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J. Exp. Med.,
Volume 188, Number 5, September 7, 1998 961-971
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T Cells by the Class Ib Gene Product, Qa-1b
By

From the * Department of Pathology and Laboratory Medicine, Emory University School of Medicine,
Atlanta, Georgia 30322; and the T cell hybridomas isolated from nonresponder H-2b mice immunized with pork insulin were
stimulated by insulin in the presence of major histocompatibility complex (MHC)-unmatched
antigen presenting cells. The restriction element used by these CD4
Division of Molecular and Clinical Rheumatology, Department of
Medicine, The Johns Hopkins School of Medicine, Baltimore, Maryland 21205
T cells was mapped to an
oligomorphic MHC class Ib protein encoded in the T region and identified as Qa-1b using
transfectants. The antigenic determinant was localized to the insulin B chain, and experiments
with truncated peptides suggested that it is unexpectedly long, comprising most or all of the 30 amino acid B chain. The antigen processing pathway used to present insulin to the Qa-1b-
restricted T cells does not require transporters associated with antigen processing (TAP), and it
is inhibited by chloroquine. A wide variety of cell lines from different tissues efficiently present
soluble insulin to Qa-1b-restricted T cells, and insulin presentation is not enhanced by phagocytic stimuli. Our results demonstrate that Qa-1b can function to present exogenous protein to
T cells in a manner similar to MHC class II molecules. Therefore, this class Ib protein may
have access to a novel antigen processing pathway that is not available to class Ia molecules.
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