SUMMARYTBEV is a flavivirus highly pathogenic for humans. By transfer of antibodies directed to the TBEV surface glycoprotein E into mice, immune protection against subsequent inoculation with free TBEV particles could be achieved. After natural TBEV infection via the skin, however, cells of the monocyte/ macrophage lineage were recently demonstrated to represent an important source of local virus replication before viraemia occurs. Whether antibodies can protect against virus challenge when contracted in the form of infected cells, however, is still unclear. In the current study, TBEV antibodies protected mice against challenge with either free virus or TBEV-infected macrophages equally well. This observation may be of more general significance.

Antibody-mediated neutralization of viruses has been extensively studied in vitro, but the precise mechanisms that account for antibody-mediated protection against viral infection in vivo still remain largely uncharacterized. The two points under discussion are antibodies conferring sterilizing immunity by neutralizing the virus inoculum or protection against the development of disease without complete inhibition of virus replication. For tick-borne encephalitis virus (TBEV), a flavivirus, transfer of neutralizing antibodies specific for envelope glycoprotein E protected mice from subsequent TBEV challenge. Nevertheless, short-term, low-level virus replication was detected in these mice. Furthermore, mice that were exposed to replicating but not to inactivated virus while passively protected developed active immunity to TBEV rechallenge. Despite the priming of TBEV-specific cytotoxic T cells, adoptive transfer of serum but not of T cells conferred immunity upon naive recipient mice. These transferred sera were not neutralizing and were predominantly specific for NS1, a nonstructural TBEV protein which is expressed in and on infected cells and which is also secreted from these cells. Results of these experiments showed that despite passive protection by neutralizing antibodies, limited virus replication occurs, indicating protection from disease rather than sterilizing immunity. The protective immunity induced by replicating virus is surprisingly not T-cell mediated but is due to antibodies against a nonstructural virus protein absent from the virion.

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