Demonstration of two distinct forms of released low-affinity type interleukin 2 receptors

The release of interleukin 2 (IL2)-binding proteins, derived from the 55-kDa low-affinity IL2 receptor (IL2R; L chain), has been observed for virtually all L chain-bearing cells in either humans, the mouse or the rat. Based on the characterization of the released human L chain as a molecule 10 kDa smaller than the cell surface receptor, either proteolytic cleavage or differential splicing of the L chain-encoding mRNA have been suggested as mechanisms underlying the receptor release. Combining affinity labeling of the L chain with 125I-labeled IL2 and immunoprecipitation with L chain-specific monoclonal antibodies applied for the detection of soluble mouse IL2R revealed the existence of two classes of soluble receptors, one being of the same size as cell surface expressed L chain, the other of 45-kDa apparent molecular mass. These findings raise the possibility of mechanisms of receptor release other than those discussed for human L chain.