Steven Kho

In chronic Plasmodium vivax (Pv) infection, the spleen accounts for over 98% of total‐body parasite biomass. Whether splenic tropism also occurs in acute infection and how the spleen influences pathogenesis have not been systematically explored. In Papua, Indonesia, we compared clinical and hematology data in 24 spleen‐intact and 25 previously splenectomized patients with acute uncomplicated vivax malaria. Plasma levels of Pv lactate dehydrogenase (PvLDH) and markers of intravascular haemolysis, inflammation, endothelial activation and neutrophil activation were measured by ELISA. Circulating parasitaemia was four times higher in splenectomized compared to spleen‐intact patients but total‐body Pv biomass (PvLDH) was three times lower in patients without a spleen. Parasite staging and greater organ‐specific symptoms suggest parasite redistribution in the absence of a spleen. Linear regression modeling adjusting for circulating parasitaemia, age, sex and fever duration demonstrated an 8.1‐fold higher PvLDH concentration in spleen‐intact patients, indicating a splenic biomass accounting for 89% of total‐body parasites. Giemsa‐based splenic histology in an untreated patient splenectomized during acute vivax malaria revealed a spleen‐to‐blood biomass ratio of 10.7. In spleen‐intact patients, splenic biomass correlated strongly with markers of disease intensity, endothelial activation and systemic inflammation, whereas circulating parasitaemia correlated weakly or not at all. Compared to spleen‐intact patients, cell‐free hemoglobin, endothelial activation and systemic inflammation were higher in splenectomized patients while inflammasome‐dependent responses were lower. In conclusion, Pv is predominantly an infection of the spleen in acute clinical vivax malaria. While the size of this hidden population correlates with disease intensity, the spleen likely modulates inflammatory pathways and heme‐associated pathology.

Background: In chronic asymptomatic Plasmodium vivax infections, the spleen accounts for more than 98% of total-body parasite biomass. Whether this splenic tropism also exists in acute infection and how the spleen influences pathogenesis have not been systematically explored. Materials and Methods: In Papua, Indonesia, we compared plasma levels of P. vivax lactate dehydrogenase [PvLDH]) and circulating parasitemia in 24 spleen-intact and 25 previously splenectomized patients with acute uncomplicated vivax malaria. Clinical and hematology data were collected and plasma markers of intravascular hemolysis (cell-free hemoglobin [CFHb]), endothelial activation (angiopoietin-2), inflammation (interleukin [IL]-1 beta, IL-6, IL-18, IL-10, tumor necrosis factor-alpha) and neutrophil activation (elastase) were measured by ELISA. Giemsa-based histology in one spleen from an untreated patient splenectomized for trauma during an episode of acute vivax malaria enabled direct assessment of splenic and circulating parasitemia and biomass microscopically. Results: Circulating parasitemia was 4-times higher in splenectomized compared to spleen-intact patients (median 21,100 vs 4,820 parasites/µL, p=0.0002) but total-body P. vivax biomass (PvLDH) was 3-times lower in patients without a spleen (median 721 vs 2,140 ng/mL, p=0.026). Parasite staging and greater organ-specific symptoms suggest redistribution of parasites in the absence of a spleen. Linear regression modeling, adjusting for circulating parasitemia, patient age, sex and duration of fever, demonstrated an 8.1-fold higher PvLDH concentration in spleen-intact patients (95% confidence interval [CI]: 3.4-19.5-fold, pP. vivax biomass correlated strongly with markers of disease intensity, endothelial activation and systemic inflammation, whereas circulating parasitemia correlated weakly or not at all. Compared to spleen-intact patients, CFHb, endothelial activation and systemic inflammation were higher in splenectomized patients while inflammasome-dependent responses were lower. Conclusions: P. vivax is predominantly an infection of the spleen, even in acute clinical vivax malaria. We conservatively estimate that 89% of total-body parasite biomass in acute infection is splenic. While the size of this hidden population correlates with disease intensity, the spleen likely regulates inflammatory pathways and heme-associated pathology.

