Antigen presentation by B lymphoma cells. Requirements for processing of exogenous antigen internalized through...

The Ag, pigeon cytochrome c, was delivered to the early endosomes in a form coupled to human ferric transferrin that entered APC through transferrin receptors. The processing of the transferrin-Ag conjugate by B lymphoma cells was compared with that of unconjugated native Ag that entered APC by a nonreceptor-mediated mechanism. Within 5 min after internalization, catabolized conjugate was detected in isolated early endosomes and did not accumulate in these organelles. Analysis of the rapid catabolism of the conjugate demonstrated that the Ag, not the transferrin, portion of the molecule was degraded by the APC, suggesting that similar proteases may mediate the processing of the conjugate and native Ag. The processing mechanisms of these molecules shared similarities. Treatment of APC with chloroquine or paraformaldehyde interfered with the stimulation of Ag-specific CD4+ T cells by both transferrin-Ag conjugate and native Ag. However, the T cell responses to the conjugate and native Ag were different in two important respects. First, T cell activation by the conjugate began at an earlier time point and occurred at a faster rate than T cell stimulation by the same concentration of native Ag during a 3-h time course. Second, the T cell response to the conjugate, but not to native Ag, was diminished by treating APC with cycloheximide, a reversible protein synthesis inhibitor. This partial inhibition of the conjugate response by cycloheximide could not be attributed to significant effects on transferrin receptor expression, or on internalization, recycling, or degradation of the conjugate. The differential cycloheximide-sensitivity of the T cell responses indicates that the processing pathways of the two molecules are different. Our findings suggest that the early endosomes may function as an Ag-processing compartment, and that more than one pathway may lead to productive processing in B lymphoma cells.