Ebola virus (EBOV) disease can result in a range of symptoms anywhere from virtually asymptomatic to severe hemorrhagic fever during acute infection. Additionally, spans of asymptomatic persistence in recovering survivors is possible, during which transmission of the virus may occur. In acute infection, substantial cytokine storm and bystander lymphocyte apoptosis take place, resulting in uncontrolled, systemic inflammation in affected individuals. Recently, studies have demonstrated the presence of EBOV proteins VP40, glycoprotein (GP), and nucleoprotein (NP) packaged into extracellular vesicles (EVs) during infection. EVs containing EBOV proteins have been shown to induce apoptosis in recipient immune cells, as well as contain pro-inflammatory cytokines. In this manuscript, we review the current field of knowledge on EBOV EVs including the mechanisms of their biogenesis, their cargo and their effects in recipient cells. Furthermore, we discuss some of the effects that may be induced by EBOV EVs that have not yet been characterized and highlight the remaining questions and future directions.

Induction of lymphopenia has been exploited therapeutically to improve immune responses to cancer therapies and vaccinations. Whereas IL-15 has well-established roles in stimulating lymphocyte responses after lymphodepletion, the mechanisms regulating these IL-15 responses are unclear. We report that cell surface IL-15 expression is upregulated during lymphopenia induced by total body irradiation (TBI), cyclophosphamide, or Thy1 Ab-mediated T cell depletion, as well as in RAG^−/− mice; interestingly, the cellular profile of surface IL-15 expression is distinct in each model. In contrast, soluble IL-15 (sIL-15) complexes are upregulated only after TBI or αThy1 Ab. Analysis of cell-specific IL-15Rα conditional knockout mice revealed that macrophages and dendritic cells are important sources of sIL-15 complexes after TBI but provide minimal contribution in response to Thy1 Ab treatment. Unlike with TBI, induction of sIL-15 complexes by αThy1 Ab is sustained and only partially dependent on type I IFNs. The stimulator of IFN genes pathway was discovered to be a potent inducer of sIL-15 complexes and was required for optimal production of sIL-15 complexes in response to Ab-mediated T cell depletion and TBI, suggesting products of cell death drive production of sIL-15 complexes after lymphodepletion. Lastly, we provide evidence that IL-15 induced by inflammatory signals in response to lymphodepletion drives lymphocyte responses, as memory CD8 T cells proliferated in an IL-15–dependent manner. Overall, these studies demonstrate that the form in which IL-15 is expressed, its kinetics and cellular sources, and the inflammatory signals involved are differentially dictated by the manner in which lymphopenia is induced.

Transcriptomic analysis of leukocytes from a severe Ebola virus disease patient revealed transitions driven by viral clearance, coagulopathy, and multiorgan dysfunction.

Until recently, immune responses in filovirus survivors remained poorly understood. Early studies revealed IgM and IgG responses to infection with various filoviruses, but recent outbreaks have greatly expanded our understanding of filovirus immune responses. Immune responses in survivors of Ebola virus (EBOV) and Sudan virus (SUDV) infections have provided the most insight, with T cell responses as well as detailed antibody responses having been characterized. Immune responses to Marburg virus (MARV), however, remain almost entirely uncharacterized. We report that immune responses in MARV survivors share characteristics with EBOV and SUDV infections but have some distinct differences. MARV survivors developed multivariate CD4+ T cell responses but limited CD8+ T cell responses, more in keeping with SUDV survivors than EBOV survivors. In stark contrast to SUDV survivors, rare neutralizing antibody responses in MARV survivors diminished rapidly after the outbreak. These results warrant serious consideration for any vaccine or therapeutic that seeks to be broadly protective, as different filoviruses may require different immune responses to achieve immunity.

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.

Background: Melioidosis is endemic in Southeast Asia and Northern Australia and is caused by the Gram-negative, facultative intracellular pathogen Burkholderia pseudomallei. Diagnosis of melioidosis is often difficult because of the protean clinical presentation of the disease, and it may mimic other diseases, such as tuberculosis. There are many different strains of B. pseudomallei that have been isolated from patients with melioidosis, but it was not clear if they could cause a similar disease in a chronic BALB/c murine model of melioidosis. Hence, we wanted to examine chronically infected mice exposed to different strains of B. pseudomallei to determine if there were differences in the host immune response to the pathogen. Results: We identified common host immune responses exhibited in chronically infected BALB/c mice, although there was some heterogeneity in the host response in chronically infected mice after exposure to different strains of B. pseudomallei. They all displayed pyogranulomatous lesions in their spleens with a large influx of monocytes/macrophages, NK cells, and neutrophils identified by flow cytometry. Sera from chronically infected mice by ELISA exhibited elevated IgG titers to the pathogen, and we detected by Luminex micro-bead array technology a significant increase in the expression of inflammatory cytokines/chemokines, such as IFN-γ, IL-1α, IL-1β, KC, and MIG. By immunohistochemical and in situ RNA hybridization analysis we found that the increased expression of proinflammatory cytokines (IL-1α, IL-1β, TNF-α, IFN-γ) was confined primarily to the area with the pathogen within pyogranulomatous lesions. We also found that cultured splenocytes from chronically infected mice could express IFN-γ, TNF-α, and MIP-1α ex vivo without the need for additional exogenous stimulation. In addition by flow cytometry, we detected significant amounts of intracellular expression of TNF-α and IFN-γ without a protein transport blocker in monocytes/macrophages, NK cells, and neutrophils but not in CD4⁺ or CD8⁺ T cells in splenocytes from chronically infected mice. Conclusion: Taken together the common features we have identified in chronically infected mice when 10 different human clinical strains of B. pseudomallei were examined could serve as biomarkers when evaluating potential therapeutic agents in mice for the treatment of chronic melioidosis in humans.