Innate lymphoid cells are activated and their levels correlate with viral load in patients with Puumala hantavirus caused hemorrhagic fever with renal syndrome
Authors: García Marina , García Anna Carrasco , Tauriainen Johanna , Maleki Kimia , Vaheri Antti , Mäkelä Satu , Mustonen Jukka , Smed-Sörensen Anna , Strandin Tomas , Mjösberg Jenny , Klingström Jonas 
Published: May 2022

ABSTRACTBackgroundInnate lymphoid cells (ILCs) are involved in immunity and homeostasis but, except for natural killer (NK) cells, their role in human viral infections is not well known. Puumala virus (PUUV) is a hantavirus that causes the acute zoonotic disease hemorrhagic fever with renal syndrome (HFRS). HFRS is characterized by strong systemic inflammation and NK cells are highly activated in HFRS, suggesting that also other ILCs might be responding to infection.MethodsHere we phenotypically analyzed peripheral ILCs in acute and convalescent PUUV-infected HFRS patients. Additionally, plasma levels of soluble factors and viral load were analyzed.FindingsOverall, the frequencies of NK cells and naïve ILCs were reduced while the frequency of ILC2, in particular the ILC2-lineage committed c-Kitlo ILC2 subset, was increased during acute HFRS. Interestingly, we observed a negative correlation between viral load and frequencies of both NK and non-NK ILCs in acute HFRS. Phenotypically, ILCs displayed an activated profile with increased proliferation, and showed altered expression of several homing markers during acute HFRS. In line with the observation of activated ILCs, plasma levels of inflammatory proteins, including the ILC-associated cytokines interleukin (IL)-13, IL-23, IL-25, IL-33, and thymic stromal lymphopoietin (TSLP), were elevated during acute HFRS.InterpretationThese findings indicate a general involvement of ILCs in response to human hantavirus infection. Further, this constitutes the first comprehensive study of ILCs in a hantavirus-caused disease, aiding in further understanding the role of these cells in disease pathogenesis and in human viral infections in general.FundingA full list of funding bodies that contributed to this study can be found in the Acknowledgements section.RESEARCH IN CONTEXTEvidence before this studyInnate lymphoid cells (ILCs) include a broad range of innate cell subsets involved, among other functions, in early response to infections. Natural killer (NK) cells have been broadly characterized in human viral infections, but much less is known in such context about other more recently discovered ILCs. Puumala virus is one of the causative agents of hemorrhagic fever with renal syndrome (HFRS), an acute zoonotic disease characterized by systemic inflammation. No current treatment or vaccines are available to date for hantavirus-caused diseases and pathogenesis is still not fully understood. A PubMed search up until April 2022 using a combination of the terms virus, Puumala virus, hantavirus, HFRS, ILCs, and NK cells shows that while studies have been performed characterizing NK cells in HFRS, no studies are available on the rest of ILCs in hantavirus-caused diseases, which in addition have been studied in only a handful of human viral infections.Added value of this studyIn this study we thoroughly characterized the ILC landscape in circulation and its milieu in acute and convalescent Puumala-infected HFRS patients. We found that the frequency of NK cells and other ILC subsets was altered in the acute HFRS patients as compared to convalescent HFRS patients and control individuals, with a decrease in NK cells and naïve ILCs, and an increase in ILC2. In particular the ILC2-lineage committed c-Kitlo ILC2 subset was found increased. Total ILCs showed increased levels of activation and proliferation, and signs of altered migration patterns in acute HFRS patients. The finding of elevated levels of several soluble inflammatory proteins associated with ILCs in acute HFRS patients further suggests an implication of ILCs in HFRS. Moreover, an association between the frequency of NK cells and non-NK ILCs and plasma viral load suggest a possible connection between viremia and the activity of these cell types during the course of disease.Implications of all the available evidenceOur research shows that all circulating ILCs, including non-NK ILCs, are activated, proliferating, and correlate with viral load in acute HFRS patients, indicating a general involvement in human hantavirus infection and disease. Being the first comprehensive study on ILCs in hantavirus infections, further research will give relevant insight into the roles of ILCs in disease pathogenesis and protection.