Distribution of CD3⁺ T lymphocytes in the adventitial layer of Echinococcus granulosus sensu stricto cysts in cattle and sheep with or without Fasciola hepatica co-infection.

Cystic Echinococcosis (CE), caused by the larval stage of Echinococcus granulosus sensu lato, elicits a complex host immune response characterized by the formation of a fibrotic capsule known as the adventitial layer. While the cellular composition of this layer has been previously studied, the spatial distribution of T lymphocytes remains poorly defined, particularly in relation to cyst fertility, host species, organ localization, and co-infection with Fasciola hepatica. In this study, we evaluated the distribution of CD3⁺ T cells in the adventitial layer of hepatic and pulmonary CE cysts collected from naturally infected cattle and sheep. Using immunohistochemistry and image analysis, we quantified CD3 expression across the adventitial layer and within three defined sub-regions (proximal, intermediate, and distal to the laminated layer). Our results show that while total CD3⁺ T-cell abundance did not vary significantly according to cyst fertility or co-infection status, their distribution patterns within the adventitial layer did. In cattle, CD3⁺ T cells were uniformly distributed throughout the adventitial layer, whereas in sheep, T cells were predominantly localized to the distal third, near the organ parenchyma. This pattern was more pronounced in non-fertile cysts. Additionally, no significant alteration in CD3 distribution or abundance was observed in the presence of F. hepatica co-infection. These findings reveal a species-specific pattern of T-cell localization within the adventitial layer and suggest that immune cell positioning, rather than quantity, may play a role in local immune modulation and cyst outcome.