A protocol to evaluate antigen-specific T-cell responses in the immunoprivileged organs of the Ifnar1-/- murine model for the Zika virus (ZIKV) infection is described. This method is pivotal for investigating the cellular mechanisms of the protection and immunopathogenesis of ZIKV vaccines and is also valuable for their efficacy evaluation.
The Zika virus (ZIKV) can induce inflammation in immunoprivileged organs (e.g., the brain and testis), leading to the Guillain-Barré syndrome and damaging the testes. During an infection with the ZIKV, immune cells have been shown to infiltrate into the tissues. However, the cellular mechanisms that define the protection and/or immunopathogenesis of these immune cells during a ZIKV infection are still largely unknown. Herein, we describe methods to evaluate the virus-specific T-cell functionality in these immunoprivileged organs of ZIKV-infected mice. These methods include a) a ZIKV infection and vaccine inoculation in Ifnar1-/- mice; b) histopathology, immunofluorescence, and immunohistochemistry assays to detect the virus infection and inflammation in the brain, testes, and spleen; c) the preparation of a tetramer of ZIKV-derived T-cell epitopes; d) the detection of ZIKV-specific T cells in the monocytes isolated from the brain, testes, and spleen. Using these approaches, it is possible to detect the antigen-specific T cells that have infiltrated into the immunoprivileged organs and to evaluate the functions of these T cells during the infection: potential immune protection via virus clearance and/or immunopathogenesis to exacerbate the inflammation. These findings may also help to clarify the contribution of T cells induced by the immunization against ZIKV.
The ZIKV is a mosquito-borne flavivirus that was first isolated in 1947 in Uganda from a febrile rhesus macaque. Recently, the ZIKV has become a public health emergency, due to its rapid dissemination in the Americas and its unexpected link to microcephaly and Guillain-Barré syndrome1,2,3. From epidemiological data, the ZIKV has been suspected to be the cause of the Guillain-Barré syndrome in around 1 per 4,000 infected adults4. Moreover, a correlation between the ZIKV and testes infection/damage in the mouse model has been demonstrated, suggesting that the ZIKV infection, under certain circumstances, can bypass the blood-testis barrier and eventually lead to male infertility5. These findings highlight the importance of completely understanding the induction of protective or pathologic immune responses during a ZIKV infection.
Much remains to be learned about the cellular immune responses to the ZIKV. CD4+ and CD8+ T-cell responses to the capsid, envelope, and nonstructural protein 1 (NS1) have been observed in ZIKV-infected monkeys and humans6,7,8. In mice, several studies have indicated that CD8+ T cells play a protective role in controlling the ZIKV replication9,10,11. Importantly, Jurado et al. demonstrated that a ZIKV infection results in the breakdown of the blood-brain barrier and perivascular infiltration of CD8+ effector T cells within the testes of Ifnar1-/- mice. Furthermore, they showed that CD8+ T cells instigate ZIKV-associated paralysis and appear to play a role in the neonatal brain immunopathology. In a previous study, we prepared the ZIKV-E294-302 tetramer and showed that ZIKV-specific CD8+ T cells exist in the brains and spinal cords of ZIKV-infected Ifnar1-/- mice, which may have important implications for the design and development of ZIKV vaccines10.
In response to the urgent need for vaccination to prevent ZIKV infection, several vaccines are in the preclinical stages of development, including RNA vaccines, recombinant vector-based vaccines, and purified protein subunit vaccines. The plasmid DNA vaccine is in phase 1 clinical trials12. The evaluation of safety and efficacy of ZIKV vaccines is, therefore, important. One advantage of the vaccines is their ability to elicit specific T-cell responses, which may be important for protection against the ZIKV. By using ZIKV-derived T-cell epitope-related tetramers, the T-cell immunoreactivity induced by an adenovirus-vector-based ZIKV vaccine could be detected in immunized mice, and both M and E glycoproteins were shown to induce robust antigen-specific CD8+ T-cell immunoreactivity13.
During a virus infection, the recognition of peptide antigens presented by major histocompatibility complex (MHC) molecules to the T-cell receptor (TCR) is an important T-cell-mediated mechanism for protecting the host14. Based on this theory, tetramer technology is a unique tool to elucidate the roles that antigen-specific T cells play in regulating the immune responses15. This protocol describes the establishment of the Ifnar1-/- mouse model for ZIKV infections, and the detection of reactive T cells in the spleen, brain, and testes of the mice by using tetramer technology. Additionally, we used the ZIKV-E294-302 tetramer to detect and evaluate T-cell immunoreactivity induced by a ZIKV vaccine (AdC7-M/E) in immunized mice. This study provides guidance for investigating T-cell responses in immunoprivileged organs and provides a reference for the potential applications in the placenta or fetal brain.
