Antigen-specific increases in the number of splenocytes expressing MHC class II molecules following restimulation with antigen in various physical forms.

To understand how a presentation system for antigens initiates an immune response and why it has a strong adjuvant activity, a number of parameters need to be analysed. In this study the frequency of spleen cells expressing MHC class II (Ia antigen) was determined after immunization of mice and restimulation of their spleen cells, in vitro, with influenza virus envelope proteins in different physical forms, namely iscoms, micelles and virus particles. All three forms of the antigen stimulated, in an antigen-specific manner, an increased proportion of spleen cells expressing MHC class II in the restimulation experiments. The induction of increased MHC class II expression was at least partly dependent on antigen-specific induction of IFN-gamma since an antibody to IFN-gamma partly inhibited the increase of MHC class II+ cells induced by iscom or by Concanavalin A. The iscom-borne antigens were superior to micelles to prime the immune response in vitro, indicating a capacity to induce memory cells. This primed immune response was readily recalled in vitro, as measured by IFN-gamma production and an increased number of MHC class II positive cells.