B cell clones that sustain long-term plasmablast growth in T-independent extrafollicular antibody responses

Abstract

Some antigens induce Ab responses without T lymphocyte help. Among these, many polysaccharide-based antigens cause marginal zone B cells to proliferate and differentiate into plasma cells. B1 cells also respond to some of these antigens. In this article, we report that antigen-specific B1b cells, in response to the T-independent antigen (4-hydroxy-3-nitrophenyl)-acetyl (NP)-Ficoll, develop into clones that sustain Ab production for months with continued production of plasma cells and the accumulation of antigen-specific B cells in follicles. The persistence of this T-independent plasmablast response contrasts with the short-term plasmablast growth associated with T-dependent extrafollicular responses. The nature of the cells responding to NP-Ficoll was probed by using chimeras that have B1 cells but lack primary B lymphopoietic capacity and have very few B2 cells or T cells. The chimeras were constructed by transferring 10(5) IgM(+) IgD(-) peritoneal exudate cells into mice unable to produce their own T and B cells because of deficiency in recombinase-activating gene 1 (RAG-1). The chimeras mounted sustained IgM and IgG3 anti-NP Ab responses to NP-Ficoll. This finding was associated with continued NP-specific extrafollicular plasmablast growth and the accumulation of NP-specific B cells in follicles. B cells were not found in the marginal zones of chimeras, and they also lacked recirculating IgD(+) cells and CD3(+) cells. The absence of B2 and T cells confirms that hemopoietic cell chimerism leading to primary lymphopoiesis had not been established.

Fig. 1.

Evidence for persistent B cell clones that sustain extrafollicular responses to NP-Ficoll. (A) Immunohistological analysis of the splenic-specific Ab response of WT mice to NP-Ficoll. Blue triangles represent the number of NP-specific plasmablasts per mm² of section at intervals after immunization; red triangles depict the total number of NP-specific plasmacytoid cells per mm² (the sum of the numbers of NP-specific plasmablasts and NP-specific plasma cells). Triangles represent values from individual mice; continuous red and dashed blue lines connect median values. The number next to the day-0 red triangle indicates the number of superimposed points. (B) Construction of chimeras that have mature B cells but no capacity for B lymphopoiesis. Flow cytometry shows IgM and IgD expression of donor B220⁺ peripheral lymph-node cells (Left); 10⁷ lymph-node cells were transferred into each RAG-1-deficient recipient. IgM and IgD expression of splenic B cells 54 days after lymph-node cell transfer is shown (Right); the red circle highlights an emergent IgM^high IgD^low population not obvious in the donor population. (C) A spleen section from a chimera immunized on days 14 and 49 after cell transfer with NP-Ficoll and analyzed 6 days after the second immunization. NP-specific plasmacytoid cells (with strong blue cytoplasmic staining) are seen in an extrafollicular focus (ExF), whereas NP-specific B cells (with less strong blue surface staining) are seen among the orange-stained IgD⁺ recirculating B cells in a primary follicle (PF). (D) A similar section, this time identifying plasmablasts with cytoplasmic NP-binding (blue) and nuclear Ki-67 expression (brown) in an ExF. Some NP-specific B cells are seen in a secondary follicle (SF), and a proportion of these are Ki-67⁺; the other Ki-67⁺ cells in the follicle are germinal center B cells. (E) Plots similar to those in A, except that the responses of chimeras are followed. Groups of chimeras were immunized 14, 14 and 49, or 14, 49, and 72 days after construction by lymph-node transfer. Red symbols show the total numbers of NP-specific plasmacytoid cells per mm², and blue symbols show the numbers of NP-specific plasmablasts per mm². Red lines and dashed blue lines connect median values. The number next to the day-0 red circle indicates the number of superimposed points. (F) Similar plots to those in E, depicting data from the same chimeras but showing the numbers, in follicles and the T zone, of NP-specific B cells (red circles) and B blasts (blue circles) per mm² of section. Red lines and dashed blue lines connect median values.

Fig. 2.

The relative ability of different B cell subsets to render RAG-1-defficient mice responsive to NP-Ficoll. (A) Flow cytometry plots of the unsorted and sorted donor populations used to construct chimeras in RAG-1^−/− mice (a plot of donor peripheral lymph-node cells is shown in Fig. 1B). (B) NP-specific serum IgM and IgG3 Ab titers of chimeras immunized with NP-Ficoll on days 14 and 42 after cell transfer. The titers were determined on blood samples taken immediately before the first immunization, just before the second immunization, and 6 days after the second immunization. The symbols correspond to the donor cells used to construct the chimeras shown in A.

Fig. 3.

Transfer of 10⁵ PEC depleted of IgD+ cells enables RAG-1^−/− mice to sustain long-term responses to NP-Ficoll without restoring the recirculating and marginal zone pools of B2 cells. (A) Analysis of NP-specific cells in the spleen of a chimera constructed 48 days before with 10⁵ IgD-depleted PEC. The photomicrograph shows IgM⁺ plasmacytoid cells (large arrow). Stromal elements in a follicle (F) and marginal sinus (M) are stained blue for mucosal addressin cell-adhesion molecule expression; There is a cluster of NP-specific B cells in the follicle (small arrow), but none of these cells are located in the marginal zone, which lies outside the marginal sinus. Flow cytometry shows analysis of IgM, IgD, GL7, and CD138 expression by NP-binding cells gated into B220^high and B220^low subsets. The spleen was taken 48 days after cell transfer with immunizations with NP-Ficoll on days 14 and 42. (B) IgM and IgD expression in the spleen of a chimera constructed with 10⁵ IgD-depleted PEC. (C) IgM and IgD expression in the spleen of a chimera constructed with 10⁷ peripheral lymph-node (LN) cells. (B and C) The chimeras were also immunized with NP-Ficoll on days 14 and 42 after cell transfer and analyzed on day 48; The arrow points to clustered IgM⁺ plasmacytoid cells in the red pulp. T, T zone.