Effector Phenotype of Plasmodium falciparum-Specific CD4+ T Cells Is Influenced by Both Age and Transmission Intensity in Naturally Exposed Populations

Naturally acquired immunity to malaria emerges slowly in malaria-endemic populations. In areas of high transmission, adults experience relatively few clinical episodes, compared with children, despite similar exposure to parasites, indicating that the adult immune system is able to control parasite burden and/or immunopathology associated with disease. Murine models have shown that cytokine-producing CD4+ T cells are critical for protection from malaria [1, 2]. In naturally exposed humans, interferon γ (IFN-γ) production by CD4+ T cells in response to merozoite [3, 4] and sporozoite [5–7] antigens has been associated with protection from disease. The importance of Plasmodium falciparum–specific CD4+ T-cell responses was further demonstrated by a study of naive adults vaccinated with an ultra-low dose of blood-stage parasites, which found that the CD4+ T-cell response was protective from malaria, even in the absence of antibody [8]. Advances in multiparametric flow cytometry have made it clear that assessment of pathogen-specific T cells by a single parameter is often inadequate for identifying correlates of protection. Antigen-specific T cells are functionally heterogeneous and can exhibit a complex variety of effector mechanisms. Mounting evidence from many infectious models suggests that the quality, rather than the quantity, of response is critical for immunity [9]. For instance, multifunctional CD4+ T cells coproducing IFN-γ, tumor necrosis factor α (TNF-α), and interleukin 2 (IL-2) correlate with vaccine-mediated protection from Leishmania major [10] and are implicated in protection induced by vaccinia virus immunization [11]. Multifunctional CD4+ T cells that coproduce TNF-α and/or IL-2 in conjunction with IFN-γ are thought to have increased function [12] and are associated with nonprogression of human immunodeficiency virus (HIV) infection [13]. In P. falciparum malaria, sterilizing immunity induced by experimental vaccination with whole sporozoites is associated with the induction of CD4+ T cells that coproduce IFN-γ, IL-2, and/or TNF-α [14–16]. CD4+ T-cell responses of a similar phenotype have been implicated in protection from malaria conferred by the vaccine RTS,S [17–19]. Among naturally exposed children, several effector phenotypes of P. falciparum–specific CD4+ T cells have been described, including distinct subsets producing inflammatory (TNF-α and IFN-γ) and regulatory (interleukin 10 [IL-10]) cytokines [20–22]. Recent studies suggest that the effector phenotype may be dependent on parasite exposure intensity, as recent symptomatic malaria is associated with increased frequencies of IFN-γ/IL-10–coproducing cells and a reduced frequency of TNF-α–producing cells [20, 21]. Murine models of malaria and other parasitic infections indicate that IFN-γ/IL-10–coproducing cells play an essential role in protecting the host from immunopathology, although this may come at the cost of reduced or delayed parasite clearance [23–25]. The identification of CD4+ T-cell responses in children susceptible to symptomatic malaria that differ from those in immune adults may provide important information regarding the mechanisms mediating protection. In this study, we performed a detailed assessment of P. falciparum–specific CD4+ T-cell responses among children and their adult caregivers in Uganda, in areas of high or low parasite transmission. We hypothesized that the frequency and functional characteristics of P. falciparum–specific CD4+ T-cell responses among clinically immune adults may differ from those of children who remain vulnerable to symptomatic malaria.