Splenic clearance of rigid erythrocytes as an inherited mechanism for splenomegaly and natural resistance to malaria

Abstract

AbstractIn malaria-endemic areas, subjects from specific groups like Fulani have a peculiar protection against malaria, with high levels of IgM but also frequent anemia and splenomegaly. The mechanisms underlying this phenotype remain elusive. In Benin, West Africa, we measured the deformability of circulating erythrocytes in genetically distinct groups (including Fulani) living in sympatry, using ektacytometry and microsphiltration, a mimic of how the spleen clears rigid erythrocytes. Compared to non-Fulani, Fulani displayed a higher deformability of circulating erythrocytes, pointing to an enhanced clearance of rigid erythrocytes by the spleen. This phenotype was observed in individuals displaying markers of Plasmodium falciparum infection. The heritability of this new trait was high, with a strong multigenic component. Five of the top 10 genes selected by a population structure-adjusted GWAS, expressed in the spleen, are potentially involved in splenic clearance of erythrocytes (CHERP, MB, PALLD, SPARC, PDE10A), through control of vascular tone, collagen synthesis and macrophage activity. In specific ethnic groups, genetically-controlled processes likely enhance the innate retention of infected and uninfected erythrocytes in the spleen, explaining splenomegaly, anemia, cryptic intrasplenic parasite loads, hyper-IgM, and partial protection against malaria. Beyond malaria-related phenotypes, inherited splenic hyper-filtration of erythrocytes may impact the pathogenesis of other hematologic diseases.Research in contextEvidence before this studyThe genetic background of individuals influences their susceptibility to infectious diseases. Specific human groups, like the Fulani in Africa, react to malaria parasites (named Plasmodium) in a specific way. Upon infection, Fulani develop a grossly enlarged spleen, and high levels of anti-Plasmodium antibodies in their blood. They also carry smaller numbers of parasites in their blood, and thus are considered partially protected against malaria. The mechanisms underlying this natural protection, different from other natural protective mechanisms such as the sickle cell trait, are not well understood.Malaria impairs the deformability of red blood cells and the spleen is a key organ to controlling red blood cell quality. We have recently demonstrated that red blood cells containing live malaria parasites accumulate intensely in the spleen of subjects with long term exposure to these parasites. Enhanced retention of infected and uninfected red blood cells in the spleen would explain why the spleen is larger and why lower numbers of parasites are left in circulation. We thus explored whether the retention of infected and uninfected red blood cells could explain why Fulani are partially protected against malaria. Because it is unethical to perform spleen puncture or biopsies for research purposes, our explorations were indirect by carefully analyzing the properties of circulating red blood cells in a large number of subjects and by assessing whether observations could be explained by their genetic make-up.Added value of this studyIn more than 500 subjects, we confirmed the high frequency of large spleens in Fulani and, through 2 different methods, we demonstrated an enhanced deformability of their circulating red blood cells, that likely stems from the more efficient removal of the less deformable ones. This enhanced deformability was found to be inheritable based on carefully collected family links and refined analysis of genetic markers.Implications of all the available evidenceOur findings indicate that genes potentially driving the filtration of red blood cells by the spleen likely influence how subjects in specific groups in Africa and elsewhere react to malaria. While most previous hypotheses pointed to conventional immunological mechanisms as the trigger, we propose that a simple physiological mechanism that controls the quality of red blood cells may drive natural protection from malaria even before the intervention of immunological cells. A better understanding of these processes is of great importance in the context of malaria elimination efforts.These findings may also have an impact on the understanding of other red blood cell-related disorders, such as inherited red cell diseases, in which splenic filtration of abnormal red blood cells may precipitate splenic complications.