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 immunogl obulin. This direct cognate interaction of T and B cells results in th e proliferation and differentiation of B cells. This concept has been established using soluble proteins. However, most of the antigens to w hich the immune system is exposed are included in complex particulate structures such as bacteria or parasites. The capacity of B cells to p resent these large and complex antigens is still unclear. To address t his question we have studied the presentation by trinitrophenyl (TNP)- specific B cells of the same antigen TNP-KLH (keyhole limpet haemocyan in), either in a soluble form or covalently linked to poly(acrolein) m icrospheres, from 0.25 to 1.5 mum in diameter. In the presence of irra diated splenocytes or purified macrophages as a source of antigen-pres enting cells (APC), KLH-specific T cells proliferated in response to s oluble TNP-KLH or to TNP-KLH coupled to beads. In contrast, TNP-specif ic memory B cells were totally ineffective in presenting the TNP-KLH b eads to KLH-specific T cells whereas they presented very efficiently s oluble 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 s ize particulate antigens such as bacteria or parasites.