-lo H. Chen

Thymic Function Can Be Accurately Monitored by the Level of Recent T Cell Emigrants in the Circulation

Expression of the avian chT1 thymocyte antigen persists on a subpopulation of peripheral T cells enriched in the DNA deletion circles created by alphabeta and gammadelta TCR gene rearrangements. The chT1+ cells are evenly distributed among all of the peripheral T lymphocyte compartments. The levels of chT1+ T cells in the periphery gradually decline in parallel with age-related thymic involution, and these cells disappear following early thymectomy. Experiments in which variable numbers of the 14 thymic lobes are removed in young chicks indicate a direct correlation between the levels of circulating chT1+ cells and residual thymic mass. Measurement of recent thymic emigrants in the periphery thus provides an accurate indication of thymic function.

Avian T cell ontogeny

Publisher Summary This chapter focusses on avian T cell development. A renewed interest in avian T cell development has emerged with the use of monoclonal and functional antibodies to elucidate T cell differentiation antigens and molecular and functional definitions of mammalian T cell receptors (TCRs). Thousands of avian species have been identified; the domestic chicken has served as the avian representative in most studies of the immune system. Chick-quail chimeras have also proved to be an especially informative model. The chapter focuses on information gained in studies of these model systems. Monoclonal antibodies are produced against a variety of functionally important molecules expressed on the surface of chicken T cells and most of these have well-defined mammalian counterparts. The chicken T cell receptors can be divided into three groups, each of which is recognized by a different monoclonal antibody. Direct evidence for thymus-directed chemotaxis of hematopoietic precursors is obtained by observing migration of cells toward the thymic epithelium in an in vitro model. Comparative analysis of avian T cell development and function reveals striking conservation of the major features defined for the mammalian T cell system. In both mammals and birds, the T cells utilize either γ/δTCR or α/βTCR together with a CD3 protein complex as signal-transducing receptors for antigen presented in the context of major histocompatibility complex restriction elements.

Chicken cathelicidin-B1, an antimicrobial guardian at the mucosal M cell gateway

Mucosal epithelial M cells provide an efficient portal of entry for microorganisms. Initially defined by their irregular microvilli and abundant transcytotic channels in the avian bursa of Fabricius, M cells also are found in the lymphoid follicle-associated epithelium of the mammalian appendix, Peyers patches, and other mucosal surface-lymphoid interfaces. We describe here a previously unrecognized cathelicidin gene in chickens, chCATH-B1, that is expressed exclusively in the epithelium of the bursa of Fabricius. Like the mature peptides of previously identified cathelicidins, the carboxyl-terminal peptide of chCATH-B1 has broad antimicrobial activity against Gram-positive and Gram-negative bacteria. chCATH-B1 expression is restricted to the secretory epithelial cell neighbors of the M cells, whereas its mature peptide is transported to become concentrated on the fibrillar network surrounding basolateral surfaces of the M cells that overlie the bursal lymphoid follicles. We conclude that chCATH-B1 is well placed to serve a protective antimicrobial role at the M cell gateway.

Reversible Disruption of Thymic Function by Steroid Treatment

The effect of steroid treatment on the thymic output of T cells was examined in an avian model. Recent thymic emigrants in chickens transiently express the chicken T cell Ag 1 thymocyte marker, and thymic function can be monitored indirectly by measuring the levels of TCR gene rearrangement excision circles in peripheral T cells. Both parameters were used to show that intensive steroid treatment induces thymic involution and a profound reduction in the supply of naive T cells to the periphery. Conversely, resident T cells in the peripheral lymphocyte pool were relatively spared. Thymopoiesis immediately recovered following cessation of steroid treatment, concurrent with restoration of the thymic output of newly formed T cells. Repopulation of the peripheral T cell pool recapitulated the ontogenetic pattern of γδ T cell replenishment before αβ T cell reseeding, thereby indicating the complete recovery of thymic function after a course of steroid treatment.

T Cell Receptors and T Cell Development

Birds provide a valuable model system for the study of early development. Studies of immune system development and function in chickens have contributed to some of the central tenets in immunology. These include:(1) allorecognition by lymphocytes in graft-vs-host (GVH) reactions (Simonsen 1985), (2) clonal reactivity of immunocompetent lymphocytes (Burnet 1957), (3) the separate differentiation pathways of T and B cells which cooperate in immune responses (Cooper et al. 1965), (4) hematopoietic stem cell origin of both lymphoid and myeloid lineages (Moore and Owen 1965), (5) the developmental migratory waves of thymocyte progenitors (Le Douarin 1978), (6) immunoglobulin (Ig) isotype switching by IgM-bearing B lymphocytes (Kincade et al. 1970; Kincade and Cooper 1973), and (7) somatic diversification of the B cell repertoire by gene conversion (Reynaud et al. 1987; Thompson and Neiman 1987).

Avian Natural Killer Cells

Natural Killer (NK) cells represent a distinct lymphoid lineage. These cells were initially defined functionally by their ability to kill certain virally infected cells and tumor cells (reviewed in Trinchieri 1989; Ritz et al. 1988). This unique function and its evolutionary conservation led to the classification of NK cells as part of the innate immune system. Recently, renewed interest in NK cells has lead to better understanding of NK cell phenotype, cytokine secretion and reactivity, developmental origin, functional capabilities, as well as the receptors mediating cytotoxicity (Moretta et al. 1994).

