, arguing against humoral immunity as the driver of the elevated sexual conversion and leaving the red blood cell environment itself as the more likely culprit.

The precise molecular mechanism by which variant hemoglobins encourage sexual commitment remains to be worked out, but the finding adds a fascinating new layer to the long evolutionary story of hemoglobin variants and malaria. Hemoglobin S and hemoglobin C are classic examples of a genetic trade-off: they cost carriers some fitness burden, including, in the case of hemoglobin S, the risk of sickle-cell disease when inherited from both parents, in exchange for protection against a killer disease. The new data suggest the ledger may be even more complicated than previously thought. If carriers of these variants, while suffering less severe illness, also harbor parasites that are more prone to becoming transmissible gametocytes, then human genetics could be quietly shaping the transmission landscape of malaria in endemic regions, influencing how intensively the parasite circulates even as it causes less disease in protected individuals.

From a public health perspective, the study opens several avenues worth pursuing. Understanding how the red blood cell environment modulates ap2-g activation could reveal new targets for transmission-blocking interventions, an area of growing importance as malaria control programs aim not merely to treat sick patients but to interrupt the parasite’s passage from human to mosquito to human. The ex vivo assay developed by the team also provides a practical tool for measuring sexual conversion directly in natural infections, something that has been difficult to achieve and that could now be applied to larger and more diverse populations. As researchers dig deeper into how host genetics sculpt the parasite’s life cycle decisions, the humble red blood cell is emerging not just as the parasite’s home and food source, but as an active participant in determining whether an infection remains a private battle between host and pathogen or becomes the next link in a chain of transmission.

Subject of Research: Effect of HBB hemoglobin variants on Plasmodium falciparum sexual conversion in naturally infected malaria patients

Article Title: Elevated Plasmodium falciparum sexual conversion in HbAC and HbAS red blood cells in naturally infected malaria patients

Article References: Elevated Plasmodium falciparum sexual conversion in HbAC and HbAS red blood cells in naturally infected malaria patients. (n.d.). https://doi.org/10.1371/journal.ppat.1014542

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014542

Keywords: malaria, Plasmodium falciparum, gametocytes, sexual conversion, hemoglobin S, hemoglobin C, HBB gene, red blood cells, Burkina Faso, transmission, ap2-g, ex vivo assay