The role of bovine γδ T cells and their WC1 co-receptor in response to bacterial pathogens and promoting vaccine efficacy: a model for cattle and humans.

γδ T cells are critical to immune surveillance and protection since they are found as resident cells in many organs and tissues, including in humans and ruminants, and circulate at substantial numbers in the blood. It is known that γδ T cells contribute to cellular immunity and protection against important pathogens including organizing granulomas in response to Mycobacteria. We have shown that IFNγ-producing bovine γδ T cells bearing the WC1 co-receptor are the major cell population responding in recall responses to Leptospira during the first month following priming by vaccination against serovar Hardjo. To date, successful vaccines largely include those to diseases that only require antibody responses for protection and attempts at creating subunit peptide vaccines to stimulate conventional αβ T cells for cellular immune responses have been mostly unsuccessful. However, activation of nonconventional T cells, such as γδ T cells that direct adaptive T cell responses, has received little attention for improving vaccines because it is not clear how best to prime γδ T cells for recall responses. Annotation of the bovine genome showed there were 13 WC1 molecules coded for by individual genes. This gene number is conserved among breeds and individuals and expression of the WC1 molecules are distributed among cells to form a number of γδ T cell subsets. Using RNA silencing, we have shown that the WC1 co-receptor contributes to the ability of γδ T cells to respond to Leptospira spp. The Leptospira-responsive γδ T cells are found within a subset of the serologically defined WC1.1(+) γδ T cell subpopulation and our data indicate that the WC1 molecules expressed act as pattern recognition receptors interacting directly with bacterial components. We are now extending this work to Mycobacteria bovis.