Antigen detection in vivo after immunization with different presentation forms of rabies virus antigen: involvement of marginal metallophilic macrophages in the uptake of immune-stimulating complexes.

Several mechanisms have been postulated to explain the relatively high immunogenicity of antigens presented in immune-stimulating complexes (iscom). Their potency can in part be explained by the specific targeting of these structures to cells presenting antigens to the immune system. However, until now no method for the subcellular detection of iscom in situ was available. In the present study, a novel, fast and simple method for the detection of iscoms in situ is demonstrated. By making use of the lipophilic fluorescent carbocyanine dyes, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) and 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO), rabies virus antigen and iscom prepared with this antigen were visualized with fluorescence microscopy. The labeled antigen and iscoms were observed in macrophages of spleen and liver of mice within 1-2 h after intravenous administration. When administered intramuscularly or in the footpad, uptake in macrophages of draining lymph nodes could be demonstrated. In the spleen, labeled inactivated virus antigen localized preferentially in the marginal zone macrophages and to a lesser extent in the red pulp macrophages. In contrast, antigen presented in iscom was taken up mainly by the marginal metallophilic macrophages and to a much lesser extend by marginal zone macrophages or follicular-dendritic and -B cells. This method enables the detection of iscom and membrane viruses and allows the analysis of their relation to antigen-presenting cells in situ. Here, we demonstrate that iscom containing rabies virus antigen are taken up by a subset of macrophages in the spleen distinct from those that take up inactivated rabies virus antigen not presented in iscom, thereby possibly explaining the observed difference in immunogenicity of these antigen preparations. Furthermore, we show a lower efficiency on the induction of humoral and cellular responses after intravenous immunization for both types of antigen when compared with subcutaneous immunization.