Ovine Immune System

Ian McConnell and John Hopkins, Department of Clinical and Veterinary Medicine, University of Cambridge, Cambridge, UK

The ovine immune system occupies a special place in immunology because the sheep is the best species for investigating the physiology of the immune system. The trend to reductionism in modern immunology, which is facilitated by the ease with which the cells of the immune system can be disaggregated from lymphoid tissue and studied in vitro, has meant that much of our knowledge of the immune system is based on the analysis of the cellular functions of in vitro immune systems divorced from any physiological limits. Although the in vitro analysis of the complex cell interactions of the immune system has considerably advanced knowledge, this needs to be integrated within the context of lymphoid tissue physiology at a whole-animal level.

The physiology of the immune system, especially in the context of the immunological compartments associated with different organ systems (e.g. lymph nodes, thymus, gut, mammary gland, respiratory system, urogenital system, central nervous system), can be studied separately in the sheep. These studies have gained much from the technique, first developed by Morris, Hall and Lascelles, of lymphatic cannulation of single lymph nodes in sheep, which provides a powerful system for analyzing immune cell popu lations within different immunological microen-vironments. The lymph node is the heart of the immune system and cannulation of nodal afferent and efferent lymphatics provides a unique approach to the in vivo analysis of the immune system not possible in other species. The cannulated lymph node retains all its vascular and neurological connections. All the cells which enter the node, either from the blood or from the afferent lymphatics to the node leave the node in the efferent lymph. Cannulation of the efferent lymphatic therefore allows the monitoring of immunological events in a single lymph node. The system is currently being used to study the pathophysiology of the immune response to lympho-tropic viruses such as the ruminant lentivirus, maedi-visna, the prototype lentivirus, which infects the accessory cells of organized lymphoid tissue. Intestinal immunity, especially to nematode parasites, can also be studied uniquely by cannulation of the lymphatic vessels draining the gut and mesenteric nodes.

In recent years, cellular and molecular markers for the ovine immune system have been produced by several laboratories. This has permitted the identification of lymphocyte populations, their subsets and receptors, major histocompatibility complex (MHO

class I and class II genes and proteins, immunoglobulins and cytokines. Two international workshops have defined a large number of ovine CD molecules, T cell subset markers, MHC class I and II polymorphisms and cytokines including most of the cytokines associated with macrophages and lymphocyte subsets (e.g. IFNs, IL-1, IL-2, IL-4, IL-6, IL-7, IL-10 and IL-12, GM-CSF, TNF). These cytokine genes are cloned, sequenced and exist as recombinant proteins and several cytokine receptors are now defined (IL-2R etc). By using combinations of monoclonal antibodies and cytokine probes TH1 and T|(2 subsets have been identified. Ovine equivalents of the inte-grins (CD11/CD18), L-selectins and lymphocyte homing receptors are also well characterised.

In terms of its overall organization, the ovine immune system is similar to that of humans but with some notable differences.

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