Splenic rupture is a recognized complication of acute Plasmodium falciparum and Plasmodium vivax malaria, but the risk of splenic rupture in chronic asymptomatic infections is unknown. In Timika, Papua, Indonesia, we determined the proportion of PCR-detectable asymptomatic peripheral parasitemia in patients undergoing trauma-related splenectomy (2015–21) and found it was more than twice the proportion compared to a 2013 household survey of the general population (87.9% [29/33] vs 38.6% [697/1807]; P < .0001). Our findings suggest asymptomatic parasitemia with either P. falciparum or P. vivax is associated with splenic rupture following trauma, pointing toward an additional consequence of chronic infection in malaria-endemic areas.

The human spleen significantly influences red blood cell (RBC) dynamics due to its ability to retain and/or remove RBCs from peripheral blood circulation. This filtering can mediate a range of malaria disease manifestations, depending on the physiological properties of the spleen. Data collected from patients undergoing splenectomy in Papua, Indonesia, revealed that in asymptomatic infections the spleen harboured substantially more infected RBCs than were circulating in the peripheral blood and that the spleen is also congested with uninfected RBCs. We hypothesise that two conditions hold for the spleen to retain such a high proportion of infected and uninfected RBCs: (i) the retention rate of RBCs is significantly higher than in uninfected patients; and (ii) phagocytosing macrophages cannot clear all of the infected RBCs from the spleen. In this paper, we present a mathematical model of RBC dynamics that includes, for the first time, the spleen as a compartment capable of retaining large numbers of infected and uninfected RBCs in Plasmodium falciparum and P. vivax infections. By calibrating the model to the Papuan data, we demon-strate that the spleen plays a significant role in removing not only infected RBCs but also uninfected RBCs. Uninfected RBC retention in the spleen, attributable to malaria, is substantially higher than circulating RBC loss due to parasitisation, for infections by both Plasmodium species. In chronic infections, the ratio of circulating uninfected RBCs lost to splenic retention per circulating uninfected RBC lost to parasitisation is 17:1 for P. falciparum and 82:1 for P. vivax . These ratios are larger than previously published estimates for acute clinical infections.

The risk of severe malaria from the zoonotic parasite Plasmodium knowlesi approximates that from P. falciparum. In severe falciparum malaria, neutrophil activation contributes to inflammatory pathogenesis, including acute lung injury (ALI). The role of neutrophil activation in the pathogenesis of severe knowlesi malaria has not been examined. We evaluated 213 patients with P. knowlesi mono-infection (138 non-severe, 75 severe) and 49 Plasmodium-negative controls from Malaysia. Markers of neutrophil activation (soluble neutrophil elastase [NE], citrullinated histone [CitH3] and circulating neutrophil extracellular traps [NETs]) were quantified in peripheral blood by microscopy and immunoassays. Findings were correlated with malaria severity, ALI clinical criteria, biomarkers of parasite biomass, haemolysis, and endothelial activation. Neutrophil activation increased with disease severity, with median levels higher in severe than non-severe malaria and controls for NE (380[IQR:210–930]ng/mL, 236[139–448]ng/mL, 218[134–307]ng/mL, respectively) and CitH3 (8.72[IQR:3.0–23.1]ng/mL, 4.29[1.46–9.49]ng/mL, 1.53[0.6–2.59]ng/mL, respectively)[all p<0.01]. NETs were higher in severe malaria compared to controls (126/μL[IQR:49–323] vs 51[20–75]/μL, p<0.001). In non-severe malaria, neutrophil activation fell significantly upon discharge from hospital (p0.24, p<0.05). Plasma NE and angiopoietin-2 were higher in knowlesi patients with ALI than those without (p<0.008); neutrophilia was associated with an increased risk of ALI (aOR 3.27, p<0.01). In conclusion, neutrophil activation is increased in ALI and in proportion to disease severity in knowlesi malaria, is associated with endothelial activation, and may contribute to disease pathogenesis. Trials of adjunctive therapies to regulate neutrophil activation are warranted in severe knowlesi malaria.

