Topological journey of parasite-derived antigens for presentation by MHC class I molecules

Within cells of their host, many bacteria and parasites inhabit specialized compartments, such as a modified phagosome for Mycobacterium tuberculosis or a parasitophorous vacuole for Toxoplasma gondii These locations could exclude microbial material from entry into the MHC class I surveillance pathway Remarkably, however, under these circumstances, cells can still signal the presence of invading pathogens to circulating CD8(+) T cells, which typically play a key role in protection against such intracellular organisms Here, we review MHC I presentation pathways in various contexts, ranging from model antigens in non-infectious settings to pathogen-infected cells We suggest that presentation of intracellular pathogens can be described as not just one but several distinct pathways, perhaps because diverse pathogens have evolved different strategies to interact with host cells