Several prior reports have identified peptides that are naturally associated with major histocompatibility complex (MHC) class II molecules on presenting cells. We have examined the delivery of a peptide from exogenous sources to MHC class II molecules. The peptide derives from the influenza virus hemagglutinin (HA) and activates a CD4+ T cell hybridoma. In functional assays of antigen presentation, this epitope is delivered effectively to T cells either in the context of influenza virus or chimeric immunoglobulin (Ig) molecules (Ig-HA) in which the peptide has replaced the CDR3 loop of the heavy chain. We find that the identical 11-mer peptide can be isolated from mouse MHC class II antigens whether the exogenous source of peptide is free HA peptide, the Ig-HA chimera, or ultraviolet-inactivated PR8 influenza virus. The Ig-HA chimera proves to be the most efficient vehicle for charging class II molecules via the exogenous route. Given the fact that self Igs represent natural long-lived carriers, we suggest that antigenized Igs have considerable potential for peptide delivery to MHC molecules in situ.

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