says Kiessling. Cell recognition in living systems instead involves binding agents that attach only weakly to any single target receptor, and thus stick to cells only when several receptors are displayed together. These weak "multivalent" interactions cut down on cases of mistaken identity, because if the agent contacts the wrong cell type, it can be easily displaced.
The team got the idea to mimic this process from efforts to transplant pig organs into primates. The surfaces of most mammalian and bacterial cells express large amounts of a carbohydrate, called alpha-Gal in scientific shorthand, while the cells of humans and other higher primates do not. What humans and primates do produce in abundance is an antibody against the carbohydrate, called anti-Gal.
When scientists tried transplanting pig organs into primates, the anti-Gal antibodies bound to the alpha-Gal on the organ's cells, unleashing a potent immune response that caused immediate organ rejection. But true to natural cell recognition, the immune response occurs only when clusters of many alpha-Gal molecules are present for anti-Gal to bind with.
Armed with this knowledge, Kiessling's group modified an agent known to bind tightly to integrin and tethered it to alpha-Gal. When they mixed this molecule with cells displaying high levels of integrin, the agent, by attaching to the receptor, decorated the cells with large amounts of alpha-Gal. In cell cultures containing human serum, the alpha-Gal then elicited the cell-destroying immune reaction.
In cells with low concentrations of integrin, the agent still bound, but the resulting levels of alpha-Gal weren't sufficient to elicit the immune response, and the cells survived. The same wasn't true if the cell-binding agent delivered doxorubicin to cells instead: They were killed regardless of the amount of integrin they carried.
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