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B cells provide one of the key mechanisms of immunological memory, which is theproduction of neutralising antibodies. How B cells respond to infections and vaccinationgives clues to how the development of the immunological memory is facilitated, and canthus lead to a deeper understanding of why the immune system sometimesmalfunctions. This thesis focuses on the human B cell responses in three differentsettings: Acute viral infection, mechanisms involved in germinal centre responses, andvaccination upon interrupted B cell depletion therapy in patients with multiple sclerosis(MS). We have found that during acute Puumala-orthohantavirus (PUUV) infection, Bcells activate on a large scale and derive a phenotype similar to previous observations inautoimmune diseases and chronic infections. Patients with PUUV infection also haddecreased expression of the complement regulatory protein Decay-Accelerating Factor(DAF) at an early stage in the disease. Here, we hypothesised that this might be a resultof a robust B cell response, and therefore we continued to assess B cells at the peripheralsites of their maturation. We found that B cells downregulated the complementinhibitory protein during the germinal centre reaction, which also primed the cells forphagocytosis. This finding shed light to the mechanisms that control B cell homeostasis.Finally, we assessed the B cell responses towards vaccination in patients with MS afterinterruption of their B cell depletion therapy. Here we showed that the patients yieldedexpansion of vaccination-specific memory B cells. However, these memory B cells didnot comprise expansion of DAFlo cells, in contrast to the non-MS control individuals.We speculated that the B cell depletion might have an impact on the formation of B cellmemory after interrupted treatment. Taken together, this thesis contributes to theoverall understanding of the life cycle of B cells, in the context of infection, vaccination,and homeostasis.

Inflammation is driven either by infection with pathogens or sterile stimuli, such as tissue damage or autoimmune diseases. Upon tissue damage, ATP is released passively from the dead or compromised cells. During stress, ATP can be secreted from the cells. Extracellular ATP (eATP) acts as an endogenous danger signal. An increase in eATP is sensed by cell surface purinergic receptors and regulates the onset and resolution of inflammation. Extracellular ATP is an important inflammatory mediator during sterile inflammation. On the other hand, the role of eATP is poorly studied during infection, both bacterial and viral. In this thesis, I present the molecular mechanisms underlying ATP secretion during bacterial infections and the role of eATP in human hantaviral infections.

During infection with certain enteropathogenic Gram-negative bacteria, intestinal epithelial cells secrete ATP via connexin hemichannels as an alert signal to activate the immune system, which triggers acute inflammation in the gut. However, neither what triggers ATP secretion nor the molecular mechanisms of ATP secretion were known. Pharmacological, genetic, and microscopy-based evidence shows that during invasive bacterial infections, the plasma membrane ruffles act as mechanical immune stimuli and activate the inherently mechanosensitive plasma membrane channel PIEZO1. Mechanically activated PIEZO1 leads to the influx of Ca2+ ions and concurrent ATP secretion. In addition, PIEZO1 also activates protective transcriptional responses. Thus, PIEZO1 acts as a sensor for invasive infection using mechanical stimuli, unlike the so-far-described immune sensors of infection, which all recognize microbial components by chemical interaction.

During human hantavirus infection, the humoral immune responses are poorly studied. Our collaborators found that atypical B cells, which do not have the surface marker CD27, show increased frequency in a cohort of hantavirus-infected patients. CD27 shedding in murine lymphocytes had been previously linked to eATP-dependent activation of a purinergic receptor7. To test whether ATP levels in the circulation of hantavirus-infected patients are elevated, an approach to perform same-day eATP quantifications in human plasma was developed. This assay was used to establish the normal eATP concentration in plasma in a cohort of healthy volunteers and to show that eATP levels are elevated in the acute and convalescent stages of hantavirus infection. Further, the addition of ATP to isolated human B cells recapitulated the observed CD27 shedding via a metallomatrix proteinase-8-dependent (MMP8) mechanism. Together, these projects provide evidence for the importance of eATP in bacterial and viral infectious diseases.