Timing of activation of CD4+ memory cells as a possible marker to establish the efficacy of vaccines against contagious agalactia in sheep

Mycoplasma agalactiae is a major pathogen of sheep and goats in many areas of the world and particularly in Mediterranean countries. It causes contagious agalactia, an infectious disease primarily affecting mammary glands. Many vaccines against the pathogen are currently under development. The aim of the study was to investigate the involvement of T cell-mediated immunity during vaccination and challenge experiments against Mycoplasma agalactiae. A comparison of the antigen-specific expansion of interferon gamma positive T cell memory and naïve subsets was performed between vaccinated and non-vaccinated sheep to identify cellular subsets whose activation was different between protected and non-protected sheep. Data reported in this manuscript demonstrated that two out of the three vaccines used in this study protected sheep from the disease. In the protected groups CD4+ memory interferon-γ+ T cells underwent an early expansion (p < 0.05 when compared to unprotected groups), whilst memory CD8+ Interferon-γ+ T cells increased in non-protected animals 7 days after infection (p < 0.05). γδ+ Interferon-γ+ T cells reached peaks of expansion in infected and in two vaccinated groups thus indicating that these cells are not preferentially involved in protection or pathogenesis (p < 0.05).Hereby we propose that the early activation of CD4+ memory Interferon-γ+ T cells could be considered as a marker of protection from the disease as well as a tool to establish vaccine efficacy.