Immune response to Plasmodium vivax has a potential to reduce malaria severity

Abstract

Plasmodium falciparum infection causes transient immunosuppression during the parasitaemic stage. However, the immune response during simultaneous infections with both P. vivax and P. falciparum has been investigated rarely. In particular, it is not clear whether the host's immune response to malaria will be different when infected with a single or mixed malaria species. Phenotypes of T cells from mixed P. vivax–P. falciparum (PV-PF) infection were characterized by flow cytometry, and anti-malarial antibodies in the plasma were determined by an enzyme-linked immunosorbent assay. We found the percentage of CD3⁺δ2⁺-T cell receptor (TCR) T cells in the acute-mixed PV-PF infection and single P. vivax infection three times higher than in the single P. falciparum infection. This implied that P. vivax might lead to the host immune response to the production of effector T killer cells. During the parasitaemic stage, the mixed PV-PF infection had the highest number of plasma antibodies against both P. vivax and P. falciparum. Interestingly, plasma from the group of single P. vivax or P. falciparum malaria infections had both anti-P. vivax and anti-P. falciparum antibodies. In addition, antigenic cross-reactivity of P. vivax or P. falciparum resulting in antibodies against both malaria species was shown in the supernatant of lymphocyte cultures cross-stimulated with either antigen of P. vivax or P. falciparum. The role of δ2 ± TCR T cells and the antibodies against both species during acute mixed malaria infection could have an impact on the immunity to malaria infection.

Introduction

Mixed malaria infection is common and has been reported in many parts of the world where malaria is endemic [1–5]. In regions with low malaria endemicity regions, especially in Thailand, mixed infection with Plasmodium falciparum (PF) and P. vivax (PV) is common. These co-infections can be either simultaneous or sequential. Previous studies have shown that mixed PV-PF malaria infection is less severe than the single P. falciparum infection in terms of lower frequency of anaemia, treatment failure and clinical outcomes for the patients [6]. Moreover, mixed PV-PF malaria infection is approximately a quarter as severe as single P. falciparum infection [7]. It is conceivable that interaction between the host's immunity and the two malaria species may take place during acute infection.

The immune mechanism plays an important role in resisting malaria and other infectious diseases [8]. Immunity to malaria induced by different plasmodia species may give various outcomes. Immunity to P. falciparum is still controversial. Previous study has shown immunosuppression in acute P. falciparum leading to a lower absolute number of CD3⁺ T cells [9], although the overall percentages of CD4⁺ and CD8⁺ T cells are not changed [9–11]. On the other hand, during acute P. vivax infection, the percentage of CD4⁺ but not CD8⁺ T cells is elevated, whereas the number of antibodies against this parasite is low [12].

γδT cells play a role in linking the innate and adaptive immunities against the broad range of parasites [13–15]. These γδT cells recognize non-peptide phosphoantigens of the microbes leading to the release of cytokines such as tumour necrosis factor (TNF)-α and interferon (IFN)-γ, and therefore exert the effector function, i.e. cytotoxicity and natural killing [16–18]. Generally, CD3⁺δ2⁺T cells predominate in the peripheral blood in response to many infectious agents such as Mycobacterium spp. [19], Pseudomonas aeroginosa and Escherichia coli[14]. In cases of P. vivax malaria, the elevation of CD3⁺δ2⁺ T cells is observed in peripheral blood but not in P. falciparum malaria infection [12]. The natural immune response against malaria in hosts during acute mixed PV-PF malaria infection has been investigated rarely. So far, only one study from Ethiopia has shown that γδT cells are increased in mixed PV-PF malaria infection and single P. falciparum infection, but not in P. vivax infection [10]. However, successful immunity to malaria required both cell-mediated and humoral immune responses. Therefore, in this study, we characterized the natural immune response during acute mixed PV-PF malaria infection in patients who live in areas of Thailand where malaria is endemic. Understanding both cell-mediated and humoral responses may disclose the roles of the host's immunity to the two malaria species.

