Rapid outer-surface protein C DNA tattoo vaccination protects against Borrelia afzelii infection

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Abstract

Borrelia afzelii is the predominant Borrelia species causing Lyme borreliosis in Europe. Currently there is no human vaccine against Lyme borreliosis, and most research focuses on recombinant protein vaccines against Borrelia burgdorferi sensu stricto. DNA tattooing is a novel vaccination method that can be applied in a rapid vaccination schedule. We vaccinated C3H/HeN mice with B. afzelii strain PKo OspC (outer-surface protein C) using a codon-optimized DNA vaccine tattoo and compared this with recombinant protein vaccination in a 0–2–4 week vaccination schedule. We also assessed protection by DNA tattoo in a 0–3–6 day schedule. DNA tattoo and recombinant OspC vaccination induced comparable total IgG responses, with a lower IgG1/IgG2a ratio after DNA tattoo. Two weeks after syringe-challenge with 5 × 105 B. afzelii spirochetes most vaccinated mice had negative B. afzelii tissue DNA loads and all were culture negative. Furthermore, DNA tattoo vaccination in a 0–3–6 day regimen also resulted in negative Borrelia loads and cultures after challenge. To conclude, DNA vaccination by tattoo was fully protective against B. afzelii challenge in mice in a rapid vaccination protocol, and induces a favorable humoral immunity compared to recombinant protein vaccination. Rapid DNA tattoo is a promising vaccination strategy against spirochetes.

Introduction

Lyme borreliosis is caused by Borrelia burgdorferi sensu lato (s.l.) spirochetes, which are transmitted by Ixodes ticks. The disease usually manifests itself as a local skin lesion, designated erythema migrans, and can disseminate and cause multiple erythema migrans, borrelial lymphocytoma, acrodermatitis chronica atrophicans, carditis, oligoarthritis or a polyradiculitis/meningoencephalitis (Lyme neuroborreliosis). In North America Lyme borreliosis is caused by B. burgdorferi sensu stricto (s.s.), while in Europe most cases are caused by Borrelia afzelii followed by Borrelia garinii and B. burgdorferi s.s.1,2

During transmission from the tick to the host B. burgdorferi s.l. spirochetes differentially express specific genes to adapt to the environment of the mammalian host. For example, they downregulate outer-surface protein A (OspA)—which facilitates egression from the tick midgut—and upregulate outer-surface protein C (OspC)—which is necessary for migration to the tick salivary glands and dissemination in the mammalian host.3, 4, 5 Moreover, OspC has been shown to bind to an Ixodes tick salivary protein (Salp15) providing B. burgdorferi s.l. spirochetes protection from antibody-mediated killing at the tick bite site.6,7

Currently, there is no human vaccine against Lyme borreliosis. Human vaccine studies have been mostly based on recombinant OspA as antigen, and a human OspA vaccine was previously on the market for 4 years but was discontinued for multiple reasons.8 Other vaccine candidates have been studied, of which OspC vaccines have been most successful. In addition, other Borrelia outer membrane proteins and vector-based vaccines have shown (partial) protection.9 Compared to conventional protein vaccines DNA vaccines have the advantage of being cheap. Furthermore, they have a long shelf-life at ambient temperatures, do not yield vector-specific immune responses in multi-boost regimens and induce potent cellular immune responses in animal models. DNA vaccination based on Borrelia OspA delivered intramuscularly and OspC delivered by a gene-gun or intradermal needle application was previously shown to be effective against challenge with B. burgdorferi s.s. spirochetes in a murine infection model.10, 11, 12, 13

In an effort to improve immunogenicity and thus facilitate translation to humans, a dermal delivery technique using a tattoo device has been developed. This method injects the DNA into the skin via thousands of skin perforations and hence locally induces an inflammatory milieu that functions as an adjuvant. It leads to more robust immune responses and allows for faster vaccination regimens than intramuscular DNA vaccination. As this technique has not previously been investigated for bacterial pathogens in general, and Borrelia in particular, in this paper we describe vaccination against B. afzelii spirochetes using a codon-optimized DNA vaccine based on B. afzelii strain PKo OspC, applied by tattoo in a normal and a rapid vaccination protocol.

