When the body’s own cells turn into ticking time bombs, as in cases of viral infection or cancerous transformation, a mechanism known as ‘cross-presentation’ enables the immune system’s dendritic cells (DCs) to sound the alarm.
Figure 1: Compared to untreated dendritic cells (top), cells treated with an HSP90 inhibitor (radicicol) (bottom) tend to retain a larger percentage of the antigen ovalbumin (red; left) within membrane bound endosomes (green; middle). Endosomes are labeled with a stain that marks the membrane protein PKH67. Copyright : 2012 the National Academy of Sciences
“Dendritic cells first internalize cancerous or virus-infected cells through a mechanism called phagocytosis, and then process cellular antigens into short peptides,” explains Heiichiro Udono of the RIKEN Center for Allergy and Immunology in Yokohama. DCs subsequently present these fragments to killer T cells, which seek out and destroy other affected cells. Phagocytosed molecules travel within sealed membrane bubbles called endosomes, and new work from Udono and his colleagues has revealed insights into how these antigens are released into the cytosol prior to cross-presentation1.
Udono’s team focused on heat-shock protein 90 (HSP90), a molecule that previous studies have linked to cross-presentation. HSP90 comes in two forms, á and â, which perform overlapping roles. Mice need at least one of these proteins to live. Udono and colleagues succeeded in generating healthy mice that exclusively lack HSP90á. They found that, although HSP90â appears to make some contribution, the loss of HSP90á had a striking effect on antigen processing. DCs isolated from these mice showed defects in their capacity for cross-presentation, and failed to activate killer T cells efficiently following exposure to ovalbumin, a model antigen.
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