Materials and methods

Sample collection

Blood samples were collected in 20 µl of heparin from 17 acutely mixed PV-PF malaria-infected individuals, 63 P. vivax- and 63 P. falciparum-infected individuals at malaria clinics in Mae Sot, Tak province, Thailand; 50 malaria naive volunteers from non-malaria endemic areas were recruited as naive controls. Diagnosis of malaria infection was based on the examination of Giemsa-stained thick and thin blood films. Recruitment criteria were: age ≥ 15 years; body temperature ≤ 40°C; systolic blood pressure ≥ 90 mm; haematocrit ≥ 25%. Clinical data are shown in Table 1. This study was approved by the Committee on Human Rights Related to Human Experimentation, Mahidol University, Bangkok. Informed consent was obtained from each individual before blood samples were taken.

Table 1.

Information and clinical data* of mixed Plasmodium vivax–P. falciparum (PV-PF), single P. vivax, single P. falciparum infections and naive controls

	Naive controls	Mixed species	P. vivax	P. falciparum
No. of patients	50	17	63	63
Age (year)*	39 ± 5·1	27 ± 9	31 ± 13	28 ± 8·2
Male	45	14	51	54
Female	5	3	12	9
Temperature (°C)*	36·5 ± 0·5	38·5 ± 0·8	37·3 ± 0·8	38 ± 1·3
Parasitaemia (%)*	0	1·8 ± 2†	0·2 ± 0·2	1·9 ± 2·1
		0·07 ± 0·1‡
Haematocrit (%)*	42 ± 5·3	37·6 ± 7·3	40 ± 4·2	41 ± 8·5

^*Mean ± standard deviation.

^†P. falciparum;

^‡P. vivax.

Preparation of peripheral blood mononuclear cells (PBMCs)

PBMCs were separated from the collected blood by gradient centrifugation at 800 g for 20 min using Lymphoprep™ (AXIS-Shield PoC AS, Oslo, Norway). PBMCs were washed twice with RPMI-1640 by centrifugation at 800 g for 10 min and resuspended in RPMI-1640 containing 10% fetal calf serum (FCS). The viability of the PBMCs was determined by trypan blue exclusion dye. PBMCs (10⁷ cells/ml) diluted in Cell banker® (Nihon Zenuaku Kohgyo, Japan) were stored in liquid nitrogen until further analysis.

Antigen preparation

White blood cells were depleted from P. vivax-infected blood by filtering through a Plasmodiper® (Whatman, Maidstone, UK). The red blood cells were washed twice with RPMI-1640 by centrifugation at 1190 g for 5 min. The parasites were cultured at 5% haematocrit in McCoy's medium (Gibco, Carlsbad, CA. USA) supplemented with 25% human antibody serum for 24–30 h in 5% CO₂ until a mature schizont of P. vivax appeared [20]. P. falciparum culture was performed as described previously [21] in RPMI-1640 medium supplemented with 10% human serum until a mature schizont of P. falciparum appeared. P. vivax and P. falciparum parasites were separated by centrifugation on 60% Percoll®. The infected red blood cell (iRBCs) pellets were pulsed for 40 s on ice at 150 watts and stored at –70°C to be used in a lymphocyte stimulation assay and enzyme-linked immunosorbent assay (ELISA). The protein concentration of the P. vivax schizont extract (PvSE), and P. falciparum schizont extract (PfSE) was determined by a Bradford assay (Bio-Rad, Hercules, CA, USA). Uninfected red blood cells (uRBC) were processed similarly and used as control protein.

Antigenic cross-stimulation of lymphocytes

To investigate the antigenic cross-reactivity between P. vivax and P. falciparum parasites. PBMCs from a single P. vivax or P. falciparum infection were stimulated with PfSE and PvSE. The PBMCs were cultured at (2 × 10⁵ cells/100 µl/well) in a HEPES-buffer RPMI-1640 supplemented with 10% FCS. The PvSE, PfSE and uRBC at 10 µg/100 µl and 100 µg/100 µl were added to each well. The cell quality control of the PBMCs was tested with a medium containing 10 µg/100 µl of phytohaemagglutinin (PHA) and incubated at 37°C in 5% CO₂ for 5 days. The culture supernatant was stored at –20°C for further investigation of cross-reactivity.