Results

Dose finding for B. afzelii strain PKo challenge

Prior to our vaccination studies, we performed a dose-finding experiment to establish an adequate number of spirochetes for challenge by intradermal needle inoculation. We inoculated 38 mice with a range of spirochetes (5 × 102–5 × 106), and assessed infection by qPCR (quantitative PCR) on DNA extracted from ear biopsies after 7 days. Mice were killed after 2 weeks and Borrelia loads in the heart and bladder were detected by qPCR. In addition, skin biopsies (taken from the inoculation site) and bladder were cultured and finally Borrelia antibody titers were assessed by enzyme-linked immunosorbant assay (ELISA), using an OspC-deficient B. burgdorferi s.s. strain as a lysate. After 7 days, most mice inoculated with 5 × 104 spirochetes or higher produced qPCR-positive ear tissue (Figure 1a), while after 14 days all mice inoculated with 5 × 104 spirochetes or higher had qPCR-positive heart tissue (Figure 1b) and bladder tissue tested positive in 8 out of 9 (Figure 1c). Cultures of skin (inoculation site) demonstrated that an inoculum of 5 × 103 spirochetes yielded viable spirochetes in all mice at 2 weeks after infection, while in the bladder this was observed at an inoculum of 5 × 104 spirochetes or higher (Figures 1d and e). Anti-Borrelia antibody titers correlated with the height of the inoculum (Figure 1f). Based on these findings, we determined that a challenge of 5 × 105 spirochetes was optimal for our vaccination studies.

Figure 1
figure1

Dose finding in C3H/HeN mice after needle inoculation with B. afzelii PKo. Inoculum describes the amount of spirochetes injected. (ac), Borrelia DNA loads in mice organs were determined by performing a qPCR (in triplicate) detecting OspA and compensating for mouse beta actin. Panel a depicts Borrelia loads in ear biopsies 7 days after needle inoculation with different amounts of Borrelia spirochetes; b and c depict loads in the heart and bladder 2 weeks after infection. Closed circles indicate positive qPCR loads, open diamonds indicate OspA-negative qPCR results where the OspA detection limit was divided by the mouse beta actin signal in the corresponding sample. (d and e) Two weeks after infection, tissues were cultured in modified Kelly medium at 33 °C and blinded samples were checked weekly for the presence of spirochetes. Time to culture positivity is portrayed in panels d and e for skin and bladder biopsies, respectively. (f) ELISA measuring cross-reactivity between mice sera and B. burgdorferi OspC-mutant lysate.

Generation of a B. afzelii PKo DNA vaccine and a regular 0, 2 and 4 weeks DNA vaccination protocol

A DNA vaccine was constructed based on the cDNA of B. afzelii PKo OspC, with its signal sequence replaced by the human tissue plasminogen activator (hTPA) signal sequence and preceded by a Kozak sequence. The resulting sequence was codon-adapted, leading to the recombinant plasmid pVAX-hTPA-OspC (Figure 2a and Supplementary Data S1).

Figure 2
figure2

Characterization of the humoral immune response after vaccination with a DNA vaccine by tattoo versus vaccination with recombinant OspC in a 0–14–28 day immunization protocol. (a) pVAX-hTPA-OspC DNA vaccine insert. It contains a Kozak sequence, a codon-optimized human tissue plasminogen activator (hTPA) signal sequence, a codon-optimized OspC gene from B. afzelii strain PKo and a double stop codon, and was cloned into a pVAX1 vector. (b) Mice were vaccinated at 0, 2 and 4 weeks with the DNA vaccine or negative control by tattoo, or with a recombinant OspC vaccine using complete and incomplete Freund’s adjuvant. Vaccination time points are indicated by arrow symbols. (c) OspC-specific IgG1/IgG2a ratios in individual mice at t=2, 4 and 6 weeks, after vaccination with rOspC or DNA tattoo at t=0, 2 and 4 weeks. Antibody titers and ratios were compared using a two-tailed Student’s t-test. n.s.: P=0.25. Error bars represent mean±s.e.m.

