Patterns of inflammatory responses and parasite tolerance vary with malaria transmission intensity

Abstract

Background: In individuals living in malaria-endemic regions, parasitaemia thresholds for the onset of clinical symptoms vary with transmission intensity. The mechanisms that mediate this relationship are however, unclear. Since inflammatory responses to parasite infection contribute to the clinical manifestation of malaria, this study investigated inflammatory cytokine responses in children with malaria from areas of different transmission intensities (ranging from low to high).

Methods: Blood samples were obtained from children confirmed with malaria at community hospitals in three areas with differing transmission intensities. Cytokine levels were assessed using the Luminex^®-based magnetic bead array system, and levels were compared across sites using appropriate statistical tests. The relative contributions of age, gender, parasitaemia and transmission intensity on cytokine levels were investigated using multivariate regression analysis.

Results: Parasite density increased with increasing transmission intensity in children presenting to hospital with symptomatic malaria, indicating that the parasitaemia threshold for clinical malaria increases with increasing transmission intensity. Furthermore, levels of pro-inflammatory cytokines, including tumour necrosis factor alpha (TNF-α), interferon-gamma (IFN-γ), interleukin (IL)-1β, IL-2, IL-6, IL-8, and IL-12, decreased with increasing transmission intensity, and correlated significantly with parasitaemia levels in the low transmission area but not in high transmission areas. Similarly, levels of anti-inflammatory cytokines, including IL-4, IL-7, IL-10 and IL-13, decreased with increasing transmission intensity, with IL-10 showing strong correlation with parasitaemia levels in the low transmission area. Multiple linear regression analyses revealed that transmission intensity was a stronger predictor of cytokine levels than age, gender and parasitaemia.

Conclusion: Taken together, the data demonstrate a strong relationship between the prevailing transmission intensity, parasitaemia levels and the magnitude of inflammatory responses induced during clinical malaria.

Keywords: Cytokines; Immunity; Malaria; Parasite tolerance; Transmission intensity.

Fig. 1

Pattern of pro-inflammatory responses to malaria infection across different transmission sites. Plasma levels of pro-inflammatory cytokines a tumour necrosis factor (TNF)-α, b interferon (IFN)-γ, c interleukin (IL)-2, d IL-1β, e IL-12, f IL-6, g IL-8, and h granulocyte macrophage colony stimulating factor (GM-CSF), were quantified in children with malaria in three areas of Ghana with varying malaria transmission intensities (Accra < Navrongo < Kintampo). Comparisons across sites were performed using Kruskal-Wallis H test with Dunn’s posthoc test (Accra N = 71; Navrongo N = 44; Kintampo N = 58). Data are presented as box plots where boxes represent the inter-quartile ranges, while the whiskers represent the 10th and 90th percentiles. The lines across the boxes indicate the median values, while closed circles represent outliers

Fig. 2

Pattern of anti-inflammatory responses to malaria infection across the different transmission areas. Plasma levels of anti-inflammatory cytokines a interleukin (IL)-10, b IL-4, c IL-13, and d IL-7, were quantified in children with malaria in three areas of Ghana with varying malaria transmission intensities (Accra < Navrongo < Kintampo). Comparisons across sites were performed using Kruskal-Wallis H test with Dunn’s posthoc test (Accra N = 71; Navrongo N = 44; Kintampo N = 58). Samples below the detection limits were assigned a concentration of zero, including 12 samples for IL4 (Accra = 3, Navrongo = 1, Kintampo = 8), and 33 samples for IL7 (Accra = 6, Navrongo = 7, Kintampo = 20). Data are presented as box plots where boxes represent the inter-quartile ranges, while the whiskers represent the 10th and 90th percentiles. The lines across the boxes indicate the median values, while closed circles represent outliers

Fig. 3

Association between pro-inflammatory cytokines and parasite density across the sites. The relationships between parasite density in children with malaria and plasma levels of pro-inflammatory cytokines a tumour necrosis factor (TNF)-α b interleukin (IL)-12, c interferon (IFN)-γ, d IL-1β, e IL-2, f IL-6 and g IL-8, were examined using Spearman’s rank correlation test. P values in bold type indicate statistical significance. Samples below the detection limits were excluded from the analysis. (ρ = Spearman’s correlation coefficient)

Fig. 4

Association between anti-inflammatory cytokines and parasite density across the sites. The relationships between parasite density in children with malaria and plasma levels of anti-inflammatory cytokines a interleukin (IL)-10, b IL-4, c IL-13, and d IL-7, were examined using Spearman’s rank correlation test. P values in bold type indicate statistical significance. Samples below the detection limits were excluded from the analysis. (ρ = Spearman’s correlation coefficient)

Fig. 5

Correlation matrix showing the relationships between cytokines across the sites. The interrelationships between the levels of inflammatory cytokines in children with malaria were examined by Spearman’s correlation test, and the results are summarized in a colour matrix. The strength of correlation between pairs of cytokines are illustrated on a colour scale, where the least statistically significant relationships are coloured green while the most significant are in red. (ρ = Spearman’s correlation coefficient)