Cloning and expression of the chicken immunoglobulin joining (J)-chain cDNA.

Abstract The J chain is a component of polymeric immunoglobulin (Ig) molecules and may play an important role in their polymerization and the transport of polymeric Ig across epithelial cells. In this study, the primary structure of the chicken J chain was determined by sequencing cDNA clones. The cDNA had an open reading frame of 476 nucleotides encoding a putative protein of 158 amino acid residues including the signal sequence. The 3′ untranslated region consisted of 1216 nucleotides and a poly(A) tail. The deduced amino acid sequence of the chicken J chain had a high degree of homology to that of human, cow, rabbit, mouse, frog, and earthworm, with eight conserved Cys residues identical to the mammalian J chains. Northern blot hybridization performed with total RNA from various chicken tissues revealed high levels of J-chain mRNA expression in spleen, intestine, Harderian gland, and bursa of Fabricius, and low levels in the thymus. The J chain was expressed in the bursa as early as day 15 of embryogenesis. These data indicated that the chicken J-chain gene displays a high degree of homology with that of other species, and is expressed at an early stage of development of the chicken immune system.

Evolution of T cell receptor (TCR) α β heterodimer assembly with the CD3 complex

T cell antigen receptors (TCR) are composed of an antigen‐recognizing unit, the TCRα β  heterodimer, and a signal transduction ensemble, the CD3 complex. Whereas mammals possess three CD3 dimers (δ ϵ , γ ϵ , and ζ 2), birds and amphibians have only two (δ /γ ‐ϵ  and ζ 2). To understand evolutionary changes in TCR/CD3 assembly,a phylogenetic approach was employed to dissect the interaction of TCRα β  heterodimers with the CD3 components. While sheep and mouse TCRα  and TCRβ  chains could replace the corresponding human chains in mutant human T cells to restore surface TCR/CD3 expression and function, chicken TCRα , TCRβ  and CD3δ /γ  chains were unable to replace the corresponding human chains in forming a chimeric TCR/CD3 complex. The inability of chicken TCR/CD3 components to replace the human molecules in T cells was found to result from the lack of interaction between chicken TCRα β  heterodimers and the human CD3 complex. In contrast, if no CD3 molecules are present (non‐T cells), TCRα ‐TCRβ  chain pairing can take place in an apparently non‐controlled way. Thus, the TCR‐CD3 interactions have changed with the evolutionary divergence of two mammalian CD3γ  and CD3δ  genes from a single prototypic chicken δ /γ  gene. Our data suggest that the structures in mammalian TCR.C regions, which distinguish between CD3δ  and CD3γ  chains, have evolved with the appearance of two separate CD3δ  and CD3γ  functions.

An avian B-lymphocyte protein associated with β2-microglobulin

A member of the family of β2-microglobulin (β2m)-associated cell surface glycoproteins was identified by the CB3 monoclonal antibody. The Mr 50 000 heavy chain of the CB3 antigen differs from conventional class I heavy chains (Mr 45 000) in the extent of glycosylation, charge, and peptide composition. Because of its selective expression on avian B cells and its similarity to mammalian class I-like molecules, we speculate that the CB3 antigen may play a role in T- and B-cell interactions. rommental stimuli is to prepare monoclonal antibodies against bursal cell surface antigens. Using this strategy we have identified several proteins that are expressed on the surface of bursal B cells (Chen and Cooper 1987). In the present study we characterize CB3, a class I-like molecule associated with β2-microglobulin (β2m) on the surface of both bursal and peripheral B cells.

Genomic Structure and Transcriptional Regulation of the Early B Cell Gene chB1

The avian B cell differentiation Ag chB1 is a membrane glycoprotein relative of the mammalian B cell differentiation Ag CD72. Unlike CD72, this C-type lectin is expressed in relatively high levels on immature B cells in the bursa of Fabricius and is down-regulated on mature B cells in the periphery. An immunoreceptor tyrosine-based inhibitory motif in the chB1 cytoplasmic tail suggests a potential regulatory role in intrabursal B cell development. To gain further insight into the selective expression and function of chB1, we determined the genomic organization of chB1 and examined the mechanism of its transcriptional regulation. The 8-exon chB1 gene proved to have very similar organization to that of mouse CD72, further supporting the idea that chB1 is a CD72 relative. As for mouse CD72, the chB1 promoter region lacks a TATA box but contains a conserved initiator element. The 131-bp region (−161 to −30) proximal to the transcriptional start site, which contains a potential early B cell factor binding site, is essential for the B lineage stage-specific transcription of chB1, whereas PU.1 and B cell-specific activator protein/Pax5 have been shown to play important roles in CD72 promoter activity and cell-type specificity. This analysis suggests that differences in transcriptional regulation of these phylogenetically related genes may determine the differences in expression pattern and, therefore, the function of avian chB1 and mammalian CD72 during B cell development.