In the first set of experiments, we compared DNA vaccination by tattoo with a subcutaneously administered recombinant protein vaccine (rOspC, recombinant OspC). The protein vaccine was emulsified in complete Freund’s adjuvant for priming, while the boosters after 2 and 4 weeks were emulsified in incomplete Freund’s adjuvant. Using this regimen the rOspC vaccine elicited a higher antibody titer (P=0.04) than the DNA vaccine (Figure 2b) at 4 weeks, which was 2 weeks after the first booster. However, eventually at t=6 weeks the titers of both vaccines plateaued at a comparable level. To determine differences in T-helper-cell polarization between the two vaccination strategies we measured IgG subclasses induced by the vaccination. The DNA vaccine resulted in a higher IgG2a antibody level after the first vaccination than the rOspC vaccine (P=0.005), indicating a rapid Th1 response (Supplementary Data S2). The OspC-specific IgG1/IgG2a ratio was significantly lower after DNA tattoo compared to rOspC vaccination after 2 weeks (P=0.04) and 6 weeks (P=0.002; Figure 2c). These lower IgG1/IgG2a ratios after DNA tattoo indicate a more Th1-skewed immune response compared to recombinant OspC vaccination.

After challenging the mice with 5x105 B. afzelii PKo spirochetes we determined B. afzelii PKo loads in the ear by qPCR at 7 days (ear biopsy) and in other tissues at 14 days, when all mice were killed. Borrelia DNA loads were negative in all tissues both in the rOspC and in the pVAX-OspC vaccinated mice, except for one positive bladder sample after pVAX-hTPA-OspC vaccination and one positive heart sample in a rOspC vaccinated mouse (Figures 3a and d). Ear biopsies taken 14 days after challenge were also negative in all vaccinated mice (data not shown). In all calculations, negative OspA values were replaced by the value of the OspA detection limit, and the difference in Borrelia loads between vaccinated and control mice remained significant in all organs. Importantly, 6-week cultures of the skin and bladder of the animals in both groups remained negative, further underscoring the observed protective effect (Table 1). Hence, using stringent detection methods a clear protective response against Borrelia infection in skin and deeper tissues was observed after both vaccination strategies. As another marker of protection we measured anti-Borrelia IgG antibodies in both groups after B. afzelii challenge. For this purpose we coated ELISA plates with a mutated B. burgdorferi s.s. strain lacking OspC, thus measuring OspC-independent anti-Borrelia IgG antibody titers. Upon inoculation with B. afzelii we could demonstrate a clear rise in antibody levels in control mice (P<0.0001), but no significant difference was found in either the rOspC or DNA tattoo vaccinated groups (P=0.08 and P=0.40; Figure 3e).

Figure 3
figure3

Protection from challenge with 5x105 Borrelia afzelii spirochetes after vaccination with recombinant OspC or DNA tattoo vaccination in a 0–14–28 day immunization schedule. (ad) Borrelia DNA loads were determined 7 days (ear biopsy) and 14 days after challenge (skin around the inoculation site, bladder and heart). Black dots depict positive Borrelia loads, open diamonds depict negative loads where the OspA detection limit was divided by the sample’s mouse beta actin load. Borrelia loads were compared using a two-sided nonparametric test (Mann–Whitney). (e) Borrelia serology independent of OspC antibodies was compared between mice before Borrelia challenge (open circles) and 2 weeks after challenge (closed circles). Serology results were compared using a two-tailed Student’s t-test. Error bars represent mean±s.e.m.

