In this part of the prospective PALUREA study [8], we evaluated the relevance of circulating levels of 8 cytokines in 278 adults hospitalized for imported malaria: 134 with UM and 144 with SM. For IL-1α, IL-1β, IL-2, IL-4, IFNγ and TNFα, more than 50% of patients had undetectable levels on hospital admission, when IL-10 and MIF were fully detectable and significantly higher in severe cases. IL-10 was also significantly associated with parasitemia.
Cytokines play a pivotal role in response to Plasmodium falciparum infection. In the present study, we found that profiles of 6 among the 8 tested cytokines were poorly associated with disease severity. Strikingly, a significant proportion of patients demonstrated indetectable levels for IL-1α, IL-1β, IL2, IL4, TNFα, and IFNγ. Those findings must be interpreted in the conditions of the study which means routine evaluation at the time of hospital admission. Indeed, plasma level of cytokine might vary depending on time from onset of symptoms to hospitalization. In our study, a few numbers of cytokines might have been blunted due to a large delay between admission and onset of symptoms which was of 4 [2–6] days. Some cytokines have shown to peak before blood stage of infection: IFNy for example is secreted very early in falciparum malaria pathogenesis even prior to onset of detectable bloodstage infection(15). TNFα and IL-1β have been shown to be at maximum within 2h following in vitro stimulation while IL-10 concentration was detected after 7h with a maximum response at 48h (16). At last, a technical issue might be evoked, but this hypothesis seems very doubtful since tubes were analyzed in duplicate and centralized in a facility with great skills in the field. In this perspective, it seems reasonable to conclude that serum cytokine level was low in most patients, for the tested cytokines, at the time of presentation.
Balance between pro and anti-inflammation is key to eliminate the parasite without inducing excessive pro-inflammatory response which might drive further organ damage. IL-10 is a fundamental anti-inflammatory cytokine and has been shown to play an important regulatory function in malarial models (17,18). IL-10 was associated with severity in various studies(7,19) and several studies conducted among African children found IL-10 to be higher in cerebral malaria compared with other forms(20–22). However, its role in malaria pathophysiology is hard to decipher. Several preclinical studies have suggested a possible protective role, notably due to its inhibition of ICAM-1 expression on endothelium which might reduce parasite sequestration(23). Conversely, IL-10 also exerts potent anti-inflammatory effects, which may paradoxically promote parasite growth. At last, IL-10 exerts strong immune suppressive effects on circulating monocytes (25,26) which might lead to increased bacterial susceptibility(27). Such association was never reported during malaria, however, our observations demonstrated that patients with persisting high levels of IL-10 (on day 0, day 1 and day 2) experienced significantly more nosocomial infections.
MIF was one of the first cytokine identified and is an important mediator of mammalian inflammatory conditions(28) It is constitutively expressed by a broad spectrum of cells and tissues and plays a role of regulator of immune response. Recently, the discovery of a Plasmodium falciparum-encoded MIF ortholog (PfMIF) raised awareness on its role in the pathophysiology of malaria(29,30). MIF exerts pro-inflammatory properties, which might lead to an increased cytoadherence of parasitized erythrocytes resulting in more severe malarial disease. MIF also inhibits erythroid, multipotential and granulocyte–macrophage progenitor-derived colony formation (G-CSF), indicating that it could be implicated in the pathophysiology of malarial anemia (31).
In our study, we found that MIF was reliably associated with disease severity, which is consistent with a previous report among Zambian children(21) and with an Indian cohort which involved both infants and adults (32). However, conversely to Thuma et al. we did not find any association between MIF level and anemia. Our results might be insufficient to conclude on the precise role of MIF in malarial anemia since we were not able to perform PfMIF dosage. In fact, to our knowledge, no study ever dosed the respective concentration of the two isoforms of MIF (hMIF and PfMIF) and their relationship with severity. This might have several applications since various therapeutic intervention aiming at neutralizing PfMIF have shown promising results (33,34).
Perspectives
Main perspective of host immune response assessment among ICU patients remains in its ability to characterize immune dysfunction and reliably identify patient vulnerable to subsequent infection. Higher levels of IL-10 may be associated with worsened immunity and increased susceptibility to secondary infections. Preclinical studies have pointed out the detrimental effect of high IL-10 concentration, reducing other cytokine synthesis and responsible for the reduction in class II major histocompatibility complex (MHC) expression on monocytes(35). In our population, persisting high level of IL-10 was observed within patients who presented subsequent nosocomial infection. Such was already observed in sepsis, but never described in malaria(26). Fine monitoring of both cytokines and membrane receptor associated with immune suppression, such as mHLA-DR, is a promising approach in sepsis, and deserves further investigations in malaria patients(27).