High responses were seen only in few animals of each haplotype and not in general. A polymorphism in the chicken CD8α gene was found in our experimental chicken lines, resulting in incapability to detect CD8α+ T cells using antibodies from the CT8 clone. Screening chickens with alternative find more antibodies showed that antibodies from the 2-398 clone were able to discriminate all CD8α+ cells from CD8α− cells, and consequently this antibody was used in a second vaccination experiment performed

with chickens of the haplotypes B13 and B130. This experiment showed a significant difference in antigen-specific proliferation of CD4+ T cells between the two lines, but not in CD8α+ T cell proliferation. Newcastle disease virus (NDV), an avian paramyxovirus, is a contagious virus causing Newcastle disease (ND). NDV is able to infect almost all species of birds, of which chickens are the most susceptible, in that some field strain infections of chickens have been shown to cause mortalities of 50% or more [1]. As the virus can be spread through domestic as well as wild birds, it is extremely important to

be able to convey Trichostatin A concentration efficient protection to chicken flocks. Vaccination is a widely used method in the control of viral diseases, including ND in the poultry production [2]. Improved knowledge of mechanisms involved in immunological protection in chickens is important in order to develop improved vaccines and optimize vaccine regimens. A standard evaluation of vaccination efficiency is measuring specific antibody titres to vaccines. However, it is also well known that the cellular immune Sitaxentan system is a key actor in vaccine-induced

antiviral immunity [3]. Thus, it was shown by Marino and Hanson [4] that ND-vaccinated bursectomized chickens that were unable to produce antibodies at a protective level still resisted challenge with ND. Furthermore, it has been shown that the level of humoral response after ND vaccination measured by HI titres does not correlate with the cellular response as measured by the under-agarose leucocyte-migration-inhibition technique [5]. In general, commercial ND vaccines are known to induce protective immunity with T cells playing a major role in clearance of the virus [6–8]. In relation to this, reliable techniques for a quantitative and qualitative evaluation of T cell responses upon vaccination are of great importance. One method to measure cellular responses in chickens is the antigen-specific T cell proliferation assay based on carboxyfluorescein diacetate-succinimidyl ester (CFSE) staining of peripheral blood mononuclear cells (PBMC) in order to trace the proliferating T cells. Measurement of proliferating cells as well as non-proliferating cells is subsequently taken by flow cytometry.