Augmented antigen presentation by mouse Ia + T clone cells BK-BI-2.6.O4.1 mediated by transferrin receptors.

The murine T clone cells BK-BI-2.6.O4.1 (BI/O4.1) synthesize and express MHC class II molecules constitutively. BI/O4.1 cells are able to present various protein antigens to antigen-specific CD4 + T cells. However, a 10-fold higher concentration of antigen is needed to activate specific T cells to lymphokine secretion by BI/O4.1 cells in comparison with spleen cells or with the more homogeneous population of bone marrow-derived macrophages (BMMph). The authors tested whether the reduced antigen presentation potential of BI/O4.1 cells was augmented by transferrin-mediated uptake of the model antigen ovalbumin (OVA) coupled to human ferric transferrin. It was shown that 240-fold less OVA was sufficient to induce proliferation of an OVA-specific T-cell clone when the conjugate and not native OVA was used. The presence of ferric TF in the cultures competitively inhibited this effect of the conjugate. A similar shift in the dose-response curve to lower doses of antigen was induced by the conjugate when B lymphoma cells were used as antigen-presenting cells. BMMph and P388D1 cells processed and presented the conjugate with similar efficiency as native OVA, although both cell types exposed transferrin receptors. These data suggest that the reduced antigen presentation potential of BI/O4.1 T clone cells is due to the inefficient uptake of OVA by pinocytosis and delivery into the processing compartment.