Differences between the in vitro combinations of secretory component (SC) and immunoglobulin polymers (Ig) by enumeration of SC epitopes.

The enumeration of the SC epitopes has been established on 125I-labelled free and combined SC, by binding to anti-SC coated beads, then by addition of 3H-labelled anti-SC Fab' fragments of various specificities. The number of moles of Fab' fragments found on the beads increases in relation to the introduced amount. The extrapolation to an infinite concentration of added Fab' fragments gives the maximal theoretical accessible number of SC epitopes. The number of hidden epitopes (cryptotopes) is established by subtracting the total number found on sIgA, IgA-SC and IgM-SC from those found for free SC. These values are confirmed with Fab' fragments specific for the inaccessible determinant of SC. There are 4 cryptotopes in the case of sIgA, 3 for IgA1-SC, 2 for dimer IgA-SC and only 1 for IgM-SC (polyclonal or monoclonal). Thus the in vitro combinations of SC with polyclonal IgA dimers are different from the in vitro combinations with polyclonal or monoclonal IgM. The structural implications of these differences are discussed.