The severity of malaria can range from asymptomatic to lethal infections involving severe anaemia and cerebral disease; however, the molecular and cellular factors responsible for these differences are poorly understood. Thus, identifying the factors that mediate virulence will contribute to developing antiparasitic therapy. Similar to the findings for other infectious diseases, DCs are essential for initiating adaptive and innate immune responses in malaria. Although DCs are known to be important for initiating immunity to malaria [
29,
30], studies describing the function of DCs in the adaptive response to infection have not explored their direct role in disease outcome, survival, anaemia, or parasitaemia. Furthermore, limited data exist on the differing outcomes of
Plasmodium infections. Interestingly, the mean incubation period of Korean
P. vivax isolates is reported to extend as long as 279 ± 41 days (range 153–452 days) [
31]. The ratio of cases with short and long incubation periods was 25 and 75 %, respectively [
32]. As such, it was hypothesized that disease in patients with long incubation periods might be due to pathogens that have undergone immune escape or tolerance. It is mainly attributable to Treg cells that have a crucial role in immune suppression during malaria infection, as well as the escape of parasites from host immune surveillance [
33,
34].
Two main DC sub-sets have been characterized in humans, myeloid (mDC, CD11c+) and plasmacytoid DCs (pDCs, CD11c
−) [
35]. A sub-set of mDCs (also known as monocyte-derived DCs, MDDCs or mo-DCs) are differentiated from monocytes (CD14
+) when cultured in the presence of GM-CSF and IL-4 [
36]. DCs express a wide repertoire of membrane receptors, including pattern recognition receptors (PRRs) that induce their terminal differentiation and activation upon engagement by DAMPs or pathogen-associated molecular patterns (PAMPs) [
37]. C-type lectin receptors (CLRs), a sub-type of PRRs, promote the activation of Syk and CARD9 and preferentially induce the polarization of naïve Th0 lymphocytes into Th17 cells [
38‐
40]. In comparison, tolerogenic DCs are able to inhibit the differentiation of naïve Th0 lymphocytes, thus suppressing the generation of T cell-mediated immune responses, and induce the generation of Treg cells [
41,
42]. In this regard, it has been theorized that different membrane receptors (e.g., PSGL-1 and PD-L1), as well as cytokines (e.g., IL-10), are able to induce the generation of tolerogenic DCs [
43,
44]. Since S100A8 is reported to promote the generation of Treg cells through the induction of tolerogenic DCs [
45], the role of S100A8 was investigated in
P. vivax patients. Notably, the amount of S100A8 in vivax malaria patients was three times higher than that in healthy volunteers (Fig.
1), and the mean value of S100A8 produced by vivax malaria patients (141.2 ng/ml) was less than that of falciparum malaria patients (3420 and 13,580 ng/ml for parasitaemia <0.2 and ≥0.2 %, respectively) [
20]. However, these levels did not correlate with the amount of antibodies produced (Fig.
2). Interestingly, antibody titres were inversely correlated with parasitaemia (Fig.
3); however, the level of serum S100A8 was significantly related to the parasitic load in patients with
P. falciparum, as well as fever episodes in children up to 6 years of age [
20]. This observation emphasizes a potential difference in the function of S100A8 in
P. vivax and
P. falciparum patients. Comparison of the induction of PD-L1 and HLA-DR double-positive iDCs by several vaccine candidate peptides and synthetic S100A8 showed that approximately half of the double-positive cells were induced by synthetic S100A8 compared to LPS. The Pv210 type of synthetic CSP induced double-positive DCs with relatively higher expression than that of AMA-1 and MSP-1 (Fig.
4). This suggests that CSP might induce immune tolerance in some patients, but not in a statistically significant manner (Fig.
5). As expected, parasite-infected RBCs from vivax malaria patients significantly induced DC maturation than in normal persons (Fig.
6). However,
P. falciparum-infected RBC appeared to inhibit DC maturation, subsequently reducing its capacity to stimulate T cells [
46]. Therefore, S100A8 in vivax malaria appears to promote its immune escape, as elevations in PD-L1 on the iDCs surface by synthetic S100A8 promotes Treg cell induction greater than that observed in controls (Fig.
7). The serum levels of S100A8 decreased with increase in parasitaemia (Fig.
3) and the ratio of Treg cells generated was inversely proportional to the concentration of S100A8 in sera (Fig.
8). This means that S100A8 function is controlled by unknown factors in the sera for maintaining the immune balance or S100A8 concentration may reflect the beginning of onset. Therefore, the next goal is to investigate these factors or investigation of S100A8 concentration according to time course of disease onset which affect the Treg generation rate. Only synthetic S100A8 was sufficient to increase Treg cell induction from healthy volunteer PBMCs, whereas the CSP, MSP-1 and AMA-1 synthetic peptides were unable to mediate this effect (Fig.
9). In addition, IL-10 production in iDCs increased significantly with the addition of synthetic S100A8 (Fig.
10). The immunoregulatory mechanisms involving IL-10 appear complex and involve both pro-inflammatory and suppressive functions, although the prime function of IL-10 is to protect the host from excessive immune and inflammatory responses [
47]. Nevertheless, S100A8 plays a clear pro-inflammatory function by facilitating the secretion of IL-1β, IL-12p70, TNF-α, and IFN-γ by mature DCs (Fig.
10). In the mouse, S100A8 acts as a potent chemo-attractant for neutrophils and monocytes in vitro and in vivo [
48‐
50] to influence leukocyte migration and transmigration into tissues by increasing their deformability [
51]. Therefore, S100A8 likely plays dual roles in the inflammatory response depending upon the environmental conditions. Thus, synthetic S100A8 up-regulated the expression of HLA-DR, PD-L1, CD86, and IL-10 in DCs necessary for generating Treg cells, which contributes to escape from the host immune defences [
52]. The presence of parasite-specific CD4
+ T cells correlates with reduced parasite burdens and disease severity in malaria, but the effect of parasites on Treg cell induction is the opposite [
28]. The source of S100A8 in malaria remains unclear, although it is known to be secreted from monocytes, macrophages and neutrophils [
53‐
58]. Petersen et al. found that a novel mechanism of immune regulation mediated by S100A8 is an important factor in orchestrating coordinated immune reactions. This observation allows for the propagation of an appropriate innate inflammatory response by acting as a classical DAMP protein, while also preventing hyper-activation of the adaptive immune system so as to avoid the tissue damage that can be caused by excessive immune responses [
19]. S100A8 may exhibit pleiotropic effects and promote IL-10 secretion to elicit synergic effects that regulate harmful inflammatory tissue damage by virtue of its capacity to act as an antioxidant [
59], thereby protecting against acute cytokine-mediated pathology.