Table 1 Culture positivity for B. afzelii 6 weeks after challenge

Vaccination against B. afzelii PKo with two 3-day boosters—a rapid DNA vaccination protocol

Since it was previously shown that DNA vaccine delivery by tattoo is able to induce protection against influenza and subcutaneous tumors using a fast vaccination protocol, we were interested to see whether this would also be the case for a bacterial pathogen.14 Therefore, we vaccinated mice with pVAX-hTPA-OspC or a negative control at t=0, 3 and 6 days and challenged the mice at t=21 days with 5x105 B. afzelii PKo spirochetes. As could be expected, assessment of OspC-specific antibody titers before challenge showed that the rise in anti-OspC IgG did not occur any faster after rapid DNA vaccination than after the standard DNA vaccination protocol (Figure 4a). At the time of challenge the anti-OspC antibody titer was lower than in the mice vaccinated in a regular protocol (P=0.04). Next, we assessed the protection induced by the fast regimen by qPCR and culture. qPCR of ear DNA 7 days after Borrelia challenge as well as of ear DNA (data not shown), skin DNA (at the inoculation site), bladder DNA and heart DNA 14 days after challenge were all negative for Borrelia in the mice vaccinated with the pVAX-hTPA-OspC DNA vaccine administered by tattoo in a 0–3–6 day regimen (Figures 4b and e). Significant differences between mice receiving rapid DNA tattoo versus controls were observed in all organs except skin, even using the OspA detection limit in negative samples. Moreover, cultures of the bladder and skin (inoculation site) remained negative, whereas cultures from mice tattooed with a negative control plasmid were all positive (Table 2). Finally, similar to our observation using the normal DNA vaccination schedule, mice vaccinated with pVAX-hTPA-OspC by the rapid protocol did not develop anti-Borrelia antibody responses after B. afzelii challenge (Figure 4f). No significant difference was found when comparing pre-and post-challenge serum (P=0.50) while control mice showed a rise in anti-Borrelia antibodies after challenge (P<0.0001).

Figure 4
figure4

Effects of DNA tattoo vaccination using pVAX-hTPA-OspC in a rapid vaccination protocol (0, 3 and 6 days). (a) Total IgG antibody titers after DNA vaccination comparing a rapid vaccination protocol (solid lines) with a regular vaccination protocol (broken lines). Vaccination time points are indicated by solid arrows (rapid protocol) and by broken arrows (regular protocol). Antibody titers were measured prior to B. afzelii challenge (2 weeks after the second booster vaccination). (be) Borrelia afzelii DNA loads in mice organs. Samples were analyzed by qPCR in triplicate. When OspA signal was not detected in any of the three reactions the detection limit for OspA was used (open diamonds). Borrelia loads were compared using a two-sided nonparametric test (Mann–Whitney). n.s.: P=0.18. (f) Anti-Borrelia IgG was measured in pre- and post-challenge serum as indicated by open and solid circles, respectively. Serology results were compared using a two-tailed Student’s t-test. Error bars represent mean±s.e.m.

Table 2 Culture positivity for B. afzelii 6 weeks after challenge

Discussion

In this study, we describe the successful application of a rapid DNA tattoo method as a vaccination technique against Borrelia and show that it induces a similar humoral immune response as recombinant protein vaccination, yet yields a more favorable IgG1/IgG2a ratio. Furthermore, we performed the first published dose-finding study for B. afzelii strain PKo and using the selected inoculum in our vaccination and challenge experiments none of the mice vaccinated with either recombinant OspC or with a DNA tattoo vaccine targeting OspC were infected as determined by qPCR, culture and serology. Finally, when the DNA tattoo was administered in a rapid vaccination protocol (0, 3 and 6 days), full protection was also obtained, to our knowledge for the first time in the setting of a DNA tattoo vaccination against a bacterial pathogen.

Humoral immunity to OspC is highly important for Borrelia clearance.15,16 Therefore, it was our goal to maximize the humoral immune response by adding an hTPA signal sequence as keratinocyte-derived OspC antigen will be secreted, probably skewing the immune response from a CD8+ T-cell response towards CD4+ T-cell and B-cell activation.12 IgG1 in mice does not activate complement due to its relative inflexibility and is indicative of a Th2 polarized immune response.17,18 In contrast, IgG2a is indicative of a Th1-skewed response, which has been shown to be important in clearance of Borrelia.19, 20, 21 In our study, a DNA tattoo elicited an immune response with a lower IgG1/IgG2a ratio than a rOspC vaccine, while maintaining high total IgG levels. This indicates a more Th1-polarized immune response and is in line with the lower IgG1/IgG2a ratios observed after needle-based DNA vaccination, as opposed to gene-gun immunization.13,18 Presumably, in our case the danger signals elicited by over 5 × 104 needle injections in the skin skew the immune response away from Th2-associated IgG1 production. Importantly, total IgG levels after 6 weeks were similarly elevated after DNA tattoo vaccination compared to recombinant OspC vaccination, which shows that DNA tattoo is also able to induce prolonged presence of transmission-blocking antibodies.

