The surface epithelium of the bursa of Fabricius consists of interfollicular (IFE) and follicleassociated epithelium (FAE). The IFE comprises (i) cylindrical-shaped secretory cells (SC) and (ii) cuboidal basal cells (BCs). The FAE provides histological and two-way functional connections between the bursal lumen and medulla of the follicle. We used a carbon solution and anticaveolin-1 (Cav-1) to study the endocytic activity of FAE. Carbon particles entered the intercellular space of FAE, but the carbon particles were not internalized by the FAE cells.

Coronavirus infection delays the development of the cortico-medullary (CM) capillary network which results in retarded development of bursal follicles. The smaller size of the medulla in the coronavirus-infected birds may lead to a lower number of B lymphocytes and bursal secretory dendritic cells, which negatively affects the reactivity and efficacy of the immune system. Contrary to the wild-type infectious bronchitis virus (IBV) strain, infection induced by H120 vaccine virus exerts only a moderate influence on caveolin-1 expression of the CM capillary web and on follicular development compared to the untreated controls.

The aim of this immunocytochemical study was to compare mannosebinding lectin (MBL) production induced by avian coronavirus in the spleen and caecal tonsil (CT). One-day-old specific-pathogen-free (SPF) chickens were experimentally infected with six QX field isolates and the H120 vaccine strain. In the negative control birds, the spleen was MBL negative, while the CT showed scattered MBL-positive cells in close proximity and within the surface epithelium and germinal centre (GC)-like cell clusters. MBL was detectable in the ellipsoidassociated cells (EACs) and cell clusters in the periarterial lymphoid sheath (PALS) by 7 days post infection (dpi). In both organs, the MBL-positive cells occupy antigen-exposed areas, indicating that GC formation depends on resident precursors of dendritic cells. The majority of MBL-positive EACs express the CD83 antigen, providing evidence that coronavirus infection facilitated the maturation of dendritic cell precursors. Surprisingly, co-localisation of MBL and CD83 was not detectable in the CT. In the spleen (associated with circulation), the EACs producing MBL and expressing CD83 are a common precursor of both follicular (FDC) and interdigitating dendritic cells (IDC). In the CT (gut-associated lymphoid tissue, GALT) the precursors of FDC and IDC are MBL-producing cells and CD83-positive cells, respectively. In the CT the two separate precursors of lymphoid dendritic cells provide some 'autonomy' for the GALT.