Porcine T lymphocytes and NK cells--an update

Natural killer (NK) cells represent an important cell population of the innate immune system with the ability to attack spontaneously pathogen-infected and malignant body cells as well as to produce immune-regulatory cytokines. T lymphocytes belong to the adaptive immune system and perform a wide array of functions in immune regulation, inflammation and protective immune responses. In this review we summarize the current knowledge about the phenotype and functional characteristics of these two cell populations in swine. Porcine NK cells can be distinguished from T cells by the complex phenotype perforin+ CD3(-)CD4(-)CD5(-)CD6(-)CD8alpha+CD8beta(-)CD11b+CD16+. Investigations so far show that these cells have the capacity to lyse virus-infected target cells and respond to various regulatory cytokines. Such cytokines can induce interferon-gamma (IFN-gamma) production in porcine NK cells, as well as the up-regulation of effector/activation molecules like perforin and CD25. Porcine T cells can be divided into a number of subpopulations, including a prominent fraction of T cells expressing T-cell receptors (TCR) with gammadelta-chains. Like TCR-alphabeta T cells, these TCR-gammadelta T cells can express CD8alpha and MHC class II, two molecules which in swine seem to be correlated with an activation status of T cells. Functional properties of these cells seem to include cytolytic activity as well as antigen presentation; however, both aspects require further investigation. Like in other species, TCR-alphabeta T cells in swine comprise MHC class-I restricted cytolytic T cells, T-helper cells and recently identified regulatory T cells. We summarize data on the phenotype and function of these cells including memory cell formation. Current knowledge suggests that MHC class-I restricted cytolytic T cells can be identified by the expression of CD8alphabeta heterodimers. T-helper cells express CD4 as well as other activation-related markers, including CD8alpha, MHC class II and CD45RC. Porcine regulatory T cells have a phenotype similar to that of mouse and humans: CD4+CD25+Foxp3+. First results indicate that these cells can suppress proliferation of other T cells and produce IL-10. Finally, the abundant expression of swine-specific activation markers CD8alpha and MHC class II on T cells and NK cells is discussed in more detail.