Porcine CD3+NKp46+ Lymphocytes Have NK-Cell Characteristics and Are Present in Increased Frequencies in the Lungs of Influenza-Infected Animals

The CD3(-)NKp46(+) phenotype is frequently used for the identification of natural killer (NK) cells in various mammalian species. Recently, NKp46 expression was analyzed in more detail in swine. It could be shown that besides CD3(-)NKp46(+) lymphocytes, a small but distinct population of CD3(+)NKp46(+) cells exists. In this study, we report low frequencies of CD3(+)NKp46(+) lymphocytes in blood, lymph nodes, and spleen, but increased frequencies in non-lymphatic organs, like liver and lung. Phenotypic analyses showed that the majority of CD3(+)NKp46(+) cells coexpressed the CD8αβ heterodimer, while a minor subset expressed the TCR-γδ, which was associated with a CD8αα(+) phenotype. Despite these T-cell associated receptors, the majority of CD3(+)NKp46(+) lymphocytes displayed a NK-related phenotype (CD2(+)CD5(-)CD6(-)CD16(+)perforin(+)) and expressed mRNA of NKp30, NKp44, and NKG2D at similar levels as NK cells. Functional tests showed that CD3(+)NKp46(+) lymphocytes produced IFN-γ and proliferated upon cytokine stimulation to a similar extent as NK cells, but did not respond to the T-cell mitogen, ConA. Likewise, CD3(+)NKp46(+) cells killed K562 cells with an efficiency comparable to NK cells. Cross-linking of NKp46 and CD3 led to degranulation of CD3(+)NKp46(+) cells, indicating functional signaling pathways for both receptors. Additionally, influenza A(H1N1)pdm09-infected pigs had reduced frequencies of CD3(+)NKp46(+) lymphocytes in blood, but increased frequencies in the lung in the early phase of infection. Thus, CD3(+)NKp46(+) cells appear to be involved in the early phase of influenza infections. In summary, we describe a lymphocyte population in swine with a mixed phenotype of NK and T cells, with results so far indicating that this cell population functionally resembles NK cells.