In humans, ∼0.1% to 0.3% of circulating red blood cells (RBCs) are present as platelet-RBC (P-RBC) complexes, and it is 1% to 2% in mice. Excessive P-RBC complexes are found in diseases that compromise RBC health (eg, sickle cell disease and malaria) and contribute to pathogenesis. However, the physiological role of P-RBC complexes in healthy blood is unknown. As a result of damage accumulated over their lifetime, RBCs nearing senescence exhibit physiological and molecular changes akin to those in platelet-binding RBCs in sickle cell disease and malaria. Therefore, we hypothesized that RBCs nearing senescence are targets for platelet binding and P-RBC formation. Confirming this hypothesis, pulse-chase labeling studies in mice revealed an approximately tenfold increase in P-RBC complexes in the most chronologically aged RBC population compared with younger cells. When reintroduced into mice, these complexes were selectively cleared from the bloodstream (in preference to platelet-free RBC) through the reticuloendothelial system and erythrophagocytes in the spleen. As a corollary, patients without a spleen had higher levels of complexes in their bloodstream. When the platelet supply was artificially reduced in mice, fewer RBC complexes were formed, fewer erythrophagocytes were generated, and more senescent RBCs remained in circulation. Similar imbalances in complex levels and senescent RBC burden were observed in humans with immune thrombocytopenia (ITP). These findings indicate that platelets are important for binding and clearing senescent RBCs, and disruptions in platelet count or complex formation and clearance may negatively affect RBC homeostasis and may contribute to the known risk of thrombosis in ITP and after splenectomy.

Splenomegaly frequently occurs in patients with Plasmodium falciparum (Pf) or P. vivax (Pv) malarial anemia, but mechanisms underlying this co‐occurrence are unclear. In malaria‐endemic Papua, Indonesia, we prospectively analyzed red blood cell (RBC) concentrations in the spleen and spleen‐mimetic retention in 37 subjects splenectomized for trauma or hyperreactive splenomegaly, most of whom were infected with Plasmodium. Splenomegaly (median 357 g [range: 80–1918 g]) was correlated positively with the proportion of red‐pulp on histological sections (median 88.1% [range: 74%–99.4%]; r = .59, p = .0003) and correlated negatively with the proportion of white‐pulp (median 8.3% [range: 0.4%–22.9%]; r = −.50, p = .002). The number of RBC per microscopic field (>95% uninfected) was correlated positively with spleen weight in both Pf‐infected (r = .73; p = .017) and Pv‐infected spleens (r = .94; p = .006). The median estimated proportion of total‐body RBCs retained in Pf‐infected spleens was 8.2% (range: 1.0%–33.6%), significantly higher than in Pv‐infected (2.6% [range: 0.6%–23.8%]; p = .015) and PCR‐negative subjects (2.5% [range: 1.0%–3.3%]; p = .006). Retained RBCs accounted for over half of circulating RBC loss seen in Pf infections. The proportion of total‐body RBC retained in Pf‐ and Pv‐infected spleens correlated negatively with hemoglobin concentrations (r = −.56, p = .0003), hematocrit (r = −.58, p = .0002), and circulating RBC counts (r = −.56, p = .0003). Splenic CD71‐positive reticulocyte concentrations correlated with spleen weight in Pf (r = 1.0; p = .003). Retention rates of peripheral and splenic RBCs were correlated negatively with circulating RBC counts (r = −.69, p = .07 and r = −.83, p = .008, respectively). In conclusion, retention of mostly uninfected RBC in the spleen, leading to marked congestion of the red‐pulp, was associated with splenomegaly and is the major mechanism of anemia in subjects infected with Plasmodium, particularly Pf.