Prevalence and protein specificity of human antibodies to Inkoo virus infection

Inkoo virus (INKV), a member of the California serogroup orthobunyaviruses, is circulating widely in northern Europe. Although the virus was discovered over 40 years ago, the disease associations and immune responses in human infection are poorly characterized. We first developed an immunofluorescence assay (IFA) for the detection of INKV antibodies in humans, and then we studied a panel of 1,292 sera in patients with a febrile illness in Finland. We found four acute (immunoglobulin M [IgM] positive) INKV infections and an IgG seroprevalence of 51.3%. The data indicate that the infection has become more common than it was in the 1960s, especially in southern Finland. Two distinct IgG IFA fluorescence patterns were observed: a granular pattern in sera from patients with acute INKV infection and a diffuse pattern in those with long-standing immunity. Further analysis with a panel of INKV-positive sera (n = 18; verified by neutralization assay) of protein-specific responses, using immunoprecipitation and IFA based on baculovirus-expressed INK N, Gn, and Gc proteins, demonstrated a strong IgG response predominantly towards N protein in the acute phase. In contrast, in patients with long-standing immunity, the Gc response was more prominent and the N response was weaker. In conclusion, a diagnostic IgG IFA pattern distinguishing between acute infection and long-standing immunity was observed. N protein seems to be the optimal antigen for the serodiagnosis of acute infection, and the Gc protein could be appropriate for the serosurveillance of INKV.