Avian Immunology

Bernd Kaspers, Karel A. Schat, Bernd Kaspers, Karel A. Schat

• Publisher: Academic Press
• Year: 2011
• ISBN:
  9780080557595
• Topic: Technik & Maschinenbau
• Subtopic: Landwirtschaft

The science underpinning avian immunology is crucial to understanding basic immunological principles and the exceptional features of the avian immune system, as different strategies birds have adopted can provide important evolutionary insights. This book provides the most complete picture of the avian immune system so far. The world-wide importance of poultry protein for the human diet, the threat of an avian influenza pandemic and heavy reliance on vaccination to protect commercial flocks world-wide demonstrates the need to review the important practical lessons in disease control presented here.

• With contributions from 33 of the foremost international experts in the field this book provides the most up-to-date and comprehensive review of avian immunology of the field so far
• Contains a detailed description of the avian innate immune system reviewing constitutive barriers, chemical and cellular responses; it includes a comprehensive review of avian Toll-like receptors
• Contains a wide-ranging review of the 'Ecoimmunology' of free-living avian species, assessing the importance of this subject for studying population dynamics and reviewing the methods and resources available for carrying out such research

1

THE IMPORTANCE OF THE AVIAN IMMUNE SYSTEM AND ITS UNIQUE FEATURES

FRED DAVISON, Formerly of Institute for Animal Health, Compton, Newbury, Berkshire, RG20 7NN, UK

INTRODUCTION

THE CONTRIBUTION FROM AVIAN LYMPHOCYTES

THE CONTRIBUTION OF THE BURSA OF FABRICIUS

THE CONTRIBUTION OF THE CHICKEN MHC

THE CONTRIBUTIONS TO VACCINOLOGY

CONCLUSIONS

REFERENCES

INTRODUCTION

The avian immune system provides an invaluable model for studies on basic immunology. Birds and mammals evolved from a common reptilian ancestor more than 200 million years ago and have inherited many common immunological systems. They have also developed a number of very different and, in some cases, remarkable strategies. Due to their economic importance, and the ready availability of inbred lines, most avian research immunology has involved the domestic chicken, Gallus gallus domesticus. A remarkable consequence of this research has been the seminal contributions it has made to understanding fundamental immunological concepts, especially the complete separation of developing bursa (B)- and thymus (T)-dependent lymphocyte lineages. Some of the observations were made by chance, while others resulted from painstaking work which took advantage of special avian features, for instance the ease of access to, and precise timing of, all the stages in embryonic development. Some of the avian findings were described before being recognized as important and subsequently explained in mainstream immunology. The story of avian immunology is fascinating and by no means complete, as there is still the need for explanations of a number of unique features and the different strategies adopted by the avian system. In this chapter some of the “firsts”, rightly attributed to avian immunology, are described and the importance of further studies in avian immunology highlighted.

THE CONTRIBUTION FROM AVIAN LYMPHOCYTES

The advent of modern cellular immunology, and the fundamental role that lymphocytes play, is generally credited to the 1950s and 1960s, when lymphocyte function became an active subject for research (Burnet, 1971). The immunological significance of lymphocytes emanated from some seminal studies carried out by Gowans, Chase and Mitchison, Simonsen and their contemporaries (see Gowans, 1959; Burnet, 1971). In elegant experiments using laboratory mammals, these workers demonstrated using cell transfers that lymphocytes are essential for generating immune responses and retaining memory of previous exposure to an antigen. However, evidence that lymphocytes play such a key role in protection against infection and in tumour rejection had been in existence for almost 40 years (Murphy, 1914a, b, 1916, 1926), though little attention had been paid to it (Silverstein, 2001). This was almost certainly because, at the time that they made, the observations could not be properly explained and, possibly, because the experimental animal involved was the chicken.

Between 1912 and 1921, James Murphy, an experimental pathologist working at the Rockefeller Institute for Medical Research in New York, performed a series of remarkable experiments using chickens and their embryos to study the growth and rejection of tumour grafts. His experiments appeared to prove beyond question that the lymphocyte is the active component in tissue graft rejection, in protection against infection and, by implication, in innate and acquired immune responses (Silverstein, 2001). Murphy (1914a) observed that the fragments of rat tumours would not grow in the adult chicken, just as they did not grow in other species (xenogenic rejection). However, they could be grown on the chorioallantoic membrane (CAM) of developing chick embryos, although only up to about 18 days incubation. In older embryos tumour grafts were rejected, just as they were if grafted onto the newly hatched chick or an adult bird. Interestingly, Murphy (1914a) observed that the grafts which grew on embryos could be transferred to fresh embryos without any evidence of them being altered. They also retained their tumorigenic capacity if re-grafted onto a rat. Murphy (1914a) commented that these cellular changes occurring when living tissue is grafted onto an unsuitable host are the same, regardless of the type of host resistance, be it natural resistance because of species differences (allogeneic or xenogeneic rejection) or acquired immunity due to recovery from a tumour implanted earlier. The histological picture consisted of oedema surrounded by fibroplasia in the host tissue, the budding out of blood vessels and infiltration of surrounding host tissues with small lymphocytic cells. The inevitable consequence was that cells in the graft died fairly quickly leaving only a scar. These were quite profound though, at the time, unappreciated observations.

