Uncovering the hideout of malaria sexual parasites

More than 30 years after a similar study solely relying on parasite morphology on a small group of Gambian children,²  Aguilar and colleagues¹  conduct a rigorous analysis with state-of-the-art tools on samples of peripheral blood and bone marrow aspirates from 174 anemic children from a region of moderate to perennial malaria transmission and provide molecular evidence confirming the preferential distribution of P falciparum immature sexual stages in the human bone marrow.

Recent renewed emphasis on the eradication of malaria has highlighted the need for novel interventions to target the human malaria parasite during transmission from the human host to the mosquito. Transmission of malaria parasites relies on the sexual stages, the gametocytes, which circulate in peripheral blood and are taken up by the mosquito during a blood meal. P falciparum gametocytes do not freely circulate in the peripheral blood for their entire intraerythrocytic development, which instead requires sequestration of the immature gametocyte-infected erythrocytes in internal organs of infected individuals for as long as 10 days. How and where P falciparum gametocytes sequester in humans are fundamental unanswered questions in malaria biology, and a better understanding of the mechanisms of gametocyte sequestration and circulation is essential to enable the development of new tools to disrupt malaria transmission.

The current work of Aguilar and colleagues uses a newly developed stage-specific quantitative reverse-transcriptase polymerase chain reaction method to quantitatively detect early, intermediate, and mature gametocyte-stage transcripts in bone marrow vs peripheral blood. Results of this work provide evidence for the higher prevalence and abundance of immature gametocytes in bone marrow samples, whereas mature gametocyte are more abundant and prevalent in samples from peripheral blood. In addition, Aguilar and colleagues¹  analyze correlations of demographic and clinical parameters with gametocyte prevalence and density in both tissues. Their results indicate that severe anemia and dyserythropoiesis are independently associated with a higher prevalence of mature gametocytes in bone marrow, suggesting a relationship between hematologic disturbances and gametocyte development in this tissue. The study, conducted on a cohort of mainly asymptomatic children, also contains relevant information for the epidemiology of malaria transmissibility. Although it is now recognized that microscopy generally underestimates gametocyte presence in peripheral blood, this work interestingly shows that the same method detects a 6-fold-higher prevalence of gametocytes in bone marrow. On one hand, this further confirms the need for molecular methods to identify the reservoir of malaria transmission in asymptomatic populations, and on the other hand it suggests the importance to reveal and possibly quantify the presence of the sequestered gametocyte nurseries to predict evolution and dynamics of malaria transmissibility in the populations under survey.

Although the virtual absence of immature gametocytes in peripheral blood is described from the early days of malariology, almost nothing is known about sites and mechanisms of immature gametocyte sequestration. Only a few early analyses of postmortem specimens and a recent clinical report provide morphologic evidence indicating bone marrow and spleen as the organs where immature gametocytes are most readily detectable.^2,,-5  These studies were of limited scale and used standard light microscopy as the only method for gametocyte detection and stage differentiation. In this respect, Aguilar and colleagues have the merit of readdressing this fundamental issue with state-of-the art tools. However, because their study does not inspect the presence of gametocytes in tissues other than bone marrow and peripheral blood, we hope that this work will fuel novel efforts in investigating if gametocytes can develop and mature in other organs of the infected host. For instance some of the early postmortem observations and a recent clinical case report on a splenectomized patient suggest that immature gametocytes may also mature in the spleen.^3,4,6  Despite the different physiological functions of such organs, sinusoids in the bone marrow and in the spleen, where blood circulation occurs at a reduced flow rate, may provide suitable sites for host-cell interactions and the establishment of the early sequestering gametocytes that do not have the same adhesive properties of asexual stages.⁷ 

The described localization of immature gametocytes in the bone marrow and the association between gametocyte presence and some functional parameters of this organ raise the question of P falciparum–host interactions in this microenvironment. A recent clinical case report from a patient with subacute P falciparum malaria importantly revealed the presence of immature gametocytes in extravascular spaces of bone marrow, in close proximity to erythroblasts and adipocytes.⁵  Although this result needs to be confirmed in a larger scale and possible confounding effects of clinical symptoms or drug treatment on the histologic results need to be ruled out, this observation surely falls on a fertile ground for new hypotheses on the physiological relevance of such previously unreported localization of P falciparum immature sexual stages. Rather than representing just one of the possible parasite sequestration sites, it is tempting to speculate that bone marrow represents instead a privileged site for a complex crosstalk of gametocyte-infected erythrocytes with endothelial and nonendothelial cells. Such an interplay could master several processes from the extravascular development of immature gametocytes to their release in circulation at maturity, taking advantage of recently described changes in their cellular mechanical properties during gametocytogenesis.^8,-10 

The renewed attention dedicated to the biology of gametocytes is significantly improving our basic understanding of how malaria parasites are maintained and propagated. One example of such progress are the observations by Aguilar and colleagues, which have important practical implications in designing how parasite transmission stages can be effectively targeted by current and future antimalarial drugs or vaccines.