Immunohistochemical Investigation of Extracellular Matrix Components in the Lymphoid Organs of Healthy Pigs and Pigs with Systemic Disease Caused by Circovirus Type 2

The extracellular matrix (ECM) provides a scaffold for cell growth, impacts on cellular behaviour and plays an important role in pathological conditions. Several components of the ECM of lymphoid tissues have been shown to be crucial in the maturation, differentiation and migration of lymphocytes and other immune cells and, therefore, in the development of immune responses. Little is known of the composition and function of the ECM in porcine lymphoid tissues. The present study characterizes immunohistochemically the expression of several ECM-related molecules (i.e. hyaluronan [HA] and its receptor CD44, tenascin-C [TN-C] and versican) in primary and secondary lymphoid organs of healthy pigs and animals affected by porcine circovirus type 2-systemic disease (PCV2-SD). These ECM molecules displayed a highly defined expression pattern in healthy animals, suggesting that they may have a role in the compartmentalization of immune cells within lymphoid tissues. HA was abundant in the medulla of the thymus and follicles of secondary organs; CD44 and TN-C were present in the thymic medulla and parafollicular areas of secondary lymphoid organs; however, there was minimal expression of versican in healthy tissues. In PCV2-SD-affected animals, HA and CD44 showed a similar but more diffuse distribution. TN-C was increased in the T-cell-dependent areas and in tonsillar crypts, and versican was more abundantly expressed, with expression restricted to vascular structures and trabeculae and also surrounding tonsillar crypts. The altered expression in PCV2-SD-affected pigs was most probably related to a higher content of connective tissue secondary to tissue destruction and remodelling attempts as part of the disease process.