Murphy (1914b) also performed a series of elegant experiments using adult chicken tissues co-grafted onto the CAM with fragments of rat tumours. He observed that chicken tissues containing an abundant supply of lymphocytes, such as the spleen or bone marrow, caused tumour grafts to be rejected; whereas these were not rejected if the co-grafted tissue lacked a rich supply of lymphocytes. Later on, Murphy (1916) showed that after grafting fragments of adult chicken spleen onto the CAM of a 7-day embryo, the embryo’s own spleen became grossly enlarged (splenomegaly). This is the first published record of a graft-versus-host response (GvHR). Much later, it was explained by Simonsen (1957) as the immunologically competent lymphocytes of the adult responding to mismatched major histocompatibility complex (MHC) molecules expressed by the embryonic cells. The embryonic cells were recognized as foreign causing the adult cells to replicate and destroy the embryonic host’s lymphocytes. Graft-versus-host disease, in which allogeneic bone marrow transplants recognize the tissues of the graft recipient and cause severe inflammatory disease, is a major problem for immunosuppressed recipients receiving bone marrow transplants. The phenomenon became a major concern with the introduction of human bone marrow transplantation. Nonetheless, the phenomenon was first described using chick embryos (Murphy, 1916).

THE CONTRIBUTION OF THE BURSA OF FABRICIUS

Without doubt, the most significant contribution that avian immunology has made to development of mainstream immunology was in delineating the two major arms of the adaptive immune system. As already pointed out, in the 1960s the significance of lymphocytes was just becoming appreciated and it was generally accepted that there are two types of adaptive immune responses: humoral responses involving antibodies and cellular responses mediated by macrophages and lymphocytes. Since antibodies are produced by plasma cells, which themselves are derived from lymphocytes, it was not understood how such lymphocytes could be of the same type as those cells involved in cell-mediated functions. The bursa of Fabricius, an obscure sac-like structure attached to the proctodeal region of the bird’s cloaca (Plate 1.1), played a crucial role in unravelling this problem.

PLATE 1.1 A dissection showing the positions of the major lymphoid tissues in the chick. Insets provide details of (a) the seven thymic lobes lying alongside the left jugular vein with the crop lying beneath; (b) the spleen can be seen below the proventriculus, the dark-coloured gall bladder lies alongside; (c) caecal tonsils are found at the proximal ends of the two blind caecae, close to the ileo-caecal junction and (d) the ovoid bursa of Fabricius is situated within the pelvic girdle immediately dorsal to the cloaca. Photographs kindly provided by Dr Sven Reese, Institute for Veterinary Anatomy, University of Munich, Munich, Germany.

The cloacal bursa, takes its name from Hieronymus Fabricius of Aquapendente (1537-1619), also known as Girolamo Fabrizi d’ Acquapendente (Fig. 1.1). He was Professor of Surgery at the University of Padua, Italy from 1565 to 1613 (Adelmann, 1942) and by all accounts a brilliant anatomist, embryologist and teacher. For his pioneering work he was later credited in Italian medical science as the “Father of Embryology”. Fabricius not only carried out dissections on human cadavers but also extended his anatomical studies to other species, providing most beautiful detailed drawings of his work. From his observations of avian anatomy he surmised that the cloacal bursa, a hollow structure connected by a duct to the proctodeal region of the cloaca in the hen, most likely acts as a receptacle for storing semen:

FIGURE 1.1 Hieronymous Fabricius of Aquapendente, Professor of Surgery at the University of Padua from 1565 to 1613. Fabricius was the first to describe the bursa, which is now known to be a primary lymphoid organ, attached by a duct to the proctodeum in birds and essential for the development of bursal-derived (B) lymphocytes and the avian antibody repertoire.

Since the sac is pervious, so that there is an open passage from the anus to the uterus itself and another from the uterus to the sac, that is, above and below, and since it is closed at the other end, I think it is the place into which the cock introduces and delivers semen so that it may be stored there (from Adelmann, 1942).

This is not the case, however, but the role of the cloacal bursa continued to puzzle researchers over the following 350 years. Some surmised that, since the bursa of Fabricius regresses with sexual maturity, its size having an inverse relationship with the size testes and the adrenals, it must be some sort of endocrine or lymphoid gland associated with growth and sexual development (Glick, 1987).

Over the years many investigated the function(s) of the bursa including one young researcher, Bruce Glick, working at the Poultry Science Department at Ohio State University, USA. Glick surgically removed bursas from young chicks to investigate the effect on growth. By chance, after one experiment was concluded a colleague, Timothy Chang, asked if he could use some of the birds for a class demonstration on antibody production. A group of...