For a long time, the cellular context in which some exogenous antigens, normally processed in endocytic organelles and presented on MHC class II molecules, were presented on MHC class I was unknown. The working model that we presented in our publication proposes a novel cellular framework that explains this phenomenon. It is also interesting to realize that two other groups, Amigorena at the Curie Institute in Paris, and Cresswell at Yale University in New Haven simultaneously published results leading to the same conclusion. The publication of three stories certainly provided solid bases for a wider acceptance of this novel pathway.

Our work provides additional information on membrane trafficking within antigen-presenting cells, and a working model that presents a cellular framework, within which antigens present in phagosomes from microorganisms, for example, can be processed and presented on both MHC class I and class II molecules. This could have an interesting potential for the development of novel methods of immunization for vaccine design.

For a long time, it was believed that exogenous antigens are processed in endocytic organelles and presented at the surface of antigen-presenting cells on MHC class II molecules. This pathway is responsible for the stimulation of a subset of T cells (CD4+) that plays key roles in helping to build a sustained immune response against infectious agents. On the other hand, endogenous peptides were processed by the proteasome in the cytoplasm, translocated in the endoplasmic reticulum (ER), where they were loaded on MHC class I molecules for presentation at the cell surface through the biosynthetic pathway. This pathway is important for the stimulation of a CD8+ T-cell immune response (cytotoxic) against virally infected cells or tumor cells, for examples. The finding, over a decade ago, that microbial proteins processed in phagosomes (exogenous proteins) could trigger a CD8+ response led to the proposal that a "cross-presentation" process had to occur to allow the presentation of exogenous peptides on the MHC class I machinery present in the cytoplasm and the ER, normally used for endogenous proteins. Our finding that phagocytosis proceeds by the recruitment of ER membrane at the cell surface, a process that we called ER-mediated phagocytosis, provides a possible mechanism by which phagosomes could take advantage of the inherited ER machines to process and load exogenous peptides on MHC class I molecules.

The use of a proteomics approach to identify novel proteins present on purified phagosomes revealed that numerous proteins of the ER were present on this organelle. This observation, likely then to be related to a contamination of our preparations by ER, led to the totally unexpected finding that the ER was fusing with the plasma membrane during phagocytosis in macrophages to provide some of its membrane to the formation of nascent phagosomes. For cell biologists, the idea of the endoplasmic reticulum fusing with the plasma membrane was, somehow, heretical. A function for ER elements on phagosomes remained to be investigated and demonstrated. Despite our very rudimentary knowledge of immunology, we realized that perhaps we were given elements to answer the question of how exogenous antigens can be presented on MHC class I molecules.