Animal experiments were approved by the Institutional Animal Care and Use Committee of National Institute for Viral Disease Control and Prevention, China CDC. All experiments were performed following the Institutional Animal Care and Use Committee-approved animal protocols.
1. Virus Infection
2. Immunization with the ZIKV Vaccine
3. Isolation of the Monocytes from the Spleen
NOTE: The isolation of the monocytes from the spleen is described as mentioned previously16.
4. Isolation of Monocytes from the Brain and Testes
5. Tetramer Preparation
6. Flow Cytometry
7. Histopathology, Immunofluorescence, and Immunohistochemistry Assay
Following these methods, we have developed a murine model for ZIKV infections. Ifnar1-/- mice at 6–8 weeks of age were infected with 1 x 104 focus-forming units (FFU) of the ZIKV by retroorbital injection. Pathological symptoms and signs (Figure 1A), as well as weight changes (Figure 1B), were observed in the Ifnar1-/- mice after an infection with the ZIKV. The murine brains showed obvious edema and hyperemia (Figure 1C). Meanwhile, the testes shrank gradually (Figure 1D). Furthermore, pathological changes and the destruction of tissue were found in the brain and testes (Figure 2A). We performed an immunofluorescence assay to detect the ZIKV in the brain and testes (Figure 2B). High viral loads were detected in the brain and testis by immunostaining (Figure 2B). Immunohistochemistry showed a robust infiltration of CD3+ T cells into the mice brain after the infection with the ZIKV (Figure 2C).
To detect and evaluate ZIKV-specific T-cell responses, we prepared a mouse MHC-I H-2Db-E294-302 tetramer. The peptide E294-302 can help the H-2Db renature properly and yield a high amount of the soluble MHC-I (Figure 3A). In the shift assay, a high efficiency in biotinylation could be observed (Figure 3B). Subsequently, three streptavidin fluorescence (APC, PE, and BV421)-tagged pMHC-I tetramers were produced to detect ZIKV-specific T cells (Figure 3C). The PE-labeled tetramer had a higher efficacy to detect the specific CD8+ T cell compared to the APC- and BV421-labeled tetramers, though the difference was not statistically significant.
Using the E294-302 tetramer, we detected ZIKV-specific T lymphocytes in the spleen of the infected mice by flow cytometry at 7 d post-inoculation of the ZIKV (3.49 ± 0.45%). Also, similar to the method described in section 3 of this protocol, with 4 weeks post-immunization of AdC7-M/E vaccine, ZIKV-specific T lymphocytes were detected in the spleen (6.89 ± 1.36%) (Figure 4).
Furthermore, we detected the lymphocytes infiltrated into the immunoprivileged organs, such as the brain and testes, after the ZIKV infection. The gates were set to select for CD3+CD8+ T cells in total lymphocytes of the brain and testes. A high ratio of the E294-302 tetramer-specific T cells could be detected in the brain (42.2% in CD3+CD8+ T cells) and the testicular (26.4% in CD3+CD8+ T cells) lymphocytes (Figure 5).
Figure 1: Characterization of the ZIKV infection in Ifnar1-/- mice. Ifnar1-/- mice at 6–8 weeks of age were infected with 104 focus-forming units (FFU) of the ZIKV by retroorbital injection, and mice injected with PBS were used as uninfected controls. The (A) morphology and (B) weight changes of infected Ifnar1-/- mice were monitored. The red arrows indicate Ifnar1-/- mice presented with myeloparalysis and motor paraparesis after the infection. (C) Brains at 7 d post-inoculation and (D) testes at 0, 7, 14, and 30 days post-inoculation were harvested. Representative images of the brain and testes from the mice are shown with a ruler to indicate the sizes. The error bars represent the mean ± SEM. n = 10 mice per group per experiment. Please click here to view a larger version of this figure.
Figure 2: Detection of the ZIKV and lymphocytes in the brain and testis of ZIKV-infected Ifnar1-/- mice. (A) This panel shows histopathological changes in the brain and testes from ZIKV-infected mice compared with negative controls at 7 d post-inoculation. Scale bar = 25 µm (left panel) and 50 µm (right panel). (B) An immunofluorescence assay was performed with the anti-ZIKV Z6 antibody (green). All tissue samples were collected from ZIKV-infected mice at 7 d post-inoculation. The nuclei were stained with DAPI (blue). Scale bar = 50 µm. (C) Immunohistochemistry shows a robust infiltration of CD3+ T cells into the brain and testis. Scale bar = 25 µm (left panel) and 50 µm (right panel). Purple indicates hematoxylin, brown represents the CD3 antibody. Please click here to view a larger version of this figure.
