Effect of recombinant malarial antigen on monocyte functionality

Infection vs. Reinfection: The Immunomodulation of Erythropoiesis

Severe malarial anemia (SMA) increases the morbidity and mortality of Plasmodium, the causative agent of malaria. SMA is mainly developed by children and pregnant women in response to the infection. It is characterized by ineffective erythropoiesis caused by impaired erythropoietin (EPO) signaling. To gain new insights into the pathogenesis of SMA, we investigated the relationship between the immune system and erythropoiesis, conducting comparative analyses in a mouse model of malaria. Red blood cell (RBC) production was evaluated in infected and reinfected animals to mimic endemic occurrences. Higher levels of circulating EPO were observed in response to (re)infection. Despite no major differences in bone marrow erythropoiesis, compensatory mechanisms of splenic RBC production were significantly reduced in reinfected mice. Concomitantly, a pronounced immune response activation was observed in erythropoietic organs of reinfected animals in relation to single-infected mice. Aged mice were also used to mimic the occurrence of malaria in the elderly. The increase in symptom severity was correlated with the enhanced activation of the immune system, which significantly impaired erythropoiesis. Immunocompromised mice further support the existence of an immune-shaping regulation of RBC production. Overall, our data reveal the strict correlation between erythropoiesis and immune cells, which ultimately dictates the severity of SMA.

Pre-vaccination monocyte-to-lymphocyte ratio as a biomarker for the efficacy of malaria candidate vaccines: A subgroup analysis of pooled clinical trial data

BACKGROUND

Pre-vaccination monocyte-to-lymphocyte ratio was previously suggested as a marker for malaria vaccine effectiveness. We investigated the potential of this cell ratio as a marker for malaria vaccine efficacy and effectiveness. Effectiveness was investigated by using clinical malaria endpoint, and efficacy was investigated by using surrogate endpoints of Plasmodium falciparum prepatent period, parasite density, and multiplication rates in a controlled human malaria infection trial (CHMI).

METHODS

We evaluated the correlation between monocyte-to-lymphocyte ratio and RTS,S vaccine effectiveness using Cox regression modeling with clinical malaria as the primary endpoint. Of the 1704 participants in the RTS,S field trial, data on monocyte-to-lymphocyte ratio was available for 842 participants, of whom our analyses were restricted. We further used Spearman Correlations and Cox regression modeling to evaluate the correlation between monocyte-to-lymphocyte ratio and Whole Sporozoite malaria vaccine efficacy using the surrogate endpoints. Of the 97 participants in the controlled human malaria infection vaccine trials, hematology and parasitology information were available for 82 participants, of whom our analyses were restricted.

RESULTS

The unadjusted efficacy of RTS,S malaria vaccine was 54% (95% CI: 37%-66%, p <0.001). No correlation was observed between monocyte-to-lymphocyte ratio and RTS,S vaccine efficacy (Hazard Rate (HR):0.90, 95%CI:0.45-1.80; p = 0.77). The unadjusted efficacy of Whole Sporozoite malaria vaccine in the appended dataset was 17.6% (95%CI:10%-28.5%, p<0.001). No association between monocyte-to-lymphocyte ratio and the Whole Sporozoite malaria vaccine was found against either the prepatent period (HR = 1.16; 95%CI:0.51-2.62, p = 0.72), parasite density (rho = 0.004, p = 0.97) or multiplication rates (rho = 0.031, p = 0.80).

CONCLUSION

Monocyte-to-lymphocyte ratio alone may not be an adequate marker for malaria vaccine efficacy. Further investigations on immune correlates and underlying mechanisms of immune protection against malaria could provide a clearer explanation of the differences between those protected in comparison with those not protected against malaria by vaccination.

The Rough Guide to Monocytes in Malaria Infection

While half of the world's population is at risk of malaria, the most vulnerable are still children under five, pregnant women and returning travelers. Anopheles mosquitoes transmit malaria parasites to the human host; but how Plasmodium interact with the innate immune system remains largely unexplored. The most recent advances prove that monocytes are a key component to control parasite burden and to protect host from disease. Monocytes' protective roles include phagocytosis, cytokine production and antigen presentation. However, monocytes can be involved in pathogenesis and drive inflammation and sequestration of infected red blood cells in organs such as the brain, placenta or lungs by secreting cytokines that upregulate expression of endothelial adhesion receptors. Plasmodium DNA, hemozoin or extracellular vesicles can impair the function of monocytes. With time, reinfections with Plasmodium change the relative proportion of monocyte subsets and their physical properties. These changes relate to clinical outcomes and might constitute informative biomarkers of immunity. More importantly, at the molecular level, transcriptional, metabolic or epigenetic changes can “prime” monocytes to alter their responses in future encounters with Plasmodium. This mechanism, known as trained immunity, challenges the traditional view of monocytes as a component of the immune system that lacks memory. Overall, this rough guide serves as an update reviewing the advances made during the past 5 years on understanding the role of monocytes in innate immunity to malaria.