Accessory cells in normal human and rodent lymph nodes: morphology, phenotype, and functional implications.

This chapter provides a brief review of the professional accessory cells present in normal human and rodent lymph nodes. When dealing with the function of accessory cells in the immune system it has to be borne in mind that in principle all cells may possess or acquire the capability of antigen presentation, which is the decisive initiating step in the specific host defense bound to the lymphoid cells. Professional accessory cells are arrayed at the afferent limb of the immune response. The majority of them have a typical dendritic morphology, so that the term "dendritic cells" is widely used instead of the term "accessory cells," and has replaced the traditional terms "reticulum cells" and "reticular cells." Professional accessory cells are characterized by: 1. The capability to stimulate specific T- or B-cell response following antigen pulsing 2. A dendritic morphology 3. Expression of major histocompatibility glycoproteins, complement, and Fc receptors as well as of various adhesion molecules. 4. Presentation of processed or unprocessed native antigens 5. Production and secretion of interleukin-1 and interleukin-6 The bimodal differentiation of lymphocytes seems to be reflected in the occurrence of two types of dendritic accessory cell: accessory cells involved in cellular (T accessory cells) and humoral (B accessory cells) immunity. T accessory cells of the lymph node comprise: 1. The lymphoid dendritic cell of Steinman, which probably represent a subset of blood monocytes with possible terminal differentiation into other T accessory cells 2. Veiled cells, which have been shown to represent Langerhans cells of the epidermal covering on their way to thymus-dependent areas of lymph node, probably giving rise to interdigitating dendritic (reticulum) cells 3. Interdigitating dendritic (reticulum) cells as the typical accessory cells of lymphoid T zone The T accessory cells share morphological, ultrastructural, immunophenotypic, and functional properties, are myeloid in origin, and very probably derive from monocyte lineage. As immune stimulatory cells they present processed antigen only and initiate T-cell proliferation and differentiation by interleukin-1 and -6 secretion. Accessory cells involved in the initiation of a primary humoral immune reaction have not been convincingly characterized. Accessory cells responsible for a secondary B-cell response, however, are the so-called follicular dendritic (reticulum) cells constituting the typical web-like structure of the primary and secondary lymphoid follicles. Unlike T accessory cells, follicular dendritic (reticulum) cells are distinguished by accentuated membrane desmosomes and extensive anastomosing branches of dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)