| Abstract: | An I-A subregion-controlled structure (I-At) characterizes some helper T cells and augmenting factors. This epitope is associated with a glycoprotein. Extended trypsin digestion removed the determinant; tunicamycin blocked its reexpression. In contrast, limited trypsinization increased the number of I-At-bearing peripheral T cells from 17 to 35%. The I-At molecule density on cells expressing this structure did not change measurably with limited enzyme treatment. Rather, some previously negative T cells (20%) expressed the epitope after mild proteolysis. A third T cell subset (60%) expressed no I-At molecules regardless of enzyme treatment. We conclude that the I-At molecule is shielded by trypsin-labile material on some T cells, whereas on others it is fully exposed. The transition from a shielded to an exposed configuration may correlate with T cell activation. Cycloheximide inhibited the biosynthesis of both the I-At molecule and the shielding substance by T cells. Unlike I-A-controlled T cell structures, B cell I-A-encoded molecules are neither shielded nor trypsin labile. The relationship between I region-controlled T cell and B cell molecules is discussed. |
