The ability of antigen-presenting cells to sample distinct intracellular compartments is crucial for microbe detection. Major histocompatibility complex class I and class II molecules sample the cytosol or the late endocytic compartment, allowing detection of microbial peptide antigens that arise in distinct intracellular compartments. In contrast, CD1a and CD1b molecules mediate the presentation of lipid and glycolipid antigens and differentially sample early recycling endosomes or late endocytic compartments, respectively, that contain distinct sets of lipid antigens. Here, we show that, unlike the other CD1 isoforms or major histocompatibility complex molecules that each sample restricted only intracellular compartments, CD1c is remarkable in that it distributes broadly throughout the endocytic system and is expressed in both recycling endosomes and late endocytic compartments. Further, in contrast to CD1b, which requires an acidic environment to function, antigen presentation by CD1c was able to overcome dependence on vesicular acidification. Because CD1c is expressed on essential antigen-presenting cells, such as epidermal Langerhans cells (in the absence of CD1b), or on B cells (without CD1a or -b), we suggest that CD1c molecules allow a comprehensive survey for lipid antigens throughout the endocytic system even in the absence of other CD1 isoforms
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