Flow cytometric analysis

Phenotyping of T cells was performed by three-colour flow cytometry (FACScan; Becton Dickinson, Oxford, UK). PBMCs (10⁵ cells) were stained with the following three-colour combinations of fluorescent dye conjugated with antibodies that were specific to T cell surface markers: fluorescein isothiocyanate (FITC)-conjugated antibody to CD4, R-phycoerythrin (R-PE)-conjugated antibody to CD3 and R-PE-cyanine 5 (R-PE-Cy5) conjugated antibody to CD8; and FITC-conjugated antibody to δ2⁺ TCR T cells and R-PE antibody to CD3 (Caltag, Burlingame, CA, USA) for 30 min at 4°C. After staining and washing, the cells were fixed with 1% paraformaldehyde in PBS pH 7·4 and analysed using CellQuest software (Becton Dickinson, San Jose, CA, USA).

Determination of antibodies against parasite antigen of P. vivax and P. falciparum antigen

Two methods were used to determine anti-malarial antibodies.

Enzyme-linked immunosorbent assay

Fifty ml of 10 µg/ml (PBS, pH 7·4) PvSE and PfSE were incubated overnight at 4°C in a 96-well polystyrene immunoplate (Corning, NY, USA). The immunoplate was blocked with 100 µl/well of blocking buffer (0·5% boiled casein in PBS and 0·05% Tween 20) for 2 h at room temperature. After three washings, 50 µl of serum (1 : 100 dilution in PBS pH 7·4) were added into duplicate wells and incubated overnight at 4°C. After washing with 0·05% Tween 20 in PBS, pH 7·4, 50 µl of horseradish peroxidase-conjugated goat anti-human immunoglobulin G (IgG) (Caltag) and 50 µl of 2,2′-azino-di-(3-ethylbenzthiaoline sulphonic acid) containing 50% hydrogen peroxide (Kirkepaard & Perry Laboratories, Gaithersburg, MD, USA) were added sequentially to each well and incubated for 1 h at room temperature. Enzyme activity was measured by microplate reader, Wallac Victor (Perkin Elmer, Jügesheim, Germany) at 405 nm. The levels of anti-P. falciparum and anti-P. vivax antibodies were expressed as a ratio increase in a median optical density (OD) compared to naive controls.

Immunofluorescence assay (IFA)

The pellets of infected red blood cells by P. vivax and P. falciparum parasites were spotted, dried and fixed on the multi-well slides. The cultured supernatant from PvSE-stimulated lymphocytes and PfSE-stimulated lymphocytes was added to the multi-well slides and incubated in a humidified box at room temperature for 1·5 h. The slides were then washed three times for 5 min. The multi-well slides were then incubated with goat anti-human IgG conjugated to FITC (Serotec, Oxford, UK). The slides were washed three times and mounted with a coverslip using 50% glycerol in PBS and then analysed with a fluorescence microscope.

Statistical analysis

The phenotypes of lymphocyte were analysed using the spss program (version 11·5; SPSS Inc., Chicago, IL, USA). To compare the phenotype of T cells in different groups, these data were log-transformed in order to obtain a normal distribution. Statistical significance was determined by one-way analysis of variance (anova). Non-parametric analysis (two independent samples) and Mann–Whitney U-test were used to determine the significance levels of anti-P. falciparum and anti-P. vivax antibodies. The results were considered significant at P < 0·05.

Results

Phenotypes of T cells

Flow cytometry was used to identify subsets of T cells, including CD4⁺, CD8⁺ and CD3⁺δ2⁺ (Fig. 1). The median percentage of CD3⁺δ2⁺ T cells was significantly higher in acute mixed PV-PF malaria infection (7·4%) compared with acute P. falciparum infection (2·3%, P < 0·01) and naive controls (1·8%, P < 0·03). There was no difference in CD3⁺δ2⁺T cells between acute P. vivax and the mixed malaria infections (7·4% versus 5·9%, P > 0·05). The median percentage of CD4⁺ and CD8⁺ T cells in the mixed malaria infection (36·7% and 24·6%, respectively) was similar to that of the single P. vivax (29·2% and 25·5%, P > 0·05) and P. falciparum infection (44·8% and 23·5%, P > 0·05) and these levels were not different from the naive controls (44·7% and 24·0%, P > 0·05).

Fig. 1.

Phenotyping of T cells in mixed Plasmodium vivax–P. falciparum (PV-PF), P. vivax, P. falciparum malaria infection and malaria naive controls determined by flow cytometry. Data are shown as interquartile ranges (box plots), maximum and minimum (upper–lower line) and bars indicate median. (a) CD3⁺δ2⁺ T cells; (b) CD4⁺ : CD8⁺ T cells.

