Bovine gamma delta T cells: cells with multiple functions and important roles in immunity

The gamma delta T-cell receptor (TCR)-positive lymphocytes are a major circulating lymphocyte population in cattle, especially in young calves. In contrast, human and mice have low levels of circulating gamma delta TCR+ T cells (gamma delta T cells). The majority of the circulating gamma delta T cells in ruminants express the workshop cluster 1 (WC1) molecule and are of the phenotype WC1(+) CD2(-) CD4(-) CD8(-). WC1 is a 220 000 molecular weight glycoprotein with homology to the scavenger receptor cysteine-rich (SRCR) family, closely related to CD163. The existence of 13 members in the bovine WC1 gene family has recently been demonstrated and although murine and human orthologues to WC1 genes exist, functional gene products have not been identified in species other than ruminants and pigs. Highly diverse TCR delta usage has been reported, with expanded variable genes in cattle compared to humans and mice. Differential gamma chain usage is evident between populations of bovine gamma delta T cells, this may have implications for functionality. There is a growing body of evidence that WC1(+) gamma delta T cells are important in immune responses to mycobacteria and may have important roles in T cell regulation and antigen presentation. In this review, we will summarize recent observations in gamma delta T cell biology and the importance of gamma delta T cells in immune responses to mycobacterial infections in cattle.