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Hantaviruses can cause two severe human diseases: hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). Hantaviruses are spread to humans mainly through inhalation of infectious virions, secreted from infected rodents. The human diseases are characterized by an increased capillary leakage syndrome. Hantaviruses are known to infect endothelial cells, but they are non-cytopathogenic. The mechanism behind human disease is not well understood, but an overactive immune response is implicated in the pathogenesis. The aim of my thesis has been to investigate parts of innate and adaptive immune responses in Puumala virus-infected patients.

In paper I we found a sex difference in the cytokine profile during acute infection. Females had significantly higher plasma levels of IL-9, FGF-2, GM-CSF and lower levels of IL-8 and IP-10 compared to males. These differences may affect the activation and function of the immune response.

In paper II we studied the phenotype and kinetics of NK cells. We observed that CD56^dim NK cells were elevated during acute infection and that these, predominantly NKG2C+ NK cells, remained elevated for at least two months after symptom debut. Our novel finding of a prolonged NK cell response, implicates that NK cells may possess adaptive immunity features. 

In paper III we observed a vigorous cytotoxic T cell (CTL) response during acute infection, which contracted in parallel with decrease in viral load. The CTL response was not balanced by an increase in regulatory T cells. The T cells expressed inhibitory immunoregulatory receptors, known to dampen intrinsic T cell activity. 

In paper IV, we found that a low IgG response in patients was significantly associated with more severe disease, while the viral load did not affect the outcome. Our findings support the use of passive immunization as a treatment alternative for hantavirus-infected patients.

In conclusion, my thesis contributes to an increased knowledge about the immune response in hantavirus-infected patients. The findings, combined with future studies, will hopefully lead to a better understanding of the pathogenesis and possible treatment alternatives.