The mechanisms of maintenance of humoral immunity to infectious pathogens, particularly the contributions of memory B cells and long-lived plasma cells in maintaining specific serum antibody titres, are not well understood. Furthermore, it is not clear whether sequential heterologous humoral immune responses and disease pathology can result in the dysregulation and loss of previously acquired antibody-mediated immune responses to unrelated antigens. Here, depletion of memory B cells using anti-hCD20 monoclonal antibodies in hCD20 transgenic mice was used to dissect the role of memory B cells and long-lived plasma cells in maintaining long-term serum antibodies after intranasal Influenza A infection. Next, an experimental model of sequential infections with Influenza A/PR/8/34 and Plasmodium chabaudi chabaudi (AS) was set up, with a 15-20 week interval between the infections, in order to investigate whether sequential infection with P. chabaudi would affect pre-established humoral immunity to Influenza A. This study demonstrates that memory B cells are essential for the maintenance of long-lived serum Ab titres to Influenza A, as depletion of memory B cells results in the eventual loss of long-lived plasma cells and serum antibodies. Sequential infection with P. chabaudi results in the loss of pre-established serum antibodies to Influenza A by inducing the loss of long-lived plasma cells in an FcγRI,II,III-dependent manner, and this renders mice susceptible to secondary infection with Influenza A. However, this loss of pre-established humoral immunity is temporary, as serum antibodies do eventually return to normal levels. These findings demonstrate a mechanism shared by memory B cells and long-lived plasma cells which ensures that serum antibodies are maintained for long periods of time in the face of continuous generation and incorporation of new specificities throughout the lifetime of the host. A more complete understanding of the parameters that affect the longevity of immunological memory and how heterologous infections influence this will be vital in our understanding of the effect of continuous exposure to infectious pathogens on the efficacy and longevity of previously established immune memory
