Splenic CD21^high IGM^high (MZ) B cells can quickly generate into plasma cells after in-vivo T-independent antigen stimulation

We have previously shown that CO21^high IgM^high mouse B cells enriched in the marginal zone (MZ) are unique in both phenotype and function compared to CD21^int. IgM^int. follicular (FO) B cells. In the VH81x transgenic mouse, a predominant heavy and light chain pair gives rise to a prevailing idiotype (Id) which is expressed in the majority of B cells in the MZ. These Id⁺ B cells function similarly to MZ B cells found in other normal mouse strains. Since Id⁺ B cells bind phosphorylcholine (PC), we investigated the response to a T-independent antigen from a heat killed Strep. pneumoniae vaccine in these transgenic mice. 24 hours following i.v. injection of the vaccine, syndecan^high, cytoplasmic μ^high, CD38^low, B220^low plasma cells were observed in the red pulp and this response peaked at 3 days. These plasma cells were derived from MZ B cells as shown by expression of the predominant Id found in the VH81x MZ B cell population. Furthermore, the MZ B cells incorporated more BrdU than FO B cells 3 days after immunization, indicating that these cells were undergoing more rapid cellular division. Finally, serum Ig levels indicated that Id⁺ Ig was secreted following immunization. Thus MZ B cells are able to quickly respond to T-independent antigens by differentiating into plasma cells and secreting antigen specific Ig.