Characterization of sheep afferent lymph dendritic cells and their role in antigen carriage

We have ablated peripheral lymph nodes in sheep and subsequently cannulated the pseudo-afferent lymphatic vessel that arises as a consequence of afferent lymphatic vessels reanastomosing with the former efferent duct. This technique allows the collection of lymph with a cellular composition that resembles true afferent fluid, and in particular, containing 1-10% dendritic cells. A 16-h collection of this lymph may contain between 10(6) and 10(7) dendritic cells. This dendritic cell population may be enriched to greater than 75% by a single-density gradient centrifugation step. We have generated a mAb that recognizes sheep CD1. This monoclonal not only reacts with afferent dendritic cells, but with dendritic cells in the skin and paracortical T cell areas of lymph nodes. The expression of CD1 suggests afferent dendritic cells are related to skin Langerhans' cells and other dendritic cells that act as accessory cells for T cell responses. Consistent with this is the high level of expression by dendritic cells of molecules involved in antigen recognition by T cells, including MHC class I and class II. Afferent dendritic cells express high levels of the cellular adhesion molecule LFA-3, and at the same time express a ligand for this molecule, namely CD2. The accessory functions of afferent dendritic cells resemble those displayed by mature Langerhans' cells and by lymph node interdigitating cells. These include clustering with resting T cells and stimulating their proliferation in a primary response to antigen. Afferent dendritic cells are capable of acquiring soluble protein antigen in vivo or in vitro and presenting the material directly to autologous T cells in an antigen-specific manner. We conclude that afferent dendritic cells represent a lymph-borne Langerhans' cell involved in antigen carriage to the lymph node.