Th cell recognition of globular proteins requires the uptake and intracellular processing of the native Ag by an APC to produce a peptide fragment containing the T cell antigenic determinant, which is recognized in conjunction with Ia. This report describes the time course of the processing and presentation of a soluble globular protein Ag, pigeon cytochrome c (Pc), and of the presentation of a C-terminal peptide fragment of Pc, residues 81 to 104 (Pc 81-104), which does not require processing. Splenic B cells, acting as APC, require 6 to 8 h incubation with native Pc to process and present it to an I-Ek-restricted Pc-specific T cell hybrid, resulting in the secretion of IL-2. Moreover, the time required for B cells to process Pc is the same whether the Ag is taken up by nonspecific fluid phase pinocytosis or by binding to surface Ig. Once processed, Ag is lost from the B cell surface by 8 to 12 h, although when provided with fresh Pc, the same B cells are still capable of processing and presenting. In contrast to native Pc, only 1 to 2 h are required for the peptide fragment Pc 81-104 to become associated with B cells in a stimulatory fashion, and this time is similar for live and paraformaldehyde-fixed B cells, which cannot internalize or process the peptide. Washed free of excess peptide after 2 h, B cells lose their ability to stimulate T cells by 8 to 12 h, with a time course indistinguishable from that for the loss of processed native Pc. Prolonged incubation of B cells with the peptide for 18 to 24 h results in a dramatic loss of the ability to present Pc 81-104. Even when provided with fresh Pc or Pc 81-104, these cells have diminished ability to present these Ag. This loss is selective, inasmuch as these B cells remain equivalent to untreated B cells in the presentation of an unrelated Ag, OVA, to an I-Ak-restricted specific T cell. However, the ability to present another I-Ek-restricted antigenic peptide of the D glycoprotein of HSV to its specific T cell is also diminished. Loss of activity is observed after incubation only with the peptide and not with the native protein and is not due to a depletion of the antigenic peptide from the incubation medium.(ABSTRACT TRUNCATED AT 400 WORDS)

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