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Time to AIDS in HIV-2 infection is approximately twice as long compared to in HIV-1 infection. Despite reduced viremia, HIV-2-infected individuals display signs of chronic immune activation. In HIV-1-infected individuals, B-cell hyperactivation is driven by continuous antigen exposure. However, the contribution of viremia to B-cell perturbations in HIV-2-infected individuals remains largely unexplored. Here, we used polychromatic flow cytometry, consensus hierarchical clustering and pseudotime trajectory inference to characterize B-cells in HIV-1- or HIV-2-infected and in HIV seronegative individuals. We observed increased frequencies of clusters containing hyperactivated T-bethighCD95highCD27int and proliferating T-bet+CD95highCD27+CD71+ memory B-cells in viremic HIV-1 (p < 0.001 and p < 0.001, respectively), viremic HIV-2 (p < 0.001 and p = 0.014, respectively) and in treatment-naïve aviremic HIV-2 (p = 0.004 and p = 0.020, respectively)-infected individuals, compared to seronegative individuals. In contrast, these expansions were not observed in successfully treated HIV-1-infected individuals. Finally, pseudotime trajectory inference showed that T-bet-expressing hyperactivated and proliferating memory B-cell populations were located at the terminal end of two trajectories, in both HIV-1 and HIV-2 infections. As the treatment-naïve aviremic HIV-2-infected individuals, but not the successfully ART-treated HIV-1-infected individuals, showed B-cell perturbations, our data suggest that aviremic HIV-2-infected individuals would also benefit from antiretroviral treatment.

Objective: Human hantavirus infections can cause haemorrhagic fever with renal syndrome (HFRS). The pathogenic mechanisms arenot fully understood, nor if they affect the humoral immune system. The objective of this study was to investigate humoral immune responses to hantavirus infection and to correlate them to the typical features of HFRS: thrombocytopenia and transient kidney dysfunction.

Methods: We performed a comprehensive characterisation of longitudinal antiviral B-cell responses of 26 hantavirus patients and combined this with paired clinical data. In addition, we measured extracellular adenosine triphosphate (ATP)and its breakdown products in circulation and performed in vitro stimulations to address its effect on B cells.

Results: We found that thrombocytopenia was correlated to an elevated frequency of plasmablasts in circulation. In contrast, kidney dysfunction was indicative of an accumulation of CD27-IgD- B cells and CD27/low plasmablasts. Finally, we provide evidence that high levels of extracellular ATP and matrix metalloproteinase 8 can contribute to shedding of CD27 during human hantavirus infection.

Conclusion:  Our findings demonstrate that thrombocytopenia and kidneydysfunction associate with distinctly different effects on the humoral immune system. Moreover, hantavirus-infectedindividuals have significantly elevated levels of extracellular ATP incirculation.

Hantaviruses are zoonotic RNA viruses that cause severe acute disease in humans. Infected individuals have strong inflammatory responses that likely cause immunopathology. Here, we studied the response of mucosal-associated invariant T (MAIT) cells in peripheral blood of individuals with hemorrhagic fever with renal syndrome (HFRS) caused by Puumala orthohantavirus, a hantavirus endemic in Europe. We show that MAIT cell levels decrease in the blood during HFRS and that residual MAIT cells are highly activated. This activation correlates with HFRS severity markers. In vitro activation of MAIT cells by hantavirus-exposed antigen-presenting cells is dependent on type I interferons (IFNs) and independent of interleukin-18 (IL-18). These findings highlight the role of type I IFNs in virus-driven MAIT cell activation and suggest a potential role of MAIT cells in the disease pathogenesis of viral infections.

Germinal centers (GC) are sites for extensive B cell proliferation and homeostasis is maintained by programmed cell death. The complement regulatory protein Decay Accelerating Factor (DAF) blocks complement deposition on host cells and therefore also phagocytosis of cells. Here, we show that B cells downregulate DAF upon BCR engagement and that T cell-dependent stimuli preferentially led to activation of DAF(lo) B cells. Consistent with this, a majority of light and dark zone GC B cells were DAF(lo) and susceptible to complement-dependent phagocytosis, as compared with DAF(hi) GC B cells. We could also show that the DAF(hi) GC B cell subset had increased expression of the plasma cell marker Blimp-1. DAF expression was also modulated during B cell hematopoiesis in the human bone marrow. Collectively, our results reveal a novel role of DAF to pre-prime activated human B cells for phagocytosis prior to apoptosis.

Hantavirus infections are rodent-borne viruses causing potential lethal infections in humans. Different hantaviruses exist worldwide, reporting a fatality rate of up to 40%. The Puumala hantavirus (PUUV) is endemic in northern Sweden. This hantavirus strain has a relatively low fatality rate but the hospitalisation rate is high. No vaccine to the virus and no treatment for the disease exist. Despite differences in severity, the immune-mediated pathogenesis of Puumala virus infection is similar to that of highly lethal strains of hantavirus. It is currently unknown how the humoral immune system is affected during hantavirus infection.

The aim of this study is to characterise how the humoral immune response is affected during Puumala virus infection. A large number of longitudinal patient samples have been collected. Here, we demonstrate the longitudinal kinetics of the B cell response during Puumala virus infection and show that there is a change in B cell populations during the course of the disease. Furthermore we show that B cells carry known hantavirus receptors. This suggests that Puumala virus may directly infect B cells. Infection of the B cells could affect their function and or phenotype explaining a different immune response. Importantly, in approximately 10–15% of Puumala infected patients we could detect antibodies that could neutralise other hantaviruses in vitro. Samples from these patients could help to generate a monoclonal antibody treatment potentially treating diseases caused by several hantavirus.