Trafficking of surface-linked and encapsulated liposomal antigens in macrophages: An immunocytochemical study

Liposomal antigens are potent adjuvants of humoral and cell-mediated immuni ty. Although this property requires as an essential condition a physical as sociation between the antigen and the phospholipid vehicle, the nature of t he association, i.e., encapsulation or surface linkage, markedly influences the outcome of the elicited response. Available evidence suggests that mac rophages are involved in this fine tuning of the immune response in a manne r that is not yet clearly established. It is postulated that this might be related to their capacity to interact differently with surface-linked and e ncapsulated formulations. Using conalbumin as a model antigen, we address t he question by analyzing the movements of encapsulated and surface-linked a ntigen as well as those of MHC-II molecules in macrophages in a pulse-chase immunoelectron microscopic study carried out over a 24-hr period. The anti gen was followed using a polyclonal serum specifically raised against fragm ented conalbumin (fCA) that allows the detection of processed antigen and o f some MHC-peptide complexes. The results indicate that, in macrophages, th e two liposomal formulations affect macrophage morphology in distinct ways and circulate through the various subcellular compartments with different k inetics. On the basis of the overall results, we conclude that surface-link ed antigen gains access less readily to the endogenous presentation pathway than encapsulated antigen but can favor a more sustained activation of the immune system through its production of exosome-like structures and its mo re thorough utilization of the MHC-II pathway.