Date Published: April 17, 2014
Publisher: Public Library of Science
Author(s): Silvia Portugal, Jacqueline Moebius, Jeff Skinner, Safiatou Doumbo, Didier Doumtabe, Younoussou Kone, Seydou Dia, Kishore Kanakabandi, Daniel E. Sturdevant, Kimmo Virtaneva, Stephen F. Porcella, Shanping Li, Ogobara K. Doumbo, Kassoum Kayentao, Aissata Ongoiba, Boubacar Traore, Peter D. Crompton, James W. Kazura.
In malaria-naïve individuals, Plasmodium falciparum infection results in high levels of parasite-infected red blood cells (iRBCs) that trigger systemic inflammation and fever. Conversely, individuals in endemic areas who are repeatedly infected are often asymptomatic and have low levels of iRBCs, even young children. We hypothesized that febrile malaria alters the immune system such that P. falciparum re-exposure results in reduced production of pro-inflammatory cytokines/chemokines and enhanced anti-parasite effector responses compared to responses induced before malaria. To test this hypothesis we used a systems biology approach to analyze PBMCs sampled from healthy children before the six-month malaria season and the same children seven days after treatment of their first febrile malaria episode of the ensuing season. PBMCs were stimulated with iRBC in vitro and various immune parameters were measured. Before the malaria season, children’s immune cells responded to iRBCs by producing pro-inflammatory mediators such as IL-1β, IL-6 and IL-8. Following malaria there was a marked shift in the response to iRBCs with the same children’s immune cells producing lower levels of pro-inflammatory cytokines and higher levels of anti-inflammatory cytokines (IL-10, TGF-β). In addition, molecules involved in phagocytosis and activation of adaptive immunity were upregulated after malaria as compared to before. This shift was accompanied by an increase in P. falciparum-specific CD4+Foxp3− T cells that co-produce IL-10, IFN-γ and TNF; however, after the subsequent six-month dry season, a period of markedly reduced malaria transmission, P. falciparum–inducible IL-10 production remained partially upregulated only in children with persistent asymptomatic infections. These findings suggest that in the face of P. falciparum re-exposure, children acquire exposure-dependent P. falciparum–specific immunoregulatory responses that dampen pathogenic inflammation while enhancing anti-parasite effector mechanisms. These data provide mechanistic insight into the observation that P. falciparum–infected children in endemic areas are often afebrile and tend to control parasite replication.
In previously unexposed individuals, blood-stage Plasmodium falciparum parasites rapidly replicate and almost invariably induce fever and other symptoms of malaria  through the production of pro-inflammatory cytokines and chemokines –. Although the initial systemic inflammatory response is crucial for setting in motion the innate and adaptive immune effector mechanisms that control blood-stage parasites , , dysregulated inflammation has been linked to severe malaria ,  which only occurs in a minority of individuals with infrequent or no prior malaria exposure . Conversely, in malaria endemic areas where individuals are repeatedly exposed, P. falciparum infections more commonly cause a mild febrile illness or no symptoms at all, and parasite numbers in the blood are generally kept in check, even in young children – who have yet to acquire a fully protective antibody repertoire . The nature of the immune response that enables most children to restrain P. falciparum-induced inflammation while maintaining control of parasite replication remains elusive , .
In our previous investigations at this study site we observed that the risk of febrile malaria slowly decreases over years as individuals are exposed to intense seasonal P. falciparum transmission such that adults rarely experience febrile malaria when infected with blood-stage parasites . The gradual acquisition of blood-stage immunity that reliably protects from the onset of febrile malaria likely reflects the need for repeated infections over years to achieve levels of broadly reactive antibodies that exceed a protective threshold , . However, even malaria-susceptible children at this study site (who by definition have yet to acquire reliably protective antibodies) experience only 1 to 2 febrile malaria episodes per six-month malaria season despite ≥100 infective mosquito bites per person each season, and generally these children manage to keep parasite numbers in the blood in check . These observations prompted us to investigate immune mechanisms beyond antibody responses that might contribute to protection from febrile malaria and parasite replication in children who are exposed to repeated P. falciparum infections, and also to investigate how children become susceptible again to febrile malaria after a period of decreased P. falciparum exposure.