Antibodies to P. falciparum and P. vivax protein extracts

Antibodies to the parasite protein extracts in the plasma were determined and are shown in Fig. 2. Both mixed malaria and single malaria infections had antibodies against P. vivax and P. falciparum antigens. Interestingly, the mixed infection had the highest plasma levels of antibodies to both P. vivax and P. falciparum antigens [13-fold (P > 0·05) and 24-fold (P < 0·04), respectively] compared with those of the naive controls. The single P. falciparum group had an increased ratio of antibodies to the P. vivax (sixfold, P > 0·05) and P. falciparum infection (10-fold, P > 0·05). The single P. vivax group had a sixfold increase of anti-P. vivax and anti-P. falciparum antibodies compared with the naive control. Of the three malaria infection groups, anti-P. falciparum antibodies were significantly higher in the mixed PV-PF malaria infection (P < 0·04) compared with each single malaria infection group, as shown in Fig. 2b. Antibodies specific to P. vivax protein extracts were low in all groups, as shown in Fig. 2a.

Fig. 2.

Antibodies to Plasmodium vivax (PV) and P. falciparum (PF) in mixed PV-PF, P. vivax, P. falciparum malaria infection, and malaria naive controls determined by enzyme-linked immunosorbant assay. Data are shown in median ratio and bars indicate median of the ratios. (a) Anti-P. vivax antibodies; (b) anti-P. falciparum antibodies.

Cross-reactivity of antibodies to P. vivax and P. falciparum parasites

The plasma antibodies of the single malaria infections, either P. vivax or P. falciparum, showed reactivity with the other malaria species regardless of the species that had currently caused the infection. Interestingly, the supernatant collected from the PvSE- or PfSE-stimulated lymphocytes of the single P. falciparum infection produced both anti-P. vivax and anti-P. falciparum antibodies. Similarly, the supernatant collected from the PfSE- or PvSE-stimulated lymphocytes of the single P. vivax infection also produced both anti-P. vivax and anti-P. falciparum antibodies. The antibodies showed strongly positive with mature stages of both parasites as determined by the IFA (data not shown). The supernatant collected from the malaria PvSE- or PfSE-stimulated lymphocytes of the malaria naives showed no reactivity with any parasite.

Discussion

The objective of this study was to characterize the profiles of T cells and the response of antibodies to blood stage antigens of both P. vivax and P. falciparum during acute mixed-malaria infection. Our study gave novel evidence of the natural immune response of γδT cells against mixed malaria infection, were modulated by P. vivax and their role in the reduction of malaria severity caused by P. falciparum. The percentage of CD3⁺δ2⁺T cells were increased significantly in acute mixed and single P. vivax infections compared with these of the naive controls, whereas only a low level of these cells was found in acute P. falciparum patients. The CD4 : CD8 ratio did not show any difference between malaria patient groups, which was in line with previous studies [9–11]. These results suggested the role of CD3⁺δ2⁺T cells in the development of the cell-mediated immunity during mixed malaria infection.

The γδT cells inhibit development of the pre-erythrocytic stage of P. yoelii in a mouse model lacking αβ T cells [22], and the α-galactosyl-ceramide (a-GalCer)-activated natural killer (NK) T cells protect mice from P. yoelii infection [23]. Moreover, γδT cells, NK, NK T cells and macrophages harboured in the liver where the pre-erythrocytic stage of malaria are presented may boost the immunity to malaria infection [24] more effectively than those malaria having only the erythrocytic stages.

The γδT cells control the expansion of P. chabaudi in a mouse model [25] and inhibit growth of P. falciparum parasites [26] through the release of granulysin [8]. The elevation of CD3⁺δ2⁺ T cells in acute P. vivax infection conceivably plays a similar role to that in P. falciparum infection. Interestingly, a greater elevation of the CD3⁺δ2⁺ T cells was shown in the mixed malaria infection and the single P. vivax infection compared with that of the P. falciparum infection. By contrast, a previous study in Ethiopia showed that there was no difference in CD3⁺δ2⁺ T cells in mixed PV-PF malaria infection compared with single P. vivax or P. falciparum infection. However, the CD3⁺δ1⁺ T cells, having a killer effector function, were more numerous in the single P. falciparum and mixed PV-PF malaria infections [10].

