RESUMO
With a prevalence of up to 35% in dogs with reproductive problems, azoospermia is one of the most important reasons for male infertility. Non-obstructive azoospermia, without clinical symptoms, but histopathological damage of the testicular tissue and immune cell infiltration is referred to as spontaneous autoimmune orchitis (AIO) in the literature. Published cases in dogs describe immune cell infiltration; however, there is no consent about the involved immune cell types. We aimed to characterize immune cells in testicular biopsies of dogs with AIO (n = 9) and to compare them to those in testicular specimens from healthy control dogs with normospermic ejaculates (CG; n = 5). Immunohistochemistry was performed using specific antibodies against CD3, PAX5, MAC387, IgG and IgM to proof the presence of T lymphocytes, B lymphocytes, macrophages and early and late plasma cells, respectively. Presence of immune cells in healthy testicular tissue was low and restricted to T lymphocytes and macrophages in the interstitium with the latter also being found within the blood vessels. Different to this, AIO samples revealed presence of all investigated immune cells, underlining lymphoplasmacytic nature of chronic asymptomatic immune-mediated orchitis. Canine spontaneous AIO is characterised by a significantly increased number of immune cells, namely ≥33 immune cells/mm2 (sensitivity/specificity: 100% based on our data). The pathogenesis of canine AIO is hypothesized to be as follows: 1. Macrophages initiate AIO via T lymphocyte activation. 2. T lymphocytes lead to a "delayed type immunological response" and development of AIO. 3. Invaded B lymphocytes later differentiating to plasma cells are responsible for the second humoral immunological response and cause progression of AIO. Different to the situation in CG, T lymphocytes and plasma cells were identified within the seminiferous tubules indicating that disruption of spermatogenesis in AIO might be related to invading immune cells. Testicular biopsies provide an essential tool in the diagnosis of spontaneous AIO.