Lassa Virus Activates Myeloid DCs but Takes Away Their Power

Based on paper published in PLOS Pathogens by a team at the Unit of Biology of Emerging Viral Infections

Lassa virus (LASV) causes lassa fever (LF), which is regularly found in West Africa, with an estimated 300,000 to 500,000 cases and 3,000 to 5,000 deaths every year. The World Health Organization believes that LASV is one of several pathogens likely to cause severe outbreaks in the future since there are no approved vaccines and very limited knowledge on how the disease develops. Recent evidence has shown that dendritic cells (DCs) are an important target of LASV. A key feature of myeloid DCs (a subset of dendritic cells) is that they take in foreign substances and present their proteins on the surface in order to activate T cells (a type of immune cell that determines the specificity of an immune response to foreign substances in the body). Furthermore, mDCs can also produce IFN-1 (an antiviral signaling molecule). Research on primates infected with LASV indicated that an early IFN-1 response along with T cells exposed to the virus contributed to increased survival. This suggested to the authors that mDCs could play a key role during LASV infection.

To further investigate the role of mDCs during LASV infection, the authors began by examining whether LASV and MOPV (a type of virus that is similar to LASV but does not cause disease) activate mDCs. They infected human mDCs with either MOPV or LASV and then used RT-PCR (a technique measures how much a gene is expressed by detecting RNA levels) and flow cytometry (a technique that measures the characteristics of cell components of a solution) to measure the amount of IFN-1 and identify any mDC activation proteins present. The mDCs infected with LASV produced similar levels of IFN-1 to those of MOPV-infected mDCs while similar levels of expression were also seen for several mDCs activation proteins. This result suggested that both MOPV and LASV activate mDCs. The authors also tracked the number of virus particles in mDCs using a Luminex assay (a test that uses color-coded beads with a specific antibody that binds to the molecule of interest followed by the addition of a second antibody that fluoresces). They observed a progressive decrease in the number of virus particles which suggested that the virus was not replicating.

After demonstrating that MOPC and LASV activate mDCs but don’t replicate, the authors sought to investigate the relationship between the IFN-1 response and the lack of viral replication. Schaeffer et al (2018) inhibited the effects of the IFN-1 response by treating mDCs with antibodies (highly specific protein that binds to and is produced in response to another specific protein) that target the receptor protein which IFN-1 binds to and infecting them with LASV or MOPV that expressed mCherry (a fluorescent protein that is inserted into a gene). The authors detected mCherry in the cell after LASV and MOPV infection which indicated that the virus did replicate when IFN-1 was not functional. They further confirmed this by examining the number of virus particles and found that the virus levels increases four days post infection. Collectively, these results suggest that the IFN-1 response inhibits the infection of MOPV or LASV by preventing virus replication.

After demonstrating that both LASV and MOPV activate mDCs, the authors’ next step was to determine whether the mDC response since a major role of mDCs in the body is to activate T cells. To investigate this, T cells were combined with infected or non-infected mDCs and RT-PCR and flow cytometry were used to determine the levels of IFN-1 and activation proteins present on active mDCs. The authors observed that mDCs infected with MOPV produced IFN-1 and expressed the activation proteins in the presence of T cells whereas the LASV-infected did not express the activation proteins or produce IFN-1. These results suggested that there are interactions between T cells and mDCs that can alter the defense response to MOPV and LASV.

After examining the mDCs response, the authors looked at the T cell response to determine if T cells are activated following MOPV or LASV infection. They used flow cytometry to identify the presence of CD69 (an activation marker expressed on activated T cells) on T cells that were exposed to infected or uninfected mDCs. Schaeffer et al (2018) noted that T cells exposed to MOPV-infected DCs expressed the CD69 marker while T cells exposed to LASV-infected mDCs did not. In addition, they found that T cells which were exposed to MOPV-infected DCs had higher levels of perforin and granzyme B (toxic chemicals produced by T cells that kills cells infected by viruses). Together, these results indicate that MOPV-infected DCs activate T cells while LASV-infected mDCs do not.

After demonstrating that LASV-infected mDCs suppress the immune response by not activating T cells, the authors’ next step was to determine what role viral proteins play. They tackled this by exchanging the viral proteins in MOPV and LASV with the corresponding proteins present in the other virus then tested the IFN-1 response. The most striking result was that the MOPV which contained LASV NP (a complex that consists of a nucleic acid bonded to a protein) behaved exactly like normal LASV which suggests that the NP in LASV is responsible for suppressing T cell activation.

The authors showed that both MOPV and LASV can activate mDCs but only MOPV-infected DCs can activate T cells. In addition, they demonstrated that the major immunosuppressive properties of LASV are carried by the NP. Future directions could involve an investigation into the mechanisms behind the crosstalk between T cells and mDCs.

References

Schaeffer, J., Carnec, X., Reynard, S., Mateo, M., Picard, C., Pietrosemoli, N., ... & Baize, S. (2018). Lassa virus activates myeloid dendritic cells but suppresses their ability to stimulate T cells. PLoS pathogens, 14(11), e1007430.

Ogbu, O., Ajuluchukwu, E., & Uneke, C. J. (2007). Lassa fever in West African sub-region: an overview. Journal of vector borne diseases, 44(1), 1.

Collin, M., McGovern, N., & Haniffa, M. (2013). Human dendritic cell subsets. Immunology, 140(1), 22-30.