Chicken Secondary Lymphoid Tissues-Structure and Relevance in Immunological Research

Abstract

Recent discoveries have indicated the importance of developing modern strategies for vaccinations, more ethical research models, and effective alternatives to antibiotic treatment in farm animals. Chickens (Gallus gallus) play a crucial role in this context given the commercial and economic relevance of poultry production worldwide and the search for analogies between the immune systems of humans and birds. Specifically, chicken secondary lymphoid tissues share similar features to their human counterparts. Chickens have several secondary or peripheral lymphoid tissues that are the sites where the adaptive immune response is initiated. The more general classification of these organs divides them into the spleen and skin-, pineal-, or mucosa-associated lymphoid tissues. Each of these tissues is further subdivided into separate lymphoid structures that perform specific and different functions along the animal's body. A review summarizing the state of the art of research on chicken secondary lymphoid organs is of great relevance for the design of future studies.

Keywords: MALT; animal model; interspecies similarity; poultry; secondary lymphoid organ.

Figure 1

Summary of the current challenges in poultry science, highlighting the pivotal role of immunological research as a key to solving the present issues.

Figure 2

Schematic representation of splenic lobules in chicken. The lineal artery enters the organ and then divides into the trabecular arteries, which, in turn, branch out and enter the pulp as central arteries. The latter divide into smaller central arterioles and finally into the penicillar capillaries associated with venous sinuses. Central arteries are surrounded by a periarteriolar lymphoid sheath (PALS). The PALS is divided into the inner PALS and the outer PALS. The inner PALS contains mainly T lymphocytes expressing CD4 and TCRαβ1 molecules. TCRαβ2⁺ CD4⁺ and TCRαβ1⁺ CD8⁺ cells are also present. Between the T cells are located interdigitating dendritic cells (IDCs), most likely the precursor of follicular dendritic cells (FDCs). In the outer PALS are small and medium lymphocytes (both B cells and T cells), macrophages (MΦ), and plasma cells after activation. At the border of the trabecular and central arteries are located the germinal centers (GCs), which are composed primarily of B cells but in which a small number of follicular dendritic cells and CD4⁺ T cells can also be found.

Figure 3

Schematic representation of cecal tonsil cross-section. The general structure includes the epithelium, subepithelial zone, germinal centers (GCs), and interfollicular zones. The B cells are found mainly in GCs and in the subepithelial zone. The interfollicular (T cell-dependent) zone consists mainly of TCRαβ1⁺ CD4⁺ T cells. CD8α⁺ cells are generally present in the epithelium and are scattered through the lamina propria. TCRγδ⁺ cells are usually found in the epithelium and in the subepithelial zone. Macrophages (MΦ) occur scattered through the lamina propria but are most prevalent directly under the epithelium.