Background
For six decades, vaccination with whole attenuated parasites has remained the gold standard in malaria vaccine development and is the only way to ensure complete protection in vaccinated individuals. Following the first report of protective immunity in mice receiving irradiated sporozoites (SPZ) [
1], the approach was transferred to human trials and initially SPZ were delivered by the bites of more than 1,000 irradiated infectious mosquitoes [
2,
3]. Since then, it has been shown that not only irradiation, but also other modes of attenuation, such as administration of protective drugs [
4,
5] and genetic modification [
6] can be used. Whole-parasite malaria vaccination had been considered unfeasible for routine vaccination until recently when it returned to the focus of anti-malarial vaccine development following the development of aseptic, purified, vialed, and cryopreserved NF54
Plasmodium falciparum SPZ (PfSPZ vaccine) [
7].
The initial clinical assessment of the irradiated PfSPZ vaccine reported limited immunogenicity and protection when administered intradermally (id) [
8]. However, intravenous (iv) administration resulted in higher immunogenicity and improved protective efficacy in a subsequent study [
9]. Complementary studies in the rodent model demonstrated that only a small number of id or subcutaneously (sc) injected parasites reach the intrahepatic stage. Moreover reduced parasitic liver load was associated with low protective efficacy following id SPZ administration [
8,
10].
Concerning the translation of whole-parasite immunization towards a routine malaria vaccine, highly protective non-iv immunization regimens would be preferred especially when the vaccine is intended for the use in paediatric populations. Thus, improving the infectivity of non-iv SPZ administration routes is an important objective for further development of whole-parasite vaccination.
In natural malaria infections, SPZ are transmitted to humans by the bite of infective female
Anopheles mosquitoes. The bite elicits an acute inflammatory reaction at the feeding site, resulting from the saliva of mosquitoes that contains pro-inflammatory [
11] as well as anti-hemostatic [
12] agents that facilitate the blood feeding process. The hypothesis of this analysis is that the local inflammatory reaction may not only benefit the feeding mosquito but also improve the chances of parasites successfully entering the blood stream and in turn, establishing a malaria infection in the mammalian host.
The objective of this study was to investigate the infectivity of sc and intramuscular (im) SPZ administration in the presence or absence of both pro-inflammatory (histamine) and anti-haemostatic (heparin) drugs in the rodent C57BL/6 Plasmodium berghei (ANKA strain) model. In addition, the protective efficacy of histamine-supplemented sc whole-parasite immunization was evaluated.
Methods
Ethics statement
All animal experiments were performed according to FELASA category B and GV-SOLAS standard guidelines. Animal experiments were approved by German authorities (Regierungspräsidium Karlsruhe, Germany), § 8 Abs. 1 Tierschutzgesetz (TierSchG).
Data assessment and statistical analysis
Data assessment and statistical analysis were performed using Stata IC/13.0 (StataCorp LP, College Station, TX, USA) and Prism Version 5.0b (GraphPad Software, San Diego California, USA). Log rank-test was used to compare survival distributions. Within groups of infected animals, Student’s t-test or Mann–Whitney-U-test were applied to compare parasite liver load detected by qRT-PCR or in vivo bioluminescence measurement, as appropriate. The clinical outcome of the final immunization experiment was assessed using Fisher’s exact test.
Infection, immunization and challenge procedure
Freshly dissected P. berghei (ANKA strain) salivary gland SPZ were injected into C57BL/6 mice either sc in the neck skin fold, im in the thigh muscle or iv into the tail vein. SPZ were administered in PBS in a total volume of 100 μl, supplemented with 5 IU of heparin (Heparin 5000 IU/ml, Ratiopharm GmbH, 89079 Ulm, Germany) and varying amounts (1, 3 or 100 μg) of histamine (10 mg/ml histamine-dihydrochlorid, ALK-prick, ALK-Abell Arzneimittel GmbH, 22876 Wedel, Germany). For im administration, the volume was reduced to 50 μl.
