Understanding the mechanisms that maintain protective antibody levels after immunisation is important for vaccine\r\ndesign. In this study, we have determined the kinetics of plasma and memory B cells detectable in the blood of\r\ncattle immunised with model T-dependent or T-independent antigens. Immunisation with the T-D antigen resulted\r\nin an expansion of TNP-specific plasma cells post-TNP primary and booster immunisations, which was associated\r\nwith increased titres of TNP-specific IgG antibodies. Although no TNP-specific memory B cells were detected in the\r\nT-D group following the primary immunisation, we detected an increase in the number of TNP-specific memory B\r\ncells post-TNP boost. In contrast, no TNP-specific plasma or memory B cells were detected after primary or\r\nsecondary immunisation with the T-I antigen. We then investigated if immunisation with a third party antigen\r\n(tetanus toxin fragment C, TTC) would result in a bystander stimulation and increase the number of TNP-specific\r\nplasma and memory B cells in the T-D and/or T-I group. TTC immunisation in the T-D group resulted in a small\r\nincrease in the number of TNP-specific plasma cells post-TTC primary immunisation and boost, and in an increase\r\nin the number of TNP-specific memory B cells post-TTC boost. This bystander effect was not observed in the\r\nanimals previously immunised with the T-I antigen. In conclusion, the present study characterised for the first time\r\nthe B cell response in cattle to immunisation with T-D and T-I antigens and showed that bystander stimulation of\r\nan established T-D B cell memory response may occur in cattle.
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