Identification of novel chicken CD4⁺ CD3^- blood population with NK cell like features

Chicken NK cells have been defined in embryonic spleen and intestinal epithelium as CD8⁺ lymphoid cells that lack BCR and TCR, whereas blood NK cells have not been phenotypically defined. Here we employed the mab, 8D12 directed against CHIR-AB1, a chicken Fc receptor, to define a previously uncharacterized lymphoid cell population in the blood. Although CHIR-AB1 expression was found on several cell populations, cells with extraordinary high CHIR-AB1 levels ranged between 0.4 and 2.8% in five different chicken lines. The widespread applicability of the CHIR-AB1 mab was unexpected, since CHIR-AB1-like genes form a polygenic and polymorphic subfamily. Surprisingly the CHIR-AB1 high cells coexpressed low MHCII, low CD4 and CD5, while other T cell markers CD3 and CD8, the B cell marker Bu1, the macrophage marker KUL01 were absent. Moreover, they stained with the mab 28-4, 20E5 and 1G7, which define chicken NK cells and they also expressed CD25, CD57, CD244 and the vitronectin receptor (αVβ3 integrin). In functional assays, PMA stimulation led to high levels of IFNγ release, while spontaneous cytotoxicity was not detectable. The expression of typical NK cell markers in the absence of characteristic B- or T-cell markers, and their IFNγ release is suggestive of a yet unidentified NK like population.