Electron microscopy is a very convenient method to localize the epitopes of monoclonal antibodies (mAbs) at the surface of macromolecules for studying their tree-dimensional organization.We applied this immuno-electron microscopic method to human ct2-macroglobulin (ct2M). 29 anti-α2M mAbs have been tested with the four different forms of a2M : native and chymotrypsin-transformed tetramers, and the corresponding dimers, obtained by dissociation with divalent cations. These mAbs can be classified in three types : those which are specific for 1) the H-like transformed molecules, 2) the native molecules, and 3) those which can react with both forms of α2M.1) Among the H-like α2M specific mAbs, several react with the 20 kD-domain which is recognized by the cellular receptor of transformed a2M. This domain is located at the carboxyterminal end of each monomer. One IgG binds to the end of two adjacent tips of the H-like form.The other mAbs of this type bind to the α2M tips at non-terminal positions. Intermolecular connections built polymers of alternating α2M and IgG molecules.2) Among the native a2M-specific mAbs some are able to inhibit the protease-induced transformation of the native α2M. The binding sites of these mAbs are demonstrated on the native half-molecules. One of these mAbs was also able to react with transformed dimers, in a region corresponding very likely to an inaccessible epitope in the tetrameric transformed α2M molecule.3) Among the mAbs of this type, only two were able to inhibit the protease-induced transformation of α2M. Obviously, their epitopes should be close to the bait region of α2M. The other mAbs reacting with both α2M forms did not inhibit the α2M transformation.All these mAbs can be distinguished by the structure of the immune complexes formed with all forms of α2M. The epitopes are more easily located on the dimers and on the H-like transformed α2M than on the native molecules.From these observations, we propose a new model of the tree-dimensional organization of the human α2M in its native and transformed configurations, and of its protease-induced transformation.
An application of light and electron microscopic method to diagnostic cytology - With special reference to the mammary lesions.
