T cell recognition of fibrinogen. A determinant on the A alpha-chain does not require processing

Murine T cell hybridomas were used to examine the requirements for processing and presentation of human fibrinogen. In contrast to most protein Ag, fibrinogen (Mr 340,000) did not need to be processed by an APC, inasmuch as intact fibrinogen was presented by both pre-fixed and chloroquine-treated macrophages. Through the use of a variety of protease inhibitors, no involvement of proteases either on the macrophage surface or in the culture medium in the presentation of fibrinogen was observed. The epitope recognized by two T cell hybridomas was localized to the peptide, A alpha (551-578), which was located on the carboxy portion of the A alpha-chain. This portion of the A alpha-chain has no defined secondary structure and must possess the conformational flexibility which allows it to directly associate with an I-Ek molecule. Thus conformational flexibility may be a major factor in determining the processing requirements of a protein Ag.