The process of T cell activation, proliferation, differentiation, and conversion into memory T cells is central to antigen-specific immune responses1. Initially, T cells are activated when their T cell receptors (TCR) recognize antigenic peptides presented on the surface of antigen-presenting cells (APCs) by major histocompatibility complex (MHC) molecules2,3. This activation process requires not only the binding of TCR to MHC molecules but also the coordination of co-stimulatory signals, which are crucial for fully activating T cell functions4. Once activated, T cells quickly enter the proliferation phase, generating numerous clones that share the same antigen specificity as the original T cell. During this proliferation phase, T cells further differentiate based on their function, primarily into CD8+ T cells and CD4+ T cells5. These differentiated cells mediate cytotoxic killing and assist immune responses, respectively. Following the initial infection, a portion of the activated T cells transform into long-lived memory T cells6. These memory T cells can persist in the immune system for extended periods, rapidly responding to the same pathogen upon re-exposure. As a result, they generate a stronger and more efficient immune response, providing long-term immune protection for the host7. This entire process is critical for the design of T-cell vaccines, as it helps improve vaccine efficacy and durability8.
In this context, cultivating antigen-specific T cells in vitro becomes essential. This protocol was developed based on established methods for in vitro culture of human T cells, with necessary adaptations for the avian immune system9. By growing these cells outside of the body, researchers can study the T cell response under different immune states, such as activation, proliferation, differentiation, and memory formation10. This in vitro approach is invaluable for understanding the roles T cells play in immune responses. Furthermore, under in vitro culture conditions, the secretion of cytokines (such as IFN-γ) by T cells can be monitored11,12. This monitoring is crucial for assessing the quality, intensity, and persistence of immune responses, as well as studying interactions between T cells and other immune cells. Additionally, in vitro expansion of antigen-specific T cells enables large-scale enrichment, increases the sensitivity of detection, and enhances the assessment of T cell response levels13. Thus, in vitro culture of antigen-specific T cells serves as a powerful tool to better understand the role of poultry T cells in immune responses, which increases the sensitivity of detection and enhances the assessment of T cell response levels. While similar in vitro methods have been well established in mammalian systems, the application of such techniques in avian species remains limited. This is especially relevant in poultry immunology, where tools for analyzing T cell responses are underdeveloped. Ducks, in particular, serve as natural reservoirs for several avian influenza viruses, yet little is known about their antigen-specific T cell immunity. Therefore, developing a standardized in vitro T cell culture protocol tailored to avian systems is essential for advancing both basic research and applied immunology in poultry.
A key cytokine involved in these processes is IFN-γ, a type II interferon primarily produced by CD8+ T cells, type 1 CD4+ T cells, and NK cells14. IFN-γ plays a crucial role in inhibiting viral replication and regulating immune responses15. The expression level of IFN-γ reflects the immune status and serves as a marker for T cell activation, enabling researchers to assess T cell response levels through its expression16,17. One common method for detecting cytokine expression in immune cells is intracellular cytokine staining (ICS)18,19. However, due to limitations in experimental materials and techniques, research on ducks has lagged behind that of mammals5. At present, many researchers rely on qPCR to measure IFN-γ expression levels, though this method has certain limitations11. In our lab, we successfully developed an antibody for duck IFN-γ that is compatible with flow cytometry. Building on that success, in this study, we established an ICS method to detect IFN-γ protein expression in duck T cells, providing a reliable tool for further study on duck T cell responses.