Monocytes and macrophages in malaria infection: a phenotypic and functional analysis

© 2012 Dr. Lin Lin ChuaMonocytes and macrophages (mono/macs) have been implicated in both protection and immunopathology in malaria; the latter is linked to excessive cytokine production, which has been associated with maternal anaemia and low birth weight (LBW) during malaria in pregnancy. To better understand the roles of mono/macs in malaria infection, a phenotypic and functional investigation was undertaken in the current study. Monocytes are heterogeneous and there is a subset of monocytes, characterised by CD16 expression, which are the main producers of pro-inflammatory cytokines. These CD16+ monocytes are present at low proportion in healthy individuals, but may be expanded during infection or inflammation. To understand how pregnancy may alter maternal monocytes and their responses to malaria, monocyte subsets in peripheral and placental blood from malaria-exposed but uninfected women in Papua New Guinea were first characterised. Pregnant women in this cohort expressed high proportions of pro-inflammatory CD16+ monocytes even without malaria infection and these were inversely correlated with maternal haemoglobin level and birth weight, suggesting a role for CD16+ monocytes in adverse clinical outcomes in pregnancy. Alterations to the proportion of CD16+ monocyte subset as a consequence of malaria infection need to be addressed in future study, given that the current study was limited by the low number of malaria-infected cases. Mono/macs can be activated by haemozoin (malaria pigment) and high levels of mono/macs that have phagocytosed haemozoin correlated with severe outcomes in malaria-infected children and non-pregnant adults. However, in this study, it was demonstrated that pigmented mono/mac density during gestation was not correlated with clinical outcomes at delivery, thus does not serve as a useful prognostic marker. In addition, a product of mono/mac activation, soluble CD163 (sCD163), was also investigated as a diagnostic marker for placental malaria. It was found that sCD163 levels were not a useful indicator of placental malaria, but higher levels of sCD163 inversely correlated with haemoglobin levels, implicating mono/mac activation in maternal anaemia. The importance of heterogeneities in parasite and macrophage populations in regulating cytokine production was also explored. Using an in vitro model of human monocyte-derived macrophages (MDMs), this study investigated how infected erythrocytes with placental-binding property (CS2 parasites) may differ from other parasite strain in activating macrophages to produce cytokines. CS2 parasites induced greater levels of pro-inflammatory cytokines compared to another parasite strain that binds CD36 (E8B parasites). In addition, to address the influence of macrophage heterogeneity on cytokine response, macrophages of different polarisation states were generated and stimulated with CS2 parasites. Macrophages generated using M-CSF and GM-CSF demonstrated highest phagocytosis ability in the uptake of CS2 parasites, while M-CSF derived macrophages induced highest pro- and anti-inflammatory cytokines in response to CS2 parasites. In conclusion, the findings presented in this thesis have demonstrated that increased mono/mac activation is associated with poor maternal and foetal outcomes, implicating mono/macs in immunopathology during malaria in pregnancy. In addition, it was demonstrated in vitro that heterogeneity in macrophage and parasite populations are important determinants of cytokine responses, which may have important implications in the ability of malaria-infected host to resolve infection in vivo