For immunization under chloroquine (CQ) chemoprophylaxis (chloroquine chemoprophylaxis with sporozoites; CQ-CPS), mice were immunized by a prime-two boost regimen administered iv or sc at weekly intervals. CQ chemoprophylaxis was continuously supplied in the drinking water (CQ-DW) as previously described [
13] with an elevated CQ concentration (300 mg/l instead of 288 mg/l). CQ-DW was introduced concomitant with the first (prime) SPZ administration and maintained until 14 days after the final (boost 2) immunization. Mice were challenged four weeks after withdrawal of CQ-DW by iv injection of 10
4 freshly dissected
P. berghei salivary gland SPZ.
Parasitaemia read-out
Thin blood smears were obtained at days 4, 5, 7, 10, and 14 post-infection. Blood smears were Giemsa-stained and 25 light fields, each representing approximately 400 single-layer erythrocytes, were assessed by light microscopy. Malaria infection was reported if at least two infected red blood cells were detected within the same slide on at least one occasion.
Quantification of parasite liver load by real-time PCR
For quantification of the parasite load in the liver by real-time qRT-PCR, C57BL/6 mice were infected with 104 SPZ iv or sc, with or without supplementation of 5 IU heparin and 3 μg or 100 μg of histamine.
Mice were sacrificed at 42 or 48 hours post-infection, and livers were removed and homogenized. Total RNA was isolated with the RNeasy kit (Qiagen), and complementary DNA (cDNA) was synthesized with the RETROScript kit (Ambion), according to the manufacturer’s instructions. Real-time PCR was performed with the ABI 7500 sequence detection system and Power SYBR Green PCRMasterMix (Applied Biosystems), according to the manufacturer’s instructions, using gene-specific primers for the P. berghei 18SrRNA [GenInfo Identifier (GI), 160641] (forward: 5′-AAGCATTAAATAAAGCGAATACATCCTTAC-3′; reverse: 5′-GGAGATTGGTTTTGACGTTTATGTG-3′) and the mouse GAPDH gene (GI, 281199965) (forward: 5′-CGTCCCGTAGACAAAATGGT-3′; reverse:5′-TTGATGGCAACAATCTCCAC-3′). Temperature profile was as follows: 95°C for 10 min, followed by 40 cycles of 95°C for 15 sec, 55°C for 45 sec and 60°C for 1 min.
In vivo quantification of parasite load
The transgenic
P. berghei line 676m1cl1 (Pb GFP-Luc
con) [
14] was used for
in vivo imaging. Luciferase activity was visualized using an
in vivo imaging System (IVIS 100; Caliper Life Sciences, USA) as previously described [
15]. In brief, animals were anesthetized using isofluorane, their belly was shaved and de-haired. D-luciferin (Synchem Laborgemeinschaft OHG, Germany) was dissolved in PBS and a total of 2.5 mg/mouse were injected intraperitoneally. Bioluminescence imaging was acquired with an exposure time of 180 sec directly following administration of D-luciferin and analysed using Living Image 2.50.1 (Xenogen Corp., Hopkinton, MA, USA).
Discussion
This study in the rodent malarial model demonstrates that the parasite liver load following sc or im SPZ injection can be increased by co-administration of heparin and histamine. The increase in parasite liver burden correlates with histamine supplementation in a dose-dependent manner, while heparin supplementation might be dispensable. Parasite liver load is thought to be associated with protective efficacy achieved by whole-parasite immunization [
8,
10]. Therefore, the concept of histamine supplementation might represent an important contribution to the ongoing efforts in the development of an efficacious non-iv whole-parasite vaccine.
Within the analyses, it was found that supplementation with 100 μg histamine and 5 IE heparin increases the parasite liver burden by approximately five-fold as compared to conventional sc SPZ administration. The parasite liver load thereby reaches approximately 10% of the level achieved by iv infection, which is superior to any previous report on im, sc or id SPZ administration [
17].
Importantly, sc histamine administration is not restricted to the rodent model but has previously been applied as an adjuvant to immunotherapy in oncologic phase II and III clinical trials. Subcutaneous administration of 1 mg histamine-dihydrochloride was found to be safe [
18], even when applied in an outpatient setting [
19]. A safety assessment in rats demonstrated that repeated doses of 500 mg/kg or higher elicited acute tissue damage, while the repeated injection of up to 100 mg/kg did not result in relevant side effects besides a local inflammatory reaction [
20]. Thus, the concept of histamine co-administration is likely to be safe and feasible not only in the rodent model but also in human whole-parasite vaccination.
