Ebola virus is a highly virulent emerging pathogen and a causative agent of viral hemorrhagic fever.¹ Studies of the pathogenesis of Ebola virus infection in humans have indicated that recovery is largely dependent on the development of an immune response.^1-3 To investigate the persistence of immune response in humans, we examined levels of cytokine expression after in vitro whole-blood stimulation in persons 12 years after infection with the Gulu strain of Sudan Ebola virus (SUDV-gul).

Ebolavirus is a highly lethal pathogen, causing a severe hemorrhagic disease with a high fatality rate. To better understand immune correlates of protection by virus specific IgG, we investigated the evolution of the Fcγ receptors (FcγRs)-activating capabilities of antiviral IgG in serum samples of long recovered survivors. To this end, longitudinal serum samples from survivors of Sudan ebolavirus (SUDV) infection, studied over years, were examined for the presence of Ebola-GP specific IgG subclasses, and for their binding to FcγRs. We developed a cell-based reporter system to quantitate pathogen-specific antibody binding to FcγRIIIA, FcγRIIA, FcγRIIB and FcγRI. With this system, we demonstrate that anti-GP-specific stimulation of the FcγRI reporter by survivors’ sera was substantially high one year after acute infection, with a slight reduction in activity over a decade post infection. We further demonstrate that GP-specific IgG1 is by far the seroprevalent subclass that retained and even enhanced its presence in the sera, over ten years post infection; the prevalence of other GP-specific IgG subclasses was considerably reduced over time. In accordance, GP-specific FcγRI reporter response and GP-specific total IgG1 subclass correlated in the studied group of Ebola survivors. These observations are important for further informing Ebola vaccine and therapeutic development.

Recovery from ebolavirus infection in humans is associated with the development of both cell-mediated and humoral immune responses. According to recent studies, individuals that did not survive infection with ebolaviruses appear to have lacked a robust adaptive immune response and the expression of several early innate response markers. However, a comprehensive protective immune profile has yet to be described. Here, we examine cellular memory immune responses among survivors of two separate Ebolavirus outbreaks (EVDs) due to Sudan virus (SUDV) infection in Uganda—Gulu 2000–2001 and Kibaale 2012. Freshly collected blood samples were stimulated with inactivated SUDV, as well as with recombinant SUDV or Ebola virus (EBOV) GP (GP_1–649). In addition, ELISA and plaque reduction neutralization assays were performed to determine anti-SUDV IgG titers and neutralization capacity. Cytokine expression was measured in whole blood cultures in response to SUDV and SUDV GP stimulation in both survivor pools, demonstrating recall responses that indicate immune memory. Cytokine responses between groups were similar but had distinct differences. Neutralizing, SUDV-specific IgG activity against irradiated SUDV and SUDV recombinant proteins were detected in both survivor cohorts. Furthermore, humoral and cell-mediated crossreactivity to EBOV and EBOV recombinant GP_1–649 was observed in both cohorts. In conclusion, immune responses in both groups of survivors demonstrate persistent recognition of relevant antigens, albeit larger cohorts are required in order to reach greater statistical significance. The differing cytokine responses between Gulu and Kibaale outbreak survivors suggests that each outbreak may not yield identical memory responses and promotes the merits of studying the immune responses among outbreaks of the same virus. Finally, our demonstration of cross-reactive immune recognition suggests that there is potential for developing cross-protective vaccines for ebolaviruses.

Robust humoral and cellular immunity are critical for survival in humans during an ebolavirus infection. However, the interplay between these two arms of immunity is poorly understood. To address this, we examined residual immune responses in survivors of the Sudan virus (SUDV) outbreak in Gulu, Uganda (2000–2001). Cytokine and chemokine expression levels in SUDV stimulated whole blood cultures were assessed by multiplex ELISA and flow cytometry. Antibody and corresponding neutralization titers were also determined. Flow cytometry and multiplex ELISA results demonstrated significantly higher levels of cytokine and chemokine responses in survivors with serological neutralizing activity. This correspondence was not detected in survivors with serum reactivity to SUDV but without neutralization activity. This previously undefined relationship between memory CD4 T cell responses and serological neutralizing capacity in SUDV survivors is key for understanding long lasting immunity in survivors of filovirus infections.

Crimean–Congo hemorrhagic fever virus (CCHFV) is an arbovirus belonging to the Nairoviridae family. The virus, as well as ticks of the Hyalomma genus, which serve as its reservoir host, are found in parts of Africa, western Asia, and southern Europe. Following sporadic zoonotic or human-to-human transmission, infection is characterized by fever, fatigue, vomiting, diarrhea, and in fatal cases, often hemorrhagic symptoms. There are currently no vaccines or targeted treatments available against CCHFV, leading the WHO to declare it a Blueprint priority pathogen in 2017. Here, we report the isolation and characterization of a panel of human monoclonal antibodies (mAbs) against CCHFV. Using a novel soluble Gn/Gc sorting antigen, we were able to isolate memory B cells specific for CCHFV from four convalescent donors. From each patient sample, we were able to derive several potently neutralizing antibodies with IC₅₀ in the nanomolar range as determined by neutralization of CCHFV virus-like particles. Neutralization by candidate hits was also confirmed using authentic CCHFV. We further show that several of the most potently neutralizing mAbs possess a breadth of neutralization spanning three clades of CCHFV strains. These broadly neutralizing mAbs are currently being tested in a mouse model of CCHFV infection, with preliminary results indicating that they have protective potential.