Comprehensive Characterization of Cellular Immune Responses Following Ebola Virus Infection

In 2016 West Africa experienced the largest and most deadly outbreak of Ebola virus (EBOV) in history. Ebola virus disease (EDV) is associated with rapid acute infection consisting of fever, fatigue, headache, myalgia, and gastrointestinal distress 3-13 days after exposure. Due to the limited number of outbreaks, high mortality rate, and limited technology present in sub-Saharan countries affected by Ebola virus, there is presently very little data on the immune response elicited by infection. In this study researchers at the University of Medical Center Hamburg-Eppendorf (UK), were able to analyze the response of immune cells in a single recovering patient. This patient, who’s blood was take 37 and 46 days post EVD symptom onset, provided an fascinating case study of lymphocyte dynamics as the patient was able to clear the virus without being administered experimental drugs. By carrying out an analysis of immune response following Ebola infection, researchers aimed to increase scientific understanding of key cellular factors that affect host cell survival, protection, and the development of persistent Ebola strains.

It is important to note that although the blood samples taken were from the patient after infection was cleared, researchers quantitatively observed an above average amount of cytotoxic T-cells accompanying persistent immune activation. T cells, like B cells, are lymphocytes, which make up the adaptive immune system, have a memory capacity that allows specific recognition of viral antigens from previous encounters to prevent future infection. In order to measure the expansion of lymphocytes in the recovering patients blood in the presence of EBOV, researchers incubated samples of patients blood with isolated Ebola proteins. After lymphocytes were allowed to bind to Ebola virus peptides, the cells were stained and analyzed for antigen-experienced T cell activation. The results of the assay indicate significantly increased CD8+ T cell activation in both the main population and effector memory population, which had previously come in contact with EBOV. CD4+ T cells also showed slightly increased T-cell activation. Levels of plasmablast B cells, capable of producing large quantities of infection fighting antibodies, were also slightly increased, and Ebola virus specific neutralizing antibodies were detected.

A comparison of T cell stimulation upon incubation with Ebola antigens

Percentage of single and double cytokine
producing T cells stimulated by different
Ebola virus antigens

This increase in CD8+ T cells, which initiate cell death and produce cytokines upon interacting infected cells, was identified to be among T cells expressing tumor necrosis factor alpha (TNFα), Interferon-γ (IFNγ). TNFα expressing T cells showed the largest increase in production and were associated with stimulation from Ebola GP1. A smaller proportion of T cells expressing both TNFα and IFNγ were stimulated by an Ebola peptides SP and GP2. This analysis was carried out Intracellular Cytokine Staining and ELIspot technique, which monitor cellular immune response through cytokine induction. The amount of IFNγ produced by T cells with antigen experience was considered modest by researchers, and was attributed mainly to the considerably late analysis of blood serum at day 37 and 46 post infection.

It is important to note that this study, while entirely specific to one individual patient who survived Ebola virus disease, provides important initial insights in to a previously uncharacterized immune response to Ebola virus. The main findings of the study suggest that the adaptive immune system was stimulated by initial Ebola infection, and that memory B cells, CD4+ T cells, and CD8+ cells participated in an activate immune response in the presence of Ebola peptides. Specifically, CD8+ T cells producing TNFα were the largest subset of lymphocytes produced in the convalescing patient upon reintroduction to viral antigens. While antigen specific immunity may be key to surviving Ebola virus infection, this study does little to illuminate how the immune system responds to initial infection, as it studies blood of a patient 37 and 46 days after the onset of the first symptoms of EVD. Similarly, many of the initial defensive anti-viral functions of the immune cells may not have presented for observation in this study because of the time frame. However, it is possible that the presence and activation potential of T-cell found in this patient’s blood indicate the continued presence of virus hidden in the body. The main criticism of this paper is that it is often contradicted by other small sample size studies of the immune systems of patients who cleared Ebola virus. The researchers recognize the incongruences and attribute them to the lack of scientific information available due to outbreak sizes and resources. Further investigation in to the pattern of adaptive immune behavior both to initial Ebola virus infection and in patients who have cleared the infection is necessary.

Source Paper:

Dahlke C, et al. Comprehensive Characterization of Cellular Immune Responses Following Ebola Virus Infection. J Infect Dis. (2016)doi: 10.1093/infdis/jiw508

Other Resources:

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Figures taken directly from source paper.