CD5-low expression lymphocytes in canine peripheral blood show characteristics of natural killer cells

NK cell markers and receptors have been discovered in many mammalian species, such as humans, mice, rats, pigs, and cows. However, there is still a lack of information concerning NK cell markers or receptors in canines. We have discovered that canine CD5-low density (CD5(lo)) cells in PBL are closely associated with NK cell characteristics. CD5(lo) cells comprised 14.9 +/- 6.68% of the total PBL. A high proportion of the CD5(lo) cell population expressed CD3 (96.6%), CD8 alpha (77.7%), CD8 beta (53%), alpha/beta TCR (83%), and CD11/18 (80%), but the expression of gamma/delta TCR (6.5%), CD4 (10.6%), and CD21 (2.4%) was low. CD5(lo) cells were larger than CD5-high density (CD5(hi)) cells. Light and electron microscopy revealed numerous large cytoplasmic granules in CD5(lo) cells, especially after IL-2 stimulation, which was in contrast to CD5hi, in which intracytoplasmic granules were not frequently seen. After IL-2 stimulation, CD5(lo) cells had significantly stronger NK cytotoxicity than CD5(hi) cells. CD5(lo) cells had much higher mRNA levels for NKG2D, CD16, CD94, CD160, perforin, and granzyme than CD5(hi). Following IL-2 stimulation, CD5(lo) cells had significantly higher mRNA levels of NKp30, NKp44, CD16, and CD94 than CD5(hi) cells. In addition, IL-2-stimulated, CD5(lo)-depleted PBL showed a loss of NK cytotoxicity. CD5(lo) cells also showed significantly lower antigen-specific cytotoxic T cell activity as compared with CD5(hi) cells. Taken together, the CD5(lo) subset in canine PBL is closely related to canine NK cells, and CD5(lo) can be used as a phenotypic marker for an IL-2-dependent canine NK cell enrichment. J. Leukoc. Biol. 84: 1501-1510; 2008.