Effective vaccination induces immune memory to protect animals upon pathogen re-encounter. Despite contradictory reports, bovine memory T cells are identified based on two isoforms of CD45, expression of CD45RO plus exclusion of CD45RA. In this report, we contrasted CD45RA/RO expression on circulatory T cells with IFNγ and IL4 expression induced by a conventional method. To our surprise, 20% of cattle from an enclosed herd did not express CD45RO on T cells without any significant difference on CD45RA expression and IFNγ or IL4 induction. In CD45RO expressing cattle, CD45RA and CD45RO expressions excluded each other, with dominant CD45RO (>90%) expression on gamma delta (γδ) followed by CD4+ (60%) but significantly higher CD45RA expression on CD8+ T cells (about 80%). Importantly, more than 80% of CD45RO expressing CD4+ and CD8+ T cells failed to produce IFNγ and IL-4; however, within the cytokine inducing cells, CD4+ T cells highly expressed CD45RO but those within CD8+ T cells mostly expressed CD45RA. Hence, CD45RO is not ubiquitously expressed in cattle, and rather than with memory phenotype, CD45RA/RO expression are more associated with distinct T cell subtypes.

Effective vaccination induces memory T cells, which protect the host against pathogen re-infections. Therefore, detection of memory T cells is essential for evaluating vaccine efficacy, which was originally dependent on cytokine induction assays. Currently, two isoforms of CD45 tyrosine phosphatase, CD45RO expression and CD45RA exclusion (CD45RO+/ CD45RA-) are used extensively for detecting memory T cells in cattle. The CD45RO+/CD45RA- markers were first established in humans around three decades ago, and were adopted in cattle soon after. However, in the last two decades, some published data in humans have challenged the initial paradigm, and required multiple markers for identifying memory T cells. On the contrary, memory T cell detection in cattle still mostly relies on CD45RO+/CD45RA- despite some controversial evidence. In this review, we summarized the current literature to examine if CD45RO+/CD45RA- are valid markers for detecting memory T cells in cattle. It seems CD45RA and CD45RO (CD45RA/RO) as markers for identifying bovine memory T cells are questionable.