B cells do not present antigen covalently linked to microspheres.

B cells have been shown to present antigen to T cells very efficiently through their capacity to capture antigens by their membrane immunoglobulin. This direct cognate interaction of T and B cells results in the proliferation and differentiation of B cells. This concept has been established using soluble proteins. However, most of the antigens to which the immune system is exposed are included in complex particulate structures such as bacteria or parasites. The capacity of B cells to present these large and complex antigens is still unclear. To address this question we have studied the presentation by trinitrophenyl (TNP)-specific B cells of the same antigen TNP-KLH (keyhole limpet haemocyanin), either in a soluble form or covalently linked to poly(acrolein) microspheres, from 0.25 to 1.5 microns in diameter. In the presence of irradiated splenocytes or purified macrophages as a source of antigen-presenting cells (APC), KLH-specific T cells proliferated in response to soluble TNP-KLH or to TNP-KLH coupled to beads. In contrast, TNP-specific memory B cells were totally ineffective in presenting the TNP-KLH beads to KLH-specific T cells whereas they presented very efficiently soluble TNP-KLH. Similar results were obtained with the A20 B lymphoma or with lipopolysaccharide (LPS)-activated TNP-specific B cells. These results therefore indicate that B cells are unable to present large size particulate antigens such as bacteria or parasites.