The increase in parasitic liver obtained through histamine co-administration has to be interpreted with caution as it might be explained by two different mechanisms. Firstly, the local inflammation and vasodilation induced by histamine injection may increase the number of SPZ entering a capillary and thus increase the total number of parasites reaching the liver stage. It is known that the majority of id administered SPZ remain in the skin for hours before entering the bloodstream and invading a hepatocyte, whereas iv-injected SPZ reach the liver within minutes [
21]. Thus, it cannot be excluded that co-administration of histamine enables SPZ to enter the blood stream at an earlier time point rather than in greater number. In this case, the increasing parasite liver load detected at a single time point post-infection might not, or only in parts, represent an increase in total liver load over time but rather a shift of the maximum liver burden to earlier time points post-infection. For the same reason, determination of the parasite liver load obtained by sc
versus iv administration at a single time point post-infection represents a minimal and thus conservative estimate of the real total liver load obtained by sc injection. While it would be desirable to determine parasite liver burden over time, such measurements remains technically challenging as
in vivo detection of intrahepatic liver development at later time points is masked by increasing blood-stage infection.
A previous study attempted to enhance the protective immunogenicity of injections either by topical application of imiquimod or by local ‘tape-stripping’ of the skin at the infection site. Interestingly, mice immunized by SPZ injected at a tape-stripped skin site demonstrated superior protection against a subsequent "by bite" SPZ challenge [
22]. Tape-stripping of the injection is likely to induce local inflammation, and the superior protection reported might indicate that local inflammation increases the number of SPZ reaching the liver and thereby augments the total parasite liver load.
In a CPS vaccination experiment, the protective efficacy of a prime-two boost sc immunization regimen supplemented with 100 μg histamine and 5 IU heparin was compared
versus a non-supplemented sc control group and an iv control group immunized by a ten-fold lower SPZ dosage. Despite the earlier observation of at least 50% parasite liver burden in histamine-supplemented sc
versus iv immunized animals, the protection achieved by histamine-supplemented sc immunization remained inferior in comparison to the iv control group and was at best slightly higher than in the non-supplemented sc immunized controls. While it cannot completely be excluded that a slightly lower parasite liver load might sufficiently explain this inferior protection, additional factors might contribute to the decreased efficacy of sc CQ-CPS immunization. It is possible that a high number of SPZ remaining in the animals’ tissue enables the development of invasion-blocking antibodies during the prime immunization, thus leading to a decreased parasite liver load in subsequent immunizations [
10]. Alternatively, immune responses induced by SPZ remaining in the skin or draining lymph nodes [
23] or a histamine-triggered increasing activity of myeloid suppressor cells [
24] might impede the development of protective cellular intrahepatic immune responses. The specific reasons for the low protection induced by repeated sc immunization are an important question for future investigations. Additional approaches and measures like e.g. intradermal SPZ administration [
22], SPZ administration at lower volumes [
21] or by microneedle patches [
25], or the use of radiation attenuated SPZ rather than CPS might be combined with the concept of histamine supplementation to develop a highly protective non-iv SPZ immunization regimen.
In the meantime, the current data suggests that even though sc immunization was insufficient to protect against blood-stage infection, it still seems to prevent the onset of experimental cerebral malaria. This observation could be explained by blood-stage directed immune responses induced by CPS [
26] and might represent an additional benefit obtained by CPS immunization. In summary, histamine supplementation represents a novel and conceptually innovative approach that could contribute to the future development of a non-iv whole-parasite malaria vaccines.
Acknowledgements
We would like to thank Miriam Ester for mosquito breeding, Marion Maier for assistance with the rodent studies and the animal facility of the German Cancer Research Center (DKFZ) for providing access to in vivo imaging facilities. We are grateful to Roland Frank, Kirsten Heiss, Matthew Lewis and Priyanka Fernandes (all University Hospital Heidelberg, Germany) for critical comments and proofreading of the manuscript. This work was supported by an award from the German Society of Paediatric Infectious Diseases (Deutsche Gesellschaft für Pädiatrische Infektiologie, DGPI) to JP. JP received funding from the German Centre for Infection Research (Deutsches Zentrum für Infektionsforschung, DZIF) (grant no. 80006013). We acknowledge the financial support of the Deutsche Forschungsgemeinschaft and Ruprecht-Karls-Universität Heidelberg within the funding program Open Access Publishing.
Competing interests
The authors have declared that they have no competing interests.
Authors’ contributions
JP conceived the study idea. JP and JFH performed the study experiments. JP wrote the first draft of the manuscript. All authors contributed to the analysis and interpretation of the data and to the final draft of the article. All authors read and approved the final manuscript.