In the current study, we show that DNA tattoo vaccination is able to induce a favorable humoral immune response that is capable of effectively preventing infection with an extracellular bacterial pathogen. However, vaccination against OspC is limited by the large sequence heterogeneity between OspC serotypes.22, 23, 24 The immunodominant OspC epitopes are believed to reside in variable regions, making it unlikely for a monovalent OspC vaccine to be cross-protective against all naturally occurring B. burgdorferi s.l. strains.25 Indeed, upon challenge of PKo-OspC vaccinated mice (using the normal protocol) with 1x104 B. burgdorferi strain N40 spirochetes we did not observe full protection (data not shown). In this respect an advantage of using DNA vaccines could be that one can easily combine multiple OspC sequences to prevent this issue. This is subject of our current and future investigations.

We have challenged mice with in vitro-cultured spirochetes and not by Borrelia-infected ticks. In vitro-cultured B. burgdorferi s.l. spirochetes lack any interaction with immunomodulating tick salivary proteins.26 It also has been shown that fewer spirochetes are required to infect mice, when these are derived from ticks or mice instead of in vitro culture.27,28 OspC is upregulated during migration of the spirochetes from the tick midgut to the tick salivary glands and plays an important role in the first stage of mammalian infection.4,5 OspC vaccination has previously been found to protect against B. burgdorferi infection both through syringe-inoculation and through tick challenge.15,29,30 In this setting, OspC vaccination was also found to diminish OspC-expressing B. burgdorferi spirochetes in the midgut of biting ticks, reducing their presence in the tick salivary glands.31 Since DNA vaccination by tattoo induces similar IgG levels compared to rOspC vaccination, with an even more favorable subclass distribution, we postulate that our technique will similarly protect from B. burgdorferi infection through tick challenge. However, more research and the establishment of a robust B. afzelii PKo tick challenge model are required to confirm this hypothesis.

DNA tattooing is a promising vaccination technique. Previous studies have shown that the DNA tattoo vaccination approach is far more effective in inducing cellular immune responses in mice and non-human primates compared to intramuscular DNA vaccination, despite the lower transfection efficiency and lower and shorter antigen expression.14,32 Superiority of the DNA tattoo has also been demonstrated in a 0–14–28 day vaccination schedule in which DNA tattoo outperformed intramuscular delivery both in inducing humoral and cellular immunity.33 Improving immunogenicity of DNA vaccines is of paramount importance for successful translation to humans. Importantly, by using the DNA tattoo it is also possible to apply a compact vaccination strategy where boosters are delivered after 3 and 6 days.

DNA tattoo vaccination could provide the key to vaccination strategies targeting pathogens in settings where quick vaccination schedules can either boost adherence or when epidemics demand quick vaccination coverage and effectiveness. Moreover, DNA vaccines can be developed quickly and can be easily deployed in developing countries due to the low cost and the long shelf-life. Since we have shown that rapid DNA tattoo vaccination can elicit protection against an extracellular spirochetal pathogen, one could speculate that DNA vaccination by tattoo could also protect against other spirochetal diseases such as syphilis, relapsing fever and leptospirosis. Moreover, the adequate humoral immune responses we show here, added to the previously described rapid induction of CD8+ T-cells, make DNA tattoo a very interesting technique to prevent other extra- and intracellular bacterial pathogens.