Our data showed no correlation between the percentage of CD3⁺δ2⁺ T cells and the number of P. vivax parasites or the number of the P. falciparum parasites. Nevertheless, the lower number of P. falciparum parasites in the mixed malaria infection, as opposed to that in the single P. falciparum infection, suggested that P. vivax might activate the effector killer function of the CD3⁺δ2⁺ T cells resulting in the inhibition of P. falciparum growth.

When P. vivax was co-infected with P. falciparum in the malaria infection, patients had a higher fever than those with single P. falciparum or single P. vivax infections [27]. High temperature is shown to kill P. falciparum parasites in in vitro studies [28,29]. In addition, serum from a P. vivax-infected donor during paroxysm inhibits maturation of P. falciparum schizonts [30]. Together, these data support the notion that a mechanism by which P. vivax controls the expansion of P. falciparum in mixed malaria infection could be via the induction and persistency of high fever in patients, particularly the high systemic temperature in the organs where P. falciparum sequestered. Therefore, if this can be established, the vaccine development imperative against P. falciparum infection and severity may derive from the combination of P. falciparum antigen and the antigen candidate from P. vivax.

Antibodies to malaria, although short-lived, are the primary mechanisms of defence against parasitic infection [31]. Induction and maintenance of anti-malarial antibodies requires repetitive infections [32,33]. Evidence such as the existence of the asymptomatic parasitaemic individual confirms development of immunity against malaria [34,35]. Recently, a study has shown that the incidence of severe malaria in patients with mixed PV-PF malaria infection is 4·2 times less than that in P. falciparum infection alone [7].

In this study we have shown that the anti-P. falciparum and anti-P. vivax antibody levels in the mixed malaria infection were higher than those of the single P. vivax or P. falciparum infections. Our data provide a new basis to support previous findings that, on one hand, development of a cross-immune reactivity between P. vivax and P. falciparum during acute mixed infection could be due to the activation of a pool of memory T cells having specificities to both P. vivax and P. falciparum antigens. These cells co-existed in the residents of the endemic areas where there is regular exposure to malaria parasites.

On the other hand, the antigenic cross-stimulation by P. vivax antigens sharing common epitopes with P. falciparum[36] results in the cell-mediated and antibody responses at high levels against P. falciparum during the acute phase of infection. Further investigations in the different geographical endemic areas are needed to verify the two categories. To stratify this finding further, we performed an in vitro T cell stimulation assay using PvSE and PfSE antigens derived from P. vivax and P. falciparum, respectively, to stimulate the PBMCs from acute single P. vivax or single P. falciparum infections with the two antigens. Both anti-P. vivax and anti-P. falciparum antibodies, tested by the IFA, were found in the supernatant collected from these assays, regardless of the parasite species which caused the infection. This suggested that the patients had developed antigen-specific memory T cells against both P. vivax and P. falciparum parasites which were activated upon re-exposure to either P. vivax or P. falciparum antigens. Supporting evidence from a study in Thailand shows the antibody cross-reactivity from a single P. vivax-infected patient against both the schizont extract of P. falciparum parasite and the PfMSP1₁₉ parasite protein [30]. In addition, the cross-reactivity between anti-PvMSP5 and anti-PfMSP5 antibodies was observed in single P. vivax or single P. falciparum infections [37]. Our findings were also supported by the evidence from an epidemiological observation and cross-sectional study [2], during a wet season, which showed that the dominant parasite is P. falciparum whereas P. vivax dominates during the dry season. Overall immunity, effector T cells and anti-malaria antibodies to malaria among the residents of endemic areas would be strengthened by the existence of P. vivax.

In summary, our results indicate the possibility of P. vivax suppressing P. falciparum parasites, because P. vivax induces CD3⁺δ2⁺T cells which are effector T killer cells. P. vivax infection also elevates anti-P. falciparum antibodies during the acute phase, and induces a very high fever. These findings suggest that the interaction between the host and P. vivax parasites could offer protection as demonstrated in the mixed PV-PF malaria infection. However, further clinical and experimental research is needed in order to verify these assumptions. Furthermore, in single P. vivax or P. falciparum infection, similar levels of T helper type 1 (Th1)/Th2 cytokine responses are shown [38]. Clarification of such responses to mixed malaria infection in man, i.e. conversion between Th1- and Th2-type responses, are of interest and require further investigation.

Disclosure

None.