Cellular Basis of Early Cytokine Response toPlasmodium falciparum

Abstract

Uncertainty remains about the cellular origins of the earliest phase of the proinflammatory cytokine response to malaria. Here we show by fluorescence-activated cell sorter analysis that ␥␦ T cells and CD14 ؉ cells from nonimmune donors produce tumor necrosis factor and that ␥␦ T cells also produce gamma interferon within 18 h of contact with mycoplasma-free Plasmodium falciparum-infected erythrocytes in vitro. This early cytokine response is more effectively induced by intact than by lysed parasitized erythrocytes. However, the IFN-␥ response to lysed parasites is considerably enhanced several days after peripheral blood mononuclear cells are primed with low numbers of intact parasitized erythrocytes, and in this case it derives from both ␣␤ and ␥␦ T cells. These data show that naïve ␥␦ T cells can respond very rapidly to malaria infection but that malaria fever may involve a multistage process in which the priming of both ␥␦ and ␣␤ T-cell populations boosts the cytokine response to lysed parasite products released at schizont rupture.Malaria-infected individuals produce large amounts of proinflammatory cytokines, such as tumor necrosis factor (TNF) and gamma interferon (IFN-␥). This innate cytokine response is responsible for the high levels of fever that occur within a few days of the onset of blood stage infection in nonimmune individuals (15,18,20,21,33). Since TNF and IFN-␥ have important antiparasitic actions but are also believed to play a major role in the pathogenesis of severe complications, such as cerebral malaria and severe malarial anemia, understanding the cellular basis of the early innate cytokine response may be of considerable importance in relation to both malaria immunity and pathogenesis.Until recently, it has been widely assumed that the early host response to malaria is broadly similar to that evoked by bacterial endotoxin, whereby parasite factors (toxins) stimulate monocytes and macrophages to release TNF and related cytokines (5,19). This model was supported by evidence that some isolates of Plasmodium falciparum strongly stimulate TNF production by human peripheral blood mononuclear cells (PBMC) (1) and by a significant literature documenting TNF production by monocytes or macrophages within a few hours of exposure to malaria parasite preparations in vitro (examples are given in references 2, 25, and 34). However, it has recently become apparent that many malaria culture preparations are contaminated with mycoplasma species which have potent macrophage-stimulatory factors (22,30,36). Mycoplasma species have been found in parasite lines obtained from different laboratories around the world, and even after infection has been eradicated, continuous cultures are susceptible to reinfection within a few months unless their mycoplasma status is continually monitored. These observations have made it necessary to reevaluate the cellular basis of the early cytokine response to malaria.We have recently observed that the pattern of early cytokine production by nonimmune human PBMC following stim...

Innate Immune Responses to Human Malaria: Heterogeneous Cytokine Responses to Blood-Stage Plasmodium falciparum Correlate with Parasitological and Clinical Outcomes

“…P. falciparum-derived hemozoin binds to TLR-9 on murine myeloid DCs that subsequently produce large amounts of TNF-␣ (60). Intriguingly, in our study, TNF-␣ was up-regulated before any significant schizont rupture leading us to speculate that, in contrast to what has previously been suggested (6,7,46), large-scale schizont rupture may not be essential for induction of innate immune responses to malaria. Possible alternative explanations include activation of monocyte-macrophages following phagocytosis of iRBC or leakage of soluble molecules from damaged-but still intact-iRBC.…”

“…Although it is possible that early, transient, and low-level IL-12 responses might have been missed (as suggested by the flow cytometric analysis in which IL-12-positive cells could be detected after 18 h of incubation with iRBC), it also seems likely that an initial wave of IFN-␥ markedly up-regulates the IL-12 response and that this in turn may further augment the IFN-␥ response, leading to a positive feedback loop from which very high concentrations of proinflammatory cytokines can result. In contrast, previous in vitro studies have reported rapid in vitro induction of IL-12p40 upon stimulation of PBMC with iRBC (4,46), raising the interesting possibility that one of the first cytokines to be induced by iRBC may be IL-23 (a heterodimer composed of p40 and p19) or possibly even IL-27, which contains a p40-related protein (53). Intriguingly, IL-27 has been shown to induce IFN-␥ production by human NK cells independently of, and before IL-12 (54), suggesting that the potential role of this cytokine in the early response to iRBC requires further investigation.…”

“…The similarity in the kinetics of the LPS and iRBC responses strongly suggested that iRBC must be able to trigger innate responses immediately on being added to the PBMC cultures. This was unexpected because it is generally assumed that schizont rupture is the key event in triggering innate immune responses (6,7,46), and large-scale schizont rupture did not occur until 3-6 h after adding of iRBC to the cultures (Fig. 6a).…”

“…The former has allowed the potential role of different cell populations such as dendritic cells (48), ␥␦ T cells (46), and NK cells (24,25,36) to be evaluated; the latter has been useful to uncover associations between immune responses and disease outcome (14, 18 -20) but cannot provide information about the sequence of events between infection and onset of disease. Here, we have taken advantage of experimental, sporozoite-induced malaria infections to describe the kinetics of the systemic cytokine response during the prepatent period of P. falciparum infection in 18 unvaccinated, previously malaria-naive subjects and have compared this with the cytokine responses triggered in vitro from whole blood or PBMC by parasite-infected RBC.…”

“…Given the timing of detectable IFN-␥ up-regulation, beginning on average 1.75 days after parasite emergence from the liver, NK cells (24) and ␥␦ T cells (46) are likely to be the primary sources of this cytokine. NK cell IFN-␥ responses may be particularly relevant to our ex vivo observations-where high IFN-␥ responses were seen in only one-third of donors and exclusively in those donors up-regulating IL-12p70 -because they can be very rapid (within 6 -15 h (24)), very heterogeneous between donors (36), and highly dependent upon IL-12p70 and IL-18 (24, 25, 52) (K. C. Newman, D. S. Korbel, and E. M. Riley, submitted for publication).…”