Materials and methods

Generation of the recombinant and DNA OspC vaccines and vaccination protocols

Groups of six mice were vaccinated at t=0 weeks, t=2 weeks and t=4 weeks, with either rOspC, a DNA vaccine coding for the hTPA signal sequence fused to OspC in a pVAX vector (pVAX-hTPA-OspC) or with a negative control plasmid. B. afzelii PKo rOspC was produced as described elsewhere and used as a positive control.7 Approximately, 10 μg of rOspC was emulsified 1:1 in 50 μl complete Freund’s adjuvant at t=0 weeks and in incomplete Freund’s adjuvant at t=2 and 4 weeks, and injected subcutaneously in two 50 μl dosages at the back of the mice. The pVAX-hTPA-OspC DNA vaccine was designed based on the OspC gene sequence in B. afzelii PKo plasmid cp27 (CP000402.1) in which we replaced the 23aa signal sequence (predicted by SignalP 4.0 web-based software, CBS, Lyngby, Denmark) with the hTPA signal sequence (genbank AAA61213.1).34 Both the OspC sequence and hTPA signal sequence were codon-optimized to mouse tRNA usage with Java Codon Adaptation tool (Braunschweig, Germany).35 At the 5′ end a BamH1 and a Kozak sequence were added, and at the 3′ end a sequence encoding a double stop codon and an Xho1 were added. The insert was synthesized (Biobasic Inc., Ontario, Canada) and ligated into a BamH1/Xho1 restricted empty pVAX vector (Invitrogen, Carlsbad, CA, USA). As a negative control, empty circular pVAX was used. Both plasmids were amplified using a Nucleobond Xtra EF kit (Macherey-Nagel, Düren, Germany) and resuspended in DNase-free water. Both in the pVAX-hTPA-OspC and in the negative control groups hair was removed from the mice abdomens using hair removal cream. Next, 20 μg of DNA vaccine was applied on the hairless abdominal skin. Subsequently a Cheyenne Hawk tattoo machine carrying a Cheyenne 13-magnum tattoo needle (both MT.DERM, Berlin, Germany) was placed on the abdominal skin and the DNA vaccines were tattooed 0.5–1 mm into the skin for 45 s at 100 Hz under isofluorane anesthesia. In a separate experiment, groups of 6 mice were vaccinated with pVAX-hTPA-OspC or negative control at t=0, 3 and 6 days.

OspC-specific total IgG, IgG1 and IgG2a:

High-binding ELISA plates (Greiner Bio-one, Kremsmünster, Austria) were coated overnight at 4 °C with 1 μg/ml rOspC PKo, washed with PBS–Tween (phosphate-buffered saline–Tween) and blocked with 1% BSA in PBS (blocking buffer) for 2 h. Mouse sera derived from either mandibular puncture or tail bleed (pre-immune, before each booster and before killing the mice) were diluted in blocking buffer and incubated for 1 h. Plates were washed and incubated for 1 h with either horseradish peroxidase (HRP)-linked anti-mouse IgG (Cell Signaling, Beverly, MA, USA) diluted 1:1000 in blocking buffer, or HRP-linked goat anti-mouse IgG2a/rat anti-mouse IgG1 (SouthernBiotech, Birmingham, AL, USA) diluted 1:3000 in blocking buffer. Plates were washed and developed in a Biotek (Winooski, VT, USA) ELISA plate reader at 450–655 nm. IgG titers were defined as the last dilution where optical density (OD) 450–655 nm was >3 s.d. above baseline signal.

B. afzelii challenge

Low-passage B. afzelii strain PKo spirochetes were cultured and counted as described before and 5x105 spirochetes in 100 μl PBS were needle-inoculated subcutaneously in the midline of the back of mice 2 weeks after the third vaccination (t=48 days in the regular and t=21 days in the rapid vaccination protocol).36 The inoculation dose was established based on a dose-finding experiment in which 38 mice received a range of doses, that is 5x102 spirochetes (n=10), 5x103 spirochetes (n=10), 5x104 spirochetes (n=9), 5x105 spirochetes (n=5), 5x106 spirochetes (n=4) and 2 mice received only PBS as a negative control. More mice were used in low-inoculum groups due to anticipated variability of the infection read-outs in individual animals. Mice were killed 2 weeks after challenge. In a separate experiment mice were infected with 1x104 B. burgdorferi s.s. strain N40 spirochetes in 100 μl PBS.

Borrelia serology

We developed an ELISA to quantitatively measure OspC-independent antibodies directed against B. afzelii PKo after infection, by measuring cross-reactivity to an OspC-deficient B. burgdorferi s.s. strain. OspC-deficient B. burgdorferi 297 (courtesy of Erol Fikrig, Yale University, New Haven, CT, USA) was cultured at 33 °C until mid-log phase and lysed using a sonicator. High-binding ELISA plates (Greiner Bio-one) were coated overnight at 4 °C with 1 μg/ml of lysate, washed three times in PBS-T and blocked for 2 h with blocking buffer. Mouse sera derived from mice 2 weeks after inoculation with B. afzelii were diluted to 1:100 in PBS and 50 μl was added and incubated for 1 h at room temperature. Pooled pre-challenge sera from triple vaccinated mice were used as a negative control. Plates were washed and incubated for 1 h with HRP-linked anti-mouse IgG (Cell Signaling) diluted 1:1000 in blocking buffer, developed and read at OD 450–655 nm.

Borrelia DNA loads

qPCR was used to quantify Borrelia DNA in mouse tissues after inoculation with B. afzelii PKo. Seven days after inoculation an ear biopsy was taken, and 14 days after inoculation an ear biopsy, skin biopsy (around the inoculation site), half of the bladder and the apex of the heart were taken. Tissues were lysed overnight and DNA was extracted using the Blood and Tissue kit (Qiagen, Venlo, The Netherlands). qPCR was performed with the Borrelia-specific OspA primers forward 5′-IndexTermAAAAATATTTATTGGGAATAGGTCT-3′ and reverse IndexTerm5′-CACCAGGCAAATCTACTGAA-3′, and with mouse beta actin primers forward 5′-AGCGGGAAATCGTGCGTG-3′ and reverse IndexTerm5′-CAGGGTACATGGTGGTGCC-3′ to correct for amount of mouse tissue in the DNA sample. qPCRs were performed using the LightCycler480 (Roche, Nutley, NJ, USA) and SYBR green dye (Roche), and reactions were performed in triplicate. PCR protocol was 95 °C 6 min, and 60 cycles of 95 °C 10 s, 60 °C 20 s and 72 °C 20 s. Results were analyzed using LinRegPCR software (Amsterdam, The Netherlands).37 Negative and positive controls were included in each qPCR run. In case of 3x negative values for OspA in a sample, the OspA value was replaced by the value of the OspA detection limit in the assay.

Borrelia cultures

Blinded samples (half of the bladder and a skin biopsy from the inoculation site) were cultured in modified Kelly–Pettenkofer medium with rifampicin (50 μg/ml) and phosphomycin (100 μg/ml) at 33 °C and 5 μl was checked weekly by dark-field microscopy for the presence of spirochetes for 6 weeks, as described before.7

Statistics

Borrelia loads were given their detection limit when negative to exclude the possibility of low qPCR sensitivity and were compared to control mice by two-sided nonparametric tests (GraphPad Prism software version 5.0, San Diego, CA, USA). Antibody titers and optical densometry data were analyzed by a two-tailed Student’s t-test. Culture positivity was analyzed using Fisher’s exact test as compared to control. Statistical significance is depicted by the following: P<0.05 (*), P<0.01 (**) and P<0.001 (***). Error bars in all figures illustrate mean±s.e.m.

Ethics

All experiments were reviewed and approved by the Animal Research Ethics Committee of the Academic Medical Center, Amsterdam, The Netherlands.

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Acknowledgements

We thank Erol Fikrig (Section of Infectious Diseases, Department of Internal Medicine, Yale University, New Haven, CT, USA) for providing the OspC-mutant Borrelia burgdorferi strain. This work was supported by a ‘Veni’ grant (91611065 and 91610095) received from The Netherlands Organisation for Health Research and Development (ZonMw).

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Correspondence to J W R Hovius.

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Wagemakers, A., Mason, L., Oei, A. et al. Rapid outer-surface protein C DNA tattoo vaccination protects against Borrelia afzelii infection. Gene Ther 21, 1051–1057 (2014) doi:10.1038/gt.2014.87

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