West Nile virus (WNV) causes asymptomatic infection in most humans, but for undefined reasons, approximately 20% of immunocompetent individuals develop West Nile fever, a potentially debilitating febrile illness, and approximately 1% develop neuroinvasive disease syndromes. Notably, since its emergence in 1999, WNV has become the leading cause of epidemic viral encephalitis in North America. We hypothesized that CD4 + Tregs might be differentially regulated in subjects with symptomatic compared with those with asymptomatic WNV infection. Here, we show that in 32 blood donors with acute WNV infection, Tregs expanded significantly in the 3 months after index (RNA + ) donations in all subjects. Symptomatic donors exhibited lower Treg frequencies from 2 weeks through 1 year after index donation yet did not show differences in systemic T cell or generalized inflammatory responses. In parallel prospective experimental studies, symptomatic WNV-infected mice also developed lower Treg frequencies compared with asymptomatic mice at 2 weeks after infection. Moreover, Treg-deficient mice developed lethal WNV infection at a higher rate than controls. Together, these results suggest that higher levels of peripheral Tregs after infection protect against severe WNV disease in immunocompetent animals and humans.

The results of this study are helpful in identifying donors at risk for adverse reactions and in understanding the contributors to reactions. Donor blood volume was an unexpectedly strong predictor of reaction. Potential interventions to reduce the frequency of reaction are discussed.

IgM and IgG develop rapidly after viremia and before RNA levels become undetectable, which occurred a mean of 13.2 days after the index donation among donors in this study. WNV RNA detection by replicate TMA rarely persists for >40 days after the index donation and is accompanied by WNV-specific neutralizing antibody, consistent with an absence of WNV transmission via transfusion of seropositive blood components.

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