Figure 3: Preparation of ZIKV-specific pMHC-I tetramers. (A) The binding of E294-302 to H-2Db is elucidated by an in vitro refolding. Blue indicates the H-2Db-E294-302 protein. Orange represents the negative control without peptide. (B) This panel shows the H-2Db-E294-302 shift assay. (C) The mock represents a PBS control. Three streptavidin fluorescence (APC, PE, and BV421)-tagged pMHC-I tetramers were used to detect ZIKV-specific T cells. Please click here to view a larger version of this figure.
Figure 4: Flow cytometric analysis of virus-specific CD8+ T cells in the spleen of ZIKV-infected and vaccine-immunized mice. Ifnar1-/- mice at 6-8 weeks of age were either infected with 104 focus-forming units (FFU) of ZIKV or received a single-dose of 4 x 1010 viral particles of AdC7-M/E or PBS as a negative control. After 7 days post-infection or 4 weeks post-vaccination, mouse splenocytes were harvested and analyzed by flow cytometry. The data are shown as mean ± SD. Statistical analysis was performed using one-way ANOVA (not significant: P > 0.05; *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001). Please click here to view a larger version of this figure.
Figure 5: Gating strategy and representative results of virus-specific CD8+ T cells in the brain and testes of ZIKV-infected mice. Representative plots are shown for the infiltrated lymphocytes in (A) the brain and (B) the testis. A succession of gates is set to select lymphoid-scatter+ and CD3+ events. Of these, CD8+ events are gated for the analysis of epitope-specific T cells. Please click here to view a larger version of this figure.
Immunogenic T-cell epitope plays a significant role in cellular immunity against pathogens23. Thus, the detection of ZIKV-specific T cells in immunoprivileged organs is a critical methodology to understand T-cell responses against the natural ZIKV infection. Meanwhile, T-cell response detection is an excellent tool to investigate the efficacy of the viral vaccine. Here, we show a comprehensive protocol to visualize the experiments, which include the isolation of lymphocytes from the spleen, brain, and testes of ZIKV-infected mice, the preparation of the immunodominant epitope E294-302 tetramer, and the recognition of ZIKV-specific CD8+ T cells in immunoprivileged organs of ZIKV-infected mice.
A previous study showed that a live ZIKV or its RNA can be detected in the semen of male patients, which indicates that the ZIKV can bypass the blood-testis barrier and replicate itself in the reproductive system24. Previously, we also showed that the ZIKV can cause testes damage and lead to male infertility in mice25. ZIKV infection can lead to viremia in rhesus monkeys, and the viral RNA can be detected in the central nervous system (CNS), as well as in the visceral organs. Immunohistochemistry revealed that ZIKV-specific antigens were presented in the CNS and the multiple peripheral organs26. Also, in murine models, ZIKV infection can induce a robust antiviral CD8+ T-cell response in the spleen and CNS26.
Compared to previous work, this study establishes systematic methods to detect ZIKV-specific CD8+ T-cell responses in the brain and testes, which are immunoprivileged sites. It is important to assess the functionality of virus-specific T cells in the immunoprivileged organs of the ZIKV-infected mice. The usage of tetramers to detect ZIKV-specific CD8+ T-cell responses in immunoprivileged organs would greatly enhance our understanding of ZIKV infections and their host immune responses. Using E294-302 tetramer, virus-specific T cells in brain and testis can be isolated by flow cytometry, to investigate the cellular mechanisms of the protection and immunopathogenesis during a ZIKV infection. Meanwhile, it is helpful for researchers to investigate further the functions of the CD8+ T cells to control the ZIKV, or to enhance the immunopathogenesis in these organs during ZIKV infection.
To analyze the antigen-specific murine CD8+ T-cell responses in the immunoprivileged organs, we prepared H-2Db-E294-302 tetramer and detected the CD8+ T cells by flow cytometry. Tetramers are a powerful tool to detect antigen-specific T cells. Here, three types of fluorochrome-conjugated streptavidin (APC, PE, and BV421) were generated. Although there are no statistically significant differences in the APC-, BV421-, and PE-labeled tetramers for detecting antigen-specific T cells, PE-labeled pMHC-I tetramers yielded the best results. Hence, the PE-labeled tetramer was used throughout this study. Interestingly, based on the PE-labeled H-2Db-E294-302 tetramer, we detected high ratios of antigen-specific T cells in both the brain and testes, which indicate the migration ability of the virus-specific T cells from the blood to immunoprivileged organs.
However, there are some limitations to the protocol. The H-2Db-E294-302 tetramer is not useful for human T-cell detection, because tetramer detection is dependent on MHC restriction. The screening of immunodominant HLA-restricted peptides is still needed. Besides, retro-orbital infection is effective for a ZIKV infection but might be not a convenient operation for some investigators. Thus, other routes of infection, including peritoneal, subcutaneous, or intravenous, are also recommended.
In the protocol described here, a critical step is the isolation of monocytes from brain and testis. It is important to acquire high-quality lymphocytes; thus, it is important to pay attention to, for example, the centrifugal speed, the strength of the grinding tissue, and the dissection of the brain and testis tissue. Besides, for tetramer preparation, protease inhibitors (PMSF, pepstatin, leupeptin) are helpful when protecting a protein from being degraded. Therefore, it makes sense to add a protease inhibitor to the refolding buffer and exchange the buffer during the process of the tetramer preparation.
In conclusion, we present the methods of detecting antigen-specific T-cell responses in the immunoprivileged organs of the Ifnar1-/- mouse model for a ZIKV infection. This platform can be widely applied to investigate the immune mechanisms of emerging and re-emerging viruses which can bypass the barriers between the blood and the immunoprivileged organs. Moreover, this study may pave the way for the future development of candidate vaccines and immunotherapies.
The authors have nothing to disclose.
The authors thank Gary Wong for the English revision. This work was supported by the National Key Research and Development Program of China (grant 2017YFC1200202), the Major Special Projects for Infectious Disease Research of China (grant 2016ZX10004222-003). George F. Gao is a leading principal investigator of the National Natural Science Foundation of China Innovative Research Group (grant 81621091).
Z6 | our lab | Ma W, Li S, Ma S, et al. Zika virus causes testis damage and leads to male infertility in mice. Cell, 2016, 167: 1511-1524 e1510 | |
CD3 | Santa Cruz | sc-1127; RRID: AB_631128 | |
Fluorescein-Conjuated Affinipure Goat anti-human IgG(H+L) | ZSGB-BIO | ZF-0308 | |
Rabbit Anti-Goat IgG, FITC Conjugated | CWBIO | CW0115 | |
FITC anti-mouse CD3 | Biolegend | 17A2 | |
APC anti-mouse CD3 | Biolegend | 100236 | |
PerCP anti-mouse CD8a | Biolegend | 100732 | |
Percoll | GE Healthcare | P8370 | |
PMSF | Ameresco | 0754-5G | Toxic and corrosive reagent. Handle with care |
Streptadivin | BD | S4762-FZ | Gives a clear solution at 10mg/ml in PBS and stored at -20 °C |
Pepstatin | Sigma | P4265-5MG | 5 mg in 2.5 ml DMSO, aliquots stored at -20ºC |
Leupeptin | Sigma | L8511 | 5 mg in 2.5 ml dH 2 O, aliquots stored at – 20ºC |
Peptides | Scilight-peptide | Must be resuspended in DMSO and stored at -20 °C | |
RPMI 1640 | Hyclone | SH30809.01 | Must be stored at 4°C |
DMSO | MP | 219605590 | Store at room temperature. |
APC-Streptadivin | BD | 554067 | Must be stored and maintained at 4°C. Do not freeze. |
FITC-Streptadivin | BD | 554060 | Must be stored and maintained at 4°C. Do not freeze. |
PE-Streptadivin | BD | 554061 | Must be stored and maintained at 4°C. Do not freeze. |
BV421-Streptadivin | BD | 563259 | Must be stored and maintained at 4°C. Do not freeze. |
PageRuler Unstained Protein Ladder | Thermo Fisher Scientific | 26614 | Must be stored at 4°C |
Cell Strainers | BF | BF10-5040 | Store at room temperature. |
Amicon Ultra 100kDa | Millipore | UFC510024 | Store at room temperature. |
Ultracentrifuge | Thermo Fisher Scientific | ||
FACS Cantol | Flow cytometer must contain lasers and filters that are compatible with the staining panel used. | ||
Superdex 200 Increase 10/300 GL | GE healthcare | 28990944 | |
AKTA PURE | GE healthcare | ||
red blood cell lysis buffer | Solarbio | R1010 | Store at 4°C. |