J Biol Chem, Vol. 274, Issue 37, 26266-26271, September 10, 1999
A Secreted Form of the Major Histocompatibility Complex Class
II-associated Invariant Chain Inhibiting T Cell Activation
Elizabeth E.
Eynon
,
Claudia
Schlax
, and
Jean
Pieters¶
From the ¶ Basel Institute for Immunology,
Grenzacherstrasse 487, CH-4005 Basel, Switzerland and the
Netherlands Cancer Institute,
Amsterdam, The Netherlands
Major histocompatibility complex (MHC) class II
molecules function at the cell surface to present antigenic peptides to
T helper cells. Intracellularly, MHC class II molecules are associated with the invariant chain (Ii). Ii can modulate MHC class
II-dependent T cell activation through (i) assistance in
the export of MHC class II molecules from the endoplasmic reticulum,
(ii) providing a targeting signal for endosomal/lysosomal compartments,
and (iii) preventing peptides from associating prematurely with MHC
class II molecules. Here we describe the generation and subsequent
secretion of a lumenal form of Ii, IiP25. IiP25 lacked the targeting
sequences for transport to MHC class II compartments but contained part of the CLIP region that is known to compete with antigenic peptides for
binding to MHC class II molecules. When added to an antigenic peptide
presentation model system, IiP25 inhibited T cell activation by
competing for the CLIP binding site at the plasma membrane. Secretion
of a lumenal Ii fragment may represent an additional mechanism to
modulate T cell